Simply put, bottomlessly deep: that is the definition of a great discovery in science. From the principle of relativity to evolution by natural selection, the concepts that govern our world are actually not that hard to state. What they mean and what they imply—well, that's another matter. And so it is with quantum entanglement. One of the most important discoveries ever made, entanglement is fairly straightforward to describe, but has yet to be understood in any serious way. Physicists have barely even gotten over their amazement that the phenomenon even exists.
The two-part video that I put together with my colleagues John Matson and Mary Karmelek, working with Sci Am's film guru Eric Olson, dramatizes entanglement. Part one presents it metaphorically; part two will show the real McCoy in a physics laboratory. The film follows in the footsteps of a steady progression of simplified versions of the original scientific arguments that has taken place over the past several decades. Not only has the theory been streamlined, so has the experimental apparatus. It could now fit on a living-room end table and should soon become a standard exercise in college physics-for-poets classes.
The basic point of entanglement is that the behavior of objects at spatially separated locations is random yet coordinated. Two (or more) particles behave as a single indivisible system, no matter how far apart they are. Indeed, even to speak of "particles" in the plural is a falsehood; we see them as individual parts, but they possess collective properties that cannot be partitioned. In the 1930s, Albert Einstein argued that for entangled particles to behave in such a coordinated way, either their behavior must be choreographed in advance or they must surreptitiously influence each other on the fly. This influence cannot pass through the intervening space—it would be, as Einstein put it, "spooky action at a distance." Three decades later, physicist John Bell devised an experiment that rules out the first possibility, leaving the spooky one as a creepy fact of nature.
The first two card tricks in the video show the basic thought-experiment that Einstein devised and published in a famous paper with Boris Podolsky and Nathan Rosen. The third trick shows Bell's elaboration. His basic insight was that it's easy enough to choreograph a simple pattern of behavior, but impossible to prearrange a sufficiently complicated one. By the way, you can use Bell's approach if any of your friends ever claims to be psychic. Ask the right types of questions, and no one will be able to respond unless they really are psychic. Humans, of course, aren't. But particles do have a telepathic power, albeit of a very limited sort.
Some technical details: For sake of getting across the idea, we neglect the role that probability plays in the actual experiment. If John and I were to exploit entanglement for real, we’d create a pair of entangled photons, he’d take one and I the other, and each of us would send his photon through a polarizing filter and see whether it emerges on the other side. The choice of “left” or “right” card in the video would correspond to the orientation of the polarizer. For John, “left” would be 0 degrees; “right,” 45 degrees. For me, “left” would be 22.5 degrees; “right,” –22.5 degrees. Assuming no experimental imperfections, the probability that we’d both see the same outcome would be about 85 percent for all possible permutations of orientations, except when both of us select “right,” in which case it would be about 15 percent. Cheaters trying to mimic entanglement could manage 75 percent at best.
I hasten to mention that some physicists and philosophers of physics doubt whether spooky action really occurs—to them, particles are no more psychic than humans are. But even in that case, something else equally weird must be going on to give the illusion of spooky action, such as a profusion of parallel universes, messages reaching us from the future, or a radically holistic view of reality. There's no way to avoid the weirdness altogether. Researchers also debate whether entanglement conflicts—in spirit if not in letter—with Einstein's special theory of relativity, as David Albert and Rivka Galchen discussed in our March 2009 cover story.
Leaving aside what the entanglement means, so much remains to be learned about the phenomenon itself. A big question is why, even though entanglement is pervasive, we don't notice it in our everyday lives. Quantum physicist Dagomir Kaszlikowski recently offered a new approach to solving this problem. The answer, ironically, may be that the very pervasiveness of entanglement camouflages it.
To help explain further what entanglement means, we've also asked quantum physicist Vlatko Vedral and physicist-historian David Kaiser to describe the long and winding road that quantum entanglement took to becoming accepted. In a sense, entanglement is so weird that we hope our video will not demystify it, but mystify it.