scientific american - 1995 04 - machines that learn from hints

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APRIL 1995

$3.95

Virtual museum of digitized art exists only inside a computer.

Machines that learn from hints.

Why frogs are vanishing.

Where the solar wind ends.

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April 1995 Volume 272 Number 4

52

64

72

106

The Puzzle of Declining Amphibian Populations

Andrew R Blaustein and David B Wake

Machines That Learn from Hints

Yaser S Abu-Mostafa

Understanding the Genetic Construction of Behavior

Ralph J Greenspan

J R Jokipii and Frank B McDonald

S CIENCE IN PICTURES The Art HistorianÕs Computer

Lillian Schwartz

Frogs, toads and salamanders survived even the catastrophes that Þnished the nosaurs, yet recent censuses suggest that many species are now mysteriouslydwindling or disappearing The destruction of their natural habitats, pollution, dis-ease, changes in the ozone layer and even tastes in haute cuisine may be at the bot-tom of this ominous development

di-Computer scientists know how to build machines that can learn from examples,but how can those machines learn more eÛciently? HereÕs a hint: give more of theright background information Although computers, unlike humans, do not intu-itively understand much about the real world, hints in the form of instructive ex-amples can teach them important principles

Beware of simplistic statements about the genes for complex human traits; the tual state of knowledge about behavioral genetics is crude Consider what has beenlaboriously discovered about one well-deÞned behaviorÑcourtshipÑin the fruit ßy

ac-Drosophila melanogaster All the results suggest that even in relatively simple

or-ganisms, behavior is inßuenced by a multitude of genes

Did Leonardo da Vinci complete the Mona Lisa as a self-portrait ? Is Queen

Eliza-beth I hiding inside an engraving of Shakespeare? Computer graphics can times Þnd the answers to questions that confound more traditional analyses

some-Far beyond PlutoÑno one yet knows how farÑthere is a discontinuity in the vacuum of space It marks the edge of the heliosphere, where the diÝuse solarwind collides sharply with the tenuous interstellar medium Some unusual cosmic

near-rays that bathe our planet originate in this region Now the Pioneer and Voyager

spacecraft, their original missions completed, are heading there

2

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118

124

The Tapestry of Power in a Mesopotamian City

Elizabeth C Stone and Paul Zimansky

50 and 100 Years Ago

1945: High-tech metallurgy

1895: The Þrst fax machine

140135

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12

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Letters to the Editors

This is no joke: real mail from real readers

Book Review: Daniel L Schacter

Critical looks at Òrepressed memoriesÓ of abuse

Essay:Jeremy Bernstein

Making Quot a fuss over intelligence

T RENDS IN HEALTH CARE The Price of Prevention

Kristin Leutwyler, staÝ writer

A Brief History of InÞnity

A W Moore

rights reserved No part of this issue may be reproduced by any mechanical, photographic or electronic process, or in the form of a phonographic recording, nor may it be stored in a retrieval system, transmitted or otherwise copied for public or private use without written permission of the publisher Second-class postage paid at New York, N.Y., and at additional mailing offices Canada Post International Publications Mail (Canadian Distribution) Sales Agreement No 242764 Canadian GST No R 127387652 Subscription rates: one year $36 (outside U.S and possessions add $11 per year for postage) Postmaster: Send address changes to Scientific American, Box 3187, Harlan, Iowa 51537 Reprints available: write Reprint Department, Scientific American, Inc., 415

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The concept of inÞnity has been boggling minds for at least 2,000 years The ophers Zeno and Aristotle did their best to sidestep it; the Pythagoreans were hor-riÞed by its inescapability; mathematician Georg Cantor came closest to taming it,but some questions remain The truly boundless may be beyond comprehension

philos-Archaeologists have usually assumed that the worldÕs Þrst cities had centralized,authoritarian social structures But detailed studies of Mashkan-shapir, a site inIraq, indicate that its rich and poor citizens lived cheek by jowl and that politicsand religion were peripheral

Health care policymakers on the lookout for medical cost-savings are in for a rude

shock: an ounce of prevention is not always worth a pound of cure The grim truth

is that treating the general population to prevent disease now is usually more pensive than paying to treat the sick later That fact leaves physicians, politiciansand the public facing some uncomfortable choices

ex-D E PARTM E N T S

The Kobe quakeÕs engineering sons Oil rigs take to deeper wa-ters Muscle maladies An anti-addiction drug Homosexuals inscience X-ray of solar ßares

les-Atoms that act like light

The Analytical Economist

A little statistics can be dangerous

Technology and BusinessThe uncertainty of gene patents

New aviation technology: still up

in the air Diamonds for machines Fish food in Calcutta

micro-ProÞleEngineer Arati Prabhakar takes on Congress

Computers rescue art

of the Young Masters

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8 SCIENTIFIC AMERICAN April 1995

THE COVER depicts a virtual museum, aninÞnitely expandable gallery of great artthat can exist only within a computer The3-D image was constructed using InfiniÐD™

2.6 Four Macintoshes took approximately

86 hours to render the image Among the

paintings are Self-Portrait and the Mona Lisa, by Leonardo da Vinci; The Siesta after Millet, by Vincent van Gogh; Self-Portrait, by Lillian Schwartz; and Portrait of My Sister,

by Jamie S Feigenbaum ( See ỊThe Art torianÕs Computer,Ĩ by Lillian Schwartz,page 106.) Image by Slim Films

His-¨

Established 1845

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L Rusting, Associate Editor; Timothy M ley ; W Wayt Gibbs; John Horgan , Senior Writer; Kristin Leutwyler; Philip Morrison , Book Editor;

Beards-Madhusree Mukerjee; Sasha Nemecek; Corey S Powell ; David A Schneider; Gary Stix ; Paul Wal- lich ; Philip M Yam; Glenn Zorpette

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Breaking Research

If you believe that at some point in

the future mankind will develop a

vir-tual-reality technology that allows a user

to experience a virtual life while

believ-ing it to be 100 percent genuine, how

can you know whether the life you are

living now is real or virtual? You canÕt

But IÕve created a formula that

deter-mines the exact chance that your life is

either real or virtual Remarkably, the

answers, using conservative values, have

generally ranged from 25 to 50

per-centÑa one-quarter to one-half chance

that our lives are virtual, not real I look

forward to working with you on this

project

DAVE PACHECO

Richmond, Calif

For every evolutionary advance, there

is an equal and opposite regressive

step to be oÝset At present, the L

biÞ-dus in motherÕs milk provides a

turn-ing point in evolution, protectturn-ing us

from returning to something like

Nean-derthal man or even the monkey The

existence of Ònegative evolution,Ó which

might broadly be described as

Òiner-tia,Ó is being ignored

J GORDON ROBERTS

Clearwater, Fla

Having discovered the ÒHarmonic

Cube,Ó I will wager $10,000 that I am

the wisest human of all time

GENE RAY, Cubic

St Petersburg, Fla

Out in Space

I am of the opinion that cosmological

attraction, or gravity, does not exist I

believe that a cosmological repulsion

force exists and that what we perceive

as gravity is merely a shadowing of this

force by one celestial body on another

The expanding universe and the

Hub-ble constant are thereby more readily

explained

BRIAN DAVIDSON

Donegal, Ireland

The news story ÒGone with a Bang,Ó

by Corey S Powell [SCIENTIFIC

AMERI-CAN, September 1994], describes away pulsarsÓ with an average velocity

Òrun-of 450 kilometers per second, enough

to escape from the Milky Way Scientistscannot explain this phenomenon Theobvious explanation: space travel by in-telligent beings By using a pulsar for agiant spaceship, an entire solar systemcan travel anywhere in space Entire civ-ilizations can have all the comforts oftheir home planet during the journey

LEROY PETERSONMesopotamia, Ohio

WhatÕs Past Is Prologue

If you and some others with a nary go into a huddle with the intent toÞnd and publish some proof that timetravel is not impossible, then the dic-tionary will prevent anything from get-ting out, unless the dictionary contra-dicts itself

dictio-MARC CUNNINGHAMBaton Rouge, La

After reading your article ÒThe tum Physics of Time Travel,Ó by DavidDeutsch and Michael Lockwood [SCIEN-TIFIC AMERICAN, March 1994], I thoughtyou would be interested in the follow-ing information A well-known psychicartist has been placed in charge of anexperimental group that is supposed to

Quan-be able to record on videotape tently clear scenes and sound from ei-ther past, present or potential future

consis-The locations of these incidents can becloseups of anywhere in our Milky Waygalaxy, even inside spacecraft and in-side dwellings on any planet

JEFFREY BLUNTSpringÞeld, Mass

The Tough Questions

I am researching the eÝects of cal notes on the chemical elements, re-lating how certain sounds of the musi-cal scale could inßuence chemicals Ex-ample: What note on the musical scalecould change or alter the molecularstructure of the element sulfur?

musi-DON DREISBloomington, Ind

We have heard that humans are afluke of the universe But what I want

to know is: Does charge-parity tion mean that the universe is a fluke

viola-of itself ?

JOHN W WALLSan Francisco, Calif

I failed to understand David Z bertÕs article ÒBohmÕs Alternative toQuantum MechanicsÓ [SCIENTIFIC AMER-ICAN, May 1994] It does not appear tocontain a description of the proposedalternative Do you have editors?

Al-MICHAEL WALSH

St Genis, France

AdamÕs Rib Revisited

As a sometime recipient of the MaleChauvinist Pig of the Year Award in thestate of Victoria (where standards arehigh), may I felicitate you on having thecourage and guile to publish the ÒTrends

in WomenÕs HealthÓ article, ÒA GlobalView,Ó by Marguerite Holloway [SCIEN-TIFIC AMERICAN, August 1994] As pro-viding evidence that women are inca-pable of objectivity or indeed any de-gree of rational thought, that they arecredulous victims of fashion and whol-

ly devoid of any critical capacity, thatthey confound sententious drivel withclear writing, that they are, in short,brainless bimbos down to the last indi-vidual, the article is faultless

MICHAEL ALDERNedlands, Western Australia

Not EverybodyÕs a CriticFor the life of me, I cannot under-stand why you people do not run anannual letters issue It would be a better

seller than the Sports Illustrated

swim-suit issue

MINAS ENSANIANBuÝalo, N.Y

Letters selected for publication may

be edited for length and clarity licited manuscripts and correspondence will not be returned or acknowledged unless accompanied by a stamped, self- addressed envelope.

Unso-10 SCIENTIFIC AMERICAN April 1995

LETTERS TO THE EDITORS

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APRIL 1945

Wool, which retains its original

ap-pearance, yet is protected against

shrinkage, even after repeated

launder-ing and dry-cleanlaunder-ing, is now

commer-cially available through the use of a

new synthetic resin known as Lanaset

When applied to the fabric, Lanaset

sta-bilizes wool and wool blends without

aÝecting the absorbency normally

char-acteristic of wool It also reduces

felt-ing and prevents fuzzfelt-ing.Ó

ÒMetallurgists, in the examination of

metal surfaces with the optical

micro-scope, have long recognized serious

weaknesses in this procedure The

sur-face under observation very frequently

does not oÝer adequate evidence of the

true shape of the details and in many

cases is even misleading However, a

newly developed microradiographic

technique is able to give an indication

of the three-dimensional contours of

the metal structure and is able to

indi-cate the distribution and identity of the

chemical components of the metal in a

precise manner.Ó

ÒA recent development that

increas-es ßight safety by keeping airplanincreas-esÕ

propellers free of ice consists in

paint-ing or spraypaint-ing the propeller blades

with a chemical lacquer called Icelac,

which, black in color, has a consistency

something like that of glycerine, and

paints or sprays freely to give a shiny,

tacky surface A satisfactory icing

pro-tective surface is maintained for

sever-al thousand hours.Ó

ÒLess than half the farms of the

Unit-ed States have electricity available True,high-tension lines have brought thisversatile servant to hundreds of thou-sands of farms, but the market has ac-tually only been barely touched High-tension lines are expensive and cannot

be run everywhere.Ó

ÒAfter standing overnight in zero gree weather, buses now receive a quickboost in temperature from a ÔJanitrolÕportable heater of the type developed

de-to preheat airplanes at Alaskan air

bas-es A heat rise of 230 degrees enablesthe appliance to deliver positive heat insub-zero temperatures.Ó

APRIL 1895

Under the combined inßuences ofgreat pressure and intense cold,hydrogen has at last surrendered andbeen liqueÞed Hydrogen has hithertomost strenuously resisted all attempts

at liquefaction, and the fact of its duracy in this respect having now beenovercome removes the only gaseous el-ement known to us which has not beenliqueÞed Until, therefore, more at-tenuated gases are added to the list

ob-of chemical simplicities, no furtherdiscoveries on this particular line

of research can be hoped for.Ó

ÒA simple pneumatic cushion,with a soft touch to the ear, hasbeen adapted to Þt all telephonereceivers It is made of softrubber, Þtted into a metal rimwhich springs or clamps overthe end of the receiver, forming acomplete air chamber designed toeffectually prevent the buzzing

or clucking sounds so annoying

to users of the telephone.Ó ÒMuch has been written as tothe picture that the compoundeye of insects produces upon thebrain or upon the nerve centers It

is obvious from the structure ofthese compound eyes that impres-sions through them must be verydifferent from those received throughour own In point of fact, experimentshave practically established that while

insects are shortsighted and perceivestationary objects imperfectly, theircompound eyes are better Þtted thanthe vertebrate eye for apprehending ob-jects in motion, and they are likewisekeenly sensitive to color.Ó

ÒAn ideal school room should vide Þfteen square feet of ßoor spacefor each pupil and a supply of 200 cu-bic feet of air per minute for every per-son in the room Such provisions wouldensure the free movement of everychild and a wholesome amount of air

pro-In France, the perfect school room, it isthought, should have a window areaequal to one-fourth the ßoor space It

is also thought best to have individualseats and desks for the pupils.ÓÒWhen the telephone was introduced

to the attention of the world, and thehuman voice was made audible milesaway, there were dreamy visions of oth-

er combinations of natural forces bywhich even sight of distant scenes might

be obtained through inanimate wire Itmay be claimed, now, that this same in-animate wire and electrical current willtransmit and engrave a copy of a pho-tograph miles away from the original

As shown in the accompanying tions, the electro-artograph, named byits inventor, Mr N S Amstutz, willtransmit copies of photographs to anydistance, and reproduce the same atthe other end of the wire, in line en-graving, ready for press printing.Ó

illustra-50 AND 100 YEARS AGO

The electro-artograph receiver The electro-artograph transmitter

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The quake that struck Kobe, Japan,

on January 17 could hardly have

been better timed for impact,

lit-erally, on the structural engineering

community When it hit, dozens of

en-gineers were jolted awake in nearby

Osaka, where a Japanese-U.S workshop

on ÒUrban Earthquake Hazard

Reduc-tionÓ was to begin later that morning

Had they not been so rudely roused, the

engineers would have heard case

stud-ies from a comparable quake that had

rocked Northridge, Calif., exactly one

year earlier

The Kobe quake was the second most

devastating in Japanese history

Barrel-ing through a densely populated port

city that had not been thought to be

es-pecially at risk, it killed more than 5,000

people and destroyed or damaged some

50,000 buildings ÒThere is 10 times as

much data as what weÕve gotten from

California, because thereÕs been 10 times

the destruction,Ó says Charles Kircher,

a structural engineering consultant in

Mountain View, Calif., who was at theOsaka meeting ÒThereÕs enormouslearning potential here.Ó

Much has been made of the

diÝerenc-es in philosophy between JapandiÝerenc-ese andAmerican engineers: the former suppos-edly emphasize strength; the latter duc-tility, the quality that lets a structurebend and deform But they share fun-damental similarities Whether in Tokyo

or Los Angeles, steel-and-concrete

con-struction combines the metalÕs ability

to withstand tension with the mortarÕsresistance to compression This ap-proach underlies nonductile concrete,

in which steel bars run longitudinallythrough structural elements

In 1971 an earthquake in the San nando Valley of California called atten-tion to the methodÕs basic shortcom-ing, namely, that powerful forces cancause the concrete to shatter and fallaway from the inner steel Within a fewyears, new building codes called forsteel-reinforced concrete, in which sep-

Fer-arate retaining hoops, known as Þnement steel, encircle the longitudinalpoles to hold the concrete in place Sim-ilar changes occurred in Japan after apair of earthquakes, one in Tokachi in

con-1968 and the other in Miyagi ture a decade later

prefec-Before these improvements, though,tens if not hundreds of thousands ofnonductile concrete buildings had beenconstructed in Japan, and KobeÕs faredpoorly in the earthquake, according toseveral U.S engineers who toured thecity The good news is that most of thestructures put up after the early 1980sÑincluding ones based on steel-reinforced concrete and otherserected around steel framesÑsurvived without substantial vis-ible destruction ÒI saw dozensand dozens of those buildingsthat had no damage apparentfrom the outside,Ó says Loring

A Wyllie, Jr., a senior principalwith Degenkolb Engineers, aSan Francisco structural engi-neering Þrm

It will be months before amore complete picture of thedamage emerges from Kobe,and there will undoubtedly besurprises Only recently has itcome to light, for example, thatsteel-frame buildings suÝeredmore harm in the Northridgequake than had been thought.Such structures are consideredmore pliant than ones based onsteel-reinforced concrete andtherefore more resistant topowerful earthquakes

A study by the Chicago-basedAmerican Institute of Steel Con-struction, however, has foundthat roughly a quarter of the

400 steel frames in the immediate area

of the Northridge quake suffered cracks

in the welds at the joints where columnsmeet girders The buildings ranged inage from one to 31 years, and althoughnone of them visibly listed or showedany other sign of distress, a number ofthem have been evacuated for repairs,and one was torn down

Improvements are needed in the waysteel frames are being built, states James

O Malley, a principal with DegenkolbEngineers Exactly how this might bedone is the point of a $2.3-million proj-

SCIENCE AND THE CITIZEN

Bracing for the Next Big One

Engineers grapple with retroÞtting Japanese and U.S buildings

SAGGING STRUCTURE in Kobe, Japan, is testament to the January earthquakeÕs ravagesÑ

and the need to combine architectural strength with ßexibility.

