update: communication and entanglement

June 8, 2009

After talking with my physics friend, I have learned that it’s basically impossible to communicate via entangles particles. So, the idea that as I, on earth, measure the spin of an electron, thus determining the spin of an entangled electron on Mars, doesn’t actually help us at all in communicating with Mars. The person on Mars still has to measure the spin of the electron (or whatever), but the problem is that he doesn’t know whether he’s looking at an electron that’s already determined or whether he’s looking at it (and its partner on Earth) for the first time and thus determining the Earth electron’s spin.

Thus, a physicist might say, the distinction between instantaneous determination (across the galaxy in an instant) and speed of light determination between two entangled particles is irrelevant because we could never confirm a successful communication faster than the speed of light.

Ok, but let’s consider the following experiment:

We entangle 10 sets of particles and then send one half to Mars and keep the other half here on Earth. We set out a timetable saying that we’re going to measure the spin of each particle one second apart, which is not enough time for us to communicate with Mars at the speed of light (about 3 minutes minimum). After finishing the experiment, the people on Mars fly back to earth, and we compare our results. If we find that all of the times an Earth particle’s spin was measured “up,” the correlated Mars particle’s spin was measured “down,” it would seem that the particles are communicating their “determinism” instantaneously, because that behavior is very improbable (i.e. impossible) for a large number of pairs of just any two particles.

If we find that when measured so closely together, the particles, don’t behave as an entangled pair, that’s certainly interesting and seems to contradict the idea of entanglement as distance-independent (which, as I understand, it’s thought to be).

Even if we prove that the entanglement-communication is instantaneous, that still doesn’t get us very far because we’ll never know (faster than the speed of light) whether a particle is determined or non-determined when we measure it. And, at this point, we have no way of setting a particle’s spin, only measuring it, so we can’t really communicate anything anyways.

Another difficulty, which I’ve hitherto overlooked, is determining when, exactly, the person on Earth and the person on Mars are going to measure their particles. Since the person on Mars had to increase his velocity relative to Earth a bit to get to Mars, his conception of “time” is a bit (not very much, though) different from that of the guy on Earth due to relativity. Perhaps this is a small difficulty, perhaps a large one.

At this point, your brain is probably hurting a lot. What’s interesting is that this kind of discussion doesn’t bother most physicists at all. It’s first year graduate coursework at most. Because physics has taken such a non-intuitive bend in the past hundred or so years, physicists are quite comfortable (because they have to be) talking about things that seem crazy in our “normal world” but actually make a good deal of sense in theoretical physics world. Thus, physicists can talk about the idea of multi-universes–in which every instant in “our” universe, another universe “branches off,” weaving an alternate path through history–because it helps explain where the collapsed states of Shrodinger’s wave equation go.

Very seldom in science–I’d say really only in physics–does the theory not make intuitive, real-life sense on some level or another. This is why we hold physicists on such a pedestal even though their work won’t actually help us in any way for at least a few hundred years.

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