Introduction to quantum mechanics 3rd edition

Preface 1The Wave Function 1.1The Schrödinger Equation 1.2The Statistical Interpretation 1.6The Uncertainty Principle Further Problems on Chapter 1 2Time-Independent Schrödinger Equation

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I N T R O D U C T I O N T O Q U A N T U M M E C H A N I C S

Third edition

Changes and additions to the new edition of this classic textbook include:

David J Griffiths received his BA (1964) and PhD (1970) from Harvard University He taught at Hampshire

College, Mount Holyoke College, and Trinity College before joining the faculty at Reed College in 1978 In2001–2002 he was visiting Professor of Physics at the Five Colleges (UMass, Amherst, Mount Holyoke,Smith, and Hampshire), and in the spring of 2007 he taught Electrodynamics at Stanford Although his PhDwas in elementary particle theory, most of his research is in electrodynamics and quantum mechanics He is

the author of over fifty articles and four books: Introduction to Electrodynamics (4th edition, Cambridge University Press, 2013), Introduction to Elementary Particles (2nd edition, Wiley-VCH, 2008), Introduction to Quantum Mechanics (2nd edition, Cambridge, 2005), and Revolutions in Twentieth-Century Physics

(Cambridge, 2013)

Darrell F Schroeter is a condensed matter theorist He received his BA (1995) from Reed College and his

PhD (2002) from Stanford University where he was a National Science Foundation Graduate ResearchFellow Before joining the Reed College faculty in 2007, Schroeter taught at both Swarthmore College andOccidental College His record of successful theoretical research with undergraduate students was recognized

in 2011 when he was named as a KITP-Anacapa scholar

A new chapter on Symmetries and Conservation Laws

New problems and examples

Improved explanations

More numerical problems to be worked on a computer

New applications to solid state physics

Consolidated treatment of time-dependent potentials

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I NT ROD UCT I ON TO Q UANT UM

MECHANI CS

Third edition

DAVID J GRIFFITHS and DARRELL F SCHROETER

Reed College, Oregon

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Second edition © David Griffiths 2017

Third edition © Cambridge University Press 2018

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This book was previously published by Pearson Education, Inc 2004 Second edition reissued by Cambridge University Press 2017

Third edition 2018

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Names: Griffiths, David J | Schroeter, Darrell F.

Title: Introduction to quantum mechanics / David J Griffiths (Reed College, Oregon), Darrell F Schroeter (Reed College, Oregon).

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Preface

1The Wave Function

1.1The Schrödinger Equation

1.2The Statistical Interpretation

1.6The Uncertainty Principle

Further Problems on Chapter 1

2Time-Independent Schrödinger Equation

2.1Stationary States

2.2The Infinite Square Well

2.3The Harmonic Oscillator

2.3.1Algebraic Method

2.3.2Analytic Method

2.4The Free Particle

2.5The Delta-Function Potential

2.5.1Bound States and Scattering States

2.5.2The Delta-Function Well

2.6The Finite Square Well

Further Problems on Chapter 2

3.4Generalized Statistical Interpretation

3.5The Uncertainty Principle

3.5.1Proof of the Generalized Uncertainty Principle

3.5.2The Minimum-Uncertainty Wave Packet

3.5.3The Energy-Time Uncertainty Principle

3.6Vectors and Operators

3.6.1Bases in Hilbert Space

3.6.2Dirac Notation

3.6.3Changing Bases in Dirac Notation

Further Problems on Chapter 3

4Quantum Mechanics in Three Dimensions

4.1The Schröger Equation

4.1.1Spherical Coordinates

4.1.2The Angular Equation

4.1.3The Radial Equation

4.2The Hydrogen Atom

4.2.1The Radial Wave Function

4.2.2The Spectrum of Hydrogen

4.4.2Electron in a Magnetic Field

4.4.3Addition of Angular Momenta

4.5Electromagnetic Interactions

4.5.1Minimal Coupling

4.5.2The Aharonov–Bohm Effect

Further Problems on Chapter 4

6Symmetries & Conservation Laws

6.1Introduction

6.1.1Transformations in Space

6.2The Translation Operator

6.2.1How Operators Transform

6.2.2Translational Symmetry

6.3Conservation Laws

6.4Parity

6.4.1Parity in One Dimension

6.4.2Parity in Three Dimensions

6.4.3Parity Selection Rules

6.5Rotational Symmetry

6.5.1Rotations About the z Axis

6.5.2Rotations in Three Dimensions

6.6Degeneracy

6.7Rotational Selection Rules

6.7.1Selection Rules for Scalar Operators

6.7.2Selection Rules for Vector Operators

7Time-Independent Perturbation Theory

7.1Nondegenerate Perturbation Theory

7.3The Fine Structure of Hydrogen

7.3.1The Relativistic Correction

7.3.2Spin-Orbit Coupling

7.4The Zeeman Effect

7.4.1Weak-Field Zeeman Effect

7.4.2Strong-Field Zeeman Effect

7.4.3Intermediate-Field Zeeman Effect

7.5Hyperfine Splitting in Hydrogen

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Further Problems on Chapter 7

