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

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the quantum story

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Witten could not formulate M-theory, he could only speculate that the theory must

exist. If it does exist it will require seven extra spatial dimensions in addition to our

familiar space–time. The seventh dimension had been missed in the earlier versions of

string theory because it is so much smaller than all the others. A further revelation from

M-theory is that it is not just a theory of one-dimensional strings. It accommodates high-

er-dimensional objects, called membranes. Two-dimensional membranes are referred to

as two-branes, three-dimensional membranes as three-branes, and so on.

The M-theory conjecture sparked a second superstring revolution. Superstrings were

‘hot’ once again, and demand from prestigious academic institutions for superstring

theorists exploded. As a new century dawned, superstring theory was fast becoming an

industry.

Advances were made in loop quantum gravity also during this time, but this was the

territory of the relativists, not particle physicists, and was therefore much less fashionable.

Loop quantum gravity was placed on a ﬁ rmer mathematical foundation. The theorists

began to understand how changes in the spin networks reﬂ ect the curvature of space–

time in the presence of mass. Masses were predicted to attract each other according to the

laws ﬁ rst devised by Newton. The graviton appeared in low-energy approximations to

the theory as a collective excitation of the spin network, much like phonons (the quantum

particle equivalent of sound waves) are collective excitations in the lattice of atoms or ions

that make up a solid.

In 1996, the theory was used to derive the Bekenstein–Hawking formula for the

entropy of a black hole, an accomplishment paralleled in superstring theory at around the

same time. The theory was also found to make the heretical prediction that the speed of

light may not after all be absolute, but may depend on the frequency of its photons. This

means that it may actually be possible to put the theory to the test. The effect is small,

but when accumulated over several billion light-years this difference may be measureable.

Observations of gamma-ray photons from bursts that signal distant cosmic explosions

are thought to provide a fertile testing ground.

Superstring theory may have dominated the landscape, but there were also other

programmes, such as topos theory, twistor theory, technicolour, topological quantum

ﬁ eld theory, to name a few. There was progress, of a kind, although the basic problems

remained stubbornly unyielding.

NASA’s Fermi Gamma-ray Telescope, launched on 11 June 2008, is believed to offer the

required sensitivity to measure these small differences.

crisis? what crisis?

401

There was, however, yet another problem to be addressed. A quantum theory of gravity

would ﬁ rst require the uniﬁ cation of general relativity and quantum theory at the funda-

mental level of interpretation.

This was not good news.

Bryce DeWitt had identiﬁ ed the problem of interpretation in his early

work on the Wheeler–DeWitt equation. He had reached for Everett’s

‘many worlds’ interpretation as a way of resolving the quantum measure-

ment problem, avoiding the need to invoke an external observer, sitting

outside the universe, to make a ‘measurement’ and collapse the wave-

function of the universe. To bridge the gulf between the quantum and

classical realms, quantum theory demands a privileged frame of refer-

ence within which classical outcomes can be obtained from quantum

measurements. General relativity removes all such privileged frames.

It was a curious juxtaposition. Just as experiments designed to

explore the nature of physical reality at the quantum level were con-

ﬁ rming the supremacy of Bohr’s complementarity and the Copen-

hagen interpretation—and so deepening the quantum measurement

problem—so quantum cosmologists were concluding that the original

Copenhagen interpretation just couldn’t be right.

But neither was many worlds. When Everett had ﬁ rst presented his the-

ory in 1957, he had written of the observer state ‘branching’ into different

states and drew an analogy with the branching of a tree. However, variants

of Everett’s original interpretation assumed that the world with which we

are familiar is but one of a very large number (possibly an inﬁ nite number)

of parallel worlds. Thus, instead of the world splitting into separate branches

as a result of a quantum event, the different terms of the superposition are

partitioned between a number of already existing parallel worlds.

We can imagine that, in one of these worlds, a version of you records

the outcome of a measurement of the spin of an electron. You observe

the result spin-up. In another world, a parallel version of you records the

result spin-down. DeWitt called it ‘schizophrenia with a vengeance’.

