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

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the quantum story

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We always return to the idealized concepts that summarize the fullest

extent of our knowledge—waves and particles.

This interpretation requires that we accept that we can never ‘know’

quantum concepts. They are simply beyond human experience and are

therefore metaphysical. A quantum entity is neither a wave nor a par-

ticle. Instead we substitute the appropriate classical concept—wave or

particle—as and when necessary.

Compare the statement credited to Bohr:

There is no quantum world. There is only an abstract quantum physical

description. It is wrong to think that the task of physics is to ﬁ nd out how

nature is. Physics concerns what we can say about nature.

with the following comment on logical positivism by British philosopher

A.J. Ayer:

The originality of the logical positivists lay in their making the impossibil-

ity of metaphysics depend not upon the nature of what could be known

but upon the nature of what could be said.

A careful analysis of Bohr’s philosophical inﬂ uences and his writings on

the Copenhagen interpretation and complementarity suggest that phil-

osophically he was closer to the tradition known as pragmatism than to

positivism.

Pragmatism, founded by the American philosopher Charles

Sanders Pierce, traces its lineage to German philosopher Georg Hegel and

has many of the characteristics of positivism in that they both roundly

reject metaphysics. There are differences, however.

The positivist doctrine of ‘seeing is believing’ means that what we can

know is limited by what we can observe. The pragmatist doctrine admits

a more practical (or, indeed, pragmatic) approach to the reality of enti-

ties—such as electrons—whose properties and behaviour are described

by theories and which produce secondary observable effects but which

themselves cannot be seen. According to the pragmatist, what we can

know is limited not by what we can see, but by what we can do.

See, for example, Murdoch, pp. 231–232.

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It seems logical that the father of modern atomic theory would want to

accept the reality of atoms. But in his Como lecture Bohr placed limits on

what a theory of the internal structure of the atom could say. He argued

that we live in a classical world and our experiments are classical experi-

ments. Go beyond these concepts and you cross the threshold between

what you can know and what you cannot.

Positivist or pragmatist, the most important feature of Bohr’s philoso-

phy is that it was principally anti-realist. It denied that quantum theory

has anything meaningful to say about an underlying physical reality that

exists independently of our measuring devices. It denied the possibility

that further development of the theory could take us closer to some as

yet unrevealed truth. ‘Accordingly, an independent reality in the ordinary

physical sense can neither be ascribed to the phenomena nor to the agen-

cies of observation,’ Bohr said.

Einstein and Schrödinger, both realists, were not going to be pleased. But

neither was present in Como. There was not long to wait, however. Much

of the illustrious group gathered in Como convened again a month later,

at the ﬁ fth Solvay Congress in Brussels, where Bohr was to repeat his

lecture on complementarity.

This time, both Einstein and Schrödinger would be present.

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PART III

Quantum

Debate

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The ﬁ rst Solvay conference on physics was held in 1911. It was sponsored by Planck and

Nernst, chaired by Lorentz, and addressed problems relating to the theories of radiation

and of speciﬁ c heat capacities. It was supported by wealthy Belgian industrialist and

philanthropist Ernest Solvay. It was the ﬁ rst invitation-only international conference on

physics. Such was its success that a year later Solvay founded the International Institutes

for Physics and Chemistry in Brussels.

The ﬁ fth Solvay conference, on ‘Electrons and Photons’, was also chaired by Lorentz.

It was to be his last public appearance. He died a few months later, in February 1928.

Despite the title of the conference, the invitation made it clear that the principal subject of

discussion was to be the new quantum mechanics.

Lorentz had begun to work on the agenda for the conference with other members of the

scientiﬁ c planning committee as early as April 1926. He had had to revise his opinions

(and extend his invitation list) in response to the rapid developments in the ﬁ eld that had

occurred in the intervening months. So it was that the founders of quantum theory—

Planck, Einstein, Bohr, de Broglie—and the new generation of quantum ‘mechanics’—

Born, Heisenberg, Pauli, Schrödinger, Dirac—were all to be in attendance.

