Light - going faster backward than forward?

[rhoenix]

I found this on Scienceblog.com, following a link from Slashdot.org.

In the past few years, scientists have found ways to make light go both faster and slower than its usual speed limit, but now researchers at the University of Rochester have published a paper today in Science on how they've gone one step further: pushing light into reverse. As if to defy common sense, the backward-moving pulse of light travels faster than light.

Confused? You're not alone.

"I've had some of the world's experts scratching their heads over this one," says Robert Boyd, the M. Parker Givens Professor of Optics at the University of Rochester. "Theory predicted that we could send light backwards, but nobody knew if the theory would hold up or even if it could be observed in laboratory conditions."

Boyd recently showed how he can slow down a pulse of light to slower than an airplane, or speed it up faster than its breakneck pace, using exotic techniques and materials. But he's now taken what was once just a mathematical oddity—negative speed—and shown it working in the real world.

"It's weird stuff," says Boyd. "We sent a pulse through an optical fiber, and before its peak even entered the fiber, it was exiting the other end. Through experiments we were able to see that the pulse inside the fiber was actually moving backward, linking the input and output pulses."

So, wouldn't Einstein shake a finger at all these strange goings-on? After all, this seems to violate Einstein's sacred tenet that nothing can travel faster than the speed of light.

"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. "The pulse of light is shaped like a hump with a peak and long leading and trailing edges. The leading edge carries with it all the information about the pulse and enters the fiber first. By the time the peak enters the fiber, the leading edge is already well ahead, exiting. From the information in that leading edge, the fiber essentially 'reconstructs' the pulse at the far end, sending one version out the fiber, and another backward toward the beginning of the fiber."

Boyd is already working on ways to see what will happen if he can design a pulse without a leading edge. Einstein says the entire faster-than-light and reverse-light phenomena will disappear. Boyd is eager to put Einstein to the test.

So How Does Light Go Backwards?

Boyd, along with Rochester graduate students George M. Gehring and Aaron Schweinsberg, and undergraduates Christopher Barsi of Manhattan College and Natalie Kostinski of the University of Michigan, sent a burst of laser light through an optical fiber that had been laced with the element erbium. As the pulse exited the laser, it was split into two. One pulse went into the erbium fiber and the second traveled along undisturbed as a reference. The peak of the pulse emerged from the other end of the fiber before the peak entered the front of the fiber, and well ahead of the peak of the reference pulse.

But to find out if the pulse was truly traveling backward within the fiber, Boyd and his students had to cut back the fiber every few inches and re-measure the pulse peaks when they exited each pared-back section of the fiber. By arranging that data and playing it back in a time sequence, Boyd was able to depict, for the first time, that the pulse of light was moving backward within the fiber.

To understand how light's speed can be manipulated, think of a funhouse mirror that makes you look fatter. As you first walk by the mirror, you look normal, but as you pass the curved portion in the center, your reflection stretches, with the far edge seeming to leap ahead of you (the reference walker) for a moment. In the same way, a pulse of light fired through special materials moves at normal speed until it hits the substance, where it is stretched out to reach and exit the material's other side [See "fast light" animation].

Conversely, if the funhouse mirror were the kind that made you look skinny, your reflection would appear to suddenly squish together, with the leading edge of your reflection slowing as you passed the curved section. Similarly, a light pulse can be made to contract and slow inside a material, exiting the other side much later than it naturally would [See "slow light" animation].

To visualize Boyd's reverse-traveling light pulse, replace the mirror with a big-screen TV and video camera. As you may have noticed when passing such a display in an electronics store window, as you walk past the camera, your on-screen image appears on the far side of the TV. It walks toward you, passes you in the middle, and continues moving in the opposite direction until it exits the other side of the screen.

A negative-speed pulse of light acts much the same way. As the pulse enters the material, a second pulse appears on the far end of the fiber and flows backward. The reversed pulse not only propagates backward, but it releases a forward pulse out the far end of the fiber. In this way, the pulse that enters the front of the fiber appears out the end almost instantly, apparently traveling faster than the regular speed of light. To use the TV analogy again—it's as if you walked by the shop window, saw your image stepping toward you from the opposite edge of the TV screen, and that TV image of you created a clone at that far edge, walking in the same direction as you, several paces ahead [See "backward light" animation].

"I know this all sounds weird, but this is the way the world works," says Boyd.

Quite interesting, I thought. What sorts of fun does this suggest?

[Captain Cyran]

*reads*

...

...

*head explodes*

My world, she is turned upside down.

[SirNitram]

Making light back up strikes me as, you know, not impossible.

But.. Am I going mad, or did he describe the light reversing in time? Emerging before it went in!

Quick, check the universe hasn't collapsed.

[Hotfoot]

Somehow this has to tie into the double slit experiment. The results seem just as insane.

