Fifty years ago this month, physicist John Bell submitted a paper on what his fellow physicists called the "Einstein Podolsky Rosen paradox," a feature of subatomic particles that made no sense whatsoever, that Albert Einstein himself called "spooky" and which even today we probably do not really understand.
Subatomic particles like electrons have certain qualities, like polarity and "spin." When physicists say these tiny bits of stuff have spin, they do not mean the kind of spinning done by a top, a planet or a four-year-old trying to get dizzy, but another kind of feature entirely that is mostly beyond my brainwaves. Anyway, if get a pair of particles together and you know that their combined spin is zero, you know that they have opposite spins from each other. And if one changes its spin, then so does the other -- instantaneously. Which is, according to what physicists know about the universe, the speed of light and the laws of nature, impossible.
It may not look impossible at short distances, because the speed of light is so fast (186,000 miles per second, if you remember) that human perception could never pick up the lag. But even when the pair of previously entangled particles is separated by significant distances, the instantaneous change still happens. No experiment has shown any reason to suppose that the two particles would not continue their linkage, even if they somehow found themselves billions of light years apart.
Prior to Bell, quantum physicists mostly ignored entanglement since it didn't have much of an impact on what they were doing. But it remained, a gaping hole in quantum theory's ability to describe the world. If my theory of the way the world works requires things to happen that contradict things we can already prove true, then I have to do one of two things: 1) Explain why what we think is happening isn't happening or 2) Explain how the theory really does cover what's happening. There is a third, but it's really just abandoning the whole theory as unworkable. This is the kind of choice you make when all of your other ones fail.
But Bell's paper began exploring entanglement and insisting on keeping it in the mix as a part of the explanation for how the world works. Later developments have put a little light on what's happening with those two particles, but Einstein may have had one thing right about the phenomenon: It's spooky.