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THE RISEOF THE MECHANICAL VIEW Thegreat mystery story Thefast clue Vectors The riddle of motion One clue remains Is heat a substance?. We know thatifthe sphere is is, ifan external force

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EVOLUTION OF PHYSICS

THE EVOLUTION OF PHYSICS

firstappeared in

OF MODERN SCIENCE,

a series ofnew books describing, in

languagesuitable for thegeneral

non-specialist reader, the present position

inmany branches ofmodern science.Theseries is editedbyDrC.P.Snow,

andispublishedby

N.W

THE SCIENTIFIC BOOK CLUB

in CHARING CROSS ROAD

BEFORE you begin reading, you rightly expect some

clearly and convincingly. This would be much easier,

find it simpler to say just what this book does not

physics Here is no systematic course in elementary

physical facts and theories Our intention was rather

to sketch in broad outline the attempts of the human

the active forces which compel science to invent ideas

representation had to be simple. Through the maze of

facts and concepts we had to choose some highway

Facts and theories not reached by this road had to be

omitted We were forced, by our general aim, to make

adefinite choiceoffactsandideas The importanceof a

VI PREFACE

the characteristics ofour idealized reader and worried

ideas and we were forced to admire the patience with

ones carefully. He knew that a scientific book, even

a novel

some idea of the eternal struggle of the inventive

human mind for a fuller understanding of the laws

A E.

Professors A G. Shenstone, Princeton, N.J., and StLoria, Lwow, Poland, for photographs onplate III

Dr M. Phillips for reading the manuscript and for

A E.

The riddle ofmotion 19

The philosophical background 55The kinetic theory of matter 59

II. THE DECLINE OF THE MECHANICAL VIEW The two electric fluids 71

The first serious difficulty 87

The velocityoflight 94

The riddle of colour 100

X CONTENTS

The realityofthe field page 148

Relativity and mechanics 202

IV QUANTA

I. THE RISE

OF THE MECHANICAL VIEW

THE RISE

OF THE MECHANICAL VIEW

Thegreat mystery story Thefast clue Vectors The riddle

of motion One clue remains Is heat a substance? The

switchback The rate of exchange The philosophical

back-ground The kinetic theoryofmatter

THE GREAT MYSTERY STORY

Such a story presents all the essential clues, and

follow the plot carefully, we arrive at the complete

solution for ourselves just before the author's disclosure

thoseofinferiormysteries,does not disappointus;

more-over, it

Can welikenthereader of such abooktothescientists,

solutions ofthe mysteries in the book ofnature? The

but it has a modicum ofjustification which may be

universe

This still unsolved We cannot

4 THE EVOLUTION OF PHYSICS

even be sure that it has a final solution The reading

with theclues already discovered. Tentativelyaccepted

theories have explained many of the facts, but no

yetbeen evolved Veryoftena seeminglyperfecttheory

New facts appear, contradicting the theory or

ad-vance

stories of Conan Doyle there comes a time when theinvestigator has collected all the facts he needs for atleastsome phase ofhisproblem These facts often seem

quite strange, incoherent, and wholly unrelated Thegreat detective, however, realizes that no further in-

vestigationisneededatthemoment, andthatonlypure

THE RISE OF THE MECHANICAL VIEW 5

con-firmationfor his theory

solutionfor himself;for he cannot, as impatientreaders

to explain, at least in part, the relation of events totheirrich context Toobtain even a partial solutionthe

scientist must collect the unordered facts available and

thought

shall be chiefly concerned with the role of thoughtsand ideas in the adventurous search for knowledge of

the physical world

human thoughtitself. Only alittle overthree hundred

years ago, however, did scientists begin to understand

Galileoand Newton, the reading has proceededrapidly

find-ing andfollowing clues, have been Some of

O THE EVOLUTION OF PHYSICS

the riddles ofnature have been solved, although many

ofthe solutions have proved temporary and superficial

in the light offurther research

All those motions we observe in nature that of a

stone thrown into the air, aship sailing the sea, a cart

with the simplest possible cases, and proceed gradually

upon it, to pushit or lift it, or let other bodies, such as

horses orsteamengines, act uponit. Ourintuitiveidea

risk the further statement that we must push harder if

the greater will be its speed A four-horse carriage

goes faster than a carriage drawn by only two horses

Intuitionthus tellsus thatspeedisessentially connectedwith action

It is a familiar fact to readers of detective fiction

solution The method of reasoning dictated by

were held for centuries Aristotle's great authority

THE RISE OF THE MECHANICAL VIEW ^

the long belief in this intuitive idea We read in the

Mechanics, for two thousand years attributedto him:

