Compared to entanglement, the double slit experiment is nothing for paradoxical behavior. Einstein (with two collaborators named Rosen and Podolsky) pointed out the strangeness created by entanglement in their famous EPR paper of 1935. John Bell revealed in 1964 that what had looked like an insoluble philosophical dilemma in fact made measurable predictions that could be tested by experiment. And several experimenters in the 1970s and 1980s actually did the test.

Here’s the basic idea. Create two particles that, due to the nature of their creation, have correlated properties. For instance, perhaps two electrons have exactly opposite spins, or two photons have exactly opposite polarization. Call the particles A and B. Let them jet away from one another toward laboratories X and Y. After some time, do an experiment in laboratory X to determine the property of particle A. This action determines the properties not just of particle A, but of particle B, as well. In other words, you’ve determined the properties of particle B without ever measuring it.

This is not a paradox at all if you assume that the particles had their particular properties from the beginning. It is no more surprising than discovering a brown and black sock in your suitcase and realizing that you must have another brown and black pair back at home. However, traditional quantum theory says this is the wrong view. Traditional quantum mechanics says that until you measure  particle A, it does not have specific properties (spin up, say, or clockwise polarization). According to quantum mechanics, the act of measuring the particle determines its properties.

Hence the paradox. How could measuring particle A in laboratory X instantly affect particle B in laboratory Y, in principle miles or even light-years away? This is called entanglement by most physicists, but Einstein called it “spooky action at a distance” and could not accept that the universe worked this way. Despite the clear mathematics of quantum theory, Einstein believed there must be “hidden variables” that instructed the particles how to behave even before they were measured.

Unfortunately, there seemed to be no way of choosing between the quantum view and Einstein’s hidden variable view. So it might have remained, but for a brilliant insight by Irish physicist John Bell. Bell showed that, far from being purely a philosophical problem, the question was amenable to experiment. Bell’s achievement was realizing that if a pair of particles had not just one or two but at least three entangled properties, Einstein’s hidden variables and traditional quantum mechanics made radically different predictions for their behaviors. And conflicting predictions about experiment are the bread and meat of physics.

For a beautiful account of Bell’s Theorem, read the section on it in Brian Greene’s Fabric of the Cosmos. While most authors just hand-wave Bell’s theorem away (as I’ve just done), Greene creates a stunningly clear description of it, one that I won’t even attempt to mimic here. Trust me; read the book (in particular, the section called “The Red and the Blue” beginning on page 84, but really getting into the heart of the matter on page 105 with the section called “Reality Testing.”)

Careful measurements show that Einstein was wrong. Local hidden variables are ruled out by experiment. Although non-local solutions are still possible, they are themselves as weird and convoluted as the traditional quantum explanation that says properties don’t exist until they’re measured. And so things stand, until we consider many worlds.

The crux of the entanglement conundrum is that somehow particles “know” about each others’ behavior instantly, even over vast distances. When the particles “decide” what property to display, they decide together, in perfect lockstep, despite the distance separating them.

In Many Worlds, the particles don’t decide. Rather, the particles choose every possible property, in direct proportion to the probabilities predicted by quantum mechanics. It is only when we experimenters compare the results of measurement in one particular history that we notice something remarkable.

Consider our original particle A. As it flies away from its origin, it spreads into David Deutsch’s multiverse. Our particle A becomes a multiversal object, existing along with innumerable shadow particles. Then, when A reaches laboratory X and is measured, the measurement entangles particular instances of A with particular histories. And in one of those particular histories, a version of you the experimenter observes A to have a particular (apparently randomly determined) property.

So far so good. But what happens next? Suppose you call your colleague in laboratory Y. Here’s where things get weird, because you discover that particle B, which was just measured there, showed a perfect correlation with particle A. The properties match up perfectly. From Bell’s theorem you know there are no local hidden variables, so how did the correlation happen?

The answer seems to be that your phone call (or telegraph, or smoke signal, or whatever system you used to communicate to laboratory Y) caused your part of the universe (laboratory X) and your colleague’s part of the universe (laboratory Y) to become entangled together. But (and here’s the weird part) the entanglement happened in just the right way to preserve the weird connection between particle A and particle B. In essence, you couldn’t reach a laboratory Y with the “wrong” answer – that version of the laboratory, while real in some other history, was unavailable to you.

OK, that probably doesn’t feel any better to you than traditional quantum mechanics. How do histories match up like that? I was bothered by the same question, and to some extent I still am. But David Deutsch himself actually answered my question about this issue:

 me: How are particles supposed to know which universe they’re allowed to be in?

David Deutsch: The same way they know what sort of particle they are, what their energy is, what their half-life is, what other particles they should decay into, and with what probabilities, where they are in space, how fast they are moving, and all their other attributes.

OK, that’s sort of like saying, “hell if I know, but that’s hardly the biggest mystery here.” But still, it was kind of thrilling for me to get an answer from the man himself, and it does put the question into perspective. Particles seem to “know” an awful lot of stuff, so why not this one more piece of information, which really could just be some kind of key code saying, “you are in my version of the universe, but you are not.”

Looked at in this way, entanglement becomes no more mysterious than the bank’s understandable (though unfortunate) practice of placing Fabio’s paycheck in his account instead of mine.

In the same way that inserting Fabio’s paycheck into my bank account would cause some sort of cosmic rift, notice that entanglement always occurs in such a way as to preserve the conservation laws – momentum, angular momentum, and so on. So whatever “code” is attached to particular instances, it must contain quite a lot of information. How is that information stored? How is it communicated to the rest of the universe? How is it that such a wild mixture of possibilities always results in the sensible, rule-following universe we observe?

Tough questions, and I’m far from satisfied. But that’s what makes this so much fun!