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ect nearing completion by a group

called the SAC Joint Venture,

consist-ing of the Structural Engineers

Associa-tion of California, the Applied

Technol-ogy Council and some California

re-search universities

With funding primarily from the

Fed-eral Emergency Management Agency,

the collaboration has investigated such

issues as joint design, welding

practic-es and materials, frame dpractic-esign and the

minimum number of beams and umns, or Òredundancy,Ó needed for ßex-ible support It plans to issue interimguidelines in May ÒA lot of stuÝ has tochange,Ó Malley notes

col-Meanwhile California has ordered theretroÞtting or at least investigation ofthousands of unreinforced masonrybuildings to ensure that they will notcollapse in a moderately strong quake

Of course, that still leaves thousands of

nonductile concrete structures, which,

so far, neither California nor Japan hasmustered the political will to address.ÒOur technological capabilities reallyexceed what societyÕs willing to pay for,Óobserves Craig Comartin, an engineer-ing consultant in northern California.ÒIn some cases, retroÞtting buildings isextremely costly IÕm not whining; itÕsjust a fact ItÕs a pay-me-now or pay-me-later kind of thing.Ó ÑGlenn Zorpette

Announcing his retirement from

competitive bicycle racing last

December, Greg LeMond brought

to a close a career marked by sublime

athletic achievement He had won the

Tour de France three times and the

world championship twice; some of

these victories came after his near death

in a 1987 hunting accident LeMond, at

age 33, has now also entered medical

record books, becoming the Þrst elite

athlete to be diagnosed with

mitochon-drial myopathy, a disorder that impairs

muscle He reported that, for

mysteri-ous reasons, his skeletal muscle cells

could no longer use oxygen to produce

the energy required for him to perform

at peak capacity

ÒGreg is the Þrst trained athlete to be

diagnosed with this condition,Ó says

LeMondÕs physician, Rochelle Taube of

the Minneapolis Sports Medicine

Cen-ter ÒUsually people with mitochondrial

myopathy can barely move, or they are

children who die of the disease.Ó Taube

emphasizes, however, that LeMondÕs

myopathy should not prevent him frompursuing an active life

The diagnosis raises questions about

a disorder whose nature and prevalenceare just now yielding to medical in-quiryÑand it underscores the diÛcul-ties physicians face when attempting

to determine the maladies of athletes

Taube believes LeMond suÝers from apreviously unknown, yet not necessari-

ly uncommon, form of this debilitating

disease She is planning to study

wheth-er the onset of mitochondrial thy in athletes is related to particularsports and levels of exertion or to theindividualÕs genetic makeup

myopa-In general, it is diÛcult to explainwhy an athlete suddenly loses the abili-

ty to compete successfully or, worse,completely breaks down physically Forinstance, a marathon runner complain-ing of fatigue might score far abovenormal on various exercise stress testsbut actually be experiencing the Þrstsymptoms of a serious ailment ÒIt ispossible,Ó Taube says, Òthat there are

changes in your metabolism and mune system if you overtrain You maychange your bodyÕs ability to adapt andleave it open to illness.Ó

im-The condition LeMond is thought tohave diÝers markedly from exertionalmyopathy, a more commonly knownmuscle disorder that can aÝect anyonewho exercises beyond his or her capac-ity High school football players andmilitary recruits seem especially suscep-tible, perhaps because many of themstart vigorous activity in hot weatherwithout much advance training Dehy-dration makes it tougher for muscles torecover and for the body to purgeitself of wastes

Reports have also tied exertionalmyopathy to HIV infection, as well

as to the use of cocaine, phetamines, LSD, alcohol and vari-ous prescription drugs Some ofthese drugs allow people to engage

metham-in repetitive activities for long ods without proper rest or ßuidconsumption As people overexer-cise, lactic acid builds up, and lev-els of muscle enzymes, such as cre-atine kinaseÑessential to musclecontractionÑrise dramatically Usu-ally the resulting soreness passes,and enzyme levels return to normal

peri-in a few days, but peri-in acute cases,the muscle cells rupture, ßoodingthe bloodstream with myoglobin,enzymes and minerals

Because exertional myopathygenerally occurs when muscles areburning energy faster than they arebeing resupplied, the problemshould vanish as a personÕs Þtness im-proves MitochondriaÑwhich use oxy-gen to produce adenosine triphosphate(ATP), which, in turn, fuels cellsÑin-crease in so-called slow-twitch musclesduring training, allowing them to pro-cess more oxygen more eÛciently forlonger periods without lactic acid build-

up and muscle cell damage

Although mitochondrial myopathyalso aÝects the skeletal muscles, it is afar diÝerent disorder, notes John ShoÝ-ner of Emory University Still a medicalenigma, mitochondrial myopathy is one

of a broad class of oxidative

phosphory-CHAMPION GREG L EMOND retired this winter because of a muscle disorder.

The End of the Road

Is a new malady aÜicting elite athletes?

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lation diseases that disrupt the cellular

energy system in skeletal muscles, the

liver, heart or brain and have been

im-plicated in diabetes, aging and a

num-ber of neurodegenerative disorders

Ba-sically, mitochondria fail to process

enough oxygen to make suÛcient ATP

Lacking aerobic capacity, some people

with mitochondrial myopathy Þnd

climbing stairs or walking the length of

a shopping mall fatiguing Others might

not even be aware that they have a

problem until they begin to exert

them-selves The condition is manifest

clini-cally in exercise intolerance, muscle

weakness, a type of cellular

degenera-tion called ragged red Þber myopathy

and increased numbers of abnormal

mitochondria A live muscle biopsy is

required to observe the last two signs

ShoÝner claims that most cases are

inherited (from the mother, the sole

source of mitochondrial DNA) In some

instances, environmental toxins such as

high levels of lead or carbon monoxide

from cigarette smoke or ingredients in

certain medicationsÑAZT, used for the

treatment of AIDS is oneÑcan induce

myopathy by damaging the

mitochon-dria Since 1988 over 30 genetic pointdeletions and even more mutationshave been found in mitochondrial DNA

By standard deÞnition, it is able that an athlete engaged in endur-ance sports could suÝer from mitochon-drial myopathy But Taube says that al-though LeMond shows none of thegenetic or enzymatic deÞciencies com-monly associated with the disorder,Òvery subtle changesÓ in his muscle cellsblock their ability to use oxygen when

inconceiv-he works hard Sinconceiv-he speculates that tinconceiv-he

lead pellets he still carries in his bodyfrom his hunting accident might relate

to the onset of the condition

While recognizing that something isadversely aÝecting LeMondÕs perfor-mance, certain specialists in the Þeldhave reacted with understandable cau-tion to TaubeÕs description of a newform of mitochondrial myopathy Right

or wrong, though, three other ance athletes have contacted Taubesince December saying they have thesame symptoms ÑMark Derr

endur-20 SCIENTIFIC AMERICAN April 1995

Like viewers watching a Chinese

shad-ow puppet play, some mers are Þnding themselves cap-tivated not by light but by darkness Theluminous stars and galaxies sprinkledacross the night sky are the obviousplayers in the cosmic drama But it turnsout that the vast stretches between gal-axies have a story to tell as well

astrono-Nadine Dinshaw of Steward

Observa-tory in Tucson, Ariz., and her ers have found that the thin gas perme-ating those voids is not a formlesssmear but rather is organized into hugeclouds Visible only in silhouette, thesenonluminous clouds may be part of aghostly network of sheets and Þlamentsthat Þlls the universe and traces theprocesses by which galaxies formed.Astronomers deduced the presence of

co-work-Astronomers in the Dark

ThereÕs more to empty space than meets the eye

X-ray images of the sun offer a new view of the nearby

star and its cycles—one that differs markedly from the

more familiar images made using visible light Recent

x-ray pictures from the Japanese Yohkoh spacecraft reveal a

striking dimming of the sun’s corona, its hot outer

atmo-sphere, between 1992 (left ) and this year (right ) This

change reflects the fluctuations of the 11-year solar cycle,

as the star evolves from its period of maximum activity in

1991 to one of minimum action, which should occur by

1997 The cycle is characterized by shifts in the number

of sunspots; these perturbations, which can be detectedwith standard imaging techniques, increase in numberwhen solar activity is greatest The x-ray data have shownthat the corona changes as well: it is 30 times dimmernow than it was three years ago near the height of the so-

Sun Spotting the DiÝerence

Trang 10

intergalactic clouds a quarter of a

cen-tury ago, in the course of analyzing the

radiation from faraway quasars (active

galaxies whose brilliant central regions

can easily be seen from across the

uni-verse) When that radiation is spread

out into a spectrum, researchers found,

certain characteristic wavelengths are

absent The pattern of the missing

wave-lengths revealed the culprit: clouds of

hydrogen atoms scattered between the

galaxies, which absorb some of the light

from any object lying farther away from

the earth Each quasar shines through

only a single part of a cloud, however,

so the structure and extent of these

cosmic will-oÕ-the-wisps remained known The absorbed radiation is most-

un-ly in the ultraviolet part of the spectrum,which does not penetrate the earthÕsatmosphere

DinshawÕs team circumvented those

problems by using the Hubble Space

Telescope to look at not one but two

light sources: two quasars that are arated in the sky by about one twenti-eth the angular diameter of the fullmoon (which, in this line of work, is a

sep-fairly wide separation) Because Hubble

orbits above the earthÕs atmosphere, itcan detect the ultraviolet rays invisible

to ground-based observatories

When Dinshaw and her collaboratorslooked at the results of their obser-vation, she recalls, they were very sur-prised: they had found a monster Pre-vious research had hinted that interga-lactic clouds were about 100,000 light-years across, or about the diameter ofthe Milky Way galaxy Yet the spectra

collected by Hubble indicated that a

single cloud formation stretched acrossboth quasars, giving it a minimum di-ameter of one million light-years ỊNo-body expected the clouds to be solarge,Ĩ Dinshaw comments The cloudalso had remarkably little internal mo-tion, suggesting it is a settled structure,not collapsing or ßying apart

ỊHow can the cloud be so large and

so quiescent?Ĩ Dinshaw wonders orists are asking the same question Ifthe clouds are held together by the grav-ity from invisible Ịdark matter,Ĩ theyshould collapse to a smaller size Ifthey are bound to a central galaxy, theyshould be moving faster If they areheld together by the pressure of the in-tergalactic medium, such large forma-tions should have rapidly dissipated.ỊThereÕs no well-developed theory toexplain the kind of cloud we see,Ĩ con-cludes Craig B Foltz of Multiple MirrorTelescope Observatory, also in Tucson,who collaborated with Dinshaw.Foltz suspects that he and his col-leagues are watching many kinds ofevents happening at the same time,among them giant shock waves com-pressing gas clouds, hydrogen wispscollapsing around young galaxies andgas collecting around clumps of darkmatter in regions where no galaxies ex-ist ỊWeÕre seeing that the process ofgalaxy formation is very complex,Ĩ and

The-so the surrounding material takes onsimilarly complex properties, Foltz ex-plains ỊBut thatÕs okay with meĐIdonÕt mind complexity!Ĩ

DinshawÕs team has already examinedtwo other quasar pairs and plans to look

at a third Such observations shouldhelp theorists Þne-tune their cosmolog-ical models by revealing what is occur-ring in all parts of the universe, not justthe well-lit corners The early indica-tions are that the dim regions are rich

in unexplored details, Ịand itÕs only cently that weÕve been able to do obser-vations like this,Ĩ Dinshaw notes.Such discoveries testify to the endur-ance of one of astronomyÕs most pow-erful but least glamorous toolsĐspec-troscopy, which Foltz describes asỊlooking at the things that you donÕtsee.Ĩ A century ago the technique en-abled astronomers to tell the poets whatthe stars are made of Now it is broad-ening awareness of the hidden order ofthe universe ĐCorey S Powell

re-A Ringside View of Stars

An unusual collision between galaxies has created a halo of stars ripe for

study—and the Hubble Space Telescope recorded it all Although the

crashing of galaxies and the subsequent formation of new stars is common,

such impacts are difficult to decipher “They often just leave behind a

mixed-up mess,” explains Kirk Borne of the Space Telescope Science Institute

Yet when the Cartwheel galaxy (left )—located some 500 million

light-years away—was jolted by one of two nearby galaxies (right ), no mess

en-sued “What makes this case unique is that the smaller galaxy basically hit

the bull’s-eye,” Borne notes “When the collision occurred, it sent a shock

wave of energy outward, like a rock making a circular ripple when it’s thrown

into a pond.” As the wave traveled, it compressed gas and matter in its

wake, spewing billions of stars in an encircling band at the point of impact

Because the ring around the galaxy is composed entirely of newly formed

stars, cosmologists have an unprecedented chance to study a uniform

popu-lation of massive stars—all born at about the same time under the same

con-ditions Identifying which of the two neighboring galaxies was responsible

for the collision, however, will prove more tricky —Steven Vames

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The more than one million

alco-holics in this country who seek

treatment every year have a new

alternative The U.S Food and Drug

Ad-ministration recently approved

naltrex-one as a medication for alcoholism; the

drug is the Þrst allowed for this purpose

since the original medication for

alco-holism, Antabuse, was introduced in

1948 The FDÃs action reßects a

grow-ing belief that substance abuse has a

strong biological component and is not

merely a character ßaw Yet experts phasize that the drug will not obviatethe need for conventional forms of ther-apy, such as counseling

em-Two recent studies, carried out at theUniversity of Pennsylvania and at YaleUniversity, demonstrated the eÝective-ness of naltrexoneĐwhich was approvedfor the treatment of opiate addiction in1984Đin the rehabilitation of alcoholics

In one study of 70 patients, 23 percent

of those given naltrexone relapsed

dur-ing the three-month-long study,

wheas 54 percent of placebo recipients sumed drinking Joseph R Volpicelli,who led the team of researchers at theUniversity of Pennsylvania, attributesthe success of naltrexone to its ability

re-to reduce the euphoria of alcohol anddampen the craving for another drink.ỊAbusers respond to alcohol diÝer-entlyĨ than do other people, Volpicellisays ỊAs they drink, the motivation tohave more increases.Ĩ Volpicelli refers

to this phenomenon as the Ịcorn chipeÝectĨĐthat is, you canÕt have just one.When alcoholics imbibe, their brains re-lease unusually elevated levels of mole-

I Get No Kick from CH 3 CH 2 OH

A new treatment for alcoholism receives FDA approval

Ianswered the telephone one night recently to hear the

voice of an old friend from graduate school in physics

After chatting about research, he mentioned his reason for

calling: he had come out as homosexual During school,

he had had an inkling of his orientation, he explained;

only recently had he felt secure enough professionally to

devote time to personal issues “I thought you had been

interested in me!” I blurted out, and he chuckled

After I put the phone down, questions lingered It had

never occurred to me that a colleague was gay Surely I

should have known better If, as surveys suggest, between

1 and 10 percent of the population is homosexual, how was

it that I was aware, now, of just one physicist who was? I

could think of great novelists who were gay but of only a

sole scientist, Alan M Turing Were homosexuals

underrep-resented in physics, in science? Were they more closeted?

None of the researchers I called in subsequent months

could identify a study of homosexual scientists To

con-duct my own unscientific survey, I asked my friend to

post questions on his electronic-mail network of some 50

gay astrophysicists I polled two other bulletin boards—

the 200-member Lesbians in Science, directed by

physi-cist Elizabeth Zita, and the 800-member National

Organi-zation of Gay and Lesbian Scientists and Technical

Profes-sionals (NOGLSTP) I also interviewed other gay scientists

The responses were bewilderingly varied; only slowly did

patterns emerge “I know only two others in my field who

are openly gay,” wrote Thomas Eads, a biophysicist at due University Yet everyone opined that the apparent rar-ity was misleading “There just seem to be fewer of uswho are out,” remarked Ron Buckmire, a mathematician atOccidental College

Pur-“Gays in science tend to be very discreet,” agreed MikelSusperregi, a cosmologist at the University of Oxford

“Those in the arts are much more conspicuous.” BarbaraBelmont of NOGLSTP noted that “science is a professionyou can immerse yourself in, forsaking all social life It pro-vides a haven from society’s pressures.” Several research-ers argued that science, in fact, attracts closeted gays.Moreover, many of those who wanted to come out sawthe sciences as unreceptive “You look around, and youthink, ‘Gosh, this is a male-dominated field I’d better keep

my mouth shut,’ ” my friend said “I find people in thehard sciences are much more intolerant,” responded JuliaGeorge of the Jet Propulsion Laboratory in Pasadena, Calif

“Undoubtedly because there are many fewer women.Women, as a rule, seem to be more accepting.”

That notion is supported by a 1982 poll of physiciansconducted by William C Matthews of the University ofCalifornia at San Diego and others Replies showed thatwomen are better disposed toward homosexuals than aremen Indeed, NOGLSTP’s “outlist” of 100 or so researchers(admittedly a small sample) showed few physicists andmathematicians, more astronomers and chemists and the

most biologists—inrough proportion tothe number of women

in each field

Gays who were outreported mixed experi-ences “My colleagues’reception has been sochilling,” Eads stated.One professor who islesbian observed thatmale colleagues re-spect her talents morereadily than otherwomen’s because “theydon’t think of me as areal woman, so it’s not

so weird that I can doscience.”