The discovery and use of scientific reasoning by

Galileo was one of the most important achievements

in the history ofhuman thought, and marks the real

intuitive conclusions based on immediate observation

must travel faster than one drawn by onlytwo?

more closely, startingwith simple everyday experiencesfamiliar to mankind since the beginning ofcivilization

We ask: how is it possible to increase this distance?

by the oiling and smoothing? Only this: the externalinfluences have been made smaller The effect ofwhat

is called friction has been diminished, both in the

8 THE EVOLUTION OF PHYSICS

evidence, an interpretation which is, in fact, arbitrary.

Onesignificantstep fartherand weshall havethe right

clue Imagine a road perfectly smooth, and wheelswith no friction at all. Then there would be nothing

to stop the cart, so that it would run for ever This

sinceit isimpossibletoeliminate allexternalinfluences.The idealized experiment shows the clue which really

the action, the greater the velocity Thus the velocity

is neither pushed, pulled, nor acted on in any otherway, or, more briefly, if no external forces act on a

same velocity along a straight line. Thus, the velocity

We have seen that law of inertia cannot be

THE RISE OF THE MECHANICAL VIEW 9

it leads to a profound understanding of real

experi-ments

From the variety of complex motions in the world

mo-tion This is the simplest, because there are no external

forces acting Uniform motion can, however, never be

realized; a stone thrown from a tower, a cart pushed

forces

often lead to the wrong suspects In our attempts to

Human thought creates an ever-changing picture of

the universe Galileo's contribution was to destroy the

intuitive view and replace it by a new one This is thesignificance ofGalileo's discovery.

But a further question concerning motion arises

im-mediately If the velocity is no indication of the ternal forces acting on a body, what is? The answer to

still more concisely by Newton, and forms a furtherclue in our investigation.

To find the correct answer we must think a little

IO THE EVOLUTION OF PHYSICS

In our idealized experiment the uniformity ofthe

a push in the direction ofthe motion What happens

ob-viously, a push in the direction opposite to that ofthe

cart is accelerated by the push, in the second case

decelerated, or slowed down A conclusion follows at

increases or decreases the velocityaccording towhether

it acts in the direction of motion or in the opposite

direction Galileo saw this clearly and wrote in his

.any velocity once imparted to a moving body will

accelerationorretardation areremoved, a conditionwhich

planes which slope downwards there is already present acause ofacceleration;whileonplanesslopingupwardsthere

is retardation; from this it follows that motion along a

horizontal plane is perpetual; for, if the velocity be

uni-form, it cannot be diminished or slackened, much lessdestroyed

By following the right clue we achieve a deeper

THE RISE OF THE MECHANICAL VIEW II

the basis of classical mechanics as formulated by Newton.

play principal roles in classical mechanics: force and

science both of these concepts are extended and

gener-alized Theymust, therefore, be examined more closely.

Whatis force? Intuitively, we feelwhatis meant by

this term The concept arose from the effort of

offorce even without picturing a horse pulling a riage! We speak of the force of attraction between

car-the sun and the earth, the earth and the moon, and of

those forces which cause the tides. We speak of theforce by which the earth compels ourselves and all theobjects about us to remain within its sphere of in-

fluence, and of the force with which the wind makes

force, in the general sense, must be held responsible.

Newtonwrotein his Principle,:

An impressed force is an action exerted upon a body,

in order to change its state, either of rest, or of moving

uniformlyforwardin arightline.