8The Varitional Principle

8.1Theory

8.2The Ground State of Helium

8.3The Hydrogen Molecule Ion

8.4The Hydrogen Molecule

Further Problems on Chapter 8

9The WKB Approximation

9.1The “Classical” Region

9.2Tunneling

9.3The Connection Formulas

Further Problems on Chapter 9

10Scattering

10.1Introduction

10.1.1Classical Scattering Theory

10.1.2Quantum Scattering Theory

10.2Partial Wave Analysis

10.2.1Formalism

10.2.2Strategy

10.3Phase Shifts

10.4The Born Approximation

10.4.1Integral Form of the Schrödinger Equation

10.4.2The First Born Approximation

10.4.3The Born Series

Further Problems on Chapter 10

11Quantum Dynamics

11.1Two-Level Systems

11.1.1The Perturbed System

11.1.2Time-Dependent Perturbation Theory

11.3.1Einstein’s A and B Coefficients

11.3.2The Lifetime of an Excited State

11.3.3Selection Rules

11.4Fermi’s Golden Rule

11.5The Adiabatic Approximation

11.5.1Adiabatic Processes

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11.5.2The Adiabatic Theorem

Further Problems on Chapter 11

A.4Changing Bases

A.5Eigenvectors and Eigenvalues

A.6Hermitian Transformations

Index

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Unlike Newton’s mechanics, or Maxwell’s electrodynamics, or Einstein’s relativity, quantum theory was notcreated—or even definitively packaged—by one individual, and it retains to this day some of the scars of itsexhilarating but traumatic youth There is no general consensus as to what its fundamental principles are, how

it should be taught, or what it really “means.” Every competent physicist can “do” quantum mechanics, but thestories we tell ourselves about what we are doing are as various as the tales of Scheherazade, and almost asimplausible Niels Bohr said, “If you are not confused by quantum physics then you haven’t really understoodit”; Richard Feynman remarked, “I think I can safely say that nobody understands quantum mechanics.”

The purpose of this book is to teach you how to do quantum mechanics Apart from some essential

background in Chapter 1, the deeper quasi-philosophical questions are saved for the end We do not believe

one can intelligently discuss what quantum mechanics means until one has a firm sense of what quantum mechanics does But if you absolutely cannot wait, by all means read the Afterword immediately after finishing

Chapter 1

Not only is quantum theory conceptually rich, it is also technically difficult, and exact solutions to all butthe most artificial textbook examples are few and far between It is therefore essential to develop specialtechniques for attacking more realistic problems Accordingly, this book is divided into two parts;1 Part I

covers the basic theory, and Part II assembles an arsenal of approximation schemes, with illustrative

applications Although it is important to keep the two parts logically separate, it is not necessary to study the

material in the order presented here Some instructors, for example, may wish to treat time-independentperturbation theory right after Chapter 2

This book is intended for a one-semester or one-year course at the junior or senior level A one-semestercourse will have to concentrate mainly on Part I; a full-year course should have room for supplementarymaterial beyond Part II The reader must be familiar with the rudiments of linear algebra (as summarized inthe Appendix), complex numbers, and calculus up through partial derivatives; some acquaintance with Fourieranalysis and the Dirac delta function would help Elementary classical mechanics is essential, of course, and alittle electrodynamics would be useful in places As always, the more physics and math you know the easier itwill be, and the more you will get out of your study But quantum mechanics is not something that flowssmoothly and naturally from earlier theories On the contrary, it represents an abrupt and revolutionarydeparture from classical ideas, calling forth a wholly new and radically counterintuitive way of thinking aboutthe world That, indeed, is what makes it such a fascinating subject

At first glance, this book may strike you as forbiddingly mathematical We encounter Legendre,Hermite, and Laguerre polynomials, spherical harmonics, Bessel, Neumann, and Hankel functions, Airyfunctions, and even the Riemann zeta function—not to mention Fourier transforms, Hilbert spaces, hermitianoperators, and Clebsch–Gordan coefficients Is all this baggage really necessary? Perhaps not, but physics is

like carpentry: Using the right tool makes the job easier, not more difficult, and teaching quantum mechanics

without the appropriate mathematical equipment is like having a tooth extracted with a pair of pliers—it’spossible, but painful (On the other hand, it can be tedious and diverting if the instructor feels obliged to giveelaborate lessons on the proper use of each tool Our instinct is to hand the students shovels and tell them to

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start digging They may develop blisters at first, but we still think this is the most efficient and exciting way to

learn.) At any rate, we can assure you that there is no deep mathematics in this book, and if you run into something unfamiliar, and you don’t find our explanation adequate, by all means ask someone about it, or look

it up There are many good books on mathematical methods—we particularly recommend Mary Boas,

Mathematical Methods in the Physical Sciences, 3rd edn, Wiley, New York (2006), or George Arfken and Jurgen Weber, Mathematical Methods for Physicists, 7th edn, Academic Press, Orlando (2013) But whatever you do, don’t let the mathematics—which, for us, is only a tool—obscure the physics.