—Madhusree Mukerjee

Coming Out in the Sciences

Cosmologist Mikel Susperregi Physicist Elizabeth Zita

Trang 12

cules known as endorphins, which

trig-ger a sensation of pleasure, enticing the

drinker to indulge further

Naltrexone mimics the shape of

en-dorphins but not their action By

block-ing the bindblock-ing of these

neurotransmit-ters to their receptors, it diminishes

al-coholÕs pleasurable eÝects and the

desire to drink more ỊNaltrexone is a

key that Þts the lock of the endorphin

receptor, but it doesnÕt open the door,Ĩ

Volpicelli explains This outcome is

sig-niÞcantly diÝerent from that of

Anta-buse, which can induce nausea and

vom-iting if patients drink during treatment

Stephanie S ÕMalley, the lead

re-searcher of the Yale study, speculates

that naltrexoneÕs gentler form of

per-suasion might be more helpful for some

patients than is Antabuse Because the

drug seems to reduce the intense desire

for alcohol, it actually helps patients to

feel better and remain abstinent,

there-by encouraging them to continue the

regimen In controlled studies of

nal-trexone, ÕMalley notes that researchers

are ỊÞnding good complianceĨ among

patients receiving the drug She tions against taking preliminary resultstoo far: ỊIn a broader population, com-pliance may not be as strong Our pa-tients tended to be fairly stable socially.ĨFurthermore, ÕMalley points out thatthe recent studies have always com-bined medication with other types oftreatment, such as counseling to devel-

cau-op ccau-oping skills Indeed, she mentions

a group of alcoholics who might ularly beneÞt from the integrated ther-apy ỊPatients with high levels of craving,who are likely to do poorly otherwise,might do better with naltrexone as part

partic-of their treatment,Ĩ ÕMalley explains

Because researchers are still in theearly stages of studying the use of nal-trexone for the treatment of alcoholism,Richard K Fuller of the National Insti-tute on Alcohol Abuse and Alcoholismexpresses Ịcautious optimismĨ regard-ing this new application of the drug

Nevertheless, he says, Ịif further ence conÞrms its eÛcacy, the approvalmay be a milestoneĨ in the treatment ofsubstance abuse Fuller adds that there

experi-has been Ịan explosion of understanding

in neuroscienceĨ in the past few years,especially in the biology of addiction.Indeed, the National Institute on DrugAbuse, which focuses primarily on abuse

of illegal drugs, has screened close totwo dozen diÝerent medications inclinical trials for their potential to treatcrack-cocaine addiction alone, accord-ing to Charles Grudzinskas of the insti-tute Volpicelli hopes future approval ofthese types of drugs Ịwill lead to a bet-ter understanding of substance abuseand lead to more funds being used intreatment programs.Ĩ

In todayÕs political climate, money isnot likely to ßood into such eÝorts Un-fortunately, Ịthe funding of treatmenthas been a bipartisan failure,Ĩ according

to Herbert D Kleber of Columbia versityÕs Center on Addiction and Sub-stance Abuse The roughly $700 millionspent by the government every year fortreatment and prevention research isdwarfed by the annual national costs ofsubstance abuse, which estimates place

Uni-at $238 billion ĐSasha Nemecek

Until recently, many medical

in-vestigators believed the AIDS

vi-rus remains more or less

dor-mant in asymptomatic persons,

step-ping up its assault on the immune

system only in late stages of infection

Now two widely hailed reports

pub-lished in January in Nature show that

the virus wages a continuous, intense

battle with the immune system from

the start The news is not all bad Those

results, and others from clinical

inves-tigations announced in February, point

to novel strategies for combating the

disease

The studies that led to the

new picture were conducted

by two groups, one by George

M Shaw of the University of

Alabama and the other by

David D Ho of the New York

University School of

Medi-cine Both teams examined

the eÝect on patients of

ex-perimental drugs that

pre-vent the human

immunode-Þciency virus (HIV) from

in-fecting cells in the body

In all subjects the drugs

caused HIV levels in the blood

to drop rapidly by factors of

100 or more At the same

time, immune system cells of

the CD4 type, which

ordinar-ily slowly fall in number

dur-ing the course of HIV infection, surged

in response That observation suggeststhe virus has an immediate impact onCD4 cell depletion It also indicatesthat Ịthe immune system is quite re-silient,Ĩ notes Robert T Schooley of theUniversity of Colorado, one of the orga-nizers of a conference on the AIDSvirus held in late January

By studying how quickly virus levelsdeclined when patients were treated, theinvestigators deduced that in untreatedpatients the rates of production anddestruction of viruses must be extreme-

ly high Individual particles survive only

about two days, but a billion or so newones, about a third of the total in thebody, are produced every day More-over, the virus destroys about a billionCD4 cells daily, which the body then at-tempts to replace

Sadly, the eÝect of the drugs did notlast After two weeks, initially rare mu-tants started to multiply in the patients,like new heads sprouting from the Hy-dra of Greek mythology The insightsmay nonetheless help researchers Þnddrugs or drug combinations that havelonger-lasting eÝectsĐand, with goodfortune, clinical beneÞts

The high turnover indicates that anyeÝective antiviral drug should have animpact on virus levels within days, ac-cording to HoÕs group That under-

standing alone could savetime that might have beenwasted looking for long-termeÝects of ineÝective drugs.The great rate of production

of new viruses also suggestsstrongly that Ịthe key is start-ing treatment early with po-tent drugs,Ĩ says Martin Mar-kowitz, one of HoÕs team Theinevitable accumulation ofmutationsĐa result of therapid turnoverĐmeans thatlate treatment faces a mon-ster with more heads.Numerous anti-HIV drugsare now in early testing.Some block the viral enzymeknown as reverse transcrip-tase; the widely prescribed

Fighting All the Time

Insights into HIV suggest ways to Þnd better AIDS treatments

HIV PARTICLES (small spheres) bud from a lymphocyte.

Trang 13

drug zidovudine (or AZT) works in the

same way Others attack diÝerent

en-zyme targets Several of the

experimen-tal drugs appear to be substantially

more eÝective than zidovudine against

HIV, at least in the short term

Interest at the January meeting

fo-cused on the combination of zidovudine

and a drug called 3TC, developed by

Glaxo Patients given the combination

still had 10-fold reductions in virus

lev-els even after 24 weeks The case is not

proved, but such sustained lowered

vi-rus levels may well delay disease

pro-gression ÒIt was the most optimistic

AIDS meeting IÕd been at for a longwhile,Ó Markowitz notes

The fervent hope is that by addingother drugs to the cocktail, it may bepossible to hold down the level of viralreplication for longer periods ÒWithcombinations, youÕre seeing more pro-nounced immunologic eÝects,Ó Schoo-ley observes ÒThe earlier pessimismabout antivirals was unfounded.ÓNor is the idea of modifying the im-mune systemÕs behavior with cyto-kinesÑnaturally occurring moleculesthat modulate its activityÑout of play

Research by Anthony S Fauci, director

of the National Institute of Allergy andInfectious Diseases, and by others hasshown that the cytokine interleukin-2,which stimulates immune responses,can increase CD4 cell levels in patients.Schooley makes a plea for more clin-ical research on drug combinations thatshow promise in laboratory tests ÒOfcourse, weÕll need to prove that theyhave clinical beneÞts,Ó he warns, ÒbutIÕm cautiously optimistic.Ó Moreover,Schooley says, the Þnding that CD4 cellscan rebound ÒoÝers hope to those sofar along that they thought it didnÕt

matter what we did.Ó ÑTim Beardsley

SCIENTIFIC AMERICAN April 1995 29

People have long criticized oil

com-panies for the accidental release

of oil into the sea, but over the

past decade a strange wrinkle has

de-veloped in the banner of environmental

protection Some scientists have begun

to wonder whether drilling in the Gulf

of Mexico could threaten marine life by,

strangely enough, reducing oil leaks

This curious twist results from two

separate advances Oil and gas

develop-ers have been moving drilling operations

farther oÝshore, enticed by immense oil

and gas Þelds previously thought to lie

in water too deep to be economically

tapped At the same time, researchers

have been uncovering

unexpect-ed richness and diversity in

deep-dwelling marine life

The work of both these groups

has recently been focused on the

Gulf ßoorÕs many natural oil

seeps Although sensitive

instru-ments are sometimes needed

to detect their subtle chemical

traces, ambient eÝusions in the

Gulf can be so great that they

leave markings on the surface

Researchers have even

discov-ered that some of these slicks

are visible from space

Most leaks, however, are hard

to Þnd In the mid-1980s oil

companies hired scientists from

Texas A&M University to survey

native petroleum seeps on the

continental slope The

research-ers stumbled on a surprisingly

plentiful biotaÑÞelds of huge

tube worms, giant mussels and

deep-sea crabs These groupings

resemble the dense seaßoor

com-munities found several years

ear-lier during exploration of the

hydrothermal vents that form at

sites of submarine volcanism

where tectonic plates separate

Like vent assemblages, seep communities are isolated fromsunlight, and their discoverers quicklyrealized that life there must rely on afood chain that begins with energy fromthe constant petrochemical bath Bacte-ria living within the gill cells of the gi-ant mussels, for example, provide fortheir hosts by metabolizing methane

petroleum-Soon after their discovery, protection

of seep dwellers became a priority tially we worried they would be so rarethat we might have an endangered-spe-cies situation,Ó remarks Ken Graham ofthe Minerals Management Service, theagency regulating oil and gas develop-

ÒIni-ment in the Gulf Because such opment involves erecting platforms,setting huge anchors and laying vaststretches of undersea pipeline, the Min-erals Management Service issued specialguidelines in 1989 for work that mightimpinge on chemosynthetic fauna.But continuing study revealed thatseep organisms are not rare after all.Chemosynthetic communities seem todevelop wherever substantial leaks oc-cur on the deep seaßoor, and there is lit-tle fear now that oÝshore drilling couldaccidentally destroy some unique spe-cies But could extraction of petroleumoÝshore reduce pressure to nearbyseeps, thereby robbing these deep-seaenclaves of their basic foodstuÝs?Although somewhat far-fetched, thequestion is not completely academic.Graham reports that Texas wild-catters often found that the ex-traction of oil on land could causenearby seeps to dry; there might

devel-be similar results oÝshore Ian R.MacDonald of Texas A&M believesdegradation of a seep by exploita-tion of its source reservoir is in-deed possible, but Òit has to beconsidered on a case-by-case basis.ÓConoco, for instance, has a plat-form sited close to one of themost extensive chemosyntheticcommunities found so far in theGulf, a place known as Bush Hill.ÒThey are producing oil a mileaway from it,Ó MacDonald re-marks, Òbut they believe they arenot tapping the reservoirÓ used bythe organisms Geologists at Shell,

a company active in exploitingdeepwater Þelds, also feel conÞ-dent that the reservoirs beingdrilled in the Gulf are not directlyconnected to any seaßoor seeps

So it would seem, for the timebeing at least, that the giant mus-sels and giant tubeworms as well

as the giant corporations can tinue to feed together happily inthe Gulf ÑDavid Schneider

con-Down and Out in the Gulf of Mexico

Oil spewing oÝshore doesnÕt always signal pollution

OIL RIG and deep-dwelling marine life both feed oÝ petroleum reservoirs.

Trang 14

Physicists may not be regarded as

people who enjoy making waves,

but some of them want to change

that By inducing atoms to act like

lightÑthat is, by turning particles into

wavesÑand passing them through an

interferometer, researchers at the

Mas-sachusetts Institute of Technology have

explored how atoms act toward one

an-other They have, in a way, split an atom

into two halves, using one half to probe

a gaseous medium and the other as a

point of reference The results promise

to lead to better measurements of

atomic behavior and improved

naviga-tional instruments

That particles behave like wavesÑ

and vice versa, for that matterÑis a

fundamental truth in physics In fact,

all objects, including beach balls and

graduate students, have a wave nature

The undulations are not noticeable,

be-cause the wavelengths of such hefty

ob-jects are too small to be relevant In

con-trast, light particles such as electrons

and neutrons have proved more

amen-able to manipulation as light [see ÒThe

Duality in Matter and Light,Ó by

Bert-hold-Georg Englert, Marlan O Scullyand Herbert Walther; SCIENTIFIC AMERI-CAN, December 1994] Indeed, the readyappearance of their waves has madethese subatomic particles indispensable

in microscopy

The advent of skilled nanofabricationand other related technologies in recentyears has enabled physicists to demon-strate the wave nature of a compara-tively heavier object ÒAtoms are morecomplex than neutrons or photons,Ó re-marks Steven Chu of Stanford Universi-

ty ÒThey oÝer another way of makingmeasurements that are potentially moreusefulÓ than those achieved with sub-atomic particles Researchers have beenespecially interested in interferome-tersÑdevices that split a wave and thenrecombine it later so that the wave in-terferes with itself

Now physicists have gone beyondmerely showcasing the atomÕs interfer-ing wave properties Jšrg Schmiedmay-

er, Michael S Chapman and David E

Pritchard, with their M.I.T colleagues,are using the interferometer as a tool toprobe how much an atom wave bends

when it passes through a particular dium In other words, they are studyingthe force exerted between diÝerenttypes of atoms ÒA whole bunch of peo-ple are starting to rethink these long-range forces,Ó Pritchard points out.ÒTheories are pretty unclear here.ÓThe workers passed sodium atomsthrough a Þnely etched diÝraction grat-ing The grating generated sodiumwaves that were then each split intotwo parts The parts moved down sep-arate paths One route traveled through

me-a gme-asÑsuch me-as xenon, helium or me-niaÑwhile the other part of the wavebypassed the gas When the sectionsrecombined, the waves interfered withthemselves The interference produced

ammo-a fringe pammo-atternÑammo-an ammo-alternammo-ating series

of bright and dark bands The brightbands indicate that the waves combinethere to achieve maximum intensity;the dark areas mark the places that thewaves cancel one another The patternshifted according to the type of gasthrough which the wave had moved.The researchers found that xenon,the heaviest gas studied, behaves most

in the way theory suggests in terms ofhow its force acts over distance Theother gases, however, had variationsthat sometimes produced results sub-

Catching That Wave

Atoms act like lightÑand get bent out of shape

Trang 15

stantially diÝerent from those

expect-ed Resolving the discrepancies may be

helpful to theoreticians trying to

pre-dict the properties of the long-sought

Bose-Einstein condensate, a collection of

atoms that supposedly acts as a single

giant atom when cooled close enough

to absolute zero

Atom interferometers might have

less obscure uses, tooÑnamely, as

gyro-scopes The interference pattern is

ex-tremely sensitive to inertial eÝects ÒWe

expect our interferometer to be about

as good as those on commercial jet

air-liners,Ó Pritchard says, Òand we should

be able to build one 500 times better

than that.Ó

PritchardÕs method is not the only

way to make atom waves interfere

ChuÕs group, for instance, relies on

la-ser bursts to excite the same atom to

two diÝerent quantum states, which

then interfere With his apparatus, Chu

has been reÞning measurements of

various physical quantities, such as

PlanckÕs constant Other workers have

been exploiting the principles behind

interfering quantum states to control

molecular dynamics as well atomic

be-havior [see ÒLaser Control of Chemical

Reactions,Ó by Paul Brumer and Moshe

Shapiro; SCIENTIFIC AMERICAN, March]

Although there is no theoretical limit

to the size of the object that can bemade wavy, there are practical limita-tions Pritchard did some calculations tosee if he could get the wave nature of a

graduate student to interfere but foundthat the transit time through the sys-tem would be longer than the studentÕslifetime It would have been a roughway to earn a degree ÑPhilip Yam

Trang 16

Economists, astrophysicists,

sociol-ogists, geologists as well as some

medical researchers spend a lot

of time looking at experiments that

God has already performed If God had

not arranged things so that some stars

were young and some were old, the

as-trophysicists would not know much

about stellar evolution Likewise, if God

had not arranged things so that the

minimum wage varied relative to the

average wage for unskilled labor from

decade to decade and state to state,

economists would have a hard time

convincing anyone that the minimum

wage puts poor people out of work

Economists and astrophysicists come

to their knowledge by Þnding

regulari-ties of some kind in the world; one

cru-cial part of their task is Þguring out

whether particular correlations point to

an important law or to the Þckle hand

of coincidence As a matter of fact,

economists are having a hard time

con-vincing people that the minimum wage

contributes to unemployment becauserecent studies show no Òstatistically sig-niÞcantÓ eÝect on jobs When Congresstakes the issue up later this year, thelivelihoods of thousands of peoplecould hang in the balance

But just what does that phrase mean,and what does it have to do with the de-bate? Go back two centuries, to PierreSimon, Marquis de Laplace, the Þrstperson to apply the notion of statisticalsigniÞcance to a serious scientiÞc prob-lem In 1773 Laplace wanted to knowwhere comets came from He reasonedthat if they originated inside the solarsystem, they would orbit in the sameplane as the planets, whereas if theycame from the far reaches of space,their paths would have no correlationwith those of bodies circling the sun

Laplace checked the motions of the last

12 comets to be discovered and Þrmlyrejected the hypothesis that cometscame from inside the solar system Ifthe comets were of local origin, one

might by chance travel at some weirdangle to the plane But the odds of get-ting two anomalies would be lower, ofthree lower yetÑthe probability, so tospeak, of rolling snake eyes three times

in a row This was a very smart idea

In the succeeding two centuries, isticians have reÞned LaplaceÕs simplenotion into Òstatistical signiÞcanceÓ anddeveloped an arsenal of formulas fordetermining whether the phenomenathat researchers observe are caused bysampling error (accidentally picking un-representative subjects) or ÒrealÓ eÝects.The gold standard for most studies isthe Ò95 percent conÞdence level,Ó whichindicates odds of only one in 20 that aresult arises from chance Economistsuse it to test whether the minimumwage has a ÒsigniÞcantÓ eÝect on em-ployment Medical researchers use it todecide whether half an aspirin a daykeeps the cardiologist away

stat-Gradually, however, it has dawned

on a few scientists that something isscrewy An obvious problem is that with

so many people doing so many studies,some of them are going to run into thatone-in-20 chance of believing in a mi-rage The converse mistake is moresubtle: scientists care about whether aresult is statistically signiÞcant, but theyshould care much more about whether

THE ANALYTICAL ECONOMIST

The InsigniÞcance of Statistical SigniÞcance

Trang 17

it is meaningfulÑwhether it has, to use

a technical term, oomph

Sadly, many scientists have started

thinking that statistical signiÞcance

measures oomph If an answer meets

the 95 percent conÞdence criteria, it

must be important; if it doesnÕt, it isnÕt

The clearest refutation of this notion

came in the study that established the

lifesaving eÝect of aspirin in men who

had already had a heart attack

Re-searchers stopped the experiment

be-fore their numbers reached ÒrealÓ

sta-tistical signiÞcance because the eÝect

of a mere half an aspirin a day was so

obvious that they considered it cal to go on giving placebos to anyone

unethi-Is this messy state of aÝairs LaplaceÕsfault? He was right about comets be-cause the relevant scale for measuringthe oomph in orbits was obvious Fur-thermore, a sample of a dozen couldyield results that were scientiÞcally aswell as statistically signiÞcant But thescale for measuring the eÝects of aspirin

or of changes in the minimum wage isnot so clear: you may get statisticallyimpeccable answers that make little dif-ference to anyone or ÒinsignificantÓones that are absolutely crucial

That conundrum is sharpest now inthe debate among economists about theminimum wage David Card and Alan

B Krueger of Princeton University haveused tests of statistical signiÞcance toargue there is no convincing evidencethat the minimum wage has a strongeÝect Most other economists disagree,both because their theory tells themotherwise and because they think Cardand Krueger are asking for too muchcertainty But because both sides aremuddled about the diÝerence betweenoomph and statistical signiÞcance, thedisagreement is not likely to get re-solved in time to help Congress De-pending on what legislators decide,many poor people (not to mention teen-agers on summer vacation) might losetheir jobs Ironically, even if they do,economic samplers may not be able toprove how many jobs were lost or thatthe minimum wage really had an eÝect

DONALD N McCLOSKEY is professor

of economics and history at the sity of Iowa.

