The author, Nicolas Gisin, is a world-class expert in the subject of the book’s subtitle: quantum “nonlocality, teleportation, and other quantum marvels”. He was a principal investigator of an experiment – performed in 1997 near Geneva, Swizerland – that gave nearly watertight evidence for one of the strangest properties of quantum theory: “nonlocality”. This is the main topic of the book, which was published in 2014 and is just about the most recent one that covers the subject for the non-professional reader.
What is “nonlocality”? Answering that question, in terms an educated layperson can understand, is the purpose of the book. All I can do here is try to indicate the gist of the matter. In 1935 Albert Eistein, with two collaborators (Boris Podolsky and Nathan Rosen) published a paper describing a thought experiment that seemed to show an inconsistency between quantum mechanics and special relativity. This became known as the “EPR paradox”. The thought experiment was based on a pair of elementary particles, such as electrons, that had been prepared in what is called an “entangled” state. That means any measurement made on certain properties of one particle must be correlated with a related measurement of properties of an entangled particle (as long as they don’t interact with any other particles). And this must be true no matter how far the particles are separated in space and time when the measurements are made.
The possibility of entanglement of quantum particles is predicted by quantum theory. However, because of Heisenberg’s uncertainty principle it leads to an apparent paradox if the particles happen to be far enough apart at the time of measurement so that no signal can pass between them in the time between the two measurements. That’s because Einstein’s Special Theory of Relativity requires that no information can be transmitted from one point to another faster than the speed of light.
When certain measurements are made on one particle of an entangled pair, then a related measurement on the other particle must turn out in a specific way no matter how far apart the particles are. And the result of measuring the second particle would probably have been different if the first particle hadn’t been measured. In other words, a measurement at one place seems able to affect instantaneously a measurement somewhere perhaps very distant, in apparent violation of the special theory of relativity.
This also seems to violate the kind of indeterminacy required by quantum mechanics, because typically measurement of quantum properties can yield a number of distinct results, each with a particular probability. More specifically, a quantum particle can be in a “superposition” of distinct “pure states”. After the measurement, the particle will be in only one of the pure states, with perhaps a different probability for each result. However, for particles that are entangled, the result of the first measurement can uniquely determine the result of the second. It is as though information about the first measurement somehow influences the other one.
Yet if the particles are so far apart that no information can pass from one particle to the other at less than or equal to the speed of light, then there is no way the second particle can know what measurement was made on the first particle or what the results were. To Einstein and many others it appeared that the only way out was that there must be some sort of unknown property (a “hidden variable”) that was shared by both particles.
In order to perform an experiment to test this hypothesis (so that quantum mechanics and special relativity would be compatible) it was necessary to be able to create appropriate pairs of entangled particles, separate them at a distance large enough that no information could propagate from one to the other in the time between measurements, and derive an estimate of how much statistical discrepancy was possible between the measurements.
At the time of the EPR paper in 1935, physicists had no idea how to meet all those conditions. There were two reasons. First, there was no adequate technology then for creating and measuring entangled particles (e. g. electrons or photons) in order to carry out the experiment. Second, there was no clear idea of what sort of correlation would show whether or not hidden variables existed. The problem is that some correlations (greater than zero) could be expected depending on the nature of the experiment. (If two people each toss a coin, the results will agree half the time, a 50% correlation, even though it’s completely random.)
The second problem was solved in 1964 by physicist John Bell. He proved mathematically that there was a numerical upper bound to a certain quantity that reflects correlations of the measurements, assuming that hidden variables exist and the rules of both special relativity and quantum mechanics applied. This numerical relation is called “Bell’s inequality”.
In the early 1980s, Alain Aspect (who much later wrote the foreword to Gisin’s book) and others were able to perform an actual experiment – and it was found that in fact Bell’s inequality was violated. When a series of many tests were performed in which one of two related measurements were performed randomly (50% probability) on separated but entangled particles, the relevant quantity was larger than allowed by Bell’s inequality if hidden variables existed. This cast substantial doubt on the idea of hidden variables (but only if they were “local”, meaning they couldn’t communicate information about their values at faster than the speed of light).
