PART of the way through Schrödinger’s Kittens, I e-mailed a friend “Read Gribbin’s new book”. A juicy, mechanical yarn is something to be shared, and that’s what John Gribbin offers us: a tale of discovery and challenge, rounded off with a “whodunit” finale.
Quantum mechanics attempts to explain the world of the very small. It works well as a set of rules for working out the behaviour of things atomic, but there is a price. That price is grasping exactly what it all means. Only the brave or foolish would claim to “understand” quantum mechanics and the reason is easy to see. It demands acceptance of a world so improbable and so at odds with what we accept as common sense that we cannot understand quantum mechanics. It bears no relationship to our everyday experiences.
Old hat it may be but the famous double-slit experiment with light beams still suffices to encapsulate the problem. A ray of light shines onto a barrier pierced by two narrow parallel slits. The light passes through the slits and forms a diffraction pattern on a screen, as is the way of waves. You might think that this proves that light really is a wave but the catch is that particles of light are doing this. Experimenters have confirmed that each light particle goes through both slits. It’s like going to the supermarket and exiting through all the checkouts simultaneously.
The problem of how to make sense of this and other weird quantum effects has been with us for seventy years. The most widely accepted explanation is called the Copenhagen interpretation, championed by the late and great Niels Bohr. It’s a viewpoint that demands a lot from its disciples, resting as it does on the acceptance of wave-particle duality and the idea that observation influences the result of an experiment. In the case of light passing through slits in a screen, your scrutiny determines its nature, as a wave or as particles.
Erwin Schrödinger contributed a great deal to quantum theory, including his mythical cat facing death in an imaginary box, unleashed on the world in 1935 as a means of exposing the difficulties with the Copenhagen interpretation. This interpretation means that Schrödinger’s cat would have to be simultaneously alive and dead until somebody peeked. At this point, it would be one or the other. To Schrödinger, that didn’t seem right.
This is the cat in the title of Gribbin’s 1984 book, In Search of Schrödinger’s Cat, in which he looks at the paradox of the dead-and-alive cat as part of an overview of quantum theory, but offers us little help with its dilemmas. In what is billed as a sequel, pitched at the same nonspecialist level, Schrödinger’s Kittens combines this cat problem with a second puzzle, and then offers us a way out of both.
The second problem is nonlocality. There are times when something over here affects something over there faster than any light signal can pass between the two places. Though this kind of action at a distance appears to contradict Einstein’s theory of relativity, nonlocality is now a fact of life following an experiment by Alain Aspect and his co-workers during the early 1980s. They measured the properties of photons from a single source – a beam of light – at two points and discovered a correlation between the two. This occurred although the transfer of information between the two points apparently violated the speed of light. The proof that nonlocality is real may turn out to be one of the great discoveries of this century.
Gribbin illustrates this second problem by replacing the cat in the box with two kittens in separate capsules which shoot off to space in opposite directions. When an alien examines the kitten in one capsule and finds it to be alive, say, the kitten on the far side of the Universe instantly becomes dead – that’s all according to the Copenhagen interpretation and nonlocality.
Cats and nonlocality are the starting point of the book – a bit of revision to ensure the book is self contained and that the reader appreciates what the problems are. Gribbin follows this with a fair chunk of history and background, pretty familiar stuff to anybody who has read Schrödinger’s Cat or any other book on quantum mechanics. Then, slowly and unobtrusively, we slide into the stuff that really matters.
Taking us past the halfway mark is a potpourri of new experiments, some of which are the practical realisation of what were just thought experiments only a few years ago. All of them confirm the predictions of quantum theory. Still, what does it all mean?
Quantum mechanics is a bit like politics. According to Gribbin, different leaders armed with the same evidence come to different and mutually exclusive conclusions and all will claim to be right. Apart from the Copenhagen interpretation, Gribbin offers us several “correct” alternatives incorporating multiple universes, magical guiding waves and more.
So which is it to be? Gribbin offers us his answer right at the end. I’m not telling you what it is, except to say that he has plucked a plum from the literature that’s certainly worth hearing about. And, who knows, maybe it is the answer.
I enjoyed Schrödinger’s Kittens enormously, and found it very stimulating. Unlike many sequels, it is better than the original, and should appeal to anybody who has even the faintest interest in quantum theory. For my part, I can’t wait to hear what my friend thought of it. When the subject is quantum theory, there’s always something to say.
Schrödinger’s Kittens and the Search for Reality, pp 261
Weidenfeld & Nicolson