The parallel worlds may interact and merge. Indeed, it has been argued

that we obtain indirect evidence for such a merging of worlds every time

we perform an interference experiment. Having initially embraced his

student’s work, Wheeler came to reject it, arguing that it carried too much

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402

‘metaphysical baggage’. Many physicists dismissed it as cheap on assump-

tions, but expensive with universes. It led Gell-Mann to complain:

One distinguished physicist, well versed in quantum mechanics, inferred

from certain commentaries on Everett’s interpretation that anyone who

accepts it should want to play Russian roulette for high stakes, because in

some of the ‘equally real’ worlds the player would survive and be rich.

Clearly some kind of alternative interpretation was needed.

In 1984, American physicist Robert Grifﬁ ths embarked on a research

programme designed to construct a new, alternative interpretation of

quantum theory, based on the notion of ‘consistent histories’. Grifﬁ ths

used the word history much as we use it in common language, as a sum-

mary of a series of connected events that unfolded in some recent past.

In this respect, Grifﬁ ths’ histories are similar to Feynman’s histories,

except that Feynman’s approach was focused on the speciﬁ c dynam-

ics associated with the motion of quantum particles from one place to

another, whereas Grifﬁ ths’ approach is more concerned with the logical

framework and interpretation of quantum theory. Alternative Feynman

histories refer to different paths a quantum particle might take from

here to there, whereas Grifﬁ ths’ histories refer to different descriptions

of events unfolding in time that, individually, serve as different but inter-

nally consistent ways of looking at the process.

In this interpretation, there is no ‘correct’ history, no right way to look

at a quantum event. There are, instead, many possible consistent histo-

ries—all equally valid. We choose which histories to apply depending on

the type of experiment we wish to perform.

If we recognize some of the possible alternative histories as ‘particle

histories’ (with which-way trajectories) and some as ‘wave histories’

(resulting in interference effects), then the consistent histories interpre-

tation is a restatement of Bohr’s principle of complementarity in the

language of probability. In fact, Grifﬁ ths claimed that the consistent his-

tories interpretation is ‘Copenhagen done right’. He wrote:

. . . just as there is no single ‘correct’ choice of consistent [history] for

describing the system, there is no single arrangement of apparatus which

can be used to verify the predictions obtained using different [histories].

crisis? what crisis?

403

In 1991 Gell-Mann and Hartle extended Grifﬁ ths’ consistency conditions and

introduced the concepts and principles of decoherence. The consistent his-

tories interpretation is now often referred to as ‘decoherent histories’. The

ambition of this interpretation was subsequently described by Gell-Mann:

We believe Everett’s work to be useful and important, but we believe that

there is much more to be done. In some cases too, his choice of vocabulary

and that of subsequent commentators on his work have created confu-

sion. For example, his interpretation is often described in terms of ‘many

worlds’, whereas we believe that ‘many alternative histories of the uni-

verse’ is what is really meant. Furthermore, the many worlds are described

as being ‘all equally real’, whereas we believe it is less confusing to speak

of ‘many histories, all treated alike by the theory except for their different

probabilities’.

That seemed to do the trick. It was no longer necessary to invoke an

external observer: wave–particle duality and other curious quantum

phenomena are simply manifestations of the different, but consistent,

histories that evolve in the quantum domain in our one, and only, uni-

verse. And the quantum domain—with all its superpositions—evolves

smoothly into our more familiar classical domain through the mecha-

nism of decoherence.

But this was not quite the end of the matter. Sensing that this was

not so straightforward, British theorist Fay Dowker, a former student of

Hawking, set to work with her colleague Adrian Kent. At a conference on

quantum gravity held at the University of Durham in the summer of 1994,

Dowker described what they had found. Her lecture left a lasting impres-

sion on Smolin, who was in the audience. It was one of the most dramatic

experiences in his scientiﬁ c career, as he explained six years later:

While the ‘classical’ world we observe, in which particles have deﬁ nite

positions, may be one of the consistent worlds described by a solution to

the theory, Dowker and Kent’s results showed that there had to be an inﬁ -

nite number of other worlds, too. Moreover, there were an inﬁ nite number

We’ll set aside for the moment our concern that decoherence explains the absence of classi-

cal superpositions but does not solve the measurement problem. See Chapter 34.

