Quantum by Manjit Kumar

Manjit Kumar’s book Quantum: Einstein, Bohr and the Great Debate about the Nature of Reality is a difficult project triumphantly accomplished. In popular history of science, the aim is to mould the history and the science together without compromising too much on either. When the science in question is quantum mechanics, an author already has his work cut out trying to explain it to the general reader. Another challenge is that the history of the quantum is about a clash of personalities and philosophical viewpoints. Turning that into a readable story is no mean feat. But Kumar has succeeded on both fronts.

The debate at the heart of Quantum is how to interpret the strange physics of the sub-atomic world. On one side was Albert Einstein. Despite the difficulty many of us have with relativity, it is actually a well-behaved physical theory that does not require us to compromise on the basic concepts of objective reality or cause and effect. Quantum mechanics, on the other hand, disposes of such foundations: it is a subjective realm where the observer appears to affect the result of experiments and where randomness is indelibly built in. Einstein could never accept this. He thought there were “hidden variables” behind quantum mechanics that would transmute it into a deterministic theory. “God does not play dice”, he said many times.

The other side of the argument was led by Niels Bohr, the greatest Dane since Tycho Brahe. Bohr developed the Copenhagen interpretation of the quantum which embraced its strangest aspects. Bohr accepted that the motion of sub-atomic particles can only be predicted as probabilities and that the experimenter is part of the same system as the thing being observed. Einstein set Bohr a number of fiendish puzzles to show that quantum mechanics was inconsistent and so incomplete. But every time, Bohr solved the problem. Eventually, after Einstein’s death, the Irish physicist John Stewart Bell developed a way to experimentally test one of the quantum paradoxes called non-locality. But, to date, the theory appears to pass even this trial.

All this has left science with a massive hangover. It is not as widely appreciated as it should be that the two crowning achievements of modern physics, relativity and quantum mechanics, are completely incompatible. It is not just that they give different results. They inhabit different metaphysical universes. Scientists have tended to assume that quantum mechanics is the more fundamental theory and string theory is an attempt, unsuccessful so far, to combine it with relativity.

I have a suspicion that current crisis in physics is a function of abandoning the metaphysical framework of a deterministic and objective universe. String theory has returned to the failed ancient Greek model of pure rationalism where clever ideas can never be tested. In the meantime, anyone who wants to understand the background to the The Trouble with Physics chronicled by Lee Smolin can do no better than to read Manjit Kumar’s Quantum.

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