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If the process of measurement by a classical observer is fundamental to creating the reality we observe, what performed the observations that brought the contents of the universe into existence? "You really need to have an observer outside the system to make sense - but there's nothing outside the universe by definition," says Brown. That's why, Brown says, cosmologists now tend to be more sympathetic to an interpretation created in the late 1950s by Princeton University physicist Hugh Everett. His "many worlds" interpretation of quantum mechanics says that reality is not bound to a concept of measurement. Instead, the myriad different possibilities inherent in a quantum system each manifest in their own universe. David Deutsch, a physicist at the University of Oxford and the person who drew up the blueprint for the first quantum computer, says he can now only think of the computer's operation in terms of these multiple universes. To him, no other interpretation makes sense. Not that many worlds is without its critics - far from it. Tim Maudlin, a philosopher of science based at Rutgers University in New Jersey, applauds its attempt to demote measurement from the status of a special process. At the same time, though, he is not convinced that many worlds provides a good framework for explaining why some quantum outcomes are more probable than others. When quantum theory predicts that one outcome of a measurement is 10 times more probable than another, repeated experiments have always borne that out. According to Maudlin, many worlds says all possible outcomes will occur, given the multiplicity of worlds, but doesn't explain why observers still see the most probable outcome. "There's a very deep problem here," he says. Deutsch says these issues have been resolved in the last year or so. "The way that Everett dealt with probabilities was deficient, but over the years many-worlds theorists have been picking away at this - and we have solved it," he says. However Deutsch's argument is abstruse and his claim has yet to convince everyone. Even more difficult to answer is what proponents of many worlds call the "incredulous stare objection". The obvious implication of many worlds is that there are multiple copies of you, for instance - and that Elvis is still performing in Vegas in another universe. Few people can stomach this idea. Persistence will be the only solution here, Brown reckons. "There is a widespread reluctance to accept the multiplicity of yourself and others," he says. "But it's just a question of getting used to it." Deutsch thinks this will happen when technology starts to use the quantum world's stranger sides. Once we have quantum computers that perform tasks by being in many states at the same time, we will not be able to think of these worlds as anything other than physically real. "It will be very difficult to maintain the idea that this is just a manner of speaking," Deutsch says. He and Brown both claim that many worlds is already gaining traction among cosmologists. Arguments from string theory, cosmology and observational astronomy have led some cosmologists to suggest we live in one of many universes.