The most revolutionary, beautiful, elegant, and important idea to be advanced in the past two centuries is the idea that reality is made up of more than one universe. By an infinity of parallel universes, in fact. By "parallel universe" I mean universes exactly like ours, containing individuals exactly like each and every one of us. There are an infinity of Frank Tiplers, individuals exactly like me, each of whom has written an essay entitled "Parallel Universes" each of which is word-for-word identical to the essay you are now reading, and each of these essays is now being read by individuals who are exactly identical to you, the reader. And more: there are other universes which are almost identical to ours, but differ in minor ways: for example, universes in which you the reader (and I the writer!) really did marry that high school sweetheart—and universes in which you didn't if you did in this universe.
A truly mind-boggling idea, because were it to be true, it would infinitely expand reality. It would expand reality infinity more than the Copernican Revolution ever did, because at most, all that Copernicus did was increase the size of this single universe to infinity. The parallel universes concept proposes to multiply that single infinite Copernican universe an infinite number of times. Actually, an uncountable infinity of times.
Several physicists in the early to mid twentieth century independently came up with the parallel universes idea—for instances, the Nobel-Prize-winning physicists Erwin Schrödinger and Murray Gell-Mann—but only a Princeton graduate student named Hugh Everett had the guts to publish, in 1957, the mathematical fact that parallel universes were an automatic consequence of quantum mechanics. That is, if you accept quantum mechanics—and more than a century of experimental evidence says you have to—then you have to accept the existence of the parallel universes.
Like the Copernican Revolution, the Everettian Revolution will take decades before it is accepted by all educated people, and it will take even longer for the full implications of the existence of an infinite number of parallel universes to be worked out. The quantum computer, invented by the Everettian physicist David Deutsch, is one of the first results of parallel universe thinking. The idea of the quantum computer is simple: since the analogues of ourselves in the parallel universes are interested in computing the same thing at the same time, why not share the computation between the universes? Let one of us do part of the calculation, another do another part, and so on with the final result being shared between us all.
Quantum mechanics is only mysterious if one ignores the other universes. For example, the Heisenberg Uncertainly Relations, which in the old days were claimed to be an expression of a breakdown in determinism, are nothing of the kind. The inability to predict the future state of our particular universe is not due to a lack of determinism in Nature, but rather due to the interaction of the other parallel universes with our own universe. The mathematics of Everett shows that if one attempts to measure a particle's position, the interaction of the particle with its analogues in the other universes will make its momentum vary enormously. (This shows, by the way, that the parallel universes are real and detectable: they interact with our own universe.) If one leaves out most of reality when trying to predict the future, then of course one's predictions are going to be incorrect.
In fact, quantum mechanics is actually more deterministic than classical mechanics! It is possible to derive quantum mechanics mathematically from classical mechanics by requiring that classical mechanics be always deterministic—and also be composed of parallel universes. So adding the parallel universes ensures the validity of Albert Einstein's dictum: "God does not play dice with the universe."
Remarkably, the other great scientist of the past two hundred years, Charles Darwin, took the opposite point of view. God, Darwin insisted, does play dice with the universe. In the last chapter of his Variation of Animals and Plants Under Domestication, Darwin correctly pointed out that anyone who truly believes in determinism will not accept his theory of evolution by natural selection acting on "random" mutations. Obviously, because there are no "random" events of any sort. All events—and mutations—are determined. In particular, if it was determined in the beginning of time that I would be here 15 billion years later writing these words, then all previous evolutionary events leading to me, like the evolution of Homo sapiens, necessarily had to occur when they did occur.
So the Everettian Revolution means that we will have to choose between Einstein and Darwin.
Many leading evolutionary biologists have recognized that there is a problem with standard Darwinian theory. For example, Lynn Margulis and Dorian Sagan, in their book Acquiring Genomes, discuss these difficulties, but their own proposed replacement does not quite eliminate the difficulties (as the great evolutionist Ernst Mayr points out in the book's Forward), because they still accept the idea that there is randomness at the microlevel. If one gives up randomness and accepts determinism, there is no reason why speciation must be gradual. It could occur in a single generation. A Homo erectus mother could give birth to a Homo sapiens male-female pair of twins. In his early work on punctuated equilibrium, the famous Harvard evolutionist Stephen J. Gould was attracted to the idea of speciation in a single generation, but he could not imagine a mechanism that would make it work. The determinism that is an implication of the Everettian Revolution provides such a mechanism, and more, shows that this mechanism necessarily is in operation.
The existence of the parallel universes means that we shall have to rethink everything. Which is why I have called this idea the Everettian Revolution.