Alice in Quantumland_ An Allegory of Quantum Physics - Robert Gilmore
Preface
n the first half of the twentieth century, our understanding of the Universe was turned upside down. The old classical theories of physics were replaced by a new way of looking at the world-quantum mechanics. This is in many ways at variance with the ideas of the older Newtonian mechanics; indeed, in many ways it is at variance with our common sense. Nevertheless, the strangest thing about these theories is their extraordinary success in predicting the observed behavior of physical systems. However nonsensical quantum mechanics may at times appear to us, that seems to be the way that Nature wants it-and so we have to play along.
This book is an allegory of quantum physics, in the dictionary sense of "a narrative describing one subject under the guise of another." The way that things behave in quantum mechanics seems very odd to our normal way of thinking and is made more acceptable when we consider analogies to situations with which we are familiar, even though the analogies may be inexact. Such analogies can never be very true to reality as quantum processes are really quite different from our normal experience.
An allegory is an extended analogy, or series of analogies. As such, this book follows more in the footsteps of Pilgrim's Progress or Gulliver's Travels than of Alice in Wonderland. "Alice" appears the more suitable model, however, when we examine the world that we inhabit.
The Quantumland in which Alice travels is rather like a theme park in which Alice is sometimes an observer, while sometimes she behaves as a sort of particle with varying electric charge. This Quantumland shows the essential features of the quantum world: the world that we all inhabit.
Much of the story is pure fiction and the characters are imaginary, although the "real-world" notes described below are true. Throughout the narrative you will find many statements that are obviously nonsensical and quite at variance with common sense. For the most part these are true. Neils Bohr, the father figure of quantum mechanics in its early days, is said to have remarked that anyone who did not feel dizzy when thinking about quantum theory had not understood it.
Seriously, Though . . .
The description of the world that is given by quantum mechanics is undoubtedly interesting and remarkable, but are we seriously expected to believe that it is true? Amazingly, we find that we must. To underline this assertion, throughout this book you will find brief notes which emphasize the importance of quantum mechanics in the real world. The notes look like this:
There are also some longer, end-of-chapter, notes. These amplify some of the trickier points in the text and are denoted thus:
See end-of-chapter note 1
Much of the way that quantum theory describes the world may seem at first sight to be nonsense-and possibly it may seem so at the second, third, and twenty-fifth sight as well. It is, however, the only game in town. The old classical mechanics of Newton and his followers is unable to give any sort of explanation for atoms and other small systems. Quantum mechanics agrees very well with observation. The calculations are often difficult and tedious, but where they have been made, they have agreed perfectly with what has really been seen.
It is impossible to stress too strongly the remarkable practical success of quantum mechanics. Although the outcome of one measurement may be random and unpredictable, the predictions of quantum theory agree consistently with the average results obtained from many measurements. Any large-scale observation will involve very many atoms and thus very many observations on the atomic scale. We again find that quantum mechanics is successful, in that it automatically agrees with the results of classical mechanics for large objects. The converse is not true.
Quantum theory was developed to explain observations made on atoms. Since its conception, it has successfully been applied to atomic nuclei, to the strongly interacting particles which derive from the nucleus, and to the behavior of the quarks of which these are composed. The application of the theory has been extended over a factor of some hundred thousand million. The systems considered have both decreased in size and increased in energy by this factor. This is a long way to extrapolate a theory from its original conception, but so far quantum mechanics appears to be quite able to deal with these extreme systems.
Insofar as it has been investigated, quantum mechanics appears to be of universal applicability. On a large scale, the predictions of quantum theory lose their random aspect and agree with those of classical mechanics, which works very well for large objects. On a small scale, however, the predictions of quantum theory are consistently borne out by experiment. Even those predictions, which seem to imply a nonsensical picture of the world, are supported by experimental evidence. Intriguingly, as is discussed in Chapter 4, quantum mechanics would appear to be in the strange position of agreeing with all observations made, while disputing that any observations can actually be made at all. It seems that the world is stranger than we imagine and perhaps stranger than we can imagine.
For the present, however, let us accompany Alice as she begins her journey into Quantumland.
Robert Gilmore