Bell derived his inequality in 1965, believing that an actual experimental test was still a long way off. But in 1972, John Clauser at the University of California, Berkeley figured out a nifty way to test it (though using paired photons instead of electrons). The result was that Bell’s inequality was violated, indicating that nature worked the way quantum physics predicted. Due to the limited size of his apparatus, and the speed of his data acquisition system, there was room for a loophole, however. The two measurements were close enough and slow enough that one could influence the other without exceeding the speed of light. So a definitive test of the locality assumption had to wait a while longer.
Ten years later, with better equipment, Alain Aspect from the Ecole Polytechnique in France was able to close that loophole. His apparatus was large enough and the detection time short enough that any “spooky” influence of one measured photon on the other had to travel about twice the speed of light. His more accurate measurements with polarized photons confirmed Clauser’s earlier results. What’s more, Aspect went on to set up a random, high-speed switching device so that he could change the path of the photons and direct them to more than one possible detector configurations. In fact, the switching could happen so fast that he could switch the photons’ paths while they were in flight from the origin to the directors.
Why would he do this? Some skeptics had maintained that the faster-than-light communication between the photons in the pair could be due to the fact that they “know” the exact configuration of the detectors downstream at the moment of their creation. By randomly switching their paths, however, they could no longer know the detector configuration to which they were heading. Aspect therefore closed another important loophole, leaving little doubt that quantum physics again made the correct prediction. The conclusion is unavoidable: nature does not have the local reality that Einstein craved.
It appears that there is no place in quantum physics for local hidden variables, and that measurement does indeed have a profound effect on wave functions. Two entangled particles remain connected through some sort of quantum channel even if they become separated by great distances. They look like they should be separable, but they are best treated as two parts of the same wave function, which can stretch over vast distances. In essence, it means that all parts of the universe are connected, which is a startling concept for most physicists.
QUANTUM QUOTE
[H]ere we have shown that in this kind of very unusual situation quantum mechanics works very well.
—Alain Aspect
Of course, it is not strictly necessary to delve into all of this interpretation stuff, if you just want to do quantum physics. The theory works, and that is enough for many scientists. It accurately predicts the results of our measurements of the real world, so long as we take enough measurements to get a good statistical sample.
On the other hand, quantum physics says nothing about the results of measurements that are not made. This apparently obvious statement is another way to understand the difficulty with objective reality. In the quantum view, things that are not observed need not be real. This is an unsettling conclusion for some, but it represents our present state of understanding, backed up by experiments. For now the question of whether quantum physics is complete has been answered in the affirmative.
This is not the end of the story, but it is a significant step. The weirdness of quantum physics does not prevent us from exploiting it in many technological applications. Entanglement, in particular, seems ripe for many creative uses, and some of these will be explored. But first, we’ll take a look at some alternatives to the Copenhagen interpretation, and confront the issue of consciousness in quantum physics.
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