Univer-LOST JOBS? Some economists say paid workers will be Þred if the minimum wage rises; others claim the evidence is statistically insigniÞcant.

Trang 18

When the London fog rolls in,

pilots ßying into Heathrow

Air-port turn on their instrument

landing system receivers to pick up the

radio beacons that will guide them

safe-ly through the soup and onto the

run-way But those beams, steady for 50

years so far, may soon begin to rock and

roll In January 1998 European FM radio

broadcasters will turn up the volume on

their transmitters, some of which are

located uncomfortably closeĐboth on

the ground and in the electromagnetic

spectrumĐto landing systems at

ma-jor airports

The worldÕs aviation agencies planned

for this contingency 20 years ago The

instrument landing system, everyone

agreed, would be replaced in the

mid-1990s with a microwave landing

sys-tem (better known as MLS) In addition

to dodging interference by moving to a

less crowded part of the spectrum, MLS

would allow pilots to make steeper and

curved descents, cutting noise and

boosting airport capacity

But in recent years that agreement

has crumbled, and the future of

poor-weather landing is up in the air At a

special international meeting this month

to settle the issue, Ronald E Morgan,

director of system architecture for the

Federal Aviation Administration, will try

to persuade other nations to abandonMLS development, as the U.S did lastsummer ỊWe would like to eliminatethe mandatory transition to MLS on in-ternational runways,Ĩ Morgan says

Heads turned from MLS when the U.S

Department of Defense encircled theearth with the Global Positioning Sys-tem (GPS), 24 satellites that broadcasttheir location and a timing signal Withany four in sight, a GPS receiver can

work out its position (plus or minus

100 meters) anywhere on or above theplanet U.S airlines were immediatelystarstruck by the technology, whichcould allow their ßeets to break out ofintercity airways and ßy more directroutes to save time and fuel Last De-cember the FAA approved their requests

to use GPS to navigate over the oceans

Low-visibility descents, of course, are

a diÝerent matter A 100-meter errorcould land a jumbo jet in a parking lot

If the GPS is to replace the instrumentlanding system, its errors must be held

to less than a meter and ening system failures to one in a bil-lion ỊWeÕve solved the accuracy prob-lem,Ĩ announces Robert D Till, an FAAtest program manager

safety-threat-Two demonstrations last October

proved that point United Parcel Serviceautopiloted a Boeing 757-200 through

50 landings using a ground station thatmeasured errors in the satellite signaland beamed the corrections to the in-coming aircraft Stanford University re-search associate Clark E Cohen hit on

an even better approach while working

on a relativity experiment CohenÕs lutionĐto boost precision, add precisesatellitesĐis more feasible than itsounds, because the additional satel-lites can be on the ground

so-The FAA tested CohenÕs idea on aUnited Airlines 737-300 Two transmit-ters, each the size of a credit card andpowered by a nine-volt battery, wereplaced about four kilometers from theend of the runway As the test planepassed over the pseudosatellites, asmall antenna on its belly picked up

weak but accurate signals andused them to reÞne to a fewcentimeters location data com-ing from four real GPS satel-lites Over several days the jetÕsautopilot made 110 successfultouch-and-go landings.The real test of StanfordÕssystem came on ßight number

37, when data uploaded to oneGPS satellite caused it to blinkout for two seconds just asthe 737 was descending Howquickly and reliably the systemwarns the pilot not to trustthe instruments in such situa-tionsĐwhat regulators call thesystemÕs ỊintegrityĨĐis at thecrux of the debate over the GPS

as a possible replacement forthe instrument landing system.Although the U.S has assuredother nations that the GPS will

be free and reliable to all usersfor at least 10 years, Ịwe have

to understand that in times ofwar, things change,Ĩ says Jo-seph F Dorfler, the FAÃs satellite pro-gram manager Software glitches andhardware failures could also shut satel-lites down without warning

To reinforce GPS integrity in the U.S.,the FAA hopes to build a $500-millionnetwork of 24 ground stations that willrelay GPS corrections to pilots and alertthem the moment something goeswrong Other countries are looking atGlonass, a Russian counterpart to GPS,and Inmarsat, which maintains a group

of marine navigation satellites, as ble backups But most observers doubtthat these systems will achieve the in-tegrity needed for low-visibility land-ings by the 1998 deadline

possi-StanfordÕs experimental system mayoÝer help here A computer on boardthe aircraft compares the six corrected

TECHNOLOGY AND BUSINESS

LANDING SYSTEM that guides airplanes down through foul weather must be replaced soon.

The U.S wants the world to rely on its military satellites, but other nations raise safety issues.

Coming in for a Landing

Should satellites or microwaves direct airplanes in bad weather?

Trang 19

signals it receives from four satellites

and two beacons If any one is too far

out of line, it disengages the autopilot

and sounds the alarm On ßight 37,

Co-hen says, all that happened within a

quarter of a second

The FAA plans further integrity tests

this spring to bolster its case at the

up-coming international meeting in

Mon-treal But U.S airlines, which are loath to

buy expensive MLS equipment,

nonethe-less Ịanticipate that we will have to have

some limited MLS capability,Ĩ says J

Roger Fleming, a senior oÛcial at the

Air Transport Association Otherwise,

when Heathrow fogs in, Americans

could be shut out ĐW Wayt Gibbs

A WidgetÕs Best Friend

Diamonds may bring a new

facet to motors and sensors

Tiny electric engines built out of

silicon chips have been making

wonderful illustrations for

gee-whiz technology stories since the early

1980s They have not been good for

much else Friction, fragility and

corro-sion are among the ailments silicon

suÝers The elementÕs major advantage

is that it is easy to form into complex

microscopic shapes because

integrated-circuit engineers have spent billions of

dollars developing tools to grow it, etch

it, dope it and alloy it with other

mate-rials Micromachinists have just tagged

along for the ride

Now a team of microfabricators

ap-pears to be taking the wheel

Research-ers at Oak Ridge National Laboratories

report in Applied Physics Letters that

they have made minute mechanical

de-vices out of diamond Diamond has a

much lower coeÛcient of friction than

silicon; it is also stronger and stiÝer

and more resistant to chemical attack

John D Hunn and his colleagues have

developed a technique for etching

pat-terns in diamond Þlms First, they

bom-bard a diamond surface with

high-ener-gy oxygen ions to produce a layer of

graphite inside the crystal (the ions do

not produce defects at the surface,

where they are traveling quickly, but

rather cause disruption when they come

to a stop) Then the researchers grow a

layer of diamond Þlm on top of the

pre-pared diamond Third, an ultraviolet

la-ser cuts a trench through the Þlm down

to the graphite layer, outlining the shape

of the parts to be made Finally, the

team places the diamond in a furnace,

where high-temperature oxygen burns

away the graphite, freeing each piece

from the substrate

The process is yielding gears as small

as four tenths of a millimeter across

Such a gear could easily serve as a tor in a small electric motor The noveldiamond part could be dropped intoone of the silicon frames that has al-ready been built, Hunn says: ỊYou could

ro-do it tomorrow.ĨFlashy though such a motor might

be, the real payoÝ of the technologywill be in diamond sensors, Hunn com-ments A diamond-Þlm membraneetched thin by a technique that usesimplanted ions could signal pressure

changes inside a corrosive liquid thatwould destroy a silicon sensor.But these microsensors will not beavailable tomorrow or even next year.The engineers must Þgure out how tointegrate electronic manufacturing ac-tivities with their machining technology.And to date, Hunn explains, neither di-amond-Þlm researchers nor microma-chinists have followed the teamÕs lead.Nevertheless, the Oak Ridge projecthas already given the Ịjewelry phaseĨ

of high-tech research and development

a new meaning ĐPaul Wallich

Trang 20

E F SchumacherÕs Small Is

Beauti-ful, the bible of the so-called

ap-propriate technology movement,

Þrst came into print more than 20 years

ago By most accounts, the small-scale

technology movement that Schumacher

championed initially failed to achieve

its promise The developing world is

Þlled with rundown wind pumps and

solar panels that were never properly

connected to a generator But although

the original implementation may have

been ßawed, appropriate

technolo-gy has staged a revival

Proof of change can be seen on

the windmill-dotted Namibian coast

or in the Kenyan countryside, where

tens of thousands of photovoltaic

systems help to cope with an

unre-liable or nonexistent electric power

grid In sheer numbers, however, the

biggest gains have come inside the

home, where more than a third of

the worldÕs populationÑsome two

billion peopleÑcook over smoky

Þres In excess of 120 million

house-holds in China, eight million in India

and 700,000 in East Africa have

be-gun to adopt stoves that more

eÛ-ciently burn wood, charcoal, coal or

livestock dung Worldwide several

hundred improved cookstove

pro-grams in more than 50 countries

got their start during the 1980s

Slight improvements in such

stove technology can have

long-ranging impacts on quality of life

and on the environment Poor city

dwellers in developing countries

may spend up to a quarter of their

$300 to $400 annual earnings on wood

or charcoal A stove that costs less than

$5 but uses 30 percent less wood can

pay for itself in a matter of months

Rural villagers who adopt enhanced

stoves can cut down on time spent (up

to one full day a week) cutting and

car-rying wood

Breathing also comes easier

Tradi-tional stoves produce a noxious mix of

hydrocarbons, carbon monoxide and

particulates that help to make acute

respiratory diseases, such as

pneumo-nia, the leading health hazard in the

developing world Such illnesses cause

an estimated 4.3 million deaths every

year Pollution levels from indoor

cook-ing can make the air in some rural

homes rival or exceed that of the

dirti-est of industrial cities

Over the past two decades physicists,

engineers and development outreach

groups have labored to create stove

de-signs that achieve eÛciencies of

be-tween 35 to 50 percent, several timesthat of a cooking Þre set between threestones on which a pot is placed TheÞres of a traditional cookstoveÑa metalcanlike enclosure stoked with wood isone exampleÑare only marginally bet-ter than such three-stone Þres

Following Schumacher to the letter,stove designers have emphasized use

of local materials One method of tering eÛciency (deÞned as the maxi-mum amount of heat delivered to a

bet-pot) comes from making ceramic stoveliners out of sand and clay The sheathspromote high cooking temperatures

An earlier generation of appropriatetechnology proselytizers made a num-ber of blunders, both technical and so-cial They would, for example, fail tomatch the circumference of the stove

top to the dimensions of common ing utensils Or they would train onlymen in how to care for the stoves Theywere also smug about how much theyneeded to know about the physics ofseemingly simple cooking implements.ÒGetting the thermodynamics right andunderstanding the heat-transfer char-acteristics are not things you do as aweekend project,Ó says Daniel M Kam-men, a physicist by training and a pro-fessor in the Woodrow Wilson School

cook-of Public and International AÝairs atPrinceton University

Selling improved cookstoves in thedeveloping world requires as much mar-ket research as does plying a newdishwashing detergent to consum-ers in suburban Chicago The cook-stove program in China, for one,gained acceptance only after designalterations gave the stoves a mod-ern look Careful administration andpricing are also essential The Chi-nese government removed many ofthe bureaucratic constraints to set-ting up stove programs It providedfew subsidies, yet most rural house-holds still decided to purchase thestoves

A World Bank report publishedlast year found that India, in con-trast to China, established a cum-bersome administrative structurefor its national program, relied onheavy subsidies and failed to con-centrate on households with thegreatest need It is estimated thathalf of the more than eight millionstoves purveyed through the Indianprogram lie unused

Schumacher made as many takes as his followers did The vi-sionary recommended that large-scaleenterprises should end up being run bythe public sector, an idea that appearssomewhat dated in light of currentevents But his notion that a revolution

mis-in simple technology can transform mis-dividual lives may have gained credence

in-even in this post-Marxist era.ÑGary Stix

Simply, the Best

Energy-eÛcient cookstove technology makes a comeback

Two commercially funded

data-bases of DNA sequences that willidentify most human genes areunlocking their computer Þles to re-searchers worldwide This boon forbiomedical work could pave the wayfor powerful new pharmaceuticals inthe next century Yet the projects, onebankrolled by SmithKline Beecham andthe other by Merck & Co., have beenthe focus of rancorous disputes

Both databases employ so-called pressed sequence tags (ESTs), short ge-netic sequences that can help workersÞnd entire genes But the similaritiesbetween the databases end there Mercksays all the information it produces will

ex-be put immediately into the public main, with no restrictions on access.The Þrst sequences generated by theeÝort, which is being conducted atWashington University, were deposited

do-Genes in the Not So Public Domain

Human DNA databanks open for businessÑand vie for users

IMPROVED STOVE with clay pot, in Kenya

Trang 21

in a public-access database in February.

It will be 18 months before the

data-base contains enough ESTs to tag the

majority of human genes

In contrast, the SmithKline Beecham

project, which began two years ago at

the nonproÞt Institute for Genomic

Re-search (TIGR) in Gaithersburg, Md., has

already tagged about half the

estimat-ed 70,000 human genes TIGR, which is

also supported by Human Genome

Sci-ences in Rockville, Md., is now testing

its database at a dozen or so research

institutions Founder of TIGR and the

pioneer of the EST technique, J Craig

Venter, along with his team, has

sub-mitted a long paper describing and

cat-egorizing TIGRÕs sequences to a

scien-tiÞc journal (rumored to be Nature).

When the paper is published, the

data-base will be made available to

academ-ic scientists who are not employees of

for-proÞt corporations, Venter says

The information, however, has strings

attached The U.S Patent and

Trade-mark Ỏce has ruled that ESTs cannot

be patented As a result, TIGRÕs

spon-sors have insisted that the most

valu-able sequences be provided only under

certain conditions The main stipulation

is that if a sequence is used to develop

a proÞtable drug, the researcherÕs tution must agree to negotiate with TIGR

insti-on a share of the royalties Moreover,although TIGR has dropped a proposal

to limit the number of queries that vestigators can send to TIGRÕs comput-ers, the database as a whole remains un-der wraps And a few genes that TIGRhas identiÞed have been withheld

in-Investigators accustomed to workingwith public information are balking atusing TIGRÕs vast database in an inter-national eÝort to map the sequences

ỊThe problem is that the sequences aresecret, and we have not been able towork out how to put them on a mapopen to everyone,Ĩ says Francis S Col-lins, director of the National Center forHuman Genome Research at the Nation-

al Institutes of Health Recently,

howev-er, Venter has quelled criticism of TIGR

by entering into a collaboration to ate a public map of ESTs

cre-MerckÕs public-domain database wasannounced in response to TIGRÕs deci-sion last fall to put restrictions on itsinformation The clash has led to some

bad blood Venter disparages the MerckeÝort as an Ịattempt to hurt usĨ andquestions the extent of MerckÕs Þnan-cial commitmentĐa topic the drug com-pany refuses to discuss For his part,Keith O Elliston, an associate directorfor bioinformatics at Merck, querieswhether TIGRÕs methods generate ac-curate results The Merck initiative di-verges enough in its technical detailsthat it will not duplicate the TIGR work,according to Elliston DNA fragmentswill be characterized at both endsĐun-like sequences in TIGRÕs database Theextra information will, Elliston says,make it far easier to construct highlyaccurate gene maps Moreover, the col-lection of sequences should containfewer duplicate entries than TIGRÕs, headds Venter counters that only TIGRÕsdatabase identiÞes all its ESTs

Nevertheless, Ịthe Merck plan is agood one,Ĩ comments Collins of the NIH.Despite the corporate public-relationsbattle, the important business of track-ing down human genes and discover-ing exactly what they do is moving full-steam aheadĐwith the help of drugcompany money ĐTim Beardsley

Voting for a Cure

The world’s toughest question blared in 117-point type

across page B5 of the New York Times on January 19.

