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Blurring Causal Lines

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Quantum experiments mix past and future on the microscopic scale.

http://fqxi.org/community/articles/display/208

EXCERPT: . . . "All events are ordered in time such that for every pair of events you can say that one event is either before or after the other," says quantum physicist Caslav Brukner. Causality is so ingrained in the texture of our lives, and our brains, that it is hard to imagine letting go of it. Yet this is exactly what Brukner proposes to do with the help of an FQXi grant of over $63,000. The implications of this idea could be enormous: we might find that space, time and causality are not the basic building blocks of nature. It would have practical consequences too—potentially helping us to build super fast quantum computers to outperform today’s devices.

[...] "In all our physical theories we assume a well-defined causality," says Brukner. That’s even true for Einstein’s special and general theories of relativity, which mess with many of our everyday conceptions by stating that time is another dimension, like the three dimensions of space. Einstein taught us that, depending on how two people move relative to one another, they may disagree on the order in which they perceive two independent events to have occurred. Causality is still sanctified in relativity, however, because there’s no way any observer could perceive an effect to have taken place before its cause.

But Brukner argues that to truly understand nature and the world we live in—and perhaps to eventually combine quantum theory and general relativity into one framework—we may have to get away from this safe sense of order. "If we believe in the validity of quantum mechanical laws, and we believe in the validity of general relativity, we need to think of a situation in which the causal order is not well-defined," says Brukner....
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