I think he's saying more that the light is coming out the back before it comes out the front, but it's late and I may be misreading it.

[Chris OFarrell]

*head explodes*

[Ariphaos]

But.. Am I going mad, or did he describe the light reversing in time? Emerging before it went in!

No, the leading edge exited before the main pulse entered, which is different. This is little different than all other such experiments that showed a pulse of light gave the illusion that it was moving faster than light. It gives the illusion of FTL by changing the nature of the pulse itself, though this seems to be a rather long such experiment.

[The_Nice_Guy]

Not only did my head explode, so did the rest of my body.

Wonder if there are any implications to this?

[Covenant]

Not only did my head explode, so did the rest of my body.

Wonder if there are any implications to this?

I'd wager you're dead.

As for the light, at the moment I think it's best to avoid a lot of speculation. Light does very odd things and it's often come out that "Lightspeed broken!" or "Message recieved before sent!" where, in about three days, a scientist comes out and explains whatever trick is being employed to make it appear that light is behaving in an irrational way, when it is not. Plus, then we'll be able to get a better picture of what it's actually doing.

[LaCroix]

Ähhhhh....

Someone give me a mop and a bucket.. My brain is resembling a puddle beneath my desk...

Could this be somehow related to the tunneling-effect? But this doesn't reverse the data, doesn't it?

[Ariphaos]

Could this be somehow related to the tunneling-effect? But this doesn't reverse the data, doesn't it?

The apparent FTL nature of the light beam is because of the tunneling effect, at least according to what I've read about it. It causes the signal to compact towards the front, which will let its beam get registered by a detector before a beam of light racing through air would.

I don't know anything about the reversal, however.

But honestly, I'm kind of tired of these claims. They are useful, but they are not FTL at all.

[Mr Bean]

*Head stays intact*
Ahah, I see, what we have here is another "FUN WITH QUANTUM SHIT!"
Can we use it for anything the defense departmant says?
No says the scientist, mostly concerned with how cool this all sounds.
Back to your gerbel launching railgun project then says the defense department funding.

[Ford Prefect]

That is frankly the oddest thing I have ever read. This goes beyond the quatum computer working best not switched on.

[Acidburns]

That is frankly the oddest thing I have ever read. This goes beyond the quatum computer working best not switched on.

Let's not bring up that computer again. I barely escaped that thread alive.

[Edi]

Interesting. But not weird enough to cause my head to explode. And the funny thing is that I always sucked at optical physics compared to e,g, thermodynamics and such. I wonder what kind of practical applications they can get out of this.

Edi

[Magnetic]

Oh, yes, what's going on in this article is VERY easy to explain. Here's what's going on. . . . . When the light is emitted from the, . . . . . what? . . . . Honey, I'm on the computer!!!. . . . . . . Okay FINE, . . . . .I'm coming... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

[dragon]

Ok thats really cool. I don't know whats worse the fact that I grasped the concept or the fact I went out and looked up more info on this and now my brain hurts. Oh well if he can develope a pulse with out the leading and trailing edges it will be interesting to see if Einstein was reight about the effect disappearing. But if the effect continues even with no edges now that will be a interesting find with some interesting applications.

[bilateralrope]

That is frankly the oddest thing I have ever read. This goes beyond the quatum computer working best not switched on.

At least that computer had a clear pratical use. For this I dont see how its will have any use even as a novelty gadget, but thats probably because I dont understand most of it

[Winston Blake]

I didn't get their explanations. My guess is that it's yet another trick with the group velocity. For example, try moving the slider in this applet back and forth and look at the apparent velocity of the superposed wave. When set to a negative value it goes backwards.

Hopefully somebody who knows their stuff will come in soon and clarify this whole thing (just like the 'off' computer).

[Hawkwings]

that's a really wierd applet... my eyes hurt now, along with my brain

[LaCroix]

At least we can confirm that most politicals are bright heads.

The are also faster in retreating from statements then forwarding the issues.

[drachefly]

They gave enough information for me to figure out what was going on. It isn't even quantum mechanics, it's just plain old optics.

The important part that you need to get is that they used a structured pulse of light here. Not only was it structured, but it had a nice continuous structure - - one that if you looked at any part of it and did an expansion about that point (either Taylor or Fourier), you'd reconstruct the whole pulse.

SO, what happened was:
the leading edge entered
the device caused some phase transformations on the various fourier components, such that the transmitted part of the wave was a miniature version of the whole wave, and it was composed of the leading edge of what came in.
What, then happens to the rest of the wave that enters later? It's reflected.

What use is this? It could be used to do certain signal processing tasks that are currently hard to do. I don't see it being a major entrant into daily life, but it could have a niche in optimizing fiber-optic network hardware. But due to its property of reflecting almost all of the pulse, I wouldn't count on it.