Readers moving between a profile of Senator Alfonse

D’Amato of New York and the latest on the O J Simpson

murder trial had their first—and probably their last—

chance to cast a ballot for a remedy for cancer For that

day, a question that Ph.D.’s have pondered for decades

was reduced to a matter of near-religious faith: “I believe

in tumor antigens,” read a checkoff box at the bottom of

the ad “I believe in cancer vaccines,” read another

Madison Avenue advertising executive Steve Fenton

cre-ated the full-page advertisement to raise the profile of his

pro bono client, the Cancer Research Institute (CRI) This

$5.4-million-a-year charity has, since 1953, raised money

to find ways to marshal the human immune system’s fight

against malignant cells Instead of the usual low-key,

anon-ymous public-service approach, Fenton decided to

com-bine the highbrow with the mundane He placed words

such as “tumor antigens” and “cytokines” in headline print

And in a direct steal from A&P store specials on asparagus

spears, he put these titles, along with a brief explanatorytext, in their own clip-out coupons that could be mailed inwith a donation

Asking people to vote on a cure for cancer arousessome skepticism among serious scientists, especially giv-

en the decidedly mixed history of the war against the ease “It has the inference that the cure for cancer isaround the corner and you can select from a menu,” com-ments Samuel S Epstein, a professor at the University ofIllinois at Chicago

dis-Still, for the CRI, tumor antigens, cancer vaccines andcytokines have proved a big hit “We’re getting more re-sponse to this ad than any we’ve ever run,” says Jill O’Don-nell-Tormey, one of the organization’s executive directors,

of the not overwhelming 40 replies received Most dents merely filled in a generic box that leaves it up to theCRI’s scientific advisory council to decide where the moneygoes Some did cast a ballot Of the six immunotherapieslisted, cancer vaccines have taken the lead in the polls.Fenton, who is an executive with D’Arcy Masius Benton

respon-& Bowles, says he was merelytrying to provoke readers intothinking, “ ‘Gee, this soundsinteresting, and the moneyisn’t going into a black hole.’ ”But the author of the slogan

“America’s Getting into ing,” for an Amtrak ad, maystill have his work cut out forhim before tumor antigensand cytokines become as fa-miliar as Metroliners and as-paragus spears —Gary Stix EXCERPTS from a Cancer Research Institute advertisement

Trang 22

Fish love sewage, and the

inhabi-tants of Calcutta love Þsh These

city dwellers also produce

sew-age: 700 million liters of it a day By

growing Þsh on this waste, Þsherfolk to

the east of the Indian metropolis

dis-covered how to link these needs at the

turn of the century But this natural

re-cycling systemĐthe largest of its kind

and possibly the oldestĐis threatened

by a state government that sees able real estate in the expanse of shal-low waters

proÞt-The Þsheries ßourish on elaboratefolk technology Diverted into a series

of ponds, the sewage sheds its cent solids Air-breathing Þsh and wa-ter plants survive in pools nearest thesewage inlet Further downstream, al-gae bloom on nutrients in the clear wa-

putres-ter Tilapia and carp, in turn, thrive onthe algae More than 20 tons of theseÞsh feed the Calcutta market every day.(Bioassays of these Þsh show that theirlevels of heavy metals and coliformbacteria are low.) The algae also pro-vide much needed oxygen to the heavi-

ly polluted city

Further, sludge from the distillationponds fertilizes extensive Ịgarbage gar-dens.Ĩ CalcuttaÕs solid waste, stripped

of its paper, plastics and metals bysome 25,000 ragpickers who roam thecityÕs noxious dumps, is ultimately al-most entirely organic Irrigated by treat-

ed sewage and then composted, thewaste becomes soil on which farmersgrow a variety of vegetables Their har-vest supplies the city with 150 tons ofproduce a day

This recycling expanse, however, hasbeen shrinking as Calcutta expandseastward In the 1960s northeasternwater bodies were Þlled in to create atownship The Dutch consultants whodrew up the plans apparently over-looked what British engineers had noted

in the 1880s: the land naturally slopes

to the east Once these pond beds weregone, the eastbound sewage canalsstopped ßowing during the rains of themonsoon season, and now they regu-larly ßood the city

In addition, thugsĐor antisocials, asIndians call themĐassociated with thecommunist state government are re-ported to have seized many Þsheries

on the pretext of distributing land toimpoverished laborers Instead the par-cels have been sold to various real-es-tate developers

Recently citizens groups sued to stopthe government from building a shinynew World Trade Center, funded by

a multinational group, on such claimedĨ terrain The case is still beingdisputed Meanwhile politicians aretouting a ỊMegacity 2011Ĩ expansionproject, the logistics of which, observesAsish K Ghosh of the Zoological Survey

Ịre-of India, have not been divulged to vironmentalists One thing is clear,though: the plan involves grand de-signs on the Þsheries

en-At the same time, these water bodiesare becoming ever more essential InFebruary the Indian Supreme Courtthreatened to close 30 factories thatwere discharging pollutants into theHoogly River, west of Calcutta Dhruba-jyoti Ghosh of the Institute for Wet-lands Management has suggested thatthese eÜuents could be treated by ex-panding the Þsheries

But the government is clearly not tening And CalcuttaÕs leap into the 21stcentury could well become a plunge into

lis-miasmic waters.ĐMadhusree Mukerjee

The Fishy Business of Waste

Development near Calcutta may thwart age-old recycling

Trang 23

Few would quibble with the value

of screening for breast cancer, but

even at its current level of

matu-rity and appreciation, the art of x-ray

mammography still shows a glaring

ßaw It often fails to detect

malignan-cies In women younger than 50 years,

for example, it misses existing cancers

nearly half the time So some medical

researchers are cautiously

contemplat-ing screencontemplat-ing for breast tumors with

magnetic resonance imaging (MRI), a

technique that probes the human body

with a combination of magnetic Þelds

and radio waves

Certain women stand to beneÞt most

from MRI: their breasts contain more

Þbrous or glandular material and less

fat than is typical, and this constitution

makes their breast tissues prone to

scatter or severely attenuate x-rays For

these women (perhaps 40 percent of

the female population), mammography

renders images that are little more than

a cloudy blur that prevents physicians

from readily seeing the subtle

architec-tural distortions indicative of cancer

Although some breast cancers signal

their presence through the fog by

creat-ing microcalciÞcations that, like bone,

leave bright spots on x-ray negatives,

many malignancies do not contain these

tiny radiographic beacons Hence, for a

woman with ÒdenseÓ breasts, a doctorÕs

normally assuring statement that no

ev-idence of cancer appears on her

mam-mogram has little weight

MRI could, at least in principle,

pro-vide the ideal diagnostic alternative

be-cause it relies on a completely diÝerent

and perhaps more promising

combina-tion of physics and physiology

Practi-tioners generally agree that MRI is

su-perior to x-rays for imaging soft tissue,

but its chief attraction for use in thebreast is that it can be applied in con-junction with tracers Images taken be-fore and after one of these magneticcompounds is infused into a patientÕsbloodstream can delineate tumorsamazingly well because the compound

is preferentially absorbed And, unlikecontrast agents employed for x-rays,those used for MRI almost never giverise to dangerous allergic reactions

Steven E Harms and his colleagues atBaylor University Medical Center havebeen carefully examining the eÝective-ness of MRI for women with breast can-cer In one of their recent studies thepatients with cancer later underwentmastectomies; thus, the exact state ofthe excised breast tissue could be de-termined Harms and his co-workersfound that mammography, even com-bined with ultrasound, depicted no can-cers that were not also detected withMRI scanning But the scans showedmany cancers not found by mammog-raphy: more than a third of the breastswith negative mammograms showedtumors with MRI

Women with radiographically densebreasts should Þnd such results partic-ularly tantalizing And should they even-tually beneÞt from it, they might alsoÞnd MRI screening relatively comfort-able The procedure only requires them

to lie face downward, with their breastsheld still below them, not painfully com-pressed as in mammographic examina-tions The patientÕs stay within the imag-ing machine is also guaranteed to bequite short, because the contrast agentmarks tumors for only a few minutesafter injection

No full-scale clinical studies, however,have yet established the seemingly rea-sonable proposition that MRI screeningwould save some womenÕs lives ÒRightnow we donÕt know,Ó remarks JeÝrey C

Weinreb of New York University cal Center Screening breasts with MRIÒcould be so great and revolutionarythat everybodyÕs getting it done,Ó henotes ÒThen again it could ßop WeÕrejust scratching the surface and learn-ing how to do it.Ó

Medi-Although a broad, long-term studycomparing MRI with mammographicscreening is needed, it would take manyyears to complete and would be im-mensely expensive A less costly and

more eÝective approach might be tofocus such studies on women who aregenetically predisposed to the develop-ment of breast cancer No formal anal-ysis of this kind has yet been started,but according to Weinreb, these eÝortsare probably only a few years away.Even if MRI screening proves clinical-

ly valuable, the cost could prevent itfrom Þnding widespread use Weinreb

is, nonetheless, optimistic: ÒIf it doeswork, breast MRI can probably be donecheaply.Ó He estimates that the cost for

a screening exam could eventually cline to within a factor of two of x-raymammography Indeed, he states, Òif Iwere doing 40 [breast MRI scans] a day,

de-I could do it [for that amount] now.ÓFor some people, price is less of aconcern than the threat of cancer, andmany women are understandably re-luctant to wait for results of long-termstudies before taking advantage of thistechnique Many manufacturers seem to

be anticipating such demand and areproducing specially designed pickupcoils for MRI scans of the breast Onenew company, Advanced Mammogra-phy Systems in Wilmington, Mass., hasdeveloped a relatively low-cost MRI ma-chine that is solely for breast exams.Weinreb fears that the proliferation ofdedicated hardware means that Òmedi-cal entrepreneursÓ may not wait for thecompletion of the proper trials beforemarketing breast MRI services to an in-creasingly worried public Noting thatcertain suburbs of New York City havesome of the highest rates of breast can-cer in the country, he speculates thatbefore long Òsomebody is going toopen up one of these facilities on LongIsland and will clean up.Ó So, as promis-ing as this technique now appears, itstands as anyoneÕs guess whether thefuture of breast MRI screening will seewomenÕs cancers, or just their fears,exposed ÑDavid Schneider

Changing the Image

Looking to MRI for diagnosing breast cancer

IMAGES OF CANCER within a ÒdenseÓ breast are foggy in an x-ray mammo- gram (left) but clear with MRI (right).

Trang 24

Gusts of sleet whip through the

air as I drive past unattended

guard stations and by the

land-scaped Þelds that form a miniature

greenbelt around the National Institute

of Standards and Technology in

Gai-thersburg, Md The Þrst big snow of

1995 landed on the nationÕs capital just

36 hours ago, throwing this Monday

morning rush hour into confusion As I

slog through the parking lot toward the

ivory-colored administrative tower

ris-ing from a complex of low, blocky labs,

I imagine the instituteÕs

direc-tor staring out the top-ßoor

window, morosely pondering

the weather as a metaphor for

the Clinton administrationÕs

political fortunes

Such a scene may have Þt

many past NIST directors, most

of them middle-aged bench

scientists who grew

comfort-able with their limited,

incon-spicuous mission: to help

in-dustrial competitors agree on

what constitutes a volt, a

me-ter, a second But from the

moment I walk into the

direc-torÕs modest oÛce, it is

appar-ent that its newest inhabitant,

Arati Prabhakar (pronounced

AR-ah-thee Pra-BOCK-er), does

not Þt the archetype for this

agencyĐand that the agency

is changing to adapt

It is not just the outward

ap-pearances that are diÝerent To

be sure, those are striking:

Pra-bhakar is the Þrst female, the

Þrst foreign-born citizen and

the youngest person to direct

the institute Gregarious and

amiable, she seems remarkably

at ease for someone who will

defend her programs before

the Senate science committee

the next day (ỊItÕs not so

diÛ-cult after the Þrst few times,Ĩ she

con-Þdes.) And the snow is hardly making

her morose ỊMy boyfriend and I had a

snowball Þght Saturday night,Ĩ she

says with a chuckle in her deep voice

ỊIt was great.Ĩ

The most germane characteristic that

distinguishes Prabhakar from her

pre-decessors, it turns out, lies in another

Þrst ỊI am the Þrst director of NIST or

NBS [the National Bureau of Standards,

its former name] who was trained as

an engineer, rather than as a scientist,Ĩshe explains ỊThat is both symbolicand meaningful, because itÕs represen-tative of what NIST is trying to do to-dayĨĐor, to be more precise, of whatthe White House is trying to do

ỊIt was very obvious when this istration came in that NIST was part ofits agenda,Ĩ Prabhakar notes ỊThatÕswhy IÕm here.Ĩ President Bill Clinton hasconsistently argued that because of aßaw in the free-market system, U.S com-panies invest so little in long-term re-

admin-search that they risk losing their logical edge to government-subsidizedcompetitors overseas The public sector,

techno-he has asserted, can compensate forthis private-sector failing by putting upseed money in certain high-risk, high-payoÝ research areas, such as digitaldata storage and DNA diagnostic tools

In NISTÕs tiny Advanced TechnologyProgram (ATP), launched during theBush administration in 1990, Clintonsaw a vehicle for his vision He plotted

a course that would nearly quadrupleNISTÕs budget to $1.4 billion by 1997and has stuck to it, requesting $1.023billion from Congress for 1996 Abouthalf that money is set aside for ATPgrants to companies willing to put anequivalent amount of their own fundsinto a long-term development project.From its genesis, the ATP has facedconservative critics who wonder whyunderpaid government analysts shouldrush in where venture capitalists fear

to treadĐand why the crown collects

no royalty when federally funded ects do lead to successful products Aspower shifted across the aisles of Con-gress this winter, ATP supporters felt achill creep into discussions of the pro-gramÕs future, which can no longer betaken for granted Even its present isunder debate: a bill put to the House of

proj-Representatives in Februarywould take back $107 million

of the $853.8 million Congressgave to NIST for 1995

As the debate unfolds, alleyes will be on Prabhakar.ỊWeÕre on a very steep ramp-

up to expand these pilot grams into national-scale ef-forts,Ĩ she says Any slip could

pro-be politically perilous And thisafternoon she bids farewell tothe ATP manager, who is retir-ing with no replacement athand The challenge of jug-gling these tasks with manda-tory house calls to Capitol Hillcould seem overwhelming forsomeone who in three dayswill turn 36 Yet Prabhakar ex-udes only conÞdent enthusi-asm, perhaps because her lifeseems to have prepared herprecisely for this job

Prabhakar is certainly tomed to being unusual Born

accus-in New Delhi, she was onlythree when her family emigrat-

ed to America, eventually tling in Texas ỊWhen youÕre anIndian kid in Lubbock, youÕreinherently diÝerent anyway,Ĩshe says with a laugh Pursuingher interest in math and sci-ence was just another way tostand out

set-Prabhakar raced through an cal engineering degree at Texas TechUniversity in three years, then headed

electri-to the California Institute of

Technolo-gy for graduate school Money was noproblemĐa pleasantly recurring pattern

in her careerĐthanks to a full ship and stipend from the Bell Labora-tories Graduate Research Program forWomen More important than the cash,she states, was the oÝer of two research-

scholar-Engineering the Future

PROFILE: ARATI PRABHAKAR

Trang 25

ers to act as mentors Robert E Nahory,

who later moved on to Bellcore, and

Martin A Pollack visited her once every

year during her graduate studies and

employed her during one summer

Nahory recalls Prabhakar struggling

with her classes and with the novelty

of being the only woman in her

depart-ment: ỊIn class the men would always

sit in the same seats, and they would

leave an empty chair between

them-selves and Arati So she began to

exper-iment by moving to diÝerent seats It

would cause some general confusion,

but everybody would move so that

again she was surrounded by empty

seats You can imagine how isolated

she must have felt.Ĩ

ỊI did not have a good time in

gradu-ate school,Ĩ Prabhakar admits, Ịbut it

did force me to think through what I

liked and what I didnÕt Recently I found

some notes I made back then, when I

was in the throes of deciding what to

do with my life Looking back, it turns

out to be a perfect road

map for what IÕve done

since What I really like

is exploring and

com-municating the

connec-tions between science,

technology and

busi-ness I love that a whole

lot more than working

applied physics that

Cal-tech had ever given to a

woman, Prabhakar was

convinced that she

want-ed Ịsomething

orthogo-nalĨ to a life of

academ-ic research, something

that felt more like

engi-neering When Pollack pointed out an

advertisement for a fellowship at the

Ỏce of Technology Assessment,

Pra-bhakar hesitated for only a second ỊI

had never conceived of working in the

government as a career,Ĩ she says,

grin-ning ỊI still feel that way.Ĩ

Nevertheless, she packed her bags

for Washington Her initial assignment

could hardly have been more formative:

to report on the state of semiconductor

research in the U.S at a time when

Ja-pan was marshaling its manufacturers

to overtake American dominance in

computer chips JapanÕs rapid success

prompted Congress to create Sematech,

a federally backed consortium that

pro-vided a seminal test of governmentÕs

ability to invigorate a ßagging industry

Years later U.S companies won back a

leading share of the market As

Prabha-kar oversaw the publicÕs investment inSematech, candidate Clinton held up theconsortium as a model for his technolo-

gy policy, even as critics derided the taxdollars spent helping an industry theysaid would have helped itself

While Prabhakar met with industryexperts around the country to educateherself about their semiconductor re-search, she inevitably educated themabout Arati Prabhakar Her report thusled directly to a job oÝer from Richard

A Reynolds, then science director at theDepartment of DefenseÕs Advanced Re-search Projects Agency (ARPA), whoasked her to run a $6-million programfunding research on gallium arsenideelectronics Prabhakar attributes her bigbreak to Ịa culture at ARPA that cele-brates taking riskĐvery similar to theculture that is growing now in our ATP

Dick took such a risk when he hired me

I was just a year out of graduate school

I didnÕt know from nothing But therewere good, smart people to show me the

ropes The secret to success in thesejobs is to work with a broad communityand to listen and add value in a collab-orative eÝort rather than trying to dothings in an arbitrary way.Ĩ

Another secret of hers is to be at theright place at the right time Within sev-eral years of her arrival, Congress shift-

ed money and power from the gon to ARPA, raising its budget for mi-croelectronics 10-fold Prabhakar rodethe wave past many senior colleagues,and by 1991 she commanded a budget

Penta-of $300 million and all Penta-of ARPÃs chipresearch Prabhakar admits to a sense

of dŽjˆ vu when she looks back to thattime, especially now that Republicansare threatening to rein in NISTÕs growth

In the late 1980s it was Congress thatwas pushing the government to pro-mote commercial research actively,

while the White House was frowning

on intervention ARPA became an logical battleground

ideo-Prabhakar recounts the lessons ofthat war ỊThe Soviet empire was break-ing up, and I remember watching thelittle Baltic countries leading the chargetoward independence They faced acritical question: How far ahead shouldyou get? Lithuania and the others need-

ed to be far enough out that they coulddrive the agenda But if they pushedtoo far, the tanks would roll in, and itwould be Prague and 1968 all over again.ThatÕs what it was like at ARPA at thatpoint It was absolutely mandatory that

we rethink the traditional approach ofworking in isolation from the commer-cial industry It wasnÕt cost-eÝective, itwasnÕt the best way to do our job, and

it wasnÕt the best way to deliver nationalsecurity And yet if you got too far outfrom the mind-set of the Pentagon, itwas clear that the tanks would roll in.And guess whatĐthe tanks rolled in.Ĩ

In 1990 then ARPA rector Craig Fields wasrolled out, ostensibly forapproving a $4-millionresearch project thatPrabhakar had arrangedwith a gallium arsenideÞrm Critics charged that

di-it was pure venture talism, but a later De-fense Department reviewvindicated PrabhakarÕsdecision

capi-As battle lines againform, this time over theATP, Prabhakar hasmoreĐand more power-fulĐallies than before.ỊAll my bosses Þrmly be-lieve that this is the way

we should be going,ĨPrabhakar exclaims withconviction Republican chairs of keysubcommittees in both houses of Con-gress have voiced more fondness forthe program than animosity toward it.But whether the industrial beneÞciaries

of the ATP will come to its rescue isless certain

Prabhakar acknowledges that life inthe political spotlight might make some

of her 3,200 employees uncomfortablywarm ỊThe days are over when NISTwas invisible,Ĩ she comments ỊThatÕsthe price you pay for stepping up tothese big challenges.Ĩ But she for one isunwilling to retreat to the former isola-tion of her ivory tower ỊYou canÕt al-low yourself to get co-opted by the sys-tem,Ĩ she says ỊThereÕs really no point

in coming to work in the morning less you can push the boundaries ofwhatÕs possible.Ĩ ĐW Wayt Gibbs

un-48 SCIENTIFIC AMERICAN April 1995

PRABHAKAR defends her programs at a Senate hearing.

