Light (or electrons or atoms) can be shined through a small hole in an opaque plate, then continuing on, through two other small holes in another plate to form interference patterns. (see Quantum Mysteries Part 1)
Imagine setting up a pair of detectors alongside of each of the holes, and sending the electrons through one at a time. Now you can see if a wave or a particle passes through one hole or both, and what the screen looks like. What you find is that each electron is seen as a particle, passing through one hole or the other, and no interference pattern develops. The pattern on the screen is like little bullets have gone through, and made the appropriate pattern. The act of observing the electron wave seems to have made it collapse and behave like a particle. You only need to look at one hole to change the pattern. How do the electrons going through one hole 'know' that we are looking at the other hole? This phenomenon is known as non-locality. Einstein called it 'spooky action at a distance.'
The Austrian Erwin Schrodinger thought that all this was ridiculous, and came up with his story of the cat in the box, which appeared in print in 1935. Albert Einstein, Boris Podolsky, and Nathan Rosen also came up with a 'thought experiment' about the same time. It is known as the 'EPR paradox.' In the mid 60's, John Bell, an Irish physicist, found a way to express the paradox in terms of an experiment that could be carried out on pairs of photons emitted from an atom simultaneously in two different directions. Several researchers took up the challenge. The most conclusive of these experiments was carried out by Alain Aspect & colleagues in the early 1980's. They demonstrated beyond any reasonable doubt that non-locality really does rule in the quantum world, and that Einstein and the others were wrong.
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