It’s an important feature of such experimenta that there are two possible measurements that can be made. A hidden variable shared by the entangled particles might determine what outcome should occur for each measurement. Also, a hidden variable, if it existed, might carry information about which measurement was made on the first particle or the result. But the hidden variables assumed in Bell’s theorem are local. Hence if the measurements are made when the particles are so far apart that no information from one measurement can reach the other before it’s measured, then there is no way the second particle to be measured can “know” what measurement was made on the first one. The second particle “sees” the first particle as it existed before any measurement was made. (The existence of “nonlocal” hidden variables, whose information could be accessible everywhere in the universe instantaneously, is conceivable, but that’s considered a very far-fetched possibility.)
More recently, Gisin’s team and others have performed more rigorous experiments that make the conclusion even more watertight. Until quite recently there had been certain “loopholes” in experiments, related to the details of the experiment, that could cast doubt on the validity of the findings. But in 2015 results were published of three separate experiments that seem to close all loopholes physicists consider plausible. The conclusion is quite clear: Bell’s inequality is violated. So unless either quantum mechanics or special relativity is wrong, there cannot be any “local hidden variables”.
Since the particles in the recent experiments are sufficiently far apart when they are measured, special relativity doesn’t allow information to pass from one measurement to the other. But since the measured correlations are found despite the large distance, the resulting correlation is called “nonlocal”. In principle, it seems as though the separation can be as great as the size of the visible universe. This is why nonlocality, though now confirmed, is such a mysterious finding.
The key element in these experiments is the entanglement between the particles being measured. The modern theory of quantum mechanics was formulated a few years before 1930, and physicists knew about entanglement very soon afterwards. But it wasn’t considered especially important or consequential before the EPR paper was published. Very few physicists saw its importance at an early date. Erwin Schrödinger was among the few, writing:
Entanglement is not one, but rather the characteristic trait of quantum mechanics, the one that enforces its entire departure from classical lines of thought.
Today, the phenomenon of entanglement is of utmost importance, not only theoretically, but also for practical applications. Without entanglement quantum communication, quantum cryptography, quantum “teleportation”, and quantum computing would all be impossible. Some of these things are already commercially available products, not just laboratory curiosities.
Gisin’s book further explains things mentioned in this review, especially the nature of the experiments that showed violations of Bell’s inequality. However, not all of these topics are covered as thoroughly as they should be for a proper understanding. Quantum computing, for example, is hardly covered at all. The book’s brevity (about 110 pages, not counting front material) is a virtue, in that it allows a focus on the central topic. But it’s also a drawback because of how much of the whole story is omitted.
Here then are some shortcomings that you should be aware of.
1. The book uses hardly any mathematics at all, except for some simple probability arguments. Relatively simple mathematics could have been used effectively to better explain entanglement, Bell’s inequality, and how things like quantum teleportation and quantum computing work.
2. The most difficult chapter in the book describes a “game” having a structure that reflects what is done in actual experiments. It may be more understandable, since the mathematics is slightly more transparent, but the game differs a lot from the physical experiments. It doesn’t involve quantum concepts at all.
3. There is a very short discussion of quantum cryptography, but it’s much too short for even a rudimentary understanding of how it works. So a reader never learns how quantum encryption makes it possible to detect any eavesdropping on a conversation.
4. There is no index or bibliography in the book. Lack of an index makes it difficult for a reader to locate where a particular topic or concept has previously been explained. Although some important references are given in footnotes, there’s little help for a reader who wants to go more deeply into various important topics. This is especially a problem, since the book itself doesn’t try to cover many topics in any real detail.
Despite these shortcomings, what the book does offer is very well done.
I’ve written another review of Gisin’s book that goes into more a little more detail on some of the relevant physics. It’s here.