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tyr...@mail.sgi.net wrote:
: Einstein's assumption that light in a vaccuum is constant.  Where and
: when did he have the chance to observe THAT.

There was an experiment made by Michelson and Morely (sp?) which measured the
difference in the observed speed of light given different speeds of the source
of light (they used the fact that the earth is moving around the sun at a
respectable velocity). It wasn't exactly their intent, BTW, but you could
interpret their experiment this way. And their results were that this observed
speed is the same, regardless of the speed of the source.

As someone already pointed out, it has been experimentally demonstrated that
information _can_ pass faster then light (from one particle to another) under a
certain set of circumstances. The same experiment shows that particles only
have attributes such as polarity (for example) when they are "needed"; if they
are not measured, they literally do not have a value for the attribute. Einstein
was aware that quantum mechanics predicted such phenomena and this was why he
insisted that it was "incomplete".

This was just just theory at the time, but a British physicist named Bell has
shown how, in principle, the effect could be demonstrated. A modified version
of the experiment has been conducted in the 80s, measuring the effect. There's
a very readable article about it in Physics Letters, called "Is the moon really
there when no one is looking". Sorry but I don't have the exact reference
handy.

Sadly, there is no way to use this effect to transmit information - the
particles do, somehow, but we can't show they did until we compare the results
of "far away" measurements, which requires normal transmission.

Just how do the particles do this trick is not known. One of the weirder
theories I heard suggested that the particles do not use FTL as such (how would
they find each other?), but send the information back in time to when they
interacted with each other.

General relativity does allow for time travel (in some forms). In fact, I read
an article in Scientific American (I think), which showed how you could build a
"time phone" given a single rotating medium-sized torus-shaped atom nucleus
(sadly, they didn't give a catalog number for ordering one) Other schemes for
time travel require less modest instruments, like rotating black holes in a
particular configurations and the like (no catalog numbers for these, either).

Given time travel (which relativity does not forbid outright) it is trivial to
simulate FTL - just go back the necessary amount of time, then go to the
destination as slow as you like - but I guess this isn't what leaps to mind
when hearing "faster then light". You found a shorter path through space-time;
on this path, you must still go slower then light.

Digressing for a moment from the main theme of this thread, it is strange that
is is distributed to 24 different newsgroups, none of which are related to the
issue, but _not_ to sci.physics. I took the liberty of adding it to the
followup list. It is a good bet the subject is covered in their FAQ, if they've
got any.

			Oren Ben-Kiki.