Light so Fast it Travels Backward

An anonymous reader writes "Slowing down light used to be considered a neat trick for physics wonks. But researchers in New York now say they've pushed light into reverse. And as if to defy common sense, the backward-moving light travels faster than light." While there's not much use to come of it yet, it will be interesting if Einstein himself is proved wrong.

Comments

Another Stupidly Confusing Physics Story

[logicnazi]

The article was pretty confusingly written so I can't be totally sure what is going on but i think this only sounds cool because we confuse the speed the actual photons travel and the speed the wave appears to travel.

It is perfectly possible to get *effects* from light that appear to travel at faster than the speed of light. Just take a flashlight in a super huge room and whip it around really fast. The spot of light on the wall may very well 'travel' faster than light but no actual photons traveled faster than light so this isn't a problem.

While this experiment is somewhat different I believe a similar confusion makes it sound way more interesting than it really is. In particular there are two different speeds one needs to talk about when you are talking about how fast light goes. There is the speed at which a crest of the wave advances and then there is the speed that a photon travels (probably some other ones too than I'm forgetting). I believe all this experiment is doing is making it so the crest of the wave appears to travel faster (or with negative speed?) than light even though all the photons in the light are not moving faster than light.

Thus it is a big analagous to the flashlight case where you have some effect (in this case the crest of the light wave) which appears to move faster than light even though no actual photons or information is really doing so.

To give an idea of how this could happen (though not the mechanism here) imagine a bunch of rods in a row like this:

_____ (time 0)

Now suppose we put activators under these rods to raise them at prearranged times. If we did this right we could get a 'wave' moving like this:

-______ (time 1)

--_____ (time 2)

_--____ (time 3)

__--___ (time 4)

Now if we timed the activators right we could make this 'wave' travel down the line arbitrarily fast (in principle even faster than the speed of light) even though no information or particle is actually being moved that fast.

While clearly the mechanism is different in this case I believe this is all that is happening. Namely the peak of electric field moves faster than light (or negative?) even though no real thing is doing so.

Re:what if you change your mind?

[JanneM]

if you get a photon out the system before you sent one, are you locked into sending one?

I read a great short story on that theme once (really short; I believe it was less than two full pages). A researcher built a time machine, and sent a brass cube five minutes back in time during a demonstration. An audience member, looking at the "two" brass cubes on the desk asked what would happen if he never sent the original cube. They tried - and the universe, except for the brass cube, ceased to exist.

Re:what if you change your mind?

[snookums]

I like to explain this type of paradox with a parallel universe theory. In your story, the universe ceased to exist because of an irresolvable paradox -- a dead-end in the timeline beyond which there was no internally-consistent state for the universe to be in. A little like what happens to the "wrong" answers when doing calculations with a quantum computer.

The thing is, there were other universes where everything was fine. The scientist put the cube into the machine and everything was okay, or the scientist never put the cube into the machine and the demonstration failed. Nobody died, and the whole of everything didn't suddenly end, they just continued along one of the consistent timelines. The versions of the people in the dead-end timeline didn't know what happened (because they ceased to exist) and the people in the continuing timeline were unaware of the existence of any others (except in a "I wonder what would have happened if..." sense).

I'll concede that this is kind of fatalistic, but if you want to allow time travel, then you really have to give up on the idea that the "forward" direction of time is special. If the second brass cube was on the table then someone must have put it there in exactly the same way that someone must have put the first one there. Cause and effect become indistinguishable because the causal relationship can run in either direction.