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

Re:Weird thought

[Y2]

Suppose that at the instant you stopped it, the coin was horizontal. You now know that, at that particular instant, the second coin was vertical

Sorry, no. If the coins aren't at the same place, then this term "at that particular instant" is not well defined.

The tantalizing notions of instant communication involve choosing which of two or more possible measurements to make on one of the photons (after they are separated) and the effects of that choice on the possible outcomes of a fixed or independently-chosen experiment on the other photon. Google "EPR Paradox" for a primer.

Re:Someone link me to an explanation?

[PaSTE]

Actually, what you've hit upon is something called the "hidden variables" theory of quantum mechanics. For a while after the spookiness of entanglement was figured out, vis. the Einstein-Podolsky-Rosen paradox, many physicists thought just like you do--that there was some hidden variable within the system that we could not measure, but which determined the state of the system exactly. They figured for instance that, if a certain decay process produced two photons, one left-polarized and one right-polarized, then there was some feature of the decay which determined which othe the photons is left-polarized and which right-polarized, so that our measurement of one did not "change the state" of the other photon, if merely revealed its pre-determined polarization.

The hidden variables theory of quantum mechanics was disproven by a physicists named John Bell. In his method, he began by assuming that these "hidden variables" existed, then, using geometric arguments and the postulates of quantum mechanics, derived a set of inequalities which showed no physical theory of local hidden variables can ever reproduce all of the predictions of quantum mechanics.

It's not intuitive at all, but Bell's argument is sound. Entanglement and action-at-a-distance is real, and not due to the system's state being pre-determined by hidden variables.