19 March 2009

Causality and Locality

There's an article in the current issue of Scientific American (unfortunately not free online) by David Z. Albert which discusses how Einstein's objections to spooky action at a distance were indeed wrong, but not for the reasons Niels Bohr thought at the time. Turns out, from research by John Bell, who's a modern day Einstein in smarts, that nonlocality is actually real. (As opposed to a mere appearance with an underlying explanation that doesn't require nonlocality, comparable to the fact that it's always night in Sydney when it's day in London. That doesn't involve a connection between them; the underlying phenomenon is their location on a spinning globe and day and night being secondary phenomena resulting from the light of the same sun).

In other words, entangled quantum phenomena remain entangled until quantum states change (for example an electron and a positron emitted from the same quantum event, with opposite spins). The distance between them (in conventional space) is irrelevant. As are considerations like the speed of light. It's not information that passes between them, it's just that, despite distance, they are part of the same thing, and its state is co-determined-- if one part changes, so does the other, instantaneously. This is so even if they are separated by millions or billions of light years, which is quite possible.

Unfortunately, the logic that dictates the impossibility of science fiction author Ursula LeGuin's ansible (subspace FTL radio) is undisturbed by these findings. There is an ironclad chain of logical reasoning as to why it is impossible to use this phenomenon to convey information. It boils down to the fact that information can act as a cause of physical events, and if it were possible to communicate FTL, it would be possible to undo causes of events that had already occurred. Causation would be violable; and in physics if it can happen, it does (sometimes); so if it doesn't happen, it can't.

Locality, as an inviolable universal principle, is defunct, but causality remains. Causality, in fact, appears to be really quite fundamental. Everything in the universe bends to its preservation.

This touches on a topic for another physics post: why any conveyance of matter or information faster than light violates causality, and whether there might be exceptions. For now, I will merely comment that, at least for ordinary distances like the distances between stars or galaxies visible from Earth, (or anything smaller than that), conveyance, including of information, faster than light, is the logical equivalent of time travel or communication backwards in time. These conjectural phenomena, which do not exist in nature, would, if they did, create causal paradoxes that are not logically resolvable. An example of how the universe bends to enforce causality: conveyance of any mass faster than light, of even so much as an electron, requires not just vast, but rather infinite, energy, according to the reliable and proven principles of general relativity, and is therefore not possible. Communication (as opposed to conveyance of matter), which can be accomplished using light or other photons themselves, can be accomplished at, but not faster than, the speed of light, without violating causality.

If none of this means anything to you, I apologize. It assumes a bit of familiarity with these ideas, which many people tend to conclude have nothing to do with them, so they take no interest. A reasonable point of view, I suppose, but not one I share.

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