Baggott J. The Quantum Story: A History in 40 Moments

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the quantum story

37 Hawking Radiation 372

Oxford, February 1974

38 The First Superstring Revolution 381

Aspen, August 1984

39 Quanta of Space and Time 391

Santa Barbara, February 1986

40 Crisis? What Crisis? 399

Durham, Summer 1994

Epilogue: A Quantum of Solace? 407

Geneva, March 2010

Notes and Sources 411

Bibliography 441

Plate Acknowledgements 448

Index 451

ABOUT THE AUTHOR

Jim Baggott is an award-winning science writer. A former academic sci-

entist, he now works as an independent business consultant but main-

tains a broad interest in science, philosophy, and history, and continues

to write on these subjects in his spare time. His previous books have been

widely acclaimed and include:

Atomic: The First War of Physics and the Secret History of the Atom Bomb 1939–49

(Icon Books, 2009);

A Beginner’s Guide to Reality (Penguin, 2005);

Beyond Measure: Modern Physics, Philosophy and the Meaning of Quantum The-

ory (Oxford University Press, 2004);

Perfect Symmetry: The Accidental Discovery of Buckminsterfullerene (Oxford

University Press, 1994); and

The Meaning of Quantum Theory: A Guide for Students of Chemistry and Physics

(Oxford University Press, 1992).

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PREFACE

The last century was deﬁ ned by physics. From the minds of the world’s

leading physicists there ﬂ owed a river of ideas that would transport man-

kind to the very pinnacle of wonder and to the very depths of despair.

This was a century that began with the certainties of absolute knowledge

and ended with the knowledge of absolute uncertainty. It was a century

in which physicists developed theories that would deny us the possibility

that we can ever properly comprehend the nature of physical reality. It

was also a century in which they built weapons with the capacity utterly

to destroy this reality.

Almost everything we think we know about the nature of our world

comes from one theory of physics. This theory was discovered and

reﬁ ned in the ﬁ rst thirty years of the twentieth century and went on to

become quite simply the most successful theory of physics ever devised.

Its concepts underpin much of the twenty-ﬁ rst century technology that

we have learned to take for granted.

But this success has come at a price, for it has at the same time com-

pletely undermined our ability to make sense of the world at the level of

its most fundamental constituents.

Rejecting the elements of uncertainty and chance implied by this new

theory, Albert Einstein once famously declared that ‘God does not play

dice’. Niels Bohr claimed that anybody who is not shocked by the theory

has not understood it. The charismatic American physicist Richard Fey-

nman went further: he claimed that nobody understands it. To anyone

tutored in the language and the logic of classical physics, this theory is at

once mathematically challenging, maddeningly bizarre, and breathtak-

ingly beautiful.

This is quantum theory, and this book tells its story.

the quantum story

xiv

If we ﬁ x on Max Planck’s discovery of his ‘quantum of action’ in Decem-

ber 1900 as the historical origin of the quantum theory, then as I write this

theory is 110 years old. Time enough, you would have thought, for physi-

cists to get to grips with it and understand what it means. Time enough

to come to terms with what quantum theory has to say about chance and

causality and the nature of physical reality. And yet, if anything, the sense

of shock has increased, not diminished, with the passing of time.

While nobody really understands how quantum theory actually works,

the rules of its application are unquestioned and the accuracy and preci-

sion of its predictions are unsurpassed in the entire history of science.

Although heated debate continues about how quantum theory should

be interpreted, there can be no debate about whether or not the theory

is fundamentally correct.

For more than four hundred years we nurtured the belief (should that,

perhaps, be faith?) that evidence-based investigation meeting scientiﬁ c

standards of rigour would reveal the true mechanisms of nature. And

yet when the mechanisms of nature were revealed to be quantum mech-

anisms, the worlds of science and philosophy were set on a collision

course. Instead of truth and comprehension, we got deeply unsettling

questions about what we can ever hope to know about the world. Quan-

tum theory pushed us to the edge of an epistemological precipice. Since

the mid-1920s we have lived in fear of stepping over the edge.

This book is a celebration of this wonderful yet wholly disconcerting

theory, from its birth in the porcelain furnaces used to study black-body

radiation in 1900 to the promise of stimulating new quantum phenom-

ena to be revealed by CERN’s Large Hadron Collider, over a century later.

It is a history told in forty ‘moments’, signiﬁ cant moments of truth or

turning points in the theory’s development.

This history takes us on a long journey. Part I deals with Planck’s dis-

covery in 1900 and traces the development of early quantum theory

through Einstein’s light-quantum hypothesis, Bohr’s quantum theory

of the atom, Louis de Broglie’s dual wave–particle hypothesis, Werner

Heisenberg’s matrix mechanics, the puzzling phenomenon of electron

spin, and Wolfgang Pauli’s exclusion principle. This section concludes

with Erwin Schrödinger’s ‘late erotic outburst’, which led him to wave

mechanics in December 1925.

