Paul Ehrenfest was in tears. He had made his decision. Soon he would attend the
week-long gathering where many of those responsible for the quantum revolution
would try to understand the meaning of what they had wrought. There he would
have to tell his old friend Albert Einstein that he had chosen to side with
Niels Bohr. Ehrenfest, the 34-year-old Austrian professor of theoretical physics
at Leiden University in Holland, was convinced that the atomic realm was as
strange and ethereal as Bohr argued.
In time it was discovered that the energy of an electron
inside an atom was ‘quantised’ it could possess only certain amounts of energy
and not others. The same was true of other physical properties, as the
microscopic realm was found to be lumpy and discontinuous and not some shrunken
version of the large-scale world that humans inhabit, where physical properties
vary smoothly and continuously, where going from A to C means passing through B.
quantum physics, however, revealed that an electron in an atom can be in one
place, and then, as if by magic, reappear in another without ever being anywhere
in between, by emitting or absorbing a quantum of energy. This was a phenomenon
beyond the ken of classical, non-quantum physics. It was as bizarre as an object
mysteriously disappearing in London and an instant later suddenly reappearing in
Paris, New York or Moscow.
In a note to Einstein as they sat around the conference
table, Ehrenfest scribbled: ‘Don’t laugh! There is a special section in
purgatory for professors of quantum theory, where they will be obliged to listen
to lectures on classical physics ten hours every day.’2 ‘I laugh only at their naiveté,’ Einstein
replied.3 ‘Who knows who
would have the [last] laugh in a few years?’ For him it was no laughing matter,
for at stake was the very nature of reality and the soul of physics.
The photograph of those gathered at the fifth Solvay
conference on ‘Electrons and Photons’, held in Brussels from 24 to 29 October
1927, encapsulates the story of the most dramatic period in the history of
physics. With seventeen of the 29 invited eventually earning a Nobel Prize, the
conference was one of the most spectacular meetings of minds ever held.4 It marked the end of a
golden age of physics, an era of scientific creativity unparalleled since the
scientific revolution in the seventeenth century led by Galileo and Newton.
Paul Ehrenfest is standing, slightly hunched forward, in the
back row, third from the left. There are nine seated in the front row. Eight men
and one woman; six have Nobel Prizes in either physics or chemistry. The woman
has two, one for physics awarded in 1903 and another for chemistry in 1911. Her
name: Marie Curie. In the centre, the place of honour, sits another Nobel
laureate, the most celebrated scientist since the age of Newton: Albert
Einstein. Looking straight ahead, gripping the chair with his right hand, he
seems ill at ease. Is it the winged collar and tie that are causing him
discomfort, or what he has heard during the preceding week? At the end of the
second row, on the right, is Niels Bohr, looking relaxed with a half-whimsical
smile. It had been a good conference for him. Nevertheless, Bohr would be
returning to Denmark disappointed that he had failed to convince Einstein to
adopt his ‘Copenhagen interpretation’ of what quantum mechanics revealed about
the nature of reality.
Instead of yielding, Einstein had spent the week attempting
to show that quantum mechanics was inconsistent, that Bohr’s Copenhagen
interpretation was flawed. Einstein said years later that ‘this theory reminds
me a little of the system of delusions of an exceedingly intelligent paranoic,
concocted of incoherent elements of thoughts’.5
It was Max Planck, sitting on Marie Curie’s right, holding
his hat and cigar, who discovered the quantum. In 1900 he was forced to accept
that the energy of light and all other forms of electromagnetic radiation could
only be emitted or absorbed by matter in bits, bundled up in various sizes.
‘quantum’ was the name Planck gave to an individual packet of energy, with
‘quanta’ being the plural. The quantum of energy was a radical break with the
long-established idea that energy was emitted or absorbed continuously, like
water flowing from a tap. In the everyday world of the macroscopic where the
physics of Newton ruled supreme, water could drip from a tap, but energy was not
exchanged in droplets of varying size. However, the atomic and subatomic level
of reality was the domain of the quantum.
By the early 1920s it had long been apparent that the advance
of quantum physics on an ad hoc, piecemeal basis had left it without solid
foundations or a logical structure. Out of this state of confusion and crisis
emerged a bold new theory known as quantum mechanics. The picture of the atom as
a tiny solar system with electrons orbiting a nucleus, still taught in schools
today, was abandoned and replaced with an atom that was impossible to visualise.
Then, in 1927, Werner Heisenberg made a discovery that was so at odds with
common sense that even he, the German wunderkind of quantum mechanics, initially
struggled to grasp its significance. The uncertainty principle said that if you
want to know the exact velocity of a particle, then you cannot know its exact
location, and vice versa.
No one knew how to interpret the equations of quantum
mechanics, what the theory was saying about the nature of reality at the quantum
level. Questions about cause and effect, or whether the moon exists when no one
is looking at it, had been the preserve of philosophers since the time of Plato
and Aristotle, but after the emergence of quantum mechanics they were being
discussed by the twentieth century’s greatest physicists.
With all the basic components of quantum physics in place,
the fifth Solvay conference opened a new chapter in the story of the quantum.
For the debate that the conference sparked between Einstein and Bohr raised
issues that continue to preoccupy many eminent physicists and philosophers to
this day: what is the nature of reality, and what kind of description of reality
should be regarded as meaningful? ‘No more profound intellectual debate has ever
been conducted’, claimed the scientist and novelist C.P. Snow. ‘It is a pity
that the debate, because of its nature, can’t be common currency.
Of the two main protagonists, Einstein is a twentieth-century
icon. He was once asked to stage his own three-week show at the London
Palladium. Women fainted in his presence. Young girls mobbed him in Geneva.
Today this sort of adulation is reserved for pop singers and movie stars. But in
the aftermath of the First World War, Einstein became the first superstar of
science when in 1919 the bending of light predicted by his theory of general
relativity was confirmed. Little had changed when in January 1931, during a
lecture tour of America, Einstein attended the premiere of Charlie Chaplin’s
movie City Limits in Los Angeles. A large crowd
cheered wildly when they saw Chaplin and Einstein. ‘They cheer me because they
all understand me,’ Chaplin told Einstein, ‘and they cheer you because no one
understands you.
Whereas the name Einstein is a byword for scientific genius, Niels Bohr was, and
remains, less well known. Yet to his contemporaries he was every inch the
scientific giant. In 1923 Max Born, who played a pivotal part in the development
of quantum mechanics, wrote that Bohr’s ‘influence on theoretical and
experimental research of our time is greater than that of any other
physicist’.8 Forty years
later, in 1963, Werner Heisenberg maintained that ‘Bohr’s influence on the
physics and the physicists of our century was stronger than that of anyone else,
even than that of Albert Einstein’.
Reference: From the prologue of the book in the following link:
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