Not faster than light

[mmcirvin]

About once a year or so, some news item pops up announcing that some experimenter has found a means of transmitting some kind of signal faster than light, thereby raising doubts about the speed-of-light limit in special relativity. Maybe 70% of the time, the story is distorted and the experimenters actually claim no such thing; they've got some sort of anomalous dispersion phenomenon that looks sort of like a thing going faster than light, but are perfectly aware that no information is being transmitted. The other 30% of the time, the investigator actually does claim faster-than-light information transfer, and invariably turns out to be somehow confused.

Given that, it's nice to see buzz about an experiment explicitly showing that an apparently faster-than-light pulse really doesn't transmit any information.

Comments

[ronebofh.livejournal.com]

Man... that sounds like a bit of hairsplitting to me. If the information wasn't transmitted, then what the heck is arriving?

(Physicists use the letter "C" as a symbol for the speed of light in a vacuum.) Shouldn't that be 'c'? Heh heh.

[mmcirvin.livejournal.com]

Imagine a bunch of people doing the wave in a stadium. If all the people in a gigantic circular stadium were looking at a central conductor giving them cues, or were working off of stopwatches according to prearranged instructions, they could make that wave go around at any speed they liked, even faster than light. But if they're getting their cues from their neighbors, then there is a limit to how fast the wave can go because it takes time to get a visual image of your neighbor. In the former case the wave does not transmit information; in the latter it does.

These "faster-than-light" waves always exist in situations where some sort of preexisting conspiracy has been set up that allows them to exist. So you couldn't use them to transmit any messages that weren't already knowable at the receiving end. It's not hairsplitting because it determines, say, whether you could in principle use this stuff to build a backwards-in-time telephone and know whether to bet on the Sox winning the pennant.