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In Part II, the book traces the development of the Copenhagen inter-

pretation of quantum theory. We move from Max Born’s interpretation

of the signiﬁ cance of Schrödinger’s wavefunction in 1926, via the intense

debates between Bohr, Heisenberg, and Schrödinger on the reality of

quantum jumps, the development of Heisenberg’s uncertainty principle,

to Bohr’s Como lecture in September 1927.

At this point, Einstein, an early champion of quantum theory, became

one of the theory’s most determined critics. Part III describes the Bohr–

Einstein debate, one of the most profound debates in the history of

science. We begin with Einstein’s ﬁ rst thought experiments, outlined

to a pensive audience during the ﬁ fth Solvay conference in October

1927, before moving on to the Einstein, Podolsky, Rosen argument and

Schrödinger’s famous cat paradox in 1935. The book pauses brieﬂ y in

this section to catch the ‘absolute wonder’ that is Paul Dirac’s relativistic

quantum theory of the electron.

To study quantum theory is to study the physicists who made it. My

original intention was to write a ‘biography’ of quantum theory based

on the biographies of the physicists who forged it and reﬁ ned it.

Many

of these same physicists also played crucial roles in the development of

the world’s ﬁ rst atomic weapons, and I had intended to include a long

section on their wartime exploits. In an already hopelessly ambitious

book, this was an ambition too far. This section became a book in itself,

titled Atomic: The First War of Physics and the Secret History of the Atom Bomb,

1939–49, published in 2009 by Icon Books. With the publisher’s permis-

sion I have included here an interlude, based on extracts from this earlier

book and focused primarily on the infamous meeting in September 1941

between Bohr and Heisenberg in Nazi-occupied Copenhagen.

As the physicists picked up the pieces of their academic careers after

the war, quantum theory was in crisis. Part IV describes the series of

crisis meetings that culminated in the development of quantum elec-

trodynamics, by Julian Schwinger, Richard Feynman, Sin-itiro Tomon-

aga, and Freeman Dyson. This was followed in 1954 by the unheralded

development of quantum ﬁ eld theory based on local gauge symmetry

I wrote the original proposal for this book at a time when ‘biographies’ of inanimate sub-

jects were still popular.

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xvi

by Chen Ning Yang and Robert Mills. Sheldon Glashow, Abdus Salaam,

and Stephen Weinberg went on to formulate early versions of a uni-

ﬁ ed electro-weak theory in 1960, and predicted the existence of ‘heavy

photons’, the W and Z particles. Although much of this effort was

summarily dismissed by the physics community, this was a time of

unprecedented fertility in theoretical physics. It culminated in Murray

Gell-Mann’s 1963 theory of quarks and the introduction of symmetry-

breaking and the Higgs mechanism in 1967.

At this point, the history of quantum theory becomes synonymous

with the history of particle physics. In Part V the book examines the

roles of ever larger and more expensive particle accelerators and collid-

ers in furnishing the evidence for the particular collection of quantum

ﬁ eld theories that has become known as the Standard Model of particle

physics. This section begins with the discovery, at the Stanford Linear

Accelerator Center (SLAC) in 1968, that the proton possesses an inter-

nal structure. This is followed by the discoveries of a hypothetical charm

quark and the colour force described by Gell-Mann and Harald Fritzsch’s

theory of quantum chromodynamics.

The experimental observation of the J/ψ meson (formed from a charm

and anti-charm quark), simultaneously at SLAC and at Brookhaven

National Laboratory in the ‘November revolution’ of 1974, and the sub-

sequent observation of the W and Z particles at CERN in 1983, set the

physicists on the path to the Standard Model. This model is constructed

from three ‘generations’ of matter particles consisting of leptons (elec-

trons and neutrinos) and quarks that interact through the exchange of

force particles—photons, W and Z particles, and colour force gluons.

There is as yet no room in the Standard Model for gravity. This section

concludes by looking over the shoulders of the physicists gathered at

CERN as they celebrate their successes in September 2003.

At this stage the book steps backwards, to 1951, and David Bohm’s

growing unease over the implications of the Copenhagen interpretation.

Encouraged by Einstein, Bohm goes on to make further reﬁ nements to the

Einstein, Podolsky, Rosen argument, bringing their thought experiment

into the realms of practicality. Bohm goes on to develop an elaborate

‘hidden variable’ alternative to conventional quantum theory.

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xvii

From these beginnings, Part VI traces the development of contem-

porary experimentation designed to probe the very nature of physical

reality itself. The book recounts the development in 1964 of John Bell’s

theorem, and Bell’s inequality, which exposed the true nature of Einstein’s

challenges and provided a straightforward test of local versus non-local

reality. The ﬁ rst deﬁ nitive experiments were performed by Alain Aspect

and his colleagues in 1981 and 1982. The quantum world was shown to be

determinedly non-local.

