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

Speed of what?

[hunterx11]

"Einstein said information can't travel faster than light, and in this case, as with all fast-light experiments, no information is truly moving faster than light," says Boyd.

I hate it when headlines use the semantics of "the speed of light" to sound sensational. "The speed of light" is just used to refer to the maximum speed of information propagation because light in a vacuum travels as that speed. I can change the speed of light by wearing glasses; while experiments similar to the one in TFA are much more complex and interesting, the point is that neither one is affecting the speed of information at all.

Re:In a related idea...

[tomstdenis]

I know your joking but "heat" doesn't care about direction.

Also consider this, what's the temperature in a vacuum where there are *no* molecules to be moving at all? :-)

Tom

Two photons travelling in opposite directions

[Danuvius]

I've been curious about this for a while... so someone please explain where I am missing the obvious.

Would not two photons/beams of light travelling in opposite directions be moving faster than the speed of light *relative* to one another?

I'm sure I'm missing something... so please, rip apart the above over-simplified statement. I hope to learn something by observing the process. ;-)

Re:Two photons travelling in opposite directions

[Phys Rev fanboy]

You've just run into one of the classic problems of people just learning relativity. Specifically, you're assuming that the light has a rest frame. In the strictest sense, it doesn't, at least not a useful one. If you tried the basic equations, from which velocity transformations are derived, and assumed they held for photons, you would find that time does not pass for a photon. Obviously, this would be weird. You don't run into the problem until you get to something travelling at the speed of light, though, and you know the physics of what happens to that photon as long as you're in a normal rest frame, so the basic answer is that you simply don't try to go into the photon's rest frame, because that is meaningless. So, if two photons are going in opposite directions, you may see them going faster than the speed of light relative to each other, but that's never been what relativity was about. Relativistic velocities are only meaningful relative to your reference frame, and your reference frame can only ever be a safe non-speed-of-light one.

Re:what if you change your mind?

[jj13]

i think the point is that...this result could make us question sense of fate... do you still have freedom of choice if you learn about actions you will perform in the future? if you get a photon out the system before you sent one, are you locked into sending one?

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:Another Stupidly Confusing Physics Story

[mrcaseyj]

The last time I saw an article like this they claimed they sent a light pulse through a chamber and it came out before it even finished entering. The trick was that the chamber was a foot long but the light pulse was 300 feet long! That's right, they weren't talking about a single photon or a single wave but a whole bunch of them, and they were interfering with each other on the way through. As they interfered with each other their amplitudes were changing.

A detector was set up at the entrance to the chamber and at the exit. The time between setting off the detectors was used to measure the speed. When the laser was turned on to create the pulse, it would take a little time to come up to full brightness. The dim little leading edge of light would pass into the chamber without setting off the detector. While this dim little leading edge was passing through the chamber the weird gasses or whatever in the chamber would cause the waves to constructively interfere with each other and some of their amplitudes would increase enough to set off the detector on the exit.

Imagine a train crossing a bridge. But this train has a couple flat cars being pushed along in the front. At each end of the bridge you set up a detector That detects anything more than 6feet (2meters) tall going down the tracks. When the train goes onto the bridge the flatcars go under the detector and don't set it off, only the locomotive behind them does. But as the flatcars are crossing the bridge someone who was laying down on the flatcar stands up at the front. As the flatcars get to the other end of the bridge the person standing up sets off the detector instead of the locomotive. Your measurements indicate that the train was going faster than it was.

The big question in these experiments is whether they're measuring the same thing on the way out as they were on the way in.

Re:Another Stupidly Confusing Physics Story

[mrcaseyj]

The detectors can be connected to the timer by two wires of equal length. The wires can be very long and slow because it only matters what the difference in time is. The timer may not receive the signals until long after the whole event has taken place, but as long as it can accurately measure the difference, then there's no problem.

The problem with these experiments, as pointed out in my above post, is that they're not measuring THE crest, they're measuring two different crests that don't necessarily have anything to do with each other. Naturally, measuring the arrival times of two different things in two different places, tells you nothing about the speed of either.

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.

Wierd idea

[randomErr]

We know that we exist in multi-dimensional universe. Not like monster from a parallel dimension, but rather dimensions such as width, length, height, and time. Is it possible that they accidentally skewed the photon of light slightly off the four dimensions we can perceive and went back on the time axis?

Asimov sez:

[Chmcginn]

In the case of the Asimov short story (Not a novel; it was 29 pages), there was a repeater that sent a one-bit signal 24 hours back in time, by having a series of some 14K automatic vials, each one putting a drop of water on its sample when it sensed the previous one dissolving. One of the researchers decided (once) he wasn't going to press the button after getting a signal. A freak storm caused severe damage to the lab, and it ended up getting pushed anyway.

Course, as it's been said - this was fiction, so it had to make sense. :)

Re:what if you change your mind?

[bar-agent]

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

My take on it is that maybe you aren't locked into sending the photon in, but a photon with the right properties will end up going in.

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.

There exist some mediums

[jfern]

Where you can send an electron faster then the speed of light. Now let me explain. Speed of light in that medium is about 0.6c, where c is the speed of light in a vaccum. Electrons go about 0.8c. Relativity says nothing about whether you can break the speed of light, what it says it that you can't exceed the speed of light in a vaccum.

Re:quote

[Frank T. Lofaro Jr.]

And if he succeeds he'll be called a crackpot and lose all his funding.

And the military will start using the technology at Area 51.