
newscientist.com - 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.
Missing ingredient
Crucially, says Page, all of these copies pose a problem for the Born rule: it’s unclear how to calculate the probability of different outcomes for a given experiment without first adding some extra ingredient that accounts for the multitude of copies (arxiv.org/abs/1003.2419). “You can’t just plug in the Born rule and get answers that make sense,” he says (see “Identity crisis”).
Andreas Albrecht of the University of California, Davis, has dubbed the problem the “Born rule crisis”. The shortcoming means we could not, in theory, calculate the probability of the outcome of any new measurement of the universe, such as the mass of the neutrino. “It’s a deep failure of something, either of quantum theory or the multiverse,” Albrecht says. “If you’re a cosmologist, you should be worried,” he adds.
Others, like physicist Mark Srednicki at the University of California, Santa Barbara, are not convinced there is a crisis. He says adjustments for missing information are fairly routine in quantum physics and should not require an overhaul of the theory.
A deeper problem, he says, is that we still don’t understand what quantum probabilities really mean. “Quantum mechanics is now 100 or so years old, but it is still deeply mysterious,” he says. “Because the concepts are so divorced from human experience, we’re still not sure we’re thinking of them in the right way.”







Moving House – Be Back Soon!
Hi all. So, as the title suggests i’m moving house – which means i’ll be without internet for a while. But naturally this will be top of my to-do list. So please bear with us, and we’ll bring you the latest news ASAP.
Thanks… see you soon!
Stephen, Paranormal Mysteries
(notyoubutste [at] gmail [dot] com)