What a Tangled Web We Weave

When we measure a tiny particle, we find it in a particular place. However, modern science shows that when we are not measuring the particle, it exists as a "wave function," in all places it might possibly be. Measuring "collapses the wave function" to one particular location.

That's fine for one particle, but what about when particles interact? Particles continually collide with and exchange energy with other particles. How does a system of many particles behave?

Modern physics predicts that interacting particles become "entangled," so that there is one wave function describing the entire system. As soon as any part of the system is measured, the entire wave function collapses and all its particles take on particular locations.

One interesting result of this is that if two particles become entangled, and then go their different ways, measuring one particle instantly results in the other particle taking on a particular value, no matter how far away it is! How does the remote particle 'know' that a measurement has taken place far away from it?

This situation predicted by modern physics is called "nonlocality" -- a particle is affected by actions that are apparently completely separated and far away. Classical physics says that such a thing is impossible. Who is right?

Many experiments have been conducted to test this theory, with the most important ones occurring in 1972, 1982, and 1997. These experiments create a pair of entangled particles, send them in opposite directions, measure one particle and then see what happened to the other one. "Local" theories and "nonlocal" theories make different predictions about what will be found.

The experiments all found that the nonlocal theories explained the actual results, and that the local theories did not. Because of the small distances involved in the 1972 and 1982 experiments, many scientists felt that the issue was still unresolved. The 1997 experiment was more definitive.

Nonlocality is a powerful concept. Once particles become entangled, their fates are inseparable. What happens to one can affect the other -- not by a chain of intermediate events, but instantly and without any exchange of matter or energy, without any connection whatsoever between them in space.

Nonlocality is an arrow pointed at the heart of science, which may be why so many scientists were reluctant to accept it for so long. If scientific experiments can be affected by what happens halfway across the universe, then the process of experimentation has a serious limitation. In a nonlocal universe, can cause and effect be separated, or are those concepts outdated and empty?

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