Finally, on Monday 24 October 1927, Einstein, Bohr, and many other leading

physicists gathered at the Institute of Physiology in Brussels to hear Lorentz’s opening

remarks and the ﬁ rst lecture of the conference, by British physicist William L. Bragg.

Bohr later remarked: ‘. . . several of us came to the conference with great anticipations

to learn his [Einstein’s] reaction to the latest stage of the development which, to our

The Debate Commences

Brussels, October 1927

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view, went far in clarifying the problems which he had himself from the outset elicited

so ingeniously.’

Bohr was about to discover just how ingenious Einstein could be.

Bragg’s lecture on X-ray reﬂ ection was followed on the Monday after-

noon by a report from Compton, recently awarded the Nobel Prize for

the discovery of the effect which carries his name.

The following afternoon de Broglie stood to present a radical alternative

approach to the interpretation of quantum phenomena based on the idea of

a ‘double solution’. In effect, de Broglie was interpreting wave–particle duality

not in terms of waves or particles, but in terms of waves and particles. In this

approach, the motions of real, individual quantum point-particles are guided

along their paths by a real wave ﬁ eld. The second ﬁ eld of the double solution

has the same statistical signiﬁ cance as the wavefunction in Schrödinger’s

wave mechanics and is open to the same probabilistic interpretation:

The wave ψ then appears as both a pilot wave (Führungsfeld of Mr Born) and

a probability wave. Since the motion of the [particle] seems to us to be strictly

determined . . . , it does not seem to us that there is any reason to renounce

believing in the determinism of individual physical phenomena, and it is in

this that our conceptions, which are very similar in other respects to those

of Mr Born, appear nevertheless to differ from them quite markedly.

It was a bold solution, one that potentially undermined complementarity

and rendered pointless all the energy that had been expended on formu-

lating and debating it. Not surprisingly, perhaps, it was not greeted with

universal enthusiasm. If de Broglie had been looking for support from

Einstein, he was disappointed. Einstein remained silent.

It seems that Einstein had explored a similar approach himself some

months earlier. On 5 May 1927 he had read a paper to the Prussian Academy

of Sciences in Berlin entitled ‘Does Schrödinger’s Wave Mechanics Deter-

mine the Motion of a System Completely or Only in the Sense of Statistics?’

His modiﬁ cation of the theory was essentially a synthesis of classical wave

and particle descriptions, with the wavefunction of Schrödinger’s mechan-

ics taking the role of a guiding ﬁ eld, guiding physically real point-particles.

Whilst Einstein was excited by his result at the beginning of May, his

enthusiasm for it had evaporated just a few weeks later. On 21 May he

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requested that his paper, by then in print, should be withdrawn. He had

begun to have doubts about attaching physical signiﬁ cance to a wave-

function deﬁ ned in a multi-dimensional conﬁ guration space. Perhaps

most importantly, he further noted that whilst in this modiﬁ cation the

particles themselves remained localized, they could still experience non-

local inﬂ uences from the guiding ﬁ eld. Such inﬂ uences had implications

for causality of just the kind he had been seeking to avoid.

Having committed to presenting a paper at the Solvay conference, on

17 June he had written to Lorentz begging forgiveness: ‘After much reﬂ ec-

tion back and forth,’ he wrote, ‘I come to the conclusion that I am not

competent [to give] such a report in the way that really corresponds to

the state of things.’

The next morning Born and Heisenberg set out the Copenhagen–

Göttingen ‘stall’. In a double-act, they described matrix mechanics,

appropriately extended to make the connection with Schrödinger’s wave

mechanics, the interpretation in terms of quantum probabilities, Heisen-

berg’s uncertainty relations, and numerous applications of the theory. They

argued provocatively that quantum theory was complete, with no further

revision of its basic physical and mathematical hypotheses required:

. . . we consider quantum mechanics to be a closed theory, whose fundamen-

tal physical and mathematical assumptions are no longer susceptible of

any modiﬁ cation.