There followed a series of experiments demonstrating the truly incom-

prehensible nature of this world, leaving the committed realist grasping

for straws. These included Marlan Scully and Kai Drühl’s quantum eraser

experiment and experiments demonstrating interference in macroscopic

quantum objects, inanimate laboratory versions of Schrödinger’s infamous

cat. This section concludes with a description of some 2006 experiments

by Anton Zeilinger and his colleagues, designed to test a further inequality

devised by Anthony Leggett. The results strongly suggest that we can no

longer assume that the particle properties we measure necessarily reﬂ ect

or represent the properties of the particles as they really are.

These experiments tell us rather emphatically that we can never

perceive reality ‘as it really is’. We can only reveal aspects of an empiri-

cal reality that depend on the nature of the instruments we use and the

questions we ask. Quantum physics, it seems, has completed its transfor-

mation into experimental philosophy.

The book closes with Part VII, which describes the efforts to forge

together the two great physical theories of the twentieth century—

quantum theory and general relativity—into a theory of quantum

gravity or, alternatively, into a ‘theory of everything’, capable in princi-

ple of describing everything in the universe. This section begins with the

development of canonical quantum gravity in the form of the Wheeler–

DeWitt equation. Applying quantum ﬁ eld theory to the curved space–

time around a black hole, Stephen Hawking discovered in 1974 that black

holes ‘ain’t so black’.

The ﬁ rst superstring revolution in August 1984 promised to provide

a theory which could not only explain all the particles of the Standard

Model but could also accommodate the graviton, the purported ﬁ eld

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xviii

particle of the gravitational force. This early promise faded, however, as

different variants of superstring theory emerged and it lost any sense of

uniqueness. Around the same time, the canonical approach was resur-

rected in the form of loop quantum gravity. Superstring theory experi-

enced something of a renaissance in March 1995, in the second superstring

revolution, and today dominates contemporary theoretical physics.

But there is growing impatience with the superstring programme’s

obsession with obscure hidden dimensions and its inability to make any

kind of testable prediction. As has happened so often in its glorious 110-

year history, quantum theory is once again in crisis. This section closes

with an exploration of the role that interpretation, an obsession among

physicists since the theory’s inception, might still have yet to play.

The book concludes optimistically with, if you will, a quantum of sol-

ace. It has cost £3.5 billion and frustratingly blew up days after it was ﬁ rst

switched on in September 2008, but the Large Hadron Collider (LHC)

at CERN in Geneva gives some hope of resolving the current crisis. At

worst, the LHC will simply conﬁ rm the existence of the Higgs boson,

validating the mechanism of spontaneous symmetry-breaking, explain-

ing how particles acquire mass, and putting the icing on the cake of the

Standard Model. At worst, the LHC will provide answers.

At best, the LHC will turn up some bizarre new experimental facts; facts

that simply can’t be accommodated in the current quantum ﬁ eld theories

that constitute the Standard Model, and the crisis will deepen. Physics will

then truly come alive once again. Only from the depths of despair are we

likely to see the breakthroughs needed to propel quantum theory on the

next stage of its journey. At best, the LHC will beg questions.

Many of the ‘moments’ I have chosen to describe in this book suggest

themselves as unambiguously key events in quantum theory’s history.

Others are a little less obvious and some have been chosen in an effort

to maintain narrative consistency and ﬂ ow. Whilst I make no apologies

for my choices, I am very conscious of the risk that, taken together, these

moments may be perceived to describe a smooth, irresistible progression

towards some inexorable scientiﬁ c truth.

This is just not how science works. It has not been possible to describe

here all the blind alleys, the dead ends, the theories that dominated for

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xix

a time only to be replaced by alternatives better able to explain the data.

The reality of scientiﬁ c endeavour is profoundly messy, often illogical,

deeply emotional, and driven by the individual personalities involved as

they ‘sleepwalk’ their way to a temporary scientiﬁ c truth.

My thanks go to Latha Menon, my editor at Oxford University Press,

for her patience, fortitude, and ability to channel my ambitions in more

practical directions. I owe a debt of thanks to Anthony Leggett, Carlo

Rovelli, and Peter Woit who read and commented on the draft manu-

script. It goes without saying that all the errors, misconceptions, and

misinterpretations that remain are entirely my fault.

I hope this book will stand as a testament to the intellectual and psy-

chological challenge posed by a quantum theory so profoundly at odds

with a common-sense conception of the world, and to those great physi-

cists who were able to rise to this challenge. A testament to what can be

achieved through the application of a theory that nobody understands.

Jim Baggott

Reading, July 2010