Hans-Several readers have noted that there are fewer worked examples in this book than is customary, and thatsome important material is relegated to the problems This is no accident We don’t believe you can learnquantum mechanics without doing many exercises for yourself Instructors should of course go over as many

problems in class as time allows, but students should be warned that this is not a subject about which anyone

has natural intuitions—you’re developing a whole new set of muscles here, and there is simply no substitutefor calisthenics Mark Semon suggested that we offer a “Michelin Guide” to the problems, with varyingnumbers of stars to indicate the level of difficulty and importance This seemed like a good idea (though, likethe quality of a restaurant, the significance of a problem is partly a matter of taste); we have adopted thefollowing rating scheme:

an essential problem that every reader should study;

a somewhat more difficult or peripheral problem;

an unusually challenging problem, that may take over an hour

(No stars at all means fast food: OK if you’re hungry, but not very nourishing.) Most of the one-star problemsappear at the end of the relevant section; most of the three-star problems are at the end of the chapter If acomputer is required, we put a mouse in the margin A solution manual is available (to instructors only) fromthe publisher

In preparing this third edition we have tried to retain as much as possible the spirit of the first andsecond Although there are now two authors, we still use the singular (“I”) in addressing the reader—it feelsmore intimate, and after all only one of us can speak at a time (“we” in the text means you, the reader, and I,the author, working together) Schroeter brings the fresh perspective of a solid state theorist, and he is largelyresponsible for the new chapter on symmetries We have added a number of problems, clarified manyexplanations, and revised the Afterword But we were determined not to allow the book to grow fat, and forthat reason we have eliminated the chapter on the adiabatic approximation (significant insights from thatchapter have been incorporated into Chapter 11), and removed material from Chapter 5 on statisticalmechanics (which properly belongs in a book on thermal physics) It goes without saying that instructors arewelcome to cover such other topics as they see fit, but we want the textbook itself to represent the essentialcore of the subject

We have benefitted from the comments and advice of many colleagues, who read the originalmanuscript, pointed out weaknesses (or errors) in the first two editions, suggested improvements in thepresentation, and supplied interesting problems We especially thank P K Aravind (Worcester Polytech),Greg Benesh (Baylor), James Bernhard (Puget Sound), Burt Brody (Bard), Ash Carter (Drew), EdwardChang (Massachusetts), Peter Collings (Swarthmore), Richard Crandall (Reed), Jeff Dunham (Middlebury),Greg Elliott (Puget Sound), John Essick (Reed), Gregg Franklin (Carnegie Mellon), Joel Franklin (Reed),

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Henry Greenside (Duke), Paul Haines (Dartmouth), J R Huddle (Navy), Larry Hunter (Amherst), DavidKaplan (Washington), Don Koks (Adelaide), Peter Leung (Portland State), Tony Liss (Illinois), JeffryMallow (Chicago Loyola), James McTavish (Liverpool), James Nearing (Miami), Dick Palas, Johnny Powell(Reed), Krishna Rajagopal (MIT), Brian Raue (Florida International), Robert Reynolds (Reed), Keith Riles(Michigan), Klaus Schmidt-Rohr (Brandeis), Kenny Scott (London), Dan Schroeder (Weber State), MarkSemon (Bates), Herschel Snodgrass (Lewis and Clark), John Taylor (Colorado), Stavros Theodorakis(Cyprus), A S Tremsin (Berkeley), Dan Velleman (Amherst), Nicholas Wheeler (Reed), Scott Willenbrock(Illinois), William Wootters (Williams), and Jens Zorn (Michigan).

1 This structure was inspired by David Park’s classic text Introduction to the Quantum Theory, 3rd edn, McGraw-Hill, New York (1992).

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Part I Theory

◈

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1 The Wave Function

◈

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(1.2)

1.1 The Schrödinger Equation

Imagine a particle of mass m, constrained to move along the x axis, subject to some specified force

(Figure 1.1) The program of classical mechanics is to determine the position of the particle at any given time:

Once we know that, we can figure out the velocity , the momentum , thekinetic energy , or any other dynamical variable of interest And how do we go aboutdetermining ? We apply Newton’s second law: (For conservative systems—the only kind we shall consider, and, fortunately, the only kind that occur at the microscopic level—the force can be expressed as

the derivative of a potential energy function,1 , and Newton’s law reads

.) This, together with appropriate initial conditions (typically the position andvelocity at ), determines

Figure 1.1: A “particle” constrained to move in one dimension under the influence of a specified force.

Quantum mechanics approaches this same problem quite differently In this case what we’re looking for

is the particle’s wave function, , and we get it by solving the Schrödinger equation:

Here i is the square root of , and is Planck’s constant—or rather, his original constant (h) divided by :

The Schrödinger equation plays a role logically analogous to Newton’s second law: Given suitable initialconditions (typically, ), the Schrödinger equation determines for all future time, just as, inclassical mechanics, Newton’s law determines for all future time.2

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1.2 The Statistical Interpretation

But what exactly is this “wave function,” and what does it do for you once you’ve got it? After all, a particle, by

its nature, is localized at a point, whereas the wave function (as its name suggests) is spread out in space (it’s a

function of x, for any given t) How can such an object represent the state of a particle? The answer is provided

by Born’s statistical interpretation, which says that gives the probability of finding the particle at point x, at time t—or, more precisely,3

Probability is the area under the graph of For the wave function in Figure 1.2, you would be quite likely

to find the particle in the vicinity of point A, where is large, and relatively unlikely to find it near point B.

Figure 1.2: A typical wave function The shaded area represents the probability of finding the particle between

a and b The particle would be relatively likely to be found near A, and unlikely to be found near B.