This force consists in the action only; and remains no

longer in the body, when the action is over For a body

maintains every new state it acquires, by its vis inertiaeonly Impressed forces are of different origins; as from

12 THE EVOLUTION OF PHYSICS

Ifa stone isdropped from the top of a tower its

the stone falls. We conclude: an external force is

act-ing inthedirectionofthe motion. Or, in other words:

the earth attracts the stone Let us take another

its highest point and begins to fall. This decrease invelocityis caused by the same force as the acceleration

of a falling body. In one case the force acts in thedirectionofthemotion,intheothercase inthe opposite

direction The forceis the same, but it causes tion or deceleration according to whether the stone is

VECTORS

farther Wegainanunderstanding ofthelaws ofnature

of our first attempts all intricate complications A

straight line is

impossible to be satisfied with an understanding of

rectilinear motion alone The motions ofthe moon, the

success, are motions along curved paths. Passing from

rectilinearmotiontomotionalong a curvedpathbrings

THE RISE OF THE MECHANICAL VIEW 13

of classical mechanics which gave us the first clues

ofscience

Let us consider another idealized experiment, in

table We know thatifthe sphere is

is, ifan external force is applied, the velocity will be

perpen-dicular to that line. What happens to the sphere?

the law of inertia, the velocities before and after theaction of the force are both perfectly uniform But

there is a difference between the uniform motion fore and after the action ofthe force: the direction is

direc-tion ofthe force are perpendicular to each other. The

final motion will be along neither ofthese two lines,

the force ifthe blow is ahard one and the initial

velo-citysmall, nearer the originallineofmotioniftheblow

is gentle and the initial velocity great Our new

con-clusion, based on the law ofinertia, is : in general the

action ofan external force changes not only the speedbut alsothe direction ofthemotion Anunderstanding

into physics by the vectors.

14 THE EVOLUTION OF PHYSICS

ofreasoning. The starting-point is again Galileo's law

ofinertia We are still far from exhausting the

Let us consider two spheres moving in different

di-rections on a smooth table So as to have a definitepicture, we may assume thetwo directionsperpendicu-

lar to each other Since there are no external forcesacting, the motions are perfectly uniform Suppose,

further, that the speeds are equal, that is, both cover

the same distance in the same interval of time But

is it correct to say that the two spheres have the same

velocity? The answer can be yes orno! Ifthe

isusualtosay thattheyhavethesamespeedorvelocity,

science must create its own language, its own

con-cepts, for its own use Scientific concepts often beginwith those used in ordinary language for the affairs,

rigor-ousness so that they may be applied to scientificthought

From the physicist's point ofviewit is advantageous

to saythat the velocities of the two spheres moving in

different directions are different Although purely a

four cars travelling away from the same traffic

THE RISE OF THE MECHANICAL VIEW 15

areallfortymilesperhour.Thisdifferentiationbetween

numberofunits.Thelengthof astickmay be3ft. 7 in.;

time interval so many minutes or seconds In each ofthese cases the result of the measurement is expressed

insuffi-cient for describing some physical concepts. The

re-cognition of this fact marked a distinct advance in

is essential for the characterization of a velocity, for

and called a vector. A suitable symbol for

l6 THE EVOLUTION OF PHYSICS

If four cars diverge with equal speed from a traffic

vectorsofthesamelength,asseenfrom ourlastdrawing

In the scale used, oneinch stands for 40m.p.h In this

way any velocity may be denoted by a vector, and

conversely, ifthe scale isknown, one may ascertain thevelocityfrom sucha vectordiagram

Iftwo cars pass each otheron the highway and their

velocities by two different vectors with arrows pointing

in opposite directions So also the arrows indicating

"uptown" and "downtown" subway trains in New

train passes or on which ofthe many parallel tracks it

THE RISE OF THE MECHANICAL VIEW

vectors alldifferent, because theydiffer either in length

or direction, or both The same four vectors may be

l8 THE EVOLUTION OF PHYSICS

a common point. Since the starting-point does not

matter, these vectorscan represent thevelocities offour

or the velocities offour cars in different parts of the

indicated directions

This vector representation may now be used to scribe the facts previously discussed concerning recti-

