Friday, November 1, 2019

John Gribbin's "Six Impossible Things"

John Gribbin, described by the Spectator as “one of the finest and most prolific writers of popular science around,” is the author of In Search of Schrödinger's Cat, The Universe: A Biography, 13.8: The Quest to Find the True Age of the Universe and the Theory of Everything, and other books. He is a Visiting Fellow in Astronomy at the University of Sussex, UK.

His new book is Six Impossible Things: The Mystery of the Quantum World. Since the book has fewer than 99 pages, Gribbin tried the Page 66 Test instead of the “Page 99 Test" and reported the following:
From page 66:
The statistics are those of the ensembles. But the ensembles are not the kind that spring to the mind of most people when they hear the term. In everyday language, an ensemble is a group of things that have some common property, or are working together – such as a musical string ensemble. To a statistician, a collection of 600 identical dice could constitute an ensemble, and if all those dice were rolled together then the laws of probability would lead us to expect to see near enough 100 sixes, 100 fives, 100 fours, 100 threes, 100 twos and 100 ones. But there is another way to get the same statistical outcome. Take a single perfect die, and roll it 600 times. You would expect 6 to come up about a hundred times, 5 to come up about a hundred times, and so on. This is the kind of ensemble the quantum physicists are referring to. A box full of molecules of gas would not constitute an ensemble in this sense; but many identical boxes of gas each experimented on in the same way would. Ideally, you would carry out exactly the same experiment on exactly the same particle many times, and monitor the outcome of each of these ‘trials’. That is the ensemble. The results would follow a probability distribution in accordance with the rules developed by Max Born.

It would be very hard to carry out such an idealised experiment, but that isn’t really the point. Instead of, say, a million electrons going through the double slit experiment at the same time and being detected on the other side, think of the same electron going round and round a million times, with the position it arrives at on the other side being noted each time it goes past. The crucial point which proponents of this interpretation like is that the particles are always real particles in the everyday use of the term.
This isn’t really the best place to start getting to grips with my book. The six impossible things of my title are six different “explanations”, or interpretations, of quantum mechanics, and my aim is to show that they are all equally crazy (although in the quantum world being crazy does not necessarily mean being wrong). This particular section puts forward the point of view of proponents of the “ensemble interpretation”, and at first sight it looks quite reasonable. But individual systems do exist in the real world, and as is often the case in quantum theory, the waters become muddier once you try to work out what happens when the system – in this case the ensemble – is studied, or otherwise interacts with the outside world. Preparing the system involves a certain amount of randomness, and observing it involves another layer of randomness. An example of this interaction with the outside world that is the so-called ‘watched pot’ experiment. Quantum physics tells us that a system (such as an atom, or array of atoms, in an energetic state) cannot change its state (in this case by giving up energy) as long as it is being “watched”, or monitored. It was Alan Turing, back in 1954, who pointed this out:
It is easy to show using standard theory that if a system starts in an eigenstate of some observable, and measurements are made of that observable N times a second, then, even if the state is not a stationary one, the probability that the system will be in the same state after, say, one second, tends to one as N tends to infinity; that is, that continual observations will prevent motion. [endnotes omitted]
And experiments have now actually been carried out which prove this to be the case. A watched quantum pot never boils (or more accurately, never freezes). So if you are baffled by quantum physics, you are in good company. It puzzled Turing, as well.
Learn more about Six Impossible Things at the MIT Press website.

--Marshal Zeringue