Gilder: The Age of Entanglement
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Louisa Gilder, The Age of Entanglement : When Quantum Physics was Reborn. New York : Alfred A. Knopf, 2008. xvi + 443 pages; illustrated with drawing by the author, some photographs; includes glossary, notes, bibliography, and index.
The title of Louisa Gilder's book is evidently meant to be literary, alluding perhaps to Auden's famous 1947 poem "The Age of Anxiety", but also to the "Age of Enlightenment", both of which lend some redolence to her subject, a quest for meaning and understanding in quantum mechanics. Also, "entanglement" can refer to several things, not least of which might refer to how any number of scientist's live have been sucked into the maelstrom of quantum mechanical hermeneutics.
It also refers to a specific concept of quantum mechanics that is straightforward enough but takes some time to present—in fact, it is the subject of the book. It refers to the fact that, in one incarnation, two particles can be emitted by a machine in such a way that their quantum states must form a certain combination, but which is in which state is not determined until one of the particles is observed and the wave-function of the other then "collapses" into the proper state of the other particle, regardless of how far apart they are (in particular, they could be arbitrarily further apart than would allow a "signal" between the particles to travel at the speed of light). The quantum states of the particles are "entangled".
This is a notion that Einstein poo-pooed as "spooky action at a distance". Einstein, along with colleagues Boris Podolsky and Nathan Rosen, in 1935 published a paper in which they proposed one of Einstein's famous "thought experiments" along the lines of the two-particle experiment. Their purpose seemed to be to mock these apparent anti-commonsensical possibilities of quantum mechanics.
The mockery didn't work. The quandary of the EPR experiment showed more life than Schrödinger's Cat and caused persistent arguments. There were false starts and blind alleys but the whole idea was put on a firm mathematical, formalistic footing by Irish physicist John Steward Bell (1928—1990) with "Bell's Theorem" and the famous "Bell Inequalities".
There are several equivalent ways of trying to state Bell's Theorem in plain English; the one I find most appealing says that "if quantum mechanics is correct then the universe is not locally deterministic".
This is subtle enough, and important enough, but he went beyond the theorem and discovered / developed the "Bell Inequalities", mathematical expressions that would be satisfied by EPR type experiments with entangled particles if the theorem were true. Remarkably, it was possible to do experiments to prove that quantum mechanics is not locally causal.
Revealed in that extraordinary paper of 1964, Bell's theorem showed that the world of quantum mechanics—the base upon which the world we see is built—is composed of entities that are either, in the jargon of physics, not locally causal, not fully separable, or even not real unless observed.
I don't grasp all the implications of those ideas, certainly; in fact, my understanding is still pretty tenuous. It takes some time to absorb the background of standard operating ideas in quantum mechanics to appreciate the problem with the EPR "paradox", then to try to incorporate Bell's ideas about nonlocality and its implications, then to take in the design and results of the EPR type experiments that verified Bell's inequalities and caused a sensation—among a few physicists, at least.
Fortunately, our author affords plenty of time to take in the ideas as she relates the earlier development of quantum mechanics, in order to show the origins and development of some central ideas and trace their growth, and to explain in some detail Bell's work and the work of the experimental scientists who competed to verify his predictions first.
Her approach draws a great deal on nonfiction narrative—this is how she wanted to bring some life to the story of these ideas and involve many of the characters (and many of them were "characters") who created and developed the ideas. I admit to a bit of skepticism at the beginning, but I was reassured by her remarks in the introduction:
This is a book of conversations, a book about how the give-and-take between physicists repeatedly changed the direction in which quantum physics developed, just as conversations, subtly or dramatically, change the world we live in and experience every day. All the conversations in this book occurred in some form, on the date specified in the text, and I have fully documented the substance of every one. [p. xiv]
I also quickly found that her writing was careful, precise, accurate, and lively reading. She has made the narrative nonfiction approach serve her purpose of illuminating the ideas and people, all the while adhering very closely to historic facts. I was fully satisfied on that count.
I often feel some trepidation when quantum mechanics is the subject. There are any number of authors who induce my scowl of disapproval when they seem to delight in obscuring or mystifying the odder aspects of quantum theory just for the sake of mysticism. There are enough aspects of the quantum world to induce awe and curiosity without turning it into a poster-child for new-age mysticism.
Ms. Gilder avoided my scowls quite adeptly. Perhaps she did not dispel all mysticisms but she comes to them honestly since she is trying to recount with historical precision the intellectual groping of the scientists involved as they tried to understand. There are oddities in quantum mechanics and inscrutable ideas and results, without having to manufacture any.
On this topic I enjoyed her excerpt from Richard Feynman giving the "Messenger Lectures" at Cornell, November 1964, on the subject "The Character of Physical Law" (later published as a book).
"There was a time when the newspapers said that only twelve men understood the theory of relativity. I do not believe there were was such a time. There might have been a time when only one man did, because he was the only guy who caught on, before he wrote his paper." (It was typical Feynman to dodge the mention of anyone but the first person singular.) "But after people read the paper a lot of people understood the theory of relativity in some way or other, certainly more than twelve. On the other hand, I think I can safely say that nobody understands quantum mechanics.
"So do not take the lecture too seriously, feeling that you really have to understand in terms of some model what I am going to describe, but just relax and enjoy it. I am going to tell you what nature behaves like. If you will simply admit that maybe she does behave like this, you will find she's a delightful, entrancing thing. Do not keep saying to yourself, 'But how can it be like that?' because you will get 'down the drain', into a blind alley from which nobody has yet escaped. Nobody knows how it can be like that." [pp. 228—229]
Oddly, I'm not sure that I understand entanglement now; I may even understand it less than I did before, suggesting that I've obviously learned some things. However, I think I now understand pretty thoroughly what it's like not to understand entanglement, and that's surely a step on the path to enlightenment.
-- Notes by JNS