The mischievous oracle
The Guardian, By Steven Poole
A brilliant guide to quantum physics impresses Steven Poole.
Quantum physics is the branch of science most irresistible to raiders from other disciplines who don’t quite understand it. The catalogue of literary-metaphorical abuses of Heisenberg’s uncertainty principle, for example, must run to thousands of entries. This ought not to be surprising — for, as this new book shows, the discipline has from its inception been intimately bound up not just with empirical investigation, but with passionate philosophical arguments about the nature of existence itself. One might not have the mathematics to follow every step of the science, but everyone has a potential stake in what it seems to imply about reality and our relationship with it.
Manjit Kumar’s book is an exhaustive and brilliant account of decades of emotionally charged discovery and argument, friendship and rivalry spanning two world wars. In what also has to operate as a kind of group biography of Planck, Einstein, Bohr, Heisenberg, Pauli, Dirac et al, the quasi-novelistic character sketches occasionally have a comic quality ("The son of a tax collector, Ludwig Boltzmann was short and stout with an impressive late 19th-century beard"); but the real meat of the book is the explanations of science and philosophical interpretation, which are pitched with an ideal clarity for the general reader. Perhaps most interestingly, although the author is admirably even-handed, it is difficult not to think of Quantum, by the end, as a resounding rehabilitation of Albert Einstein.
You might have thought that Einstein, the most famous scientist who ever lived, was not much in need of rehabilitation. But for a long time, the standard story of his reaction to quantum theory painted him as a grouchy old man, whose great work was long in the past, and who could no longer accept novel ideas. The truth, as Kumar shows, is very different.
For a start, Einstein was himself a pioneer of quantum theory, having suggested in 1913 that light was quantised — in other words, that it was not smoothly continuous, but could only exist in multiples of very small packets, or quanta. At the time, Kumar relates, this was "just too radical for physicists to accept". Two decades later, the great Danish physicist Niels Bohr and his colleagues, who had taken this idea and run with it, had become too radical for Einstein to accept.
But Einstein did not merely snipe ineffectually from the sidelines at those who were doing important science. He was taken very seriously at the time, as Kumar’s thrilling narrative of a series of epic thought-experiment battles between Einstein and Bohr shows. The popular misconception of his role was in part Einstein’s own fault, as he liked to repeat his slogan "God does not play dice" at every opportunity — yet, as Kumar demonstrates, his real objection was not to the probabilistic or statistical interpretation of quantum mechanics, but to its radical denial of an independent reality.
Bohr’s "Copenhagen interpretation", which became orthodoxy for most of the century, still has the power to shock. What it states, baldly, is that reality is determined by the experiment the scientist chooses to perform. One kind of experiment will cause light to behave like a particle; another kind will make it act as a wave. There is no underlying truth about what light "really" is. And an electron doesn’t have a definite position in space before you choose to measure it: in measuring it, you somehow oblige it to make up its mind as to where it is. (Heisenberg’s uncertainty or "indeterminacy" principle, by the way, says that if you come to know an electron’s position in this way, its momentum cannot also be accurately known; and, crucially, that this is not just a function of our experimental limitations, but also of the truth about electrons. If an electron has a definite position, it simply does not have a definite momentum, and vice-versa.)
So reality is a kind of mischievous oracle, answering only when directly questioned. Erwin Schrödinger’s famous imaginary cat (inspired in part by an exchange of letters with Einstein) was intended as a reductio ad absurdum of this conception. The cat in question is locked in a box, together with a crumb of radioactive material, which may or may not decay in the course of an hour. If the crumb decays, it will set off a mechanism that poisons the cat. After an hour, what can you say about how the cat is doing without opening the box? Intuitively, it’s obvious: either the cat is alive, or it isn’t. But, said Schrödinger, a quantum theorist would be obliged to say that the box contained "the living and the dead cat […] mixed or smeared out in equal parts". According to Copenhagen, there is no truth about the cat’s alive-or-deadness until someone opens the box and observes it.
Perhaps the most embarrassing question asked of the Copenhagen interpretation was a version of the cat paradox scaled up to cosmic size. Copenhagen says that nothing is definitively so until an act of observation "collapses the wave-function" of the system in question (condensing, so to speak, a cloud of probability into one thing or another). If that is true, how did the universe itself begin? What monstrous (or maybe divine) act of observation could have collapsed the wave-function of the entire universe so as to promote it to physical reality? Some physicists subsequently adopted a "many worlds interpretation", which envisioned an endlessly branching multiplicity of universes. It may have shut down the original observation question, but only at the price of an uncomfortable ontological profligacy.
Kumar’s story finishes by noting the results of a 1999 poll of physicists at a Cambridge conference as to which interpretation of quantum mechanics they preferred. Of 90 respondents, "only four voted for the Copenhagen interpretation, but 30 favoured the modern version of […] many worlds. Significantly, 50 ticked the box labelled ‘none of the above or undecided’". The question of the ultimate nature of reality is, it seems, still a live problem. Somewhere, Einstein is puffing on his pipe and smiling ironically.