The statistical interpretation introduces a kind of indeterminacy into quantum mechanics, for even if you

know everything the theory has to tell you about the particle (to wit: its wave function), still you cannotpredict with certainty the outcome of a simple experiment to measure its position—all quantum mechanics

has to offer is statistical information about the possible results This indeterminacy has been profoundly

disturbing to physicists and philosophers alike, and it is natural to wonder whether it is a fact of nature, or adefect in the theory

Suppose I do measure the position of the particle, and I find it to be at point C.4 Question: Where was the particle just before I made the measurement? There are three plausible answers to this question, and they serve

to characterize the main schools of thought regarding quantum indeterminacy:

1 The realist position: The particle was at C This certainly seems reasonable, and it is the response Einstein advocated Note, however,

that if this is true then quantum mechanics is an incomplete theory, since the particle really was at C, and yet quantum mechanics was unable

to tell us so To the realist, indeterminacy is not a fact of nature, but a reflection of our ignorance As d’Espagnat put it, “the position of the particle was never indeterminate, but was merely unknown to the experimenter.”5 Evidently is not the whole story—some additional

information (known as a hidden variable) is needed to provide a complete description of the particle.

2 The orthodox position: The particle wasn’t really anywhere It was the act of measurement that forced it to “take a stand” (though how

and why it decided on the point C we dare not ask) Jordan said it most starkly: “Observations not only disturb what is to be measured, they

produce it …We compel [the particle] to assume a definite position.”6 This view (the so-called Copenhagen interpretation), is associated

with Bohr and his followers Among physicists it has always been the most widely accepted position Note, however, that if it is correct there is something very peculiar about the act of measurement—something that almost a century of debate has done precious little to illuminate.

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3 The agnostic position: Refuse to answer This is not quite as silly as it sounds—after all, what sense can there be in making assertions

about the status of a particle before a measurement, when the only way of knowing whether you were right is precisely to make a

measurement, in which case what you get is no longer “before the measurement”? It is metaphysics (in the pejorative sense of the word) to worry about something that cannot, by its nature, be tested Pauli said: “One should no more rack one’s brain about the problem of whether something one cannot know anything about exists all the same, than about the ancient question of how many angels are able to sit

on the point of a needle.”7 For decades this was the “fall-back” position of most physicists: they’d try to sell you the orthodox answer, but if you were persistent they’d retreat to the agnostic response, and terminate the conversation.

Until fairly recently, all three positions (realist, orthodox, and agnostic) had their partisans But in 1964

John Bell astonished the physics community by showing that it makes an observable difference whether the

particle had a precise (though unknown) position prior to the measurement, or not Bell’s discovery effectively

eliminated agnosticism as a viable option, and made it an experimental question whether 1 or 2 is the correct

choice I’ll return to this story at the end of the book, when you will be in a better position to appreciate Bell’sargument; for now, suffice it to say that the experiments have decisively confirmed the orthodoxinterpretation:8 a particle simply does not have a precise position prior to measurement, any more than theripples on a pond do; it is the measurement process that insists on one particular number, and thereby in a

sense creates the specific result, limited only by the statistical weighting imposed by the wave function.

What if I made a second measurement, immediately after the first? Would I get C again, or does the act

of measurement cough up some completely new number each time? On this question everyone is inagreement: A repeated measurement (on the same particle) must return the same value Indeed, it would be

tough to prove that the particle was really found at C in the first instance, if this could not be confirmed by

immediate repetition of the measurement How does the orthodox interpretation account for the fact that the

second measurement is bound to yield the value C? It must be that the first measurement radically alters the wave function, so that it is now sharply peaked about C (Figure 1.3) We say that the wave function collapses,

upon measurement, to a spike at the point C (it soon spreads out again, in accordance with the Schrödinger

equation, so the second measurement must be made quickly) There are, then, two entirely distinct kinds ofphysical processes: “ordinary” ones, in which the wave function evolves in a leisurely fashion under theSchrödinger equation, and “measurements,” in which suddenly and discontinuously collapses.9

Figure 1.3: Collapse of the wave function: graph of immediately after a measurement has found the particle at point C.

Example 1.1

Electron Interference I have asserted that particles (electrons, for example) have a wave nature,

encoded in How might we check this, in the laboratory?

The classic signature of a wave phenomenon is interference: two waves in phase interfere

constructively, and out of phase they interfere destructively The wave nature of light was confirmed in

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1801 by Young’s famous double-slit experiment, showing interference “fringes” on a distant screenwhen a monochromatic beam passes through two slits If essentially the same experiment is done with

electrons, the same pattern develops,10 confirming the wave nature of electrons

Now suppose we decrease the intensity of the electron beam, until only one electron is present inthe apparatus at any particular time According to the statistical interpretation each electron will

produce a spot on the screen Quantum mechanics cannot predict the precise location of that spot—all

it can tell us is the probability of a given electron landing at a particular place But if we are patient,

and wait for a hundred thousand electrons—one at a time—to make the trip, the accumulating spotsreveal the classic two-slit interference pattern (Figure 1.4) 11

Figure 1.4: Build-up of the electron interference pattern (a) Eight electrons, (b) 270 electrons, (c)

2000 electrons, (d) 160,000 electrons Reprinted courtesy of the Central Research Laboratory,

Hitachi, Ltd., Japan

Of course, if you close off one slit, or somehow contrive to detect which slit each electron passesthrough, the interference pattern disappears; the wave function of the emerging particle is now entirelydifferent (in the first case because the boundary conditions for the Schrödinger equation have beenchanged, and in the second because of the collapse of the wave function upon measurement) But withboth slits open, and no interruption of the electron in flight, each electron interferes with itself; itdidn’t pass through one slit or the other, but through both at once, just as a water wave, impinging on

a jetty with two openings, interferes with itself There is nothing mysterious about this, once you have

accepted the notion that particles obey a wave equation The truly astonishing thing is the blip-by-blip

assembly of the pattern In any classical wave theory the pattern would develop smoothly andcontinuously, simply getting more intense as time goes on The quantum process is more like thepointillist painting of Seurat: The picture emerges from the cumulative contributions of all theindividual dots.12