In the afternoon, Schrödinger delivered the last of the formal lectures, on

wave mechanics. Wary of a showdown with the ‘matricians’, he kept to

points of principle, elaborating those aspects of wave mechanics that had

caused concern, such as the meaning of waves in conﬁ guration space,

and suggesting different approaches to interpretation. In the subsequent

discussion, Heisenberg rejected Schrödinger’s suggestion that future

developments might result in a more conventional theory in four space–

time dimensions: ‘I see nothing in Mr Schrödinger’s calculations that

would justify this hope,’ he said.

There followed an interruption to the proceedings in Brussels, due

to an unfortunate clash with another scientiﬁ c conference organized

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by the Académie des Sciences in Paris to commemorate the centenary

of the death of French physicist Augustin Fresnel. By the time Lorentz

had discovered the clash, it was too late to reschedule. He proposed a

compromise. The Solvay conference was suspended for a day. Those con-

ference participants who wished to attend the Fresnel celebrations could

do so, returning to Brussels on Friday to resume their discussions.

Consequently, Lorentz opened the meeting for general discussion on

Friday morning, expressing some of his own views and making an old

physicists’ plea for causality and determinism before calling on Bohr to

address the conference. Bohr described his concept of complementarity,

much as he had presented it at Como, but now directing his statements

to Einstein who was hearing this argument for the ﬁ rst time.

Einstein did

not respond immediately.

Eventually, Einstein stood to make some comments. ‘Despite being

conscious of the fact that I have not entered deeply enough into the

essence of quantum mechanics, nevertheless I want to present here some

general remarks,’ he said.

He referred to a general experiment involving the diffraction of a beam

of electrons or photons through a narrow slit.

The diffraction pattern

appears on a second screen and is recorded (for example using photo-

graphic ﬁ lm). If quantum theory is assumed to be a complete theory of

individual processes, the behaviour of each individual quantum particle

is described by an appropriate wavefunction and it is the properties of

the wavefunction that give rise to the diffraction pattern. However, at the

moment the wavefunction impinges on the second screen, it ‘collapses’

instantaneously, producing a localized spot on the screen which indi-

cates ‘a particle struck here’.

Bohr did not deliver a formal lecture to the ﬁ fth Solvay conference. At his request, a translation

of his Nature paper on complementarity was appended to the formal conference proceedings.

I have elaborated Einstein’s example somewhat to make it more readily understandable.

Photographic emulsion is made up of millions of tiny crystals of a silver salt (called sil-

ver halides). The photon interacts with a silver salt crystal, and the crystal (and some of its

neighbours) break down to give a black silver deposit. Developing chemicals are then used to

break down more crystals and so amplify the initial deposit to create a visible image. The ﬁ lm

is treated with further chemicals to convert any remaining silver halides into colourless salts,

so the ﬁ lm is no longer sensitive to light. This is the negative. Light is then passed through the

negative onto light-sensitive paper to create the positive.

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Einstein objected to this way of looking at the process. Suppose, he

said, that the particle is observed to arrive at some position A on the sec-

ond screen. In making this observation, we learn not only that the particle

arrived at A but also that it deﬁ nitely did not arrive at a second location

on the screen, B. What’s more, we learn of the particle’s non-arrival at B

instantaneously with the observation of its arrival at A. Before observa-

tion, the probability of ﬁ nding the particle is, supposedly, ‘smeared out’

over the whole screen.

He sensed that the collapse of the wavefunction implies a peculiar

‘action at a distance’. The particle, which is somehow distributed over a

large region of space, becomes localized instantaneously, the act of meas-

urement appearing to change the physical state of the system far from the

point where the measurement is actually recorded. He felt that this kind

of action at a distance violated the postulates of special relativity.

With a nod to de Broglie’s double solution approach, he said: ‘In my

opinion, one can remove this objection only in the following way, that

one does not describe the process solely by the Schrödinger wave, but

fig 4 The simple electron or photon diffraction experiment cited by Einstein in his

debate with Bohr. When we detect the particle at position A on the second screen, we

learn instantaneously that it did not arrive at a second location, (B). Einstein argued

that this ‘collapse of the wavefunction’ violates the postulates of special relativity.