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404

of consistent worlds that have been classical up to this point but will not

be anything like our world in ﬁ ve minutes’ time. Even more disturbing,

there were worlds that were classical now that were arbitrarily mixed up

superpositions of classical at any point in the past. Dowker concluded

that, if the consistent histories interpretation is correct, we have no right

to deduce from the existence of fossils now that dinosaurs roamed the

planet a hundred million years ago.

There is no ‘correct’ family of histories that emerges as a result of some

law of nature. The theory regards all possible histories to be equally valid

and our choice of history therefore depends on the kinds of questions

we ask. This leaves us with a signiﬁ cant context-dependence, in which

our ability to make sense of the theory depends on our ability to ask the

‘right’ questions.

If we want a history in which dinosaurs roamed the earth, then we

must ask a question for which such a history is appropriate. Of course,

this is just the same as observing that if we want to prove that the elec-

tron is a wave, then we must set up an experiment designed to reveal

electron diffraction or interference.

The multiplicity of worlds or parallel universes implied by Everett’s

original interpretation are simply traded in the decoherent histories inter-

pretation for many histories, all equally ‘real’. ‘So conventional quantum

cosmology,’ Smolin concluded, ‘seems to be a theory in which we can

formulate the answers, but not the questions.’

After a century of great achievement, we still don’t have a deﬁ nitive, uni-

ﬁ ed quantum theory which accommodates all the known fundamental

particles and all the known forces between them. The twenty parameters

of the Standard Model must still be determined by experiment. We still

do not know for sure how particles acquire mass. And, whilst we must

acknowledge that physical reality at the quantum level is far stranger than

we could have possibly imagined, we’re still unsure how it should be inter-

preted. And the situation has changed little in the last thirty years or so.

Crisis.

Perhaps we’ve been here before.

In 1934 Robert Oppenheimer had complained bitterly about the state

of theoretical physics in a letter to his brother Frank:

crisis? what crisis?

405

As you undoubtedly know, theoretical physics—what with the haunting

ghosts of neutrinos, the Copenhagen conviction, against all evidence, that

cosmic rays are protons, Born’s absolutely unquantizable ﬁ eld theory, the

divergence difﬁ culties with the positron, and the utter impossibility of

making a rigorous calculation of anything at all—is in a hell of a way.

Just thirteen years later Isidor Rabi was declaring that the previous eight-

een years had been the most sterile of the century.

Physics, like all the sciences, has a tendency to stumble from one crisis

to another. Great moments of inspired illumination may be followed by

long periods of confused fumbling in the dark. It might be argued that

crisis is a natural and preferred state. For only a crisis is likely to sponsor

the kinds of outrageous ideas needed to take our understanding of the

world forward to the next stage. Only a crisis could push Heisenberg to

wander aimlessly around Fælled Park late at night, wondering if nature

could possibly be as absurd as it seemed. Only in moments of despera-

tion can we ﬁ nd inspiration.

But this crisis is rather different.

The problem with superstring theory is that although it is the domi-

nant physical theory of modern times, it has not so far been used to make

a single prediction that could be conﬁ rmed or otherwise in a laboratory,

or in a particle accelerator. This is a unique situation. Richard Feynman

was already expressing his concerns in the late 1980s:

I don’t like that they’re not calculating anything. I don’t like that they don’t check

their ideas. I don’t like that for anything that disagrees with an experiment, they

cook up an explanation—a ﬁ x up to say ‘Well, it still might be true.’

