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Nary a week goes by that does not bring news of another feat of quantum trickery once only dreamed of in thought experiments: particles (or at least all their properties) being teleported across the room in a microscopic version of Star Trek beaming; electrical "cat" currents that circle a loop in opposite directions at the same time; more and more particles farther and farther apart bound together in Einstein's spooky embrace now known as "entanglement." At the University of California, Santa Barbara, researchers are planning an experiment in which a small mirror will be in two places at once.
Niels Bohr, the Danish philosopher king of quantum theory, dismissed any attempts to lift the quantum veil as meaningless, saying that science was about the results of
experiments, not ultimate reality. But now that quantum weirdness is not confined to thought experiments, physicists have begun arguing again about what this weirdness means, whether the theory needs changing, and whether in fact there is any problem.
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The Silly Theory
From the day 100 years ago that he breathed life into quantum theory by deducing that light behaved like a particle as well as like a wave, Einstein never stopped warning that it was dangerous to the age-old dream of an orderly universe.
If light was a particle, how did it know which way to go when it was issued from an atom?
"The more success the quantum theory has, the sillier it seems," Einstein once wrote to friend.
The full extent of its silliness came in the 1920's when quantum theory became quantum mechanics.
In this new view of the world, as encapsulated in a famous equation by the Austrian Erwin Schrödinger, objects are represented by waves that extend throughout space, containing all the possible outcomes of an observation - here,
there, up or down, dead or alive. The amplitude of this wave is a measure of the
probability that the object will actually be found to be in one state or another, a suggestion that led Einstein to grumble famously that God doesn't throw dice.
Worst of all from Einstein's point of view was the uncertainty principle, enunciated by Werner Heisenberg in 1927.
Certain types of knowledge, of a particle's position and velocity, for example, are
incompatible: the more precisely you measure one property, the blurrier and more
uncertain the other becomes.
In the 1935 paper, Einstein and his colleagues, usually referred to as E.P.R., argued that the uncertainty principle could not be the final word about nature. There must be a deeper theory that looked behind the quantum veil.
Imagine that a pair of electrons are shot out from the disintegration of some other particle, like fragments from an explosion. By law certain properties of these two fragments should be correlated. If one goes left, the other goes right; if one spins clockwise, the other spins counterclockwise.
That means, Einstein said, that by measuring the velocity of, say, the left hand electron, we would know the velocity of the right hand electron without ever touching it.
Conversely, by measuring the position of the left electron, we would know the position of the right hand one.
Since neither of these operations would have involved touching or disturbing the right hand electron in any way, Einstein, Podolsky and Rosen argued that the right hand electron must have had those properties of both velocity and position all along. That left only two possibilities, they concluded. Either quantum mechanics was "incomplete," or measuring the left hand particle somehow disturbed the right
hand one.
But the latter alternative violated common sense. Such an influence, or disturbance, would have to travel faster than the speed of light. "My physical instincts bristle at that suggestion," Einstein later wrote.
Bohr responded with a six-page essay in Physical Review that contained but one simple equation, Heisenberg's uncertainty relation. In essence, he said, it all depends on what you mean by "reality."
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