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1.3 Probability

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1.3.1 Discrete Variables

Because of the statistical interpretation, probability plays a central role in quantum mechanics, so I digressnow for a brief discussion of probability theory It is mainly a question of introducing some notation andterminology, and I shall do it in the context of a simple example

Imagine a room containing fourteen people, whose ages are as follows:

one person aged 14,

one person aged 15,

three people aged 16,

two people aged 22,

two people aged 24,

five people aged 25

If we let represent the number of people of age j, then

while , for instance, is zero The total number of people in the room is

(In the example, of course, ) Figure 1.5 is a histogram of the data The following are some questionsone might ask about this distribution

Figure 1.5: Histogram showing the number of people, , with age j, for the example in Section 1.3.1

Question 1 If you selected one individual at random from this group, what is the probability that this

person’s age would be 15?

Answer One chance in 14, since there are 14 possible choices, all equally likely, of whom only one has that

particular age If is the probability of getting age j, then

, and so on In general,

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Question 2 What is the most probable age?

Answer 25, obviously; five people share this age, whereas at most three have any other age The most

probable j is the j for which is a maximum

Question 3 What is the median age?

Answer 23, for 7 people are younger than 23, and 7 are older (The median is that value of j such that the

probability of getting a larger result is the same as the probability of getting a smaller result.)

Question 4 What is the average (or mean) age?

Answer

In general, the average value of j (which we shall write thus: ) is

Notice that there need not be anyone with the average age or the median age—in this example nobodyhappens to be 21 or 23 In quantum mechanics the average is usually the quantity of interest; in that context it

has come to be called the expectation value It’s a misleading term, since it suggests that this is the outcome

you would be most likely to get if you made a single measurement (that would be the most probable value, not

the average value)—but I’m afraid we’re stuck with it

Question 5 What is the average of the squares of the ages?

, with probability 3/14, and so on The average, then, is

In general, the average value of some function of j is given by

(Equations 1.6, 1.7, and 1.8 are, if you like, special cases of this formula.) Beware: The average of the squares, , is not equal, in general, to the square of the average, For instance, if the room contains just twobabies, aged 1 and 3, then , but

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(1.10)

Now, there is a conspicuous difference between the two histograms in Figure 1.6, even though they havethe same median, the same average, the same most probable value, and the same number of elements: Thefirst is sharply peaked about the average value, whereas the second is broad and flat (The first might representthe age profile for students in a big-city classroom, the second, perhaps, a rural one-room schoolhouse.) Weneed a numerical measure of the amount of “spread” in a distribution, with respect to the average The mostobvious way to do this would be to find out how far each individual is from the average,

and compute the average of Trouble is, of course, that you get zero:

(Note that is constant—it does not change as you go from one member of the sample to another—so it can

be taken outside the summation.) To avoid this irritating problem you might decide to average the absolute value of But absolute values are nasty to work with; instead, we get around the sign problem by squaring

before averaging:

This quantity is known as the variance of the distribution; σ itself (the square root of the average of the square

of the deviation from the average—gulp!) is called the standard deviation The latter is the customary measure

of the spread about

Figure 1.6: Two histograms with the same median, same average, and same most probable value, but different

standard deviations

There is a useful little theorem on variances:

Taking the square root, the standard deviation itself can be written as

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(1.13)

In practice, this is a much faster way to get σ than by direct application of Equation 1.11: simply calculate and , subtract, and take the square root Incidentally, I warned you a moment ago that is not, ingeneral, equal to Since is plainly non-negative (from its definition 1.11), Equation 1.12 implies that

and the two are equal only when , which is to say, for distributions with no spread at all (every memberhaving the same value)

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to generalize to continuous distributions If I select a random person off the street, the probability that her age

is precisely 16 years, 4 hours, 27 minutes, and 3.333… seconds is zero The only sensible thing to speak about

is the probability that her age lies in some interval—say, between 16 and 17 If the interval is sufficiently short, this probability is proportional to the length of the interval For example, the chance that her age is between 16 and 16 plus two days is presumably twice the probability that it is between 16 and 16 plus one day.

(Unless, I suppose, there was some extraordinary baby boom 16 years ago, on exactly that day—in which case

we have simply chosen an interval too long for the rule to apply If the baby boom lasted six hours, we’ll take

intervals of a second or less, to be on the safe side Technically, we’re talking about infinitesimal intervals.)

Thus

The proportionality factor, , is often loosely called “the probability of getting x,” but this is sloppy

language; a better term is probability density The probability that x lies between a and b (a finite interval) is

given by the integral of :

and the rules we deduced for discrete distributions translate in the obvious way:

Example 1.2

Suppose someone drops a rock off a cliff of height h As it falls, I snap a million photographs, at random intervals On each picture I measure the distance the rock has fallen Question: What is the average of all these distances? That is to say, what is the time average of the distance traveled?13

Solution: The rock starts out at rest, and picks up speed as it falls; it spends more time near the top, so

the average distance will surely be less than Ignoring air resistance, the distance x at time t is

The velocity is , and the total flight time is The probability that a

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The velocity is , and the total flight time is The probability that a

particular photograph was taken between t and is , so the probability that it shows a

distance in the corresponding range x to is

Thus the probability density (Equation 1.14) is

(outside this range, of course, the probability density is zero)

We can check this result, using Equation 1.16:

The average distance (Equation 1.17) is

which is somewhat less than , as anticipated

Figure 1.7 shows the graph of Notice that a probability density can be infinite, though probability itself (the integral of ρ) must of course be finite (indeed, less than or equal to 1).