In his book Facts and Mysteries in Elementary Particle Physics, published in

2003, Martinus Veltman did not even wish to acknowledge supersym-

metry and superstrings:

The fact is that this is a book about physics, and this implies that the theo-

retical ideas discussed must be supported by experimental facts. Neither

supersymmetry nor string theory satisfy this criterion. They are ﬁ gments

of the theoretical mind. To quote Pauli: they are not even wrong. They

have no place here.

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If this were a few theoreticians plugging away in an unfashionable area of

physics, as Yang, Mills, Glashow, Salam, and Weinberg had plugged away

with quantum ﬁ eld theory in the 1950s and 1960s, then we might agree

that no real harm is done. But such is the fashion for superstring theory

that it has come to dominate modern theoretical physics, particularly in

America. Mathematician Peter Woit used Pauli’s acerbic quote as the title

for his 2006 book denouncing the failure of superstring theory:

The failure of the superstring theory programme must be recognized and

lessons learned from this failure before there can be much hope of mov-

ing forward. As long as the leadership of the particle theory community

refuses to face up to what has happened and continues to train young

theorists to work on a failed project, there is little likelihood of new ideas

ﬁ nding fertile ground in which to grow.

Smolin added his voice to the growing chorus in 2006, with his book The

Trouble With Physics.

In July 2009, Weinberg delivered a lecture at CERN on the varying for-

tunes of quantum ﬁ eld theory. He had this to say:

I don’t want to discourage string theorists, but there’s just the possibility

that maybe that isn’t the way the world is, that the world is much more like

we’ve always known, that is, the Standard Model and general relativity.

407

EPILOGUE

A Quantum of Solace?

Geneva, March 2010

‘It’s a fantastic moment,’ declared Lyn Evans, LHC project manager at CERN in Geneva.

‘We can now look forward to a new era of understanding about the origins and evolution

of the universe.’

Sadly, Evans’ delight was to be rather short-lived. The LHC was switched on at 10:28 am

local time on 10 September 2008. Physicists crammed into the small control room cheered

as a single ﬂ ash of light appeared on a monitor, signifying that high-speed protons had been

steered all the way around the machine’s 27-kilometre ring at the operating temperature

of −271 °C, just two degrees above absolute zero. Though somewhat unspectacular (and

something of an anti-climax for the estimated one billion people thought to have watched

the moment on television), it represented the culmination of two decades of unstinting effort

by armies of physicists, designers, engineers, and construction workers.

Another proton beam was sent around the second, anticlockwise, ring, at 3 pm later

that day. Trouble began shortly afterwards. Just nine days later an electrical bus con-

nection between two of the superconducting magnets short-circuited. Electricity arced,

punching a hole in the magnets’ helium enclosure. Helium gas leaked into sector 3–4 of

the LHC tunnel, and in the subsequent explosion 53 magnets were damaged and the pro-

ton tubes were contaminated with soot. There was no hope of repair before the scheduled

winter shut-down, and a restart was tentatively ﬁ xed for spring 2009. But there were

more problems, and at a meeting in Chamonix in February 2009 CERN managers took

the decision to commission further work. The restart date was pushed back.

At the beginning of September 2009, almost a year after it had ﬁ rst been switched on,

the last of the LHC’s eight sectors began its cool-down procedure. All eight sectors were

back at their operating temperature by the end of October, with a restart scheduled for

November. Despite the increased cost of electricity during the winter months, the LHC

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408

was operated through the winter of 2009–10, primarily so that CERN physicists could

stay ahead of their rivals at Fermilab’s Tevatron, who were snapping at their heels.

Through the ﬁ rst few months of 2010, protons around the two rings were accelerated

to 3.5 TeV before being brought into head-on collision. The ﬁ rst 7-TeV proton–proton

collisions—the highest energy collisions engineered on earth—were recorded on 30

March, representing the beginning of the LHC research programme. Candidate events

involving W bosons were already being reported in early April; candidate Z boson events

were reported in May. This collision energy will be maintained for between 18 and 24

months as data are gathered. Researchers will ﬁ rst look for particles already known to

the Standard Model, before pushing on in search of new physics, with the ﬁ rst prelimi-

nary scientiﬁ c reports anticipated in the summer of 2010. There will follow a year-long

shut-down, after which the proton energies will be increased to achieve the LHC’s target

collision energy of 14 TeV.

