Probabilities of the Quantum World (Pt-11)

[To refresh, text in such square brackets is my commentary. Rest of it is a faithful documentation of the most fascinating story ever told of the Quantum Revolution]

[At the end of Part-10, we saw how the Principle of Complementarity is essential to understand Nature. The apparent opposite points of view, Wave Mechanics or v/s Matrix Mechanics, are indeed complementary and both are essentially for a more complete understanding of Nature.]

Almost all the leading participants of our good story met in the late autumn of 1927 as if to provide the story with an impressive finale.

The occasion was the Fifth Solvay Congress. The thirty two participants included Antoon Lorentz and Max Planck, Albert Einstein and Paul Ehrenfest, Niels Bhor and Max Born, Louis de Broglie and Erwin Schrodinger, William Bragg and Peter Debye, Werner Heisenberg and Wolfgang Pauli, Paul Dirac and … but enough names. It was perhaps the most representative meeting of physicists belonging to different generations — from seventy- to twenty-year-olds — in the Strum and Drang period.

One recalls how Lorentz had stated the problem of developing the mechanics of the submicroscopic world at the First Solvay Congress, when the old men present had been in their prime and those now in their twenties had been mere boys. Sixteen years had passed and the desired mechanics had been developed; Lorentz was again the chairman of the congress; it would be natural for him to praise gratefully those who had created the mechanics. However no words of praise were forthcoming — this mechanics proved to be undesirable for the great classical physicist. Lorentz put his objections against it more clearly and concisely than other scientists holding the same view:

“I wish to have a quite definite picture of any phenomenon. An electron for me is a particle which is at a given point in space at a given time. If the electron collides with an atom, penetrates it and after numerous adventures leave it then I visualize a certain line along which the electron travelled in the atom.”

Einstein was the first to speak of the drama of ideas. It was later that he remembered this drama of the Fifth Solvay Congress in which he was the main character. In contrast to Lorentz, he did not just reject the probabilistic quantum description of the world, he tried to refute its inevitability.

[Imagine this for a moment. The most progressive thinking mind, the one that refuted Newton’s absolute time and space physics, that same mind worked actively to refute the probabilistic description of Nature at the smallest of scales! History is eternally grateful for Einstein’s opposition. His arguments brought us closer to a more thorough understanding of the hidden implications of Quantum Mechanics. When great minds oppose something that is correct at its essence, it brings out the full flavor of the idea.]

The Congress took place in Brussels from October 24th to 29th.

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Almost a year had passed since Einstein had written ironically and yet deeply seriously that God did not ‘play dice’. Heisenberg later recalled:

“God does not play dice was his inviolable principle, one that he would not allow anybody to question.”

Every day in Brussels was filled with discussions: after sessions scientists talked in the hotel restaurant, in parks, on the streets. These discussions were not reported in papers or in the new bulletins over the radio. Photographers and reporters did not flock to the battlefield. Nevertheless, it was a battle whose consequences were immeasurable. It was a major, if not decisive, battle of ideas in the vase field where the atomic age had to grow and mature.

Now looking back we understand that the battle raging in Brussels for a few days was basically an unprecedented combat of two forces. But maybe a better parallel would be the Match of the Century between two great chess players — each day a new game, each game adjourned, careful analysis at night to find some brilliant and unexpected move because that was the only way to win.

Each time Einstein made the first move and planned the offensive. Bohr had to choose the defensive strategy.

When Einstein made the declaration about a God who did not play dice, those present noticed a victorious gleam in his eyes. Heisenberg recalls Bohr’s rejoinder:

“Indeed, it is no business of ours to tell God how he must govern the world!”

Thus their dispute started with the exchange of aphorisms. That was a short exploratory game, the only one that ended in a draw, it did not really contain any physics. Einstein had the advantage of having prepared some of them beforehand. His guiding idea was clear.

He knew the derivation of the uncertainty relation could not be refuted. It was derived from the fundamentals of quantum mechanics and reliably substantiated in imaginary experiment with a super-sensitive microscope. But if the uncertainty relation was true then all was lost — the classical absolute determinacy was forever expelled from the physical picture of nature. Since this could not be allowed to happen, a system of arguments refuting it had to be found. The arguments of course must be physical. No other arguments — philosophical, religious, or psychological — could be used in discussing the laws of nature. Where could he find such arguments?

The answer was unexpected — he tried to find the arguments in quantum mechanics itself. But it proved to be not so easy. Einstein started to invent various intricate paradoxical cases — imaginary experiments — in which the uncertainties inevitably reduced to zero; they vanished as if the action quantum h never existed.

By the end of the day Bohr had taken apart the imaginary experiment suggested by Einstein in the morning as a watchmaker takes apart a clock mechanism of unknown origin and demonstrates that it does not work because of this and that faulty part… Now the gleam was in Bohr’s eyes.

Yet not for long… But let is hear what the most interested of witnesses, Werner Heisenberg, has to say:

“The discussions usually started in the early morning when at breakfast Einstein suggested a new imaginary experiment .. Naturally, we immediately started analyzing it .. As a rule the same evening at supper Niels Bohr successfully demonstrated to Einstein that even this newest conjecture of his could not shake the uncertainty relation. Einstein started to fret: but the next morning at breakfast he had ready one more imaginary experiment, more intricate than the previous one, which he believed would at least irrefutably demonstrate the invalidity of the principle of uncertainty. But by the evening the same day this attempt proved no more successful than previous ones …”

Thus Bohr won game after game: his watchful analysis revealed each time some slight error in the current paradox of Einstein, demonstrating that the uncertaintainties could not be eliminated. Quantum mechanics won each game with the most powerful of possible opponents. Nature was on the side of Bohr — on the side of quantum physics.

Paul Ehrenfest, a close friend of Einstein, said bitterly to him during the discussion:

“I am ashamed of you Einstein; you attack the new quantum theory in just the same way as your enemies attacked the relativity theory!”

[How right was Bernard Shaw when he said that the only lesson of history is that nobody learns from it. Even Einstein, great man that he was, could not become an exception to this eternal rule.]

It was not only Einstein and Bohr who were locked in a bitter dispute. De Broglie insisted on his theory of pilot wave, Schrodinger on the supremacy of waves and Pauli rejected all kinds of models. And the first did not agree with the second, or the second with the third, or the third with the first and the second and so it went on… This confusion of tongues at the congress was awaiting quantum mechanics in the decades that loomed ahead.

Fortunately the framework of the tower had been completed by the autumn of 1927 by scientists from different countries. Nothing could demolish it now. Remarkably, it was its most forceful opponent Einstein (definitely not of the old school) who unwittingly helped to strengthen it, first with his quantum ideas and then with his unyielding opposition.

Yes, it was his opposition that contributed to establishing quantum mechanics. In 1963, thirty six years after the Fifth Solvay Congress, the old veteran Heisenberg explain to the historians that the outcome of the dispute between Bohr and Einstein gave rise to the feeling that a turning point had been passed in the development of quantum physics.

Here is where our good story ends — at the very summit of the crowing year in the Strum und Drang period.

But does it really end? Of course not, it just breaks off…

[Hopefully, one day, we will embark on a similar journey towards the Strings. I asked my spiritual Guru, Sri Sri Ravi Shankar, one day as he passed by me on his way out after a talk: “Guruji, what do you think about String Theory?” He stopped, looked me in the eye and said, “Finally, the scientist is figuring out that it is all vibrations!” And, he had a gleam in his eye.]

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