Problem 1.1 For the distribution of ages in the example in Section 1.3.1:

(b) Determine for each j, and use Equation 1.11 to compute the standarddeviation

(c) Use your results in (a) and (b) to check Equation 1.12

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Problem 1.2

(a) Find the standard deviation of the distribution in Example 1.2

(b) What is the probability that a photograph, selected at random, would

show a distance x more than one standard deviation away from the

average?

Problem 1.3 Consider the gaussian distribution

where A, a, and are positive real constants (The necessary integrals are

inside the back cover.)

(a) Use Equation 1.16 to determine A.

(b) Find , , and σ

(c) Sketch the graph of

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We return now to the statistical interpretation of the wave function (Equation 1.3), which says that

is the probability density for finding the particle at point x, at time t It follows (Equation 1.16)that the integral of over all x must be 1 (the particle’s got to be somewhere):

Without this, the statistical interpretation would be nonsense

However, this requirement should disturb you: After all, the wave function is supposed to be determined

by the Schrödinger equation—we can’t go imposing an extraneous condition on without checking that thetwo are consistent Well, a glance at Equation 1.1 reveals that if is a solution, so too is ,

where A is any (complex) constant What we must do, then, is pick this undetermined multiplicative factor so

as to ensure that Equation 1.20 is satisfied This process is called normalizing the wave function For some

solutions to the Schrödinger equation the integral is infinite; in that case no multiplicative factor is going to

make it 1 The same goes for the trivial solution Such non-normalizable solutions cannot represent particles, and must be rejected Physically realizable states correspond to the square-integrable solutions to

Schrödinger’s equation.14

But wait a minute! Suppose I have normalized the wave function at time How do I know that it

will stay normalized, as time goes on, and evolves? (You can’t keep renormalizing the wave function, for then A becomes a function of t, and you no longer have a solution to the Schrödinger equation.) Fortunately,

the Schrödinger equation has the remarkable property that it automatically preserves the normalization of thewave function—without this crucial feature the Schrödinger equation would be incompatible with thestatistical interpretation, and the whole theory would crumble

This is important, so we’d better pause for a careful proof To begin with,

(Note that the integral is a function only of t, so I use a total derivative on the left, but the integrand is

a function of x as well as t, so it’s a partial derivative on the right.) By the product rule,

Now the Schrödinger equation says that

and hence also (taking the complex conjugate of Equation 1.23)

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The integral in Equation 1.21 can now be evaluated explicitly:

But must go to zero as x goes to infinity—otherwise the wave function would not benormalizable.15 It follows that

and hence that the integral is constant (independent of time); if is normalized at , it stays normalized

for all future time QED

Problem 1.4 At time a particle is represented by the wave function

where A, a, and b are (positive) constants.

(a) Normalize (that is, find A, in terms of a and b).

(b) Sketch , as a function of x.

(c) Where is the particle most likely to be found, at ?

(d) What is the probability of finding the particle to the left of a? Check your

result in the limiting cases and

(e) What is the expectation value of x?

Problem 1.5 Consider the wave function

where A, , and ω are positive real constants (We’ll see in Chapter 2 for what

potential (V) this wave function satisfies the Schrödinger equation.)

(a) Normalize

(b) Determine the expectation values of x and

(c) Find the standard deviation of x Sketch the graph of , as a function

of x, and mark the points and , to illustrate the sense

in which σ represents the “spread” in x What is the probability that the

particle would be found outside this range?

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For a particle in state , the expectation value of x is

What exactly does this mean? It emphatically does not mean that if you measure the position of one particle

over and over again, is the average of the results you’ll get On the contrary: The firstmeasurement (whose outcome is indeterminate) will collapse the wave function to a spike at the value actuallyobtained, and the subsequent measurements (if they’re performed quickly) will simply repeat that same result.Rather, is the average of measurements performed on particles all in the state , which means that either

you must find some way of returning the particle to its original state after each measurement, or else you have

to prepare a whole ensemble of particles, each in the same state , and measure the positions of all of them:

is the average of these results I like to picture a row of bottles on a shelf, each containing a particle in the

state (relative to the center of the bottle) A graduate student with a ruler is assigned to each bottle, and at asignal they all measure the positions of their respective particles We then construct a histogram of the results,which should match , and compute the average, which should agree with (Of course, since we’re onlyusing a finite sample, we can’t expect perfect agreement, but the more bottles we use, the closer we ought to

come.) In short, the expectation value is the average of measurements on an ensemble of identically-prepared systems,

not the average of repeated measurements on one and the same system

Now, as time goes on, will change (because of the time dependence of ), and we might beinterested in knowing how fast it moves Referring to Equations 1.25 and 1.28, we see that16

This expression can be simplified using integration-by-parts:17

(I used the fact that , and threw away the boundary term, on the ground that goes to zero at infinity.) Performing another integration-by-parts, on the second term, we conclude:

What are we to make of this result? Note that we’re talking about the “velocity” of the expectation value of

x, which is not the same thing as the velocity of the particle Nothing we have seen so far would enable us to calculate the velocity of a particle It’s not even clear what velocity means in quantum mechanics: If the particle

doesn’t have a determinate position (prior to measurement), neither does it have a well-defined velocity All

we could reasonably ask for is the probability of getting a particular value We’ll see in Chapter 3 how toconstruct the probability density for velocity, given ; for the moment it will suffice to postulate that the

expectation value of the velocity is equal to the time derivative of the expectation value of position:

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Equation 1.31 tells us, then, how to calculate directly from

Actually, it is customary to work with momentum , rather than velocity:

Let me write the expressions for and in a more suggestive way:

We say that the operator18 x “represents” position, and the operator “represents” momentum; tocalculate expectation values we “sandwich” the appropriate operator between and , and integrate

That’s cute, but what about other quantities? The fact is, all classical dynamical variables can be expressed

in terms of position and momentum Kinetic energy, for example, is

and angular momentum is

(the latter, of course, does not occur for motion in one dimension) To calculate the expectation value of any

such quantity, , we simply replace every p by , insert the resulting operator between and , and integrate:

For example, the expectation value of the kinetic energy is

Equation 1.36 is a recipe for computing the expectation value of any dynamical quantity, for a particle instate ; it subsumes Equations 1.34 and 1.35 as special cases I have tried to make Equation 1.36 seemplausible, given Born’s statistical interpretation, but in truth this represents such a radically new way of doing

business (as compared with classical mechanics) that it’s a good idea to get some practice using it before we

come back (in Chapter 3) and put it on a firmer theoretical foundation In the mean time, if you prefer to

think of it as an axiom, that’s fine with me.

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(1.38)

Problem 1.6 Why can’t you do integration-by-parts directly on the middle

expression in Equation 1.29—pull the time derivative over onto x, note that

, and conclude that ?

This is an instance of Ehrenfest’s theorem, which asserts that expectation values

obey the classical laws.19

Problem 1.8 Suppose you add a constant to the potential energy (by “constant”

I mean independent of x as well as t) In classical mechanics this doesn’t change anything, but what about quantum mechanics? Show that the wave function picks

up a time-dependent phase factor: What effect does this have onthe expectation value of a dynamical variable?

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(1.40)

1.6 The Uncertainty Principle

Imagine that you’re holding one end of a very long rope, and you generate a wave by shaking it up and downrhythmically (Figure 1.8) If someone asked you “Precisely where is that wave?” you’d probably think he was a little bit nutty: The wave isn’t precisely anywhere—it’s spread out over 50 feet or so On the other hand, if he asked you what its wavelength is, you could give him a reasonable answer: it looks like about 6 feet By

contrast, if you gave the rope a sudden jerk (Figure 1.9), you’d get a relatively narrow bump traveling downthe line This time the first question (Where precisely is the wave?) is a sensible one, and the second (What isits wavelength?) seems nutty—it isn’t even vaguely periodic, so how can you assign a wavelength to it? Of

course, you can draw intermediate cases, in which the wave is fairly well localized and the wavelength is fairly

well defined, but there is an inescapable trade-off here: the more precise a wave’s position is, the less precise isits wavelength, and vice versa.20 A theorem in Fourier analysis makes all this rigorous, but for the moment I

am only concerned with the qualitative argument

Figure 1.8: A wave with a (fairly) well-defined wavelength, but an ill-defined position.

Figure 1.9: A wave with a (fairly) well-defined position, but an ill-defined wavelength.

This applies, of course, to any wave phenomenon, and hence in particular to the quantum mechanical

wave function But the wavelength of is related to the momentum of the particle by the de Broglie

formula:21

Thus a spread in wavelength corresponds to a spread in momentum, and our general observation now says that

the more precisely determined a particle’s position is, the less precisely is its momentum Quantitatively,

where is the standard deviation in x, and is the standard deviation in p This is Heisenberg’s famous

uncertainty principle (We’ll prove it in Chapter 3, but I wanted to mention it right away, so you can test itout on the examples in Chapter 2.)

Please understand what the uncertainty principle means: Like position measurements, momentum

measurements yield precise answers—the “spread” here refers to the fact that measurements made onidentically prepared systems do not yield identical results You can, if you want, construct a state such that

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position measurements will be very close together (by making a localized “spike”), but you will pay a price:Momentum measurements on this state will be widely scattered Or you can prepare a state with a definitemomentum (by making a long sinusoidal wave), but in that case position measurements will be widelyscattered And, of course, if you’re in a really bad mood you can create a state for which neither position normomentum is well defined: Equation 1.40 is an inequality, and there’s no limit on how big and can be—just make some long wiggly line with lots of bumps and potholes and no periodic structure

Problem 1.9 A particle of mass m has the wave function

where A and a are positive real constants.

(a) Find A.

(b) For what potential energy function, , is this a solution to theSchrödinger equation?

(c) Calculate the expectation values of , and

(d) Find and Is their product consistent with the uncertaintyprinciple?

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(1.42)

(1.43)

Further Problems on Chapter 1

Problem 1.10 Consider the first 25 digits in the decimal expansion of π (3, 1,

4, 1, 5, 9, … )

(a) If you selected one number at random, from this set, what are the

probabilities of getting each of the 10 digits?

