Faster than light, light

Daniel Davies d_squared_2002 at yahoo.co.uk
Fri Jul 21 01:56:06 PDT 2000


I heard about this years ago, but never knew how until today .....

Seriously, the abstract is a bit misleading.

It starts off by talking about "Einstein's theory of special relativity and the principle of causality(1-4) imply that the speed of any moving object cannot exceed that of light in a vacuum " (basically; special relativity implies that if there is any frame of reference in which information, matter or energy is propagated faster than light, then there exists a frame of reference in which it arrives before it is sent. So to be totally anal about this, special rel, plus no-backward-causality, plus frames-of-reference don't matter is needed.)

But when we get to the interesting part, we're told:

"The group velocity of a laser pulse in this region exceeds c and can even become negative(16, 17), while the shape of the pulse is preserved."

And group velocities above c are not ruled out by special relativity. Special relativity's restriction to c or below is for the "front velocity" of a wave; the first stirring that tells you there's a wave there.

The basic controversy seems to be whether you can convey a signal at the group velocity (this is the "velocity of the center of mass of a wave package", and the sonic boom you just heard was me reaching the limit of my understanding). It looks to me like it's going to turn on the interpretation of the words "signal" and "information". My guess is that you wouldn't be able to tell whether superluminal "signals" were causally connected to anything else in the world, so they wouldn't reduce your uncertainty, so no information is being carried and special relativity lives on. But I am now babbling

dd, terrible pop science junkie.

===== "The banker must at all times affect a respectability that is more than human ... for this reason, he is typically the most romantic and least realistic of men"

John Maynard Keynes

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