There is a lot at stake. For the politicians, bureaucrats, and science man-

agers, getting some results (any results) from an experiment that has so

far cost £3.5 billion will no doubt help to provide some justiﬁ cation for

the endeavour.

But the greatest source of value to be derived from the LHC is what it

promises to do for the future of physics.

The current crisis in physics is the direct result of a lack of experimen-

tal data. Theory must have the discipline of experiment if it is to remain

focused on the things that really matter, the things that manifestly hap-

pen in the real world. Without experiment, theory risks a retreat into

metaphysics, into relatively idle speculation about how many angels can

ﬁ t on the head of a pin, or how many unseen (and unseeable) spatial

dimensions are required for superstrings to vibrate in.

Science is not philosophy. Yet, without recourse to experiment, it is per-

haps inevitable that science becomes more speculative and metaphysical.

This drift into metaphysics is not always apparent. It can sometimes be

obscured by the language of the abstruse mathematics that is deployed.

Modern theoretical physics is ﬁ lled with dense, impenetrable, complex

mathematical structures. These are not for the uninitiated. Those on the

outside looking in may be deluded into thinking that such high math-

ematics must bring with it rigour, a sense of the absolute, the clarity of

the difference between right and wrong.

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409

But this is not so. Yes, there is rigour, there is right and wrong within

the rules of the esoteric language, but the answers you can hope to get

will still depend on the questions you choose to ask, and the way you ask

them. And while the structures may be internally rigorous, they can still

be inappropriately applied. The history of science is littered with failed

theories, all of which no doubt were mathematically rigorous and looked

good on paper at the time they were written down.

The LHC has been called a ‘big bang’ machine, capable of reproduc-

ing conditions not seen since the birth of our universe. For the Higgs

boson to reveal itself, it will be necessary not just for protons to collide

at high energy, but for their quark constituents to collide head-on. In

theory, the conditions created in the LHC should generate a Higgs boson

every couple of hours. The mass of the Higgs boson is hard to pin down

theoretically, but the LHC’s ATLAS

and Compact Muon Solenoid (CMS)

experiments are set up to search for tell-tale decay signatures (for exam-

ple, into bottom–anti-bottom pairs, and into combinations of two high-

energy photons, Z particles, W particles, and tau particles).

Detection is no simple matter. ATLAS, for example, is about half as big

as Notre Dame Cathedral in Paris and weighs as much as the Eiffel Tower.

The CMS detector is 21 metres long, 15 metres in diameter, and weighs as

much as 30 jumbo jets. The ATLAS and CMS collaborations each employ

about 3000 scientists and engineers.

If the Higgs weighs between 150 and 400 GeV, then it should be found

quite quickly and its mass could be measured with a precision of the

order of one per cent. Its discovery will indeed be a famous result.

The

Higgs will formally enter the lexicon of the Standard Model, transformed

ATLAS stands for A Toroidal LHC ApparatuS. Aside from ATLAS and CMS, the LHC is also

home to four other detector facilities. The purpose of ALICE (A Large Ion Collider Experiment)

is to look for quark–gluon plasmas. LHCb (LHC-beauty) is designed to study CP-violation in

bottom-quark decays. LHCf (LHC-forward) will be used to test devices designed to detect cos-

mic rays. Finally, TOTEM (Total Elastic and diffractive cross-section Measurement) is designed

to carry out high-precision measurements on protons.

Tantalizing glimpses of the Higgs boson were observed with a mass of 115 GeV in the ﬁ nal

days of CERN’s Large Electron–Positron collider. However, the evidence was not deemed to be

sufﬁ ciently substantial to warrant a continuation of the LEP programme, beyond the point at

which the development of the LHC would have been jeopardized. After much agonizing discus-

sion, the decision was taken to shut the LEP down. See Sample, pp. 202–23.