Beginner's Guide to Quantum Entanglement

No Fortune writes "Einstein called it 'Spooky action at a distance.' This article describes, in scientific layman's terms, how spooky action is created." From the article: "Normally the photons exit the crystal such that one is aligned in a horizontally (H) polarized light cone, the other aligned vertically (V). By adjusting the experiment, the horizontal and vertical light cones can be made to overlap. Even though the polarization of the individual photons is unknown, the nature of quantum mechanics demands they differ."

Comments

It has some errors ...

[Y2]

Aside from the inconsistent use of plural and singular quanta/quantum, I'd be very surprised if no one was baffed by this: "In all cases, a photon's axis must be 90 degrees to its motion." Axis (of spin) and direction of polarization are linked, but not in a way simple enough to fit in this zero-math article.

"Figure 5.2 is an enhanced photograph of a photon ..." - That is more than just misleading.

Re:sigh, digg

[njh]

I just had a look through digg and it lacks the one thing that makes me come back to slashdot - insightful comments. The comments were 50% juvenile drunken-louts-at-school level and the other 50% were people who didn't understand the question.

Thanks

[achurch]

Okay, I've got it now. Or at least, I've got enough of it to realize that I need to go take some more physics classes to have a decent chance of getting it . . .

ach, no

[Quadraginta]

Goodness, no, this is not an accurate analogy. Here's a formally equivalent analogy that should raise your suspicions:

Take two bullets and fire them in opposite directions from identical guns. If you measure the distance of bullet #1 at time t, you will find, amazingly, that bullet #2 has traveled exactly the same distance, but in the opposite direction. Hopefully that doesn't strike you as an amazing result.

What both of our thought experiments say is that if you establish a correlation in a composite system (the two bullets, or your two coins), and you expand the system without doing anything to mess the correlation up, then, amazingly (not!) the correlation will be preserved no matter how large the system gets.

What QE involves is something different: it says you can create a correlation after you have expanded the system, and in less time than it would take any kind of signal or force to cross the distance involved (in fact, instantaneously as far as anyone knows). The correlation can't be used for communication because you can only verify the results of the correlation by communicating the results of measurements on the two parts of the system, which, of course, you can only do at the speed of light.

The whole business arises from the fact that we don't yet understand what happens when the "wavefunction collapses." We know that measuring a quantum system instantly transfers it from the quantum state it was in into a new one (the one consistent with our measurement). So far as we know, this happens instantly over the entire volume that the wavefunction occupies. The problem with this is that it seems dangerously close to violating relativity, because it seems something is being transmitted instantaneously over finite, possibly large, distances.

Relativity is not yet in trouble because we have no good theory of quantum measurement, no knowledge of how a wavefunction collapses, so we can't apply the restrictions of relativity to the internal workings of the collapse. Relativity may never be in trouble, because the collapse may be an epiphenomenon, an event that seems to involve transmission of information but which really doesn't.

Here's an example of an epiphenomenon: point the world's biggest laser at the Moon and look through a telescope at the dot. Aim the laser at one side of the Moon, and then swing it over to the other side quickly. If it takes you 0.25 seconds to move the laser's aim, how long will it take the dot to "travel" across the face of the Moon? 0.25 seconds, clearly, for a "speed" of 22,000 km/s. If you can change the aim of the laser in less than about 0.15 seconds your dot will "travel" across the surface of the Moon faster than the speed of light.

But that's because nothing is really moving. The "motion" of the dot is just a fiction in your mind you create to help describe what you're seeing, because what you are seeing looks superficially similar to what you see when a real object moves. But there's no more real motion here than there is horizontal motion when a group in a stadium does "the wave". In the same way, the "transmission" of information in a QE experiment may turn out to be an epiphenomenon of a higher order, something that "looks" like transmission but really isn't.