(b) What is the most probable digit? What is the median digit? What is the

average value?

(c) Find the standard deviation for this distribution.

Problem 1.11 [This problem generalizes Example 1.2.] Imagine a particle of mass

m and energy E in a potential well , sliding frictionlessly back and forth

between the classical turning points (a and b in Figure 1.10) Classically, the

probability of finding the particle in the range dx (if, for example, you took a snapshot at a random time t) is equal to the fraction of the time T it takes to get from a to b that it spends in the interval dx:

where is the speed, and

Thus

This is perhaps the closest classical analog22 to

(a) Use conservation of energy to express in terms of E and

(b) As an example, find for the simple harmonic oscillator,

Plot , and check that it is correctly normalized

(c) For the classical harmonic oscillator in part (b), find , , and

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(1.44)

Figure 1.10: Classical particle in a potential well.

Problem 1.12 What if we were interested in the distribution of momenta

, for the classical harmonic oscillator (Problem 1.11(b))

(a) Find the classical probability distribution (note that p ranges from

(b) Calculate , , and

(c) What’s the classical uncertainty product, , for this system? Noticethat this product can be as small as you like, classically, simply by sending But in quantum mechanics, as we shall see in Chapter 2, theenergy of a simple harmonic oscillator cannot be less than , where

is the classical frequency In that case what can you say aboutthe product ?

Problem 1.13 Check your results in Problem 1.11(b) with the following

“numerical experiment.” The position of the oscillator at time t is

You might as well take (that sets the scale for time) and (that

sets the scale for length) Make a plot of x at 10,000 random times, and

compare it with

Hint: In Mathematica, first define

then construct a table of positions:

and finally, make a histogram of the data:

Meanwhile, make a plot of the density function, , and, using Show,

superimpose the two

Problem 1.14 Let be the probability of finding the particle in the range

What are the units of ? Comment: J is called the probability

current, because it tells you the rate at which probability is “flowing” past

the point x If is increasing, then more probability is flowing intothe region at one end than flows out at the other

(b) Find the probability current for the wave function in Problem 1.9 (This

is not a very pithy example, I’m afraid; we’ll encounter more substantialones in due course.)

Problem 1.15 Show that

for any two (normalizable) solutions to the Schrödinger equation (with thesame ), and

Problem 1.16 A particle is represented (at time ) by the wave function

(a) Determine the normalization constant A.

(b) What is the expectation value of x?

(c) What is the expectation value of p? (Note that you cannot get it from

Why not?)

(d) Find the expectation value of

(e) Find the expectation value of

(f) Find the uncertainty in

(g) Find the uncertainty in

(h) Check that your results are consistent with the uncertainty principle Problem 1.17 Suppose you wanted to describe an unstable particle, that

spontaneously disintegrates with a “lifetime” τ In that case the total

probability of finding the particle somewhere should not be constant, but

should decrease at (say) an exponential rate:

A crude way of achieving this result is as follows In Equation 1.24 we tacitly

assumed that V (the potential energy) is real That is certainly reasonable, but

it leads to the “conservation of probability” enshrined in Equation 1.27 What

if we assign to V an imaginary part:

where is the true potential energy and Γ is a positive real constant?

(a) Show that (in place of Equation 1.27) we now get

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(b) Solve for , and find the lifetime of the particle in terms of Γ

Problem 1.18 Very roughly speaking, quantum mechanics is relevant when the de

Broglie wavelength of the particle in question is greater than thecharacteristic size of the system In thermal equilibrium at (Kelvin)

temperature T, the average kinetic energy of a particle is

(where is Boltzmann’s constant), so the typical de Broglie wavelength is

The purpose of this problem is to determine which systems will have to betreated quantum mechanically, and which can safely be described classically

(a) Solids The lattice spacing in a typical solid is around nm Findthe temperature below which the unbound23 electrons in a solid are quantum mechanical Below what temperature are the nuclei in a solid

quantum mechanical? (Use silicon as an example.)

Moral: The free electrons in a solid are always quantum mechanical; the nuclei are generally not quantum mechanical The same goes for liquids

(for which the interatomic spacing is roughly the same), with theexception of helium below 4 K

(b) Gases For what temperatures are the atoms in an ideal gas at pressure P

quantum mechanical? Hint: Use the ideal gas law todeduce the interatomic spacing

quantum behavior) we want m to be as small as possible, and P as large as

possible Put in the numbers for helium at atmospheric pressure Ishydrogen in outer space (where the interatomic spacing is about 1 cm andthe temperature is 3 K) quantum mechanical? (Assume it’s monatomichydrogen, not H )

1 Magnetic forces are an exception, but let’s not worry about them just yet By the way, we shall assume throughout this book that the motion

is nonrelativistic

2 For a delightful first-hand account of the origins of the Schrödinger equation see the article by Felix Bloch in Physics Today, December

1976.

3 The wave function itself is complex, but (where is the complex conjugate of ) is real and non-negative—as a

probability, of course, must be.

4 Of course, no measuring instrument is perfectly precise; what I mean is that the particle was found in the vicinity of C, as defined by the

precision of the equipment.

5 Bernard d’Espagnat, “The Quantum Theory and Reality” (Scientific American, November 1979, p 165).

6 Quoted in a lovely article by N David Mermin, “Is the moon there when nobody looks?” (Physics Today, April 1985, p 38).

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