Light that travels faster than light.

[DarrenS]

According to this story, a team of researchers has managed to create conditions in which light travels faster than the speed of light itself, in a vacuum. I know it’s nothing unusual for a body to travel faster than light in a particular medium (Cerenkov radiation and all that), but this is surprising. They do say in the article that

And even though this seems to violate all sorts of cherished physical assumptions, Einstein needn’t move over – relativity isn’t called into question, because only a portion of the signal is affected.

Well, thanks for that dumbed-down explanation. I assume they’re saying that no individual photon or ‘influence’ exceeds c - that it’s some trick whereby only the wavefronts exceed the limit, and no information could be transmitted that way, thereby not violating Relativity. (this was the case with previous experiments demonstrating the exceeding of c.) But then they go on to say how important this is for telecommunications etc., implying that it could be used for superluminal transmission of information.

So, after all this rambling, my General Question is: what is going on here?

[Squink]

@DarrenS

So, after all this rambling, my General Question is: what is going on here?

In digging on the story, I ran across this:

A reporter’s PressPass is required to access this story until the embargo expires on 22-Aug-2005 at 02:00.

So a less confused account may be available on monday.

Wiki on Brillouin scattering.
Sure enough, this article suggests that this is yet more of the pulse/phase/group velocity stuff:

This seeming paradox can be resolved because a pulse of light is actually made up of many separate frequency components, each of which moves at their own velocities. This is known as the pulse’s phase velocity. If all the frequency components have the same phase velocity, then the overall pulse will also appear to move at that velocity.
However, if the components have different phase velocities, then the pulse’s overall velocity will depend on the relationships between the velocities of the separate components. If the velocities differ, the pulse is said to be moving at the group velocity.

So information still isn’t travelling faster than light.

[Chronos]

The linked site is taking a long time to load for me, so I can’t address it specifically. But at a guess, what’s going on here is the same as what was going on with every previous claim of this sort: Some scientist produces an entirely unremarkable result, and some reporter blows it completely out of proportion to make it look a lot more hyped than it really is. At best, this new development (whatever it is, and if it’s really new) might have some practical implications on telecommunications which have nothing to do with information propagation speed. It might, for instance, provide a means of reducing noise on a communication line, but it won’t get a message to the antipodes any quicker than a fourteenth of a second.

[Napier]

As another example of traveling faster than c, consider standing in the middle of a big round room and shining a laser pointer onto the wall by bouncing it off a mirror on the end of a motor shaft. You can run the motor faster and faster until the bright spot on the wall is moving faster than c.

Somebody else observing this and moving differently than you can observe the spot existing in multiple places at once, or sometimes vanishing, and so forth.

Still no way for two guys on the wall to modify the spot between them and communicate with it.

I think a better way of expressing c is “the limiting speed at which influences can spread”.

[DarrenS]

Thanks guys - this is what the SD is all about, fighting ignorance.

I guess we’ll have to wait to see if it’s just more media hype. Seems that Einstein can still rest peacefully for now.

I still have trouble getting my mind around the difference between transmitting an influence faster than light - like the aforementioned laser pointer example, or a shadow, versus actually using it to transmit information. Something in the back of my mind keeps thinking there must be some way to modify the experiment to transmit information FTL. Clearly I’m wrong, but I can’t see how.

(I’m tempted to hijack my own thread into EPR / action-at-a-distance / entanglement, because I never understood how that isn’t FTL information transmission either.)

[Exapno_Mapcase]

The difference with the laser pointer and shadows and other equivalent experiments (and there are many) and real FTL is that you can always see the the laser pointer signal itself never moves faster than light. Each individual signal is at light speed or slower. The effect is like frames of film. You perceive an illusion of motion in the final movie, but nothing actually moves. Similarly, the end product of the laser pointer or other beam is the illusion of FTL movement made up of many individual but separate non-FTL events.

[Saint_Cad]

@Napier

I think a better way of expressing c is “the limiting speed at which influences can spread”.

Then how would you explain the results of J.S. Bell’s experiment?

[Exapno_Mapcase]

A good description of Bell’s Theorem.

And from that page:

Sometimes one sees statements that Bell’s Theorem says that information is being transmitted at speeds greater than the speed of light. So far I have not seen such an argument that I believe is correct. If we are sitting by either of the polarisers we see that one-half the electrons pass and one-half do not; which is going to be the case for an individual electron appears to be random. Thus, the behavior at our polariser does not allow us to gain any information about the orientation of the other polariser. It is only in the correlation of the electron spins that we see something strange. d’Espagnat uses the word influence to describe what may be traveling at superluminal speeds.

There is no necessary problem. Superluminal passage of information is considered by some (See David Bohm) but is not currently popular as an explanation of non-locality.