de-linear motion We talked ofa cart, moving uniformly

in a straight line and receiving a push in the direction

in the same direction denoting the velocity after the

represents thechangein velocityforwhich, aswe know,the push is responsible For the case where the force

is directed against the motion, where the motion is

velocity, but in this case its directionis different It is

clear that not only velocities themselves but also their

THE RISE OF THE MECHANICAL VIEW IQ

due to the action of an external force; thus the force

must also be represented by a vector In order to

characterize a force it is not sufficient to state how

hard we pushthecart; we must also say inwhich

direc-tionwepush. Theforce, like the velocity orits change,

alone Therefore:the external forceisalsoavector, and

of the force as truly as they indicate the change in

velocity

Here the sceptic may remark that he sees no

vectors appear to be essential

THE RIDDLE OF MOTION

case of rectilinear motion our concepts of velocity,

we do not see how we can them to

2O THE EVOLUTION OF PHYSICS

de-scription of general motion, and that new ones must

be created Should we try to follow our old path, or

seek a new one?

The generalization of a concept is a process very

often used in science A method of generalization is

must be rigorously satisfied: any generalized concept

con-ditions are fulfilled.

we are now dealing. We can try to generalize the old

the case ofmotion along a curved path Technically,

The straight line is a special and trivial example ofa

curve If, therefore, velocity, change in velocity, and

force are introduced for motion along a curved line,

contra-dict those results previously obtained Ifthe curve

mo-tion But this restriction is not sufficient to determine

the generalization uniquely It leaves open many

pos-sibilities. The history ofscienceshows that the simplestgeneralizations sometimes prove successful and some-

THE RISE OF THE MECHANICAL VIEW 21

generaliza-tion The new concepts prove very successful and help

as that ofthe planets.

velocity, and force mean in the general case ofmotion

right Such a small body is often called aparticle. The

the particle atsomeinstantoftime Whatis the velocity

cluehints atawayof introducingthevelocity. Wemust,

idealized experiment The particle moves along thecurve, from left to right, under the influence of ex-ternal forces Imagine that at a given time, and at

the point indicated bythe dot, all theseforces suddenly

cease to act Then, the motion must, according to the

law of inertia, be uniform In practice we can, of

course, never completely free a body from all external

influences We can only surmise "what would happen

conclusions which can be drawn from it and by their

22 THE EVOLUTION OF PHYSICS

The vector in thenext drawingindicates theguesseddirection of the uniform motion if all external forces

were to vanish It is the direction of the so-called

tangent Looking at a moving particle through a

prolongation Thus the vector drawn represents the

velocity at agiven instant The velocity vectorlies on

the tangent Its length represents themagnitudeof the

velocity, or the speed asindicated, for instance, bythe

Our idealized experiment about destroying the

different positions of a particle moving along a curve

THE RISE OF THE MECHANICAL VIEW 23

a straight line? Obviously it does The tangent to a

straight line is the line itself. The velocity vector lies in

cartor the rolling spheres.

The next step is the introduction of the change invelocity ofa particle moving along a curve. This also

may be done in various ways, from which we choose

the simplest and most convenient The last drawing

mo-tion at various points along the path The first two of

these may be drawn againso that theyhaveacommon

starting-point, as we have seen is possible with vectors

starting-point is the end of the first vector and its

andmeaningless. Itbecomes muchclearer in thespecialcase in whichvectors (i) and (2) have the same direc-tion This, ofcourse, means going over to the case of

straight-line motion If both vectors have the same

24 THE EVOLUTION OF PHYSICS

*-> >

and the previous concept is regained as a special

separate the two lines in our drawing, since otherwise

We now have to take the last step in our process of

generalization Itisthemostimportant ofallthe guesses

force and change in velocity has to be established so

in velocity; the force vector has the same direction as

clue to curvilinear motion? Exactly the same ! The

only difference is that change of velocity has now a

vectorsofthelasttwodrawings showsthis pointclearly

Ifthevelocity is knownfor all points along the curve,

the direction ofthe force at any point can be deduced

at once One must draw the velocity vectors for two

instants separated by a very short time interval and

therefore corresponding to positions very near each

other The vector from the end-point of the first to

force Butv essential that the two