Trang 26

Perhaps our fascination with frogs

and other amphibians starts in

childhood, with the discovery of

tadpoles and the observation of their

metamorphosis But for many adults

today, interest stems more from

obser-vation of another type of change:

am-phibian populations in many parts of

the world seem to be dwindling, and

some groups are disappearing from

their native habitats completely The

lossÑÞrst recognized as a global

phe-nomenon in 1990Ñdeserves attention

not only because it is disturbing in its

own right but also because frogs and

their kin (mainly toads and

salaman-ders) may serve as indicators of the

overall condition of the environment

Amphibians are valuable as gauges

of the planetÕs health for a few reasons

First, they are in intimate contact with

many components of their natural roundings For example, as larvae, frogslive in water, but as adults most Þndthemselves at least partially on land

sur-Their moist, delicate skins are thinenough to allow respiration, and theirunshelled eggs are directly exposed tosoil, water and sunlight As larvae, theyare herbivores and as adults, carnivores

Because amphibians sample many parts

of the environment, their health reßectsthe combined eÝects of many separateinßuences in their ecosystem Second,these animals are good monitors of lo-cal conditions because they are home-bodies, remaining in fairly conÞned regions for their entire lives What hap-pens to frogs and their brethren is hap-pening where humans live and mightaÝect our species as well

Finally, amphibians are so varied thatany single characteristic, unique to theclass, can be dismissed as the cause ofthe dwindling numbers; hence, we sus-pect that environmental factors are in-deed the main cause for their decline

Amphibians are diverse in color, form,behavior and natural history They vary

in physical size, reproductive capacityand population density And they arefound in many ecosystems and habi-tats, including deserts, grasslands andforests, from sea level to high mountain-tops Although these creatures are mostabundant in the tropics, they are alsocommon in temperate zones and caneven be found at higher latitudes, such

as in Alaska and northern Canada

Which environmental factors might

account for the rapid decline of mals that have managed, over hundreds

ani-of millions ani-of years, to survive eventsthat led to the mass extinction of manyspecies, including the dinosaurs? Theexplanations that have been proposedare almost as diverse as the amphibianspecies in jeopardy, ranging from de-struction of habitat to natural ßuctua-tions in population size

One or more of the suggestions doseem to explain the shrinkage of manypopulations But in other cases, the rea-sons for the declines are not obvious

In those instances, the damage may becaused by subtle, interacting aspects ofregional or even global conditions Inparticular, recent work, completed lastspring, has led to the surprising discov-ery that stratospheric ozone depletionmay well be harming amphibian species

in some parts of the world

We began to suspect that the ozoneproblem might play a role as a result ofstudies that one of us ( Blaustein) and

The Puzzle of Declining

Amphibian Populations

The number of frogs, toads and salamanders is dropping

in many areas of the world The causes range from destruction

of their local habitats to global depletion of the ozone layer

by Andrew R Blaustein and David B Wake

ANDREW R BLAUSTEIN and DAVID B

WAKE combine their interests in

behav-ioral ecology and evolutionary biology in

their studies of amphibians Blaustein, an

ecologist, is a professor at Oregon State

University Wake, an evolutionary

biolo-gist, is a professor at the University of

California, Berkeley Both Blaustein and

Wake belong to the Species Survival

Commission of the World Conservation

Union Blaustein is co-chairman of the

Pacific Northwest Section of the Task

Force on Declining Amphibian

Popula-tions Wake was the founding chairman

of the International Task Force on

De-clining Amphibian Populations

ULTRAVIOLET RADIATION breakingthrough the dwindling ozone shield inthe stratosphere is a newly discovereddanger to amphibians who lay their eggs

in the open The radiation, which can

damage DNA (inset ), has recently been

shown to account for severe losses offertilized eggs in at least two such spe-cies in the Cascade Mountains of Ore-

gon, including the Cascades frog (Rana

cascadae) depicted here The egg

loss-es, in turn, may have led to declines in

Trang 27

his students began in Oregon in 1979.

The most recent experiments, often

conducted at relatively high elevations

(above 4,000 feet) in remote,

undis-turbed parts of the Cascade Mountains,

examined various aspects of life for

sev-eral species of amphibians monitored

from the egg, or embryonic, stage

through the tadpole phase and into

adulthood Although the group

intend-ed to carry out a straightforward survey

of amphibian behavior and ecology, it

discovered some unexpected results

The Role of Ultraviolet Rays

As part of the research, the team

doc-umented massive die-oÝs of

fertil-ized eggs in two species in particular:

the Cascades frog (Rana cascadae) and

the western toad (Bufo boreas)

Addi-tionally, over the course of 10 years, thegroup noticed that the numbers of adults of these species were dropping

The investigators guessed that theshrinking numbers of adult frogs andtoads could result from the fact that sofew fertilized eggs survived, and thusthey began to explore the reasons forthe damage to the eggs

The researchers quickly ruled outthe possibility that the chemistry

of the water where the animalswere laying their eggs was atfault They brought eggs intothe laboratory and rearedthe resulting embryos in

a sample of the samelake water in whichother eggs left be-

ULTRAVIOLET RAYS

ABSORBED ULTRAVIOLET RAYS

UNABSORBED ULTRAVIOLET RAYS

SITE OF

DAMAGE

OZONE LAYER

Trang 28

hind had perished The embryonic frogs

and toads developed and hatched

nor-mally in the laboratory Furthermore,

chemical analyses of the lakes and

ponds where eggs died revealed no

ob-vious pollution or excess acidity

By the late 1980s another possible

cause of egg destruction had presented

itself Scientists in several disciplines

documented a decrease in the

strato-spheric ozone shield that blocks most

ultraviolet rays from reaching the

ground These observations led

Blau-stein and his co-workers to wonder

whether increased exposure to

ultravi-olet radiation could explain the

repro-ductive problems they had seen They

also thought it might explain why many

of the amphibian species known to be

in decline were mountain dwellers that

lay their eggs in open, often shallow,

wa-ter Such eggs undergo prolonged

expo-sure to sunlight and thus to any

ultra-violet radiation that passes through the

ozone shield

The researchers speculated that

ex-cessive exposure to ultraviolet radiation

could be contributing to the problems

of the Cascades frog and western toad

because they were aware of evidence

showing that ultraviolet rays can

dam-age plant and animal life In particular,

ultraviolet-B radiation (with a

wave-length ranging from 280 to 320

nano-meters), in the middle of the ultraviolet

spectrum, is especially harmful to living

organisms In humans, for example, it

can suppress the immune system, cause

cataracts and contribute to skin cancer

What is more, as early as the mid-1970s,

Robert C Worrest of Oregon State

Uni-versity had shown that ultraviolet-B rays

could cause amphibian embryos to velop abnormally in the laboratory

de-Few experiments, however, had sidered the consequences of ultravioletradiation on amphibians or other ani-mals in nature Blaustein and his col-leagues therefore set out to determinewhether increasing levels of ultraviolet-Bradiation could play a role in the decline

con-of amphibian populations in the wild

The team for this enterprise includedecologist Susan C Walls and moleculargeneticists John B Hays and Peter D

HoÝman, as well as graduate students

D Grant Hokit and Joseph M

Kieseck-er, all then at Oregon State University

The crew based its procedure on anunderstanding of how ultraviolet radia-tion aÝects DNA When DNA absorbsenergy from such rays, the bonds thathold the molecule together break, andnew structures are formed The changes

in DNA can disrupt the functioning ofcells and may even kill them But manyorganisms have the capacity to repairDNA damage caused by ultraviolet ra-diation As part of this process, some ofthose organismsÑincluding certain spe-cies of algae, plants, Þsh, marsupialsand amphibiansÑactivate an enzymeknown as photolyase, which removesthe harmful structures

By measuring the amount of lyase produced in the eggs of variousamphibians, the workers found that lev-els varied among species Most impor-tant, they determined that species withfalling populations were generally thosewith eggs that produced low levels ofphotolyaseÑand therefore had littleprotection from ultraviolet radiation

photo-The species with the most photolyase,

the PaciÞc treefrog (Hyla regilla), was

not suÝering from a decrease in lation PaciÞc treefrog eggs have aboutthree times as much photolyase as doCascades frog eggs and six times asmuch as western toad eggs Hence, itseems that because the embryonic Cas-cades frogs and western toads producelow levels of photolyase, they do notmake enough of the enzyme to counter-act exposure to unusually high amounts

popu-of ultraviolet radiation This lack popu-of tection in turn may lead to the high mor-tality observed for the eggs and explainwhy these two species are candidatesfor threatened status in some states.Once the group noticed a correlationbetween lack of protection from ultra-violet radiation and declining popula-tion, the next challenge was to Þnd sup-porting evidence that the rays were ac-tually at fault The team collected freshlylaid eggs of Cascades frogs, westerntoads, PaciÞc treefrogs and northwest-

pro-ern salamanders (Ambystoma gracile).

The salamanders, like the other threegroups, lay their fertilized eggs in open,shallow water Additionally, these sala-manders produce extremely low levels

of photolyase

Exposed Eggs Fail to Hatch

The researchers placed the eggs inthe bottom of screened enclosures.Atop one third of the containers theyplaced a cover of clear plastic ( Mylar )that shielded the eggs from ultraviolet-

B radiation A second set remainedopen, fully exposing the eggs On theremaining third of the boxes, theyplaced a clear cover of plastic acetate

CAUSES OF AMPHIBIAN DECLINES arevaried The most signiÞcant threat topopulations remains habitat destruc-tion, such as the burning of Brazilian

rain forests ( far left ) Other proposed

causes include diseases, such as

infec-tion by the Saprolegnia fungus (left ),

Trang 29

that allowed transmission of radiation.

This treatment served as a control to

en-sure that the outcome observed in

shad-ed boxes was not causshad-ed by the covers

The workers placed a total of 48

box-es randomly around lakbox-es and ponds at

several diÝerent sites where each

spe-cies normally lays its eggs The

experi-ments on frog and toad eggs were

con-ducted in the spring of 1993 at

rela-tively high altitudes ( greater than 4,000

feet) in the Cascade Range of Oregon

The team studied the eggs of the

north-western salamanders in the foothills of

the Oregon Coast Range (600-foot

ele-vation) during 1994 The research

con-tinued until all the eggs either hatched

or perished, a process that took from

one to two weeks because of varying

weather conditions

If it were true that an inability to

combat the harm caused by excessive

exposure to ultraviolet radiation was

destroying the eggs of many

amphib-ian species, the producers of the lower

amounts of photolyase would be

ex-pected to fare worse, and the

produc-ers of higher levels, better The results

of the Þeld experiments were dramatic

More than 90 percent of the

northwest-ern salamander eggs exposed to

ultra-violet-B radiation died (compared with

45 percent of eggs protected from the

rays) More than 40 percent of the

ex-posed western toad and Cascades frog

eggs died (compared with 10 to 20

per-cent of the shielded eggs) In contrast,

almost all the eggs of PaciÞc treefrogs

in all three experimental treatments

hatched successfully

Clearly, amphibian eggs in wild

pop-ulations were dying from exposure to

ultraviolet-B radiation And this age to the eggs was very possibly con-tributing to the decline in adult popula-tions that had been observed earlier In-vestigators do not know whethernorthwestern salamanders are disap-pearing, but if these experiments areany indication, chances are good thatthose creatures, too, are in jeopardy

dam-By what mechanism does ultravioletradiation lead to the destruction of am-phibian eggs and embryos? Other re-search by Blaustein and his colleaguesmay have uncovered a partial explana-tion It turns out that since the late1980s, increasing numbers of amphib-ians in Oregon have been sickened by

the fungus Saprolegnia, which is found

naturally in lakes and ponds The gus is also known to infect hatchery-reared Þshes, especially salmon andtrout Perhaps Þsh that have been re-leased into lakes and are infected with

fun-Saprolegnia contaminate amphibian

eggs in those waters Because let rays can impair immune function inmany animals, it seems reasonable toguess that some amount of egg damage

in amphibians is caused by an let-induced breakdown in the ability ofamphibian embryos to resist infection

ultravio-by the fungus

Aside from harming fertilized eggs,ultraviolet radiation may contribute todeclines in amphibian populations byreducing the supply of aquatic insects

on which frogs and their relatives feed

High levels of such radiation have beenknown to kill insect larvae as well asaquatic algae

The work in Oregon has provided onepotentially important clue to the mys-

tery of amphibian disappearance Butmany questions still remain How manyeggs can fail to hatch before a popula-tion itself begins to decline? Does ultra-violet radiation harm growing tadpolesthat congregate in shallow water? Andare adults that bask in sunlight affect-

ed directly by ultraviolet radiation? Thetwo of us are now beginning to focus

on these issues

The Threat of Habitat Destruction

As worrisome as the increase in traviolet radiation seems to be, it isnot the only potentially signiÞcantcause of shrinkage of amphibian popu-lations In the Monteverde cloud forest

ul-of Costa Rica and in the Australian rainforests, for example, amphibians typi-cally live under a dense foliage canopyand hide their eggs Yet many of theirnumbers are also in decline

One of us ( Wake) has been gating causes of dwindling amphibianpopulations since the 1970s, when theÞrst hints of a problem began to emerge.The issue is indeed compelling, for al-though evidence of falling numbers isstrong in various parts of the world, inother areas amphibians appear to bedoing well This puzzling situation hasprompted us and others to examineclosely the possible reasons for the de-clines we have seen

investi-No single explanation Þts every case,but all seem to be important to one de-gree or another Destruction and modi-Þcation of habitat are probably the mostserious causes of falling amphibian pop-ulations Like other animals, amphibiansare threatened when forests are de-

and human consumption of frogs (right ).

Such consumption was a particular

prob-lem in the U.S in the early part of this

century, before protective measures

be-gan taking eÝect Pollution ( far right )

of waterways and of the air also

Trang 30

stroyed and wetlands are Þlled in or

paved Indeed, such activities probably

account for the decrease in a majority

of species threatened today

In one striking example of this

phe-nomenon, a recent survey in western

North Carolina showed that

clear-cut-ting of national forests leads to the

deaths of enormous numbers of

sala-manders every year Although most of

the species involved have relatively large

geographic ranges and are not in

dan-ger of extinction, the Þndings have

dis-tressing implications for amphibians

living in tropical America, Africa and

Asia There many amphibians are more

vulnerable because they have very

lim-ited geographic ranges

Pollution Plays a Part

Pollutants, too, may have altered

am-phibian populations in some parts

of the world, although data on the

ef-fects of pollution on these creatures are

sparse Some evidence suggests that

acid rain and snow, fungicides,

herbi-cides, insecticides and industrial

chem-icals may all act by impairing the

re-production and development of

am-phibians Certain synthetic compounds

can mimic the activity of naturally

oc-curring hormones Examination of birds,

Þsh and reptiles indicates that these

substances can have drastic

consequenc-es, such as a reduction in sperm count

and the alteration of male genitalia

DiseasesÑpossibly related to mental pollutionÑseem to jeopardizesome amphibians as well Recall, for in-stance, that eggs of the Cascades frogand western toad are vulnerable to the

environ-fungus Saprolegnia and that

suscepti-bility to the fungal infection is bly increased by exposure to excessiveultraviolet radiation Further, the lateArthur N Bragg, when he was at theUniversity of Oklahoma, showed that

proba-Saprolegnia can destroy whole

popula-tions of tadpoles, although this ery has been largely overlooked as acause of amphibian deaths

discov-So far only a few studies have linked

a disease to the extinction of an entirepopulation of amphibians Investigatorshave found, however, that the bacteri-

um Aeromonas hydrophila may have

triggered the disappearance of severalpopulations of western toads in Colora-

do The bacterium is highly contagiousand has been implicated as well in thedeath of adult frogs, toads and sala-manders in several other states

Some scientists attribute the ent shrinkage of amphibian popula-tions to natural ßuctuations in popula-tion size Yet certain long-term investi-gations show a more or less steadydecline in the number of amphibiansover the past 20 to 30 yearsÑan indi-cation that in some populations otherforces are at work

appar-Additional causes may explain

isolat-ed cases of dropping numbers of phibians Some populations may be de-creasing because they are collected forhuman consumption In France, for in-stance, the demand for frog legs is tre-mendous: the French eat 3,000 to4,000 metric tons of them a year Some20,000 frogs must be sacriÞced in order

am-to supply a single metric am-ton of legs.And before the turn of the century, red-

legged frogs (R aurora) were probably

overharvested as a food source in gon and California

Ore-Ironically, eÝorts to boost amphibianpopulations in the western U.S proba-bly created more problems for the na-tive amphibians there and provided anillustration of yet another possible cause

of population declines: the introduction

of nonnative species to an area

To make up for decreases in the ber of red-legged frogs, inhabitants ofOregon and California introduced the

num-bullfrog (R catesbeiana) This animal,

with its voracious appetite, competedwith or preyed on native amphibians

in its new habitat Noting that the troduced bullfrogs have become quiteabundant in some places where the orig-inal frog species have declined, manybiologists have recently suggested thatbullfrogs are a major cause of fallingnumbers And at least two ongoingstudies have directly linked the intro-duction of bullfrogs to the dwindling ofnative frog species

in-Similarly, introduction of Þsh into anecosystem may hurt amphibians, espe-cially in regions with few species of Þsh,low numbers of individual Þsh or noÞsh at all In the southern Sierra NevadaMountains of California, the introduc-tion of salmon and trout into streamshas been implicated in the demise of

mountain yellow-legged frogs (R

mus-cosa) These Þsh species directly harm

Sharp-Snouted Torrent Frog

(Taudacty-lus acutirostris), found in Australia

Cause of decline unknown

Common Toad (Bufo bufo),

found in Europe

Tiger Salamander (Ambystoma tigrinum),

found in North America

Pollution, possibly acid precipitation

Harlequin Frog (Atelopus varius),

found in Costa Rica

AMPHIBIAN SPECIES shown here areamong the many with dwindling popu-lationsÑor are already extinctÑfor rea-sons that are now being investigated in

