Rachel Courtland in New Scientist:
AN IDENTICAL copy of you is also reading this story. This twin is the same in every way, living on an Earth and in a universe that looks exactly like our own. And there may be an infinite number of them. Such doppelgängers could be a natural consequence of our present conception of the universe. Now, some physicists say they could pose a serious problem for quantum mechanics. But a possible fix may also be in sight, and it could help tie abstract quantum concepts to concrete physical causes.
In the uncertain, fuzzy world of quantum mechanics, particles do not have fixed properties until they are observed. Instead, objects that obey quantum rules exist in a “superposition” of all their possible states simultaneously. Schrödinger's famous cat, for example, is both alive and dead until we take a peek inside the booby-trapped box in which it has been placed.
Because the probability that the cat will be found alive is based on a quantum event – the decay of a radioactive substance within the box – it can be calculated using a principle called the Born rule. The rule is used to transform the vague “wave function” of a quantum state, which is essentially a mixture of all possible outcomes, into concrete probabilities of particular observations (in this case, the cat being alive or dead). But this staple of quantum mechanics fails when it is applied to the universe at large, says Don Page at the University of Alberta in Edmonton, Canada.
At issue is the possibility that there could be a multiplicity of copies of any particular experiment floating about the universe, just as there could be a multiplicity of yous. There could even be an infinite number of them if, as is thought, the early universe underwent a period of exponential growth, called inflation. Although this period ended very soon after the big bang in our observable region of space, inflation may have continued elsewhere, giving rise to a “multiverse”, an infinite space containing infinite copies of our Earth. “In an infinite universe, every possible thing would happen, and it would happen an infinite number of times,” says cosmologist Alex Vilenkin of Tufts University in Medford, Massachusetts.