Mountain Yellow-Legged Frog (Rana

muscosa), found in California

Introduction of fish to habitat

Trang 31

amphibians by eating eggs, tadpoles and

even adults, but they also have a broader

and potentially more profound result

Many separate amphibian populations

are linked to one another by streams

patrolled by few or no Þsh These links

are important because frogs and their

relatives are extremely vulnerable to

changes in their local habitat, and they

rely on the appearance of occasional

migrants to help them rebuild

dimin-ished communities The addition of new

Þsh species into an area can block

mi-gration between communities and thus

prevent the reconstitution of

endan-gered populations

Dangerous Consequences

The disappearance of amphibians

represents more than just a loss of

esthetically and behaviorally appealing

creatures These animals are crucial

components of many ecological

com-munities, and they can directly beneÞt

humans In some ecosystems,

amphib-ians are the most abundant vertebrates,

and so their absence can seriously

dis-rupt the functioning of the rest of the

ecological community Adult

amphib-ians are hunters of various animals,

in-cluding mosquitoes, ßies, Þsh, birds and

even small mammals Also, amphibian

larvae serve as a food supply for aquatic

insects, Þsh, mammals and birds

De-struction of frogs, toads and

salaman-ders thus has repercussions elsewhere

in the food chain

From the perspective of humans,

am-phibians represent a storehouse of

pharmaceutical products waiting to be

tapped fully Hundreds of chemical

se-cretions have been isolated from

am-phibian skin, and scientists are just

be-ginning to learn how valuable these

substances may be Some of these

com-pounds are already used as painkillers

and in treatment of victims of traumasranging from burns to heart attacks

Others are being investigated for theirantibacterial and antiviral properties Asamphibians disappear, potential curesfor a number of maladies go with them

The evidence that depletion of theozone shield in the stratosphere canharm the developing embryos of am-phibians highlights the complexity ofthe forces leading to the elimination ofspecies Nevertheless, habitat degrada-tion and destruction clearly remain themost powerful causes of amphibian dis-appearance around the world If habitatmodiÞcation occurs slowly enoughÑas

it did for 3,000 years in western ropeÑamphibians can adjust and even

Eu-adapt to human-induced alterations.But many of the changes we have dis-cussed, such as rises in ultraviolet levelsand in the amounts of pollutants in theenvironment, have occurred so rapidlythat species with long generation timesoften cannot adapt quickly enough.There are a lot more species of am-phibians than scientists studying them

Of those that are known, many havebeen seen only once, at the time of theirdiscovery The number of species de-scribed continues to increase at a rate of

1 to 2 percent a year If, as we believe,many of these species are at risk, awonderfully diverse group of creatures

is vanishing from the planet at a timewhen study of them has just begun

DECLINING AMPHIBIAN POPULATIONS

Da-vid B Wake in Science, Vol 253, page 860;

August 23, 1991

AMPHIBIAN DECLINES: JUDGING STABILITY,PERSISTENCE, AND SUSCEPTIBILITY OF POP-ULATION TO LOCAL AND GLOBAL EXTINC-TION Andrew R Blaustein, David B Wake

and Wayne P Sousa in Conservation

Biolo-gy, Vol 8, No 1, pages 60Ð71; March 1,

1994

AMPHIBIAN DECLINES AND CLIMATE TURBANCE: THE CASE OF THE GOLDENTOAD AND THE HARLEQUIN FROG J Alan

DIS-Pounds and Martha L Crump in vation Biology, Vol 8, No 1, pages 72Ð85;

Conser-March 1, 1994

PUTTING DECLINING AMPHIBIAN TIONS IN PERSPECTIVE: NATURAL FLUCTU-ATIONS AND HUMAN IMPACTS Joseph H

POPULA-K Pechmann and Henry M Wilbur in petologica, Vol 50, No 1, pages 65Ð84;

Her-March 1994

UV REPAIR AND RESISTANCE TO SOLAR

UV-B IN AMPHIUV-BIAN EGGS: A LINK TO TION DECLINES? Andrew R Blaustein, Pe-ter D Hoffman, D Grant Hokit, Joseph M.Kiesecker, Susan C Walls and John B Hays

POPULA-in ProceedPOPULA-ings of the National Academy of Sciences, Vol 91, No 5, pages 1791Ð1795;

Natterjack Toad (Bufo calamita),

found in Great Britain

Pollution, particularly acid rain

Gastric Brooding Frog (Rheobatrachus

silus), found in Australia (possibly extinct) Cause of decline unknown

Golden Toad (Bufo periglenes),

found in Costa Rica (possibly extinct)

detail In many cases, the declines

re-main puzzling For each species, the

causes of trouble implicated are given

next to each picture

Western Toad (Bufo boreas),

found in western North America

Disease related to ultraviolet radiation

Trang 32

Glowing comets and brilliant

au-roras are visible reminders that

space in the inner solar system

is far from empty This region is

per-meated by swiftly ßowing charged

par-ticles emanating from the sun, a

con-tinuous torrent of solar wind that often

blows in sudden gusts The fetch of

this wind extends well past the orbit of

the earth or the range of visible comets

The outward rush of particles and the

solar magnetic Þeld carried with them

carve an enormous spherical cavity in

the interstellar medium that reaches

far beyond the orbit of the most

dis-tant planets of the solar system This

immense region, a bubble of solar

dom-inance within the vastness of space, is

called the heliosphere

One might imagine that with

increas-ing distance from the sun the

helio-sphere gradually fades to a diÝuse

boundary wherein particles of the solar

wind gently mix with the interstellar

breeze of dust and gas But this is not

at all the case: near the limits of the

out-er heliosphout-ere lies an abrupt nuity at which a myriad of intriguingphysical phenomena are thought likely

disconti-to occur As of yet, however, icists have no direct measurements ofthe heliosphereÕs outer margins and somust infer, theorize or simply specu-late on its exact nature We do not evenknow with any certainty how far fromthe sun this boundary forms But ourignorance of the distant reaches of theheliosphere may last only a few moreyears, when space probes Þnally breakthrough this Þrst barrier toward inter-stellar space

astrophys-Past the orbits of Neptune and Pluto,

on trajectories taking them beyond theedges of the solar system, drifts a smallßotilla of spacecraft This modest scien-

tiÞc armada consists of Pioneer 10 and

11, along with Voyager 1 and 2, all of

which were launched about two decadesago If we were to look back at the solarsystem from any of these spacecrafttoday, the sun would be the brightestobject in view, but it would nonethelessappear more than 1,000 times dimmerthan as seen from the earth Even atthese great distances, though, the fourspacecraft remain well within the helio-sphere Onboard instruments continue

to register disturbances originating onthe sunÕs surface that propagate out-ward at about 400 kilometers per sec-ond Despite this enormous velocity,these sudden gusts still take manymonths to reach the probes

The original mission of the Pioneer and Voyager spacecraftÑto study the

giant planets Jupiter, Saturn, Uranusand NeptuneÑstands as one of the en-during triumphs of space exploration

But the continuing vitality of these fourprobes after the celebrated planetaryßy-bys and our growing awareness ofthe complex and dynamic behavior of

the distant solar wind have engendered

an important second mission for theseversatile scientiÞc minions: to study themost remote parts of the heliosphereand its interface with the interstellarmedium The success of this newly es-tablished mission depends not only onthe technical capabilities of the space-craft and their earth-based controllersbut also on the nature of the helio-sphere itself

The Outer Heliosphere

The general structure of the solarwind and the heliosphere was Þrstoutlined three decades ago by Eugene

N Parker of the University of Chicago

in a series of revolutionary theoreticalpapers Observations have since veriÞedthe structure he predicted, at least out

to the position of the farthest

space-craft, Pioneer 10 We now have

con-Þrmed that the solar wind, as it movesradially outward from the sun, bringswith it the imprint of the solar atmo-sphere: certain parts of the sun emithigh-speed wind in vast streams thatßow hundreds of kilometers per secondfaster than the typical wind If the sunwere stationary, those streams wouldsimply form linear rays, but because itrotates every 27 days, fast streams orig-inating near the solar equator can over-take slower streams emanating fromadjacent areas on the surface Some-times this conÞguration lasts for sever-

al solar rotations, setting up regions inspace where the interface between fastand slow streams of solar plasma be-comes spiral in shape These irregulari-ties rotate with the sun and are known

to space physicists as co-rotating action regions

inter-In addition, some parts of the solaratmosphere can eject irregular puÝsand gusts that travel as transient distur-bances in the wind Occasionally, huge

Quest for the Limits

of the Heliosphere

Four aging spacecraft are racing to the outer reaches

of the solar system Soon they may break through

the last barriers to interstellar space

by J R Jokipii and Frank B McDonald

CDON-ALD have been involved in the study of

energetic particles in the heliosphere for

many years Jokipii has been professor

of planetary sciences and astronomy at

the University of Arizona since 1974 and

has served as an interdisciplinary

inves-tigator on the Ulysses mission His major

scientiÞc interest is the origin and

prop-agation of energetic particles in space

McDonald is a senior research scientist

at the University of MarylandÕs Institute

for Physical Science and Technology and

has been a principal investigator on

many National Aeronautics and Space

Administration projects, in particular the

cosmic-ray experiments on the Pioneer

and Voyager spacecraft He has also been

active in planning other scientiÞc

mis-sions, such as the Interplanetary

Moni-toring Platform and High Energy

Astro-nomical Observatory series as well as

the Compton Gamma Ray Observatory.

Trang 33

eruptions on the sun produce blast

waves that severely disrupt the solar

windÕs more steady currents

The many varied interactions of the

solar-wind plasma produce shock waves,

which heat the wind and also generate

energetic particles Co-rotating

interac-tion regions and their associated shock

waves are a major feature of the solar

wind out to more than 10 astronomical

units ( One astronomical unit, or AU, is

the radius of the earthÕs orbit around

the sun, some 150 million kilometers,

or 93 million miles.) Farther out, such

interaction regions combine, forming

so-called global merged interaction gions, which populate space to the out-

re-er reaches of the heliosphre-ere

The Interplanetary Magnetic Field

Embedded within the stream ture of the heliosphere lies a com-plex interplanetary magnetic Þeld Theinterplay of the magnetic Þeld and so-lar wind can be rather complex; some

struc-of this behavior, however, can be

readi-ly visualized in terms of the familiarconcept of magnetic lines of force andthe properties these Þeld lines give to

the solar wind as it expands outward

By earthly standards the plasma thatconstitutes the solar wind might seemrather insubstantial and formless Yetbecause it is a good electrical conduc-tor and because the kinetic energy ofthe ßow is so much greater than theenergy of the magnetic Þeld, the mag-netic-Þeld lines in the heliosphere can

be treated as though they move withthe solar wind, being eÝectively Òfroz-

en in.Ó This frozen magnetic ßux lendsthe tenuous plasma added pressureand viscosity These properties developfrom magnetic forces rather than from

HELIOSPHERE encompasses that part of space dominated by

the solar wind Near its outer margins, outßowing solar

plas-ma (red arrows) is deßected by the ßow of interstellar gas at

the heliopause (purple), but only after slowing abruptly at

the spheroidal termination-shock front A bow shock (white) may also form in the interstellar gas Data from Pioneer (yel-

low arrows) and Voyager (orange arrows) should improve

understanding of the heliosphereÕs farthest boundaries

Trang 34

the more familiar molecular

interac-tions found in denser ßuids So the

he-liosphere contains a tangle of

magnet-ic-Þeld lines, stretched out by the wind

into an enormous spiral whorl, which

until recently had hardly been explored

Early observations indicated that the

magnetic-Þeld lines alternated between

being directed inward or outward from

the sun at diÝerent solar longitudes

The Þrst chance to measure the Þeld at

relatively high heliographic latitudes

came in 1974, after Pioneer 11 deßected

its trajectory out of the ecliptic plane

us-ing JupiterÕs large gravitational

attrac-tion to exchange momentum It came as

a surprise (although Michael Schulz, a

space physicist at Aerospace

Corpora-tion, had predicted it ) when Pioneer 11

and Voyager 1 reached the

heliograph-ic latitude of 16 degrees north and

dis-covered that the magnetic Þeld was

nearly always directed outward Edward

J Smith of the Jet Propulsion

Laborato-ry in Pasadena, Calif., concluded that

the spacecraft were observing magnetic

Þelds carried by the solar wind and that

at northern solar latitudes this Þeld was

oriented away from the sun

The Pioneer and Voyager missions

showed in 1976 that the sunÕs magnetic

Þeld was organized such that the Þeld

lines in the northern hemisphere

gen-erally pointed outward from the sun;

those in the opposite hemisphere

point-ed inward Because the polarity of the

sunÕs Þeld changes every 11 years (atthe time of the sunspot maximum), amagnetic cycle lasting 22 years results

So in 1986 the Pioneer 11 and Voyager

1 space probes detected that the

north-ern Þeld was pointed duly inward

In the heliosphere the transition tween the inward- and outward-directedmagnetic Þelds has the shape of a verythin, warped surface that is carried out-ward by the solar wind to form a vastinterplanetary current sheet Solar rota-tion twists the sheet so that the wrin-kles lie along spiral magnetic-Þeld linesand rotate with the sun Space probesnear the solar equatorial plane detectmagnetic Þelds that are alternately di-rected inward and outward as the cur-rent sheet rotates past them in space

be-The sheet is least wrinkled during solarminima, the periods of fewer sunspotsand lowered activity that occur every

11 years Its geometry becomes so voluted during solar maxima that thenormal sheetlike structure becomes en-tirely unrecognizable

con-But in whatever conÞguration, themagnetic-Þeld pattern originating atthe sunÕs surface is carried to the dis-tant margins of the heliosphere by thesolar wind over about a yearÕs time Dur-ing this period, fast plasma streamscontinue to merge with slower ones,spawning regions of enhanced plasmadensity and magnetic-Þeld strength

Out to some great but as yet unknown

distance, the large-scale ture of the solar wind and themagnetic Þeld is fundamentallypreserved

struc-The Termination Shock

As it travels, the solar wind pands over an increasinglylarge volume Eventually the so-lar-wind plasma is spread sothinly that it can no longer pushoutward against the small inwardpressure of the local interstellarmedium The wind does not slowdown gradually at this point, be-cause its velocity is greater thanthat at which disturbances canmove within it Instead the solarwind undergoes a sudden, violentchange in speed

ex-This behavior follows from thefundamentals of supersonic ßuidßow For the motion to diminishincrementally, the downstreammaterial must signal the up-stream ßuid to slow These sig-nals must be carried by soundwaves moving through the medi-

um But such waves cannot agate against ßows moving fasterthan sound As a result, the up-stream ßuid crashes into the ßuid ahead,setting up a confrontation called ashock wave Something similar occurs

prop-in a highway accident when cars hind cannot slow down fast enough toavoid hitting those ahead

be-Much as with a multicar accident, weexpect the solar-wind termination shock

to be irregular and turbulent As the lar-wind gas passes through the shock,its outward velocity should slow toabout one quarter of its original value.Some of the windÕs kinetic energy isconverted to heat, raising the tempera-ture of the interstellar gas to more than

so-a million degrees Celsius Some kineticenergy goes into compressing the mag-netic Þeld: we expect that Þeld strengthshould jump to about four times its val-

ue inside the shock So at this ary, where the solar wind trades out-ward velocity for heat and turbulence,

bound-we expect to Þnd a giant, spheroidalshock front with a complex but stillsomewhat mysterious structure.Evidence from Cosmic Rays

Before spacecraft oÝered direct surements, astrophysicists reliedmainly on the study of cosmic rays todeduce something of the nature of theouter heliosphere Galactic cosmic raysare subatomic particles (electrons, pro-tons, all the heavier nuclei from helium

mea-to uranium, positrons and a small

num-NEUTRAL CURRENT SHEET forms in the solar wind under the inßuence of the

opposite-ly directed magnetic Þelds above and below the sunÕs magnetic equator During the solar

cycle, the tilt of the magnetic equatorial plane changes along with rising and falling solar

activity This tilt, combined with the rotation of the sun, creates a spiral geometry (shown

here in idealized form) as the solar wind carries the trapped magnetic Þeld outward

Trang 35

Probes Built to Go the Distance

Deep-space missions present major technical

chal-lenges to spacecraft designers in the areas of weight,

power and communications The Pioneer missions called

not only for escape from the earth’s gravity but also for

sufficient energy to reach Jupiter with a travel time of two

years or less The solution was to use a high-energy

rock-et booster and to keep the spacecraft as light as

possi-ble—at launch Pioneer 10 weighed only 250 kilograms.

Reliable sources of electricity were also critical: far from

the sun, the only practical ones are radioisotope

thermo-electric generators (RTGs), which use the decay of

ra-dioactive materials to produce electricity from heat Each

Pioneer spacecraft has four RTG units, which initially

gen-erated a total of 155 watts The ultimate lifetime of these

missions will probably be defined by the radioactive

de-cay of the plutonium oxide fuel and the degradation of

the conversion elements within the RTG units

The Voyager probes were designed in the mid-1970s

using what had been learned from the earlier Pioneer

mis-sions A more powerful launch vehicle made it possible to

deploy heavier (825 kilograms) and more complex

space-craft: the Voyager design includes significant onboard

computer capability, an experiment platform with sion pointing and improved RTG units that supplied 470watts at launch The sophistication and flexibility of the

preci-Voyager system have been demonstrated by the

consider-able reengineering that was done in flight to prepare

Voy-ager 2 for its late-scheduled encounter with Uranus [see

“Engineering Voyager 2’s Encounter with Uranus,” by

Richard P Laeser, William I McLaughlin and Donna M.Wolff; SCIENTIFICAMERICAN, November 1986 ]

Communication with the Pioneer and Voyager probes

demands a large onboard antenna, the dominant

fea-ture The Pioneer craft support a 2.7-meter parabolic dish

antenna, somewhat smaller than the 3.7-meter dish found

on the Voyager probes The Pioneer antenna spins about

the spacecraft’s axis, which is kept pointed toward the

earth by the occasional use of small thrusters The

Voy-ager spacecraft do not spin but are stabilized about all

three axes so that the high-gain antenna can be kept rected toward giant antennae on the earth

di-The three sites of the National Aeronautics and SpaceAdministration’s Deep Space Network (in California, Aus-tralia and Spain) are among the most critical components

of the Pioneer and Voyager programs During the 20 yearssince the Pioneer 10 mission, the Jet Propulsion Laborato-

ry in Pasadena, Calif., has significantly upgraded this tem by adding new receivers and increasing antenna size.But such improvements cannot compensate for the falter-ing signal levels caused by the ever increasing distance tothe four space probes For more reliable communication,

sys-the total rate of data transmission from Pioneer 10 has been

slowed to its minimum value of 16 bits a second—about

as fast as one might send messages by Morse code The

Voyagers’ larger antenna and higher transmission

frequen-cy make possible the use of the more available 34-meterantenna of the Deep Space Network and allow communi-cation at 160 bits a second—slower than most computer

modems but still better than the telegraph from Pioneer.

RADIOISOTOPETHERMOELECTRICGENERATORS

8.415-GIGAHERTZHIGH-GAINANTENNA

COSMIC-RAYDETECTOR

PLASMADETECTOR

VOYAGER

GOLDSTONE 70-METER ANTENNA

RADIO-ASTRONOMYAND PLASMA-WAVEANTENNA

LOW-ENERGYPARTICLEDETECTOR

Trang 36

ber of antiprotons) that travel at close

to the speed of light and appear to

pop-ulate all parts of the universe Their

ubiquitous presence in the cosmos is

inferred from their ability to produce

high-energy gamma rays and radio

waves (which can be detected on the

earth) Within our galaxy, cosmic rays

commonly originate where the

shock-wave remnants of supernova explosions

are thought to accelerate the particles

to extremely high energies In addition,

during periods of heightened activity,

the sun, too, can occasionally produce

signiÞcant numbers of solar ÒcosmicÓ

rays of lower energy

The heliosphere is constantly bathed

by galactic cosmic rays These

cosmic-ray particles can diÝuse upwind against

the solar plasma because of their

ex-tremely high speeds and the presence

of irregularities in the Þeld Because of

their electrical charge, the particles

gy-rate tightly around magnetic-Þeld lines,

and as a result, cosmic rays also tend to

travel out of the heliosphere along with

the frozen-in magnetic ßux In general,

the solar wind acts to modulate the

in-tensity of cosmic rays impinging on the

earth, making it diÛcult for them to

reach the inner heliosphere This

exclu-sion is most eÝective at low energies;

highly energetic cosmic rays proceedlargely unaÝected Because the fractionexcluded varies with solar activity, cos-mic-ray intensity follows the 11-yearsunspot cycle: it peaks when solar ac-tivity is at a minimum

Galactic cosmic rays pass through theouter heliosphere, and so they can pro-vide valuable information about thisunexplored region Much of our under-standing of the outer reaches of the so-lar system has been derived by compar-ing models of how cosmic rays traversethe heliosphere with observations Forexample, data from the four deep-spaceprobes have shown that the cosmic-raygradientÑÑthe rate at which the intensity

of galactic cosmic rays increases withheliocentric distanceÑÑis much smallerthan was expected This Þnding indi-cates that the heliosphere is larger thanwas predicted before the launch of the

Pioneer and Voyager probes.

Anomalous Cosmic Rays

In the early 1970s, as the Pioneer

spacecraft moved toward Jupiter, tectors on a number of spacecraft re-vealed the existence of an unexpected

de-low-energy cosmic-ray component tinuing study has demonstrated an en-hancement at low energies for rays con-sisting of helium, nitrogen, oxygen,neon, argon and, most recently, hydro-gen nuclei This peculiar compositionand energy spectrum deÞne the anom-alous cosmic-ray component Observa-

Con-tions from the Pioneer and Voyager

spacecraft have shown that the

intensi-ty of the anomalous cosmic rays creases with distance from the sun.What is the origin of this mysteriouscosmic-ray component? Work over thepast two decades has painted a com-pelling picture of how these cosmic raysare generated, although its accuracy isnot completely proved

in-In 1974 Lennard A Fisk, Benzion lovsky and Reuven Ramaty, while at theNational Aeronautics and Space Admin-istration Goddard Space Flight Center,suggested that the anomalous compo-nent originates as neutral atoms in inter-stellar space As the heliosphere movesthrough the interstellar gas, neutralatoms, which are not aÝected by mag-netic Þelds or other forces of the plas-

Koz-ma, stream freely into the inner sphere Those that pass near the sun areionized by solar radiation or by the so-lar wind itself to become singly chargedions Once the neutral atoms becomeions, the magnetic-Þeld lines in the so-lar wind snare them and convect theseparticles outward Fisk and his col-leagues speculated that subsequent ac-celeration to higher energies turns theseions into the anomalous cosmic rays.The original basis for this suggestionwas that most of the carbon in the in-terstellar medium cannot take part inthis process, because carbon is almostcompletely ionized in interstellar space(which explains its very low abundance

helio-in the anomalous component) Recentobservations near the earth by a num-ber of space missions have demonstrat-

ed that the anomalous oxygen (and sumably also the other components) issingly charged This result supports themodel of Fisk and his co-workers: cos-mic-ray nuclei from a nearby source(within the heliosphere) can retain someelectrons, whereas normal cosmic raysare fully stripped of all their electronsduring their passage through the galaxy.But how were these newly formedions accelerated to the observed cos-mic-ray energies? During the 1970s, anumber of proposals were put forth.None, however, successfully predictedthe steady increase in the intensity ofthe anomalous cosmic rays registered

pre-by the Pioneer and Voyager probes as

they moved far out into the heliosphere.Then, in 1981, one of us ( Jokipii ), alongwith Mark E Pesses and David Eichler,

TERMINATION SHOCKS are not unique to the heliosphere; one can be seen in

wa-ter running in a kitchen sink The streaming wawa-ter initially radiates outward faswa-ter

than waves can propagate through it As a result, the surrounding ßuid cannot send

an inward signal that its motion has slowed A shock front forms where the fast- and

slow-moving parts of the ßuid abruptly collide The termination-shock boundary of

the heliosphere, like its water analogue, is likely to be irregular and turbulent

Trang 37

both then at the University of Maryland,

suggested that the acceleration of

sin-gly charged ions occurs at the

termina-tion-shock boundary Plasma shocks

can accelerate charged particles, and

this location seemed a likely site for

en-ergizing the anomalous cosmic raysÑ

it contains the strongest, most

long-lived shock anywhere in the heliosphere

Detailed computer modeling has since

shown that most observed features of

the anomalous component follow

natu-rally from this notion

Observations of the Shock Front

Important clues about the nature of

the termination region have been

col-lected by Donald A Gurnett and

Wil-liam S Kurth of the University of Iowa

Since 1983 they have registered

low-fre-quency bursts of radio noise (at two

to three kilohertz) using detectors on

board both the Voyager spacecraft The

signals persist for many months and

then gradually drift to higher

frequen-cies In July 1992 these researchers

ob-served the onset of a particularly strong

radio event and noted that it occurred

more than 400 days after an unusually

intense period of solar activity This

se-quence followed the same pattern as

an-other large noise burst in 1983 These

remarkable radio signals probably

orig-inate just beyond the termination shockand, along with the anomalous cosmicrays, provide tantalizing informationabout this vast unexplored frontier

As the Pioneer and Voyager space

probes speed farther and farther fromthe sun, there is an increasing likeli-hood that they will soon encounter thetermination shock Estimates based onwhat was then known about the inter-stellar medium had originally put thetermination-shock boundary anywherefrom 75 to 150 AU from the sun, butdata collected so far from the probeswould suggest considerably smaller val-ues So it is entirely possible that one ormore of the probes will reach the shockwithin the next decade

NASA scientists have therefore takensteps to ensure that the proper mea-surements will be made during passagethrough the shock Indeed, they believethey may have several opportunities toobserve it, as gusts and turbulence inthe solar wind move the terminationshock in and outÑperhaps leading tomultiple crossings as the front movesback and forth past the spacecraft Oncethe spacecraft Þnally pass beyond theshock, the wind will slacken, and, forthe Þrst time, an artifact of humanitywill begin to experience directly theeÝects of the interstellar plasma Per-haps then the true nature of the inter-

stellar medium will Þnally be clariÞed.Sometime in the 21st century, afterhaving reported the physical conditions

of the outer heliosphere and possiblythe termination shock itself, the fourspacecraft will continue their journey to

the stars Pioneer 10 should remain

op-erational until the turn of the century

(at about 70 AU ), and Voyager 2 has

enough consumables to last until about

2015 (at about 130 AU ) But even aftersteerage and communication are lost,for eons to come these probes will fol-low a well-charted course through ourgalaxy as four small man-made objectsadded to the gaseous clouds of inter-stellar space They go as the Þrst voy-agers from planet Earth, like small bot-tles tossed into an inÞnite sea

COSMIC-RAY PARTICLES generally show a smoothly varying

distribution of energies (left ), but certain nuclei are

strange-ly abundant at low energies (darker shading) This anomastrange-ly

arises from neutral particles that stream into the heliosphere,

become ionized and convect outward Particles accelerate tohigher energies by scattering oÝ magnetic irregularities on

both sides of the shock front (right ) Accelerated particles

deßect inward and are detected as anomalous cosmic rays

FURTHER READINGTHE HELIOPAUSE Steven T Suess in Re-

views of Geophysics, Vol 28, No 1,

pag-es 97Ð115; February 1990

PHYSICS OF THE OUTER HELIOSPHERE ited by S Grzedzielski and D E Page.COSPAR Colloquia Series, Vol 1 Perga-mon Press, 1990

Ed-EXPLORING THE SUN Karl Hufbauer.Johns Hopkins University Press, 1991

SOLAR WIND SEVEN Edited by E Marschand R Schwenn COSPAR Colloquia Se-ries, Vol 3 Pergamon Press, 1992

PARTICLE ENERGY(MILLIONS OF ELECTRON VOLTS)

SHOCK

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Why is an elephant big, dark and

strangely shaped?Ĩ the

ques-tion goes ỊBecause if it was

small, white and round, it would be an

aspirin.Ĩ This answer may ring funny to

human ears, but it could well prove

in-formative to a computer trying to

iden-tify such objects as elephants or aspirin

Knowledge we commonly take for

grant-ed is not available to machines unless

carefully spelled out For machines,

learning is not at all simple

Despite the challenges, machine

learn-ing is one of the fastest-growlearn-ing

tech-nologies today The past few years have

witnessed an explosion of applications,

ranging from automated reading of

handwritten zip codes at the post oÛce

to predicting seat demand in the airline

industry Indeed, the last time you

re-ceived a credit card from a bank,

chanc-es are it was approved by a machine

that learned on its own how to evaluate

credit risk And the future of machine

learning is on the rise

Designing a computer program to

handle a particular job almost invariably

demands a thorough understanding of

that task and its solution Machine ing therefore has a fundamental appeal

learn-Instead of devising a specialized gram, one could merely provide train-ing examples to a versatile machine thatwould learn on its own

pro-A self-learning credit-card approvalsystem would, for instance, use histori-cal data about ỊgoodĨ and ỊbadĨ cus-tomers to judge applicants The ma-chine does not care about the details ofthe problem All it does is take matchedpairs of inputs ( in this case, personalinformation) and outputs (credit behav-ior ) and absorb whatever informationtheir relation contains The trained ma-chine then serves to evaluate new ap-plicants This kind of procedure takesautomation one step further than nor-mally envisaged It not only applies acomputer to a repetitive task, it auto-mates the very problem of designing asystem to perform that task

One can, in principle, apply the odology of machine learning to a widearray of problems If, however, the in-put-output examples available lack vi-tal information, the machine may fail

meth-to acquire proÞciency Fortunately, onecan often append the needed informa-tion in the form of an intelligent hint

The hints used in machine learningrange from simple observations to so-phisticated knowledge

In computer-vision applications, forinstance, in which the goal is to recog-nize objects, there are many invariancehints These assert that an object re-mains the same object when it shiftsposition in the range of view or changes

in size In Þnancial-market applications,there are many monotonicity hints,which state that if an input consistentlyshifts in one sense or direction, the out-put must also consistently move justone way Each particular applicationhas its own hints that can aid the learn-ing process

If one knows enough about a givenapplication to oÝer hints, why botherwith machine learning in the Þrst place?Why not employ this knowledge to de-sign a specialized machine for the job?

In some instances one can do so, butthe fact of the matter is that usually toolittle is known about a problem to spec-ify a method for its solution according

to a well-deÞned set of rules

Applications range between two tremes: structured problems that aretotally deÞned and require no examples,and random problems that are com-pletely undeÞned and depend entirely

ex-on training examples for their solutiex-on.Machine learning using intelligent hints

is the way to handle the vast middleground

Machine-Learning Paradigm

How do machines learn? Many ferent models for machine learn-ing have been devised Typically the im-plementation used will have a generalstructure that is broadly tailored to theproblem, but it will also have many freeparametersĐthese might be thought of

dif-as the knobs and dials for tuning themachine The values given to these ad-justments determine how the machinewill ultimately act; diÝerent settings willproduce completely diÝerent results.The behavior of a machine can beviewed mathematically as a functionthat associates input values (the specif-ics of a problem to be solved ) with cor-responding output values (the decision

or action to be made) The goal in chine learning is to make the machineemulate the target function, the desiredmapping of inputs to outputs We canuse training examples from the targetfunction to guide the selection of valuesfor the machineÕs free parameters Witheach example, the machine reÞnes itsinternal settings so that it matches the

ma-YASER S ABU-MOSTAFA is professor

of electrical engineering and computer

science at the California Institute of

Technology and chairman of

NeuroDol-lars, a California-based corporation He

received a B.Sc from Cairo University in

1979, an M.S.E.E from the Georgia

Insti-tute of Technology in 1981 and a Ph.D

from Caltech in 1983, where he now

heads the Learning Systems Group and

serves as one of the principal

investiga-tors for the National Science Foundation

Center for Neuromorphic Systems

Engi-neering Abu-Mostafa has been a

techni-cal consultant for Citibank since 1988

He has published widely in the areas of

learning theory, neural networks,

pat-tern recognition, information theory and

computational complexity

Machines That Learn

from Hints

Machine learning improves significantly

by taking advantage of information available from intelligent hints

by Yaser S Abu-Mostafa

Trang 39

CAN YOU SOLVE IT ? These objects have been sorted into two

classes, indicated by either a blue or brown border Which

characteristic distinguishes them? Computers programmed

to learn from examples often face similar puzzles Providingthe machine with hints can make learning faster and easier.For a hint to help with this puzzle, turn the page

Trang 40

inputs and outputs appropriately Whenthe machine reaches a setting that cor-responds as closely as possible to thetarget function, it will have in eÝectÒlearnedÓ it Machine learning is simplythe search for the right positions forthe knobs Because the search is guid-

ed by the training examples, this digm is called, naturally enough, learn-ing from examples

para-The most widely applied form of suchmachine learning is the neural network[see ÒHow Neural Networks Learn fromExperience,Ó by GeoÝrey E Hinton; SCI-ENTIFIC AMERICAN, September 1992].Neural networks were inspired by thepower of real neurobiological systems.They consist of many computationalelements interconnected in such a waythat each elementÕs output reßects in-puts from a number of other elements.The adjustable parameters of a neuralnetwork are called synaptic weights after their biological counterparts, thesynapses that connect nerve cells inthe brain The ßexibility of neural net-works and the simplicity of their train-ing have made them the machine-learn-ing model of choice for the past 10years; neural networks now Þnd uses

in a broad range of machine-learningapplications Although specialized elec-tronic and even optical networks havebeen built [see ÒOptical Neural Comput-ers,Ó by Yaser S Abu-Mostafa and De-metri Psaltis; SCIENTIFIC AMERICAN,March 1987], in most cases, one imple-ments a neural network simply as aprogram running on a personal com-puter or workstation

With all the training required, wemight imagine the need for tedious late-night sessions at the computer, super-vising the machine as it learns Fortu-nately, responsibility for Þnding the op-timal adjustments usually falls on alearning algorithm, a method that re-duces the process to a series of simple,repetitive steps that the computer canperform independently One of themost common learning systems in usetoday is the back-propagation algorithmfor training neural networks This tech-nique was popularized primarily by Da-vid E Rumelhart while at the Universi-

ty of California at San Diego

Back-propagation uses simple lus to decide how to change the param-eters of the neural network It takes atraining exampleÑan input and its cor-responding outputÑand makes smallmodiÞcations to the network parame-ters to minimize the diÝerence betweenthe current response of the network andthe target response This step is repeat-

calcu-ed over and over, each time nudging thenetwork a bit closer to the desired eÝect.After going through all the examples

VISUAL HINT aids both machines and people in solving the puzzle Drawing the

axis makes it clear that the top six objects lack the mirror-image symmetry

exhibit-ed by the bottom three This characteristic distinguishes brown and blue categories

INVARIANCE HINTS can help machines recognize that objects do not lose their

identity when viewed in a new way A machine attempting to identify trees, for

in-stance, would not inherently know that size and position did not matter (top).

Training on ÒvirtualÓ examples of quite different subjectsÑsuch as a face or a

chairÑcould prompt the machine to grasp these principles

Tiêu đề	Machines that learn from hints
Tác giả	Yaser S. Abu-Mostafa
Trường học	Scientific American
Thể loại	Essay
Năm xuất bản	1995
Thành phố	New York

Định dạng
Số trang	80
Dung lượng	5,89 MB