Light's Most Exotic Trick Yet: So Fast it Goes ... Backwards?

University of Rochester ^ | 11 May 2006 | Staff (press release)

[PatrickHenry]

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.

Robert Boyd, professor of optics (PHOTO CREDIT: University of Rochester)

"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.

[Ichneumon]

I'm not a physicist, but this smells like BS to me.

That's because you're making the mistake of using common sense, and thinking that it has to apply to the quantum level as well. It doesn't. *Lots* of things at the quantum level blatantly violate "common sense" notions of how things "ought" to behave.

[BadAndy]

"That's because you're making the mistake of using common sense, and thinking that it has to apply to the quantum level as well. It doesn't. *Lots* of things at the quantum level blatantly violate "common sense" notions of how things "ought" to behave."

I think I finally understand the behavior of politicians.

[harrowup]

I hope in the paper they managed to use the word "endochronic".

Take this thing down to the Arizona-Mexico border and the President won't have to waste his time Monday night.

[Political Junkie Too]

I can't wait for Sharper Image to sell a mirror that makes me look younger.

-PJ

[Physicist]

First off, light does have mass.

Not that we know of.

[PatrickHenry]

Well, that was provocative. I'll just sit back and wait to see what develops.

[GOPJ]

seY, "sdrawrof"

[Daus]

My '82 Pontiac Bonneville always seemed faster off the line in reverse than in drive. Now I know why. Thanks. :)

[<1/1,000,000th%]

I think I can see myself.

[LuxMaker]

http://math.ucr.edu/home/baez/physics/Relativity/SR/light_mass.html

What is the meaning of the equation E=mc2? You can interpret it to mean that energy is the same thing as mass except for a conversion factor equal to the square of the speed of light. Then wherever there is mass there is energy and wherever there is energy there is mass. In that case photons have mass but we call it relativistic mass. Another way to use Einstein's equation would be to keep mass and energy as separate and use it as an equation which applies when mass is converted in energy or energy is converted to mass as in nuclear reactions. The mass is then independent of velocity and is closer to the old Newtonian concept. In that case only total of energy and mass would be conserved but it seems better to try to keep conservation of energy. The interpretation most widely used is a compromise in which mass is invariant and always has energy so that total energy is conserved but kinetic energy and radiation does not have mass. The distinction is purely a matter of semantic convention.

The above article says light has no mass but with a qualifier that it has "relativistic mass".

More here on relativistic mass:
http://math.ucr.edu/home/baez/physics/ParticleAndNuclear/photon_mass.html

[LuxMaker]

Also here is the anatomy of an electron described as a photon trapped in a self resonant loop:
http://www.photontheory.com/willis.htm

We know that electrons have rest mass.

[Physicist]

As Baez correctly states:

The interpretation most widely used is a compromise in which mass is invariant and always has energy so that total energy is conserved but kinetic energy and radiation does not have mass. The distinction is purely a matter of semantic convention.

Translation: When physicists say "mass" they mean the observer-invariant quantity, of which light has none. Those who argue with them are playing word games.

To which I can only add: "...or are wrong."

[RightWingAtheist]

Actually, this reminds me of James Blish's classic short story "Bleep".

[freedumb2003]

Hey that's not possible!

The "Theory of Light" is both testable and repeatable. I turn on my flashlight and I see light.

This is complete athiestic, anti-Christian, Commie nonsense.

You evil "lightists." We should be able to discuss in school alterate theories of light. That God created it on the First Day and that is it!

(/best imitation of a CRIDer I could do)

[ ;) ]

[SlowBoat407]

I have a few questions about this:

First, doping usually increases the RI, and is necessary for fiber optics to work. I'm not sure what erbium does as opposed to the more standard doping materials, but the light is still traveling through, basically, glass. What effect does the erbium have?

Second, was the reference shot through a standard glass fiber, or through air?

Third, was Fresnel reflection off the endface of the opposite end of the fiber taken into account? It seems to me that we have a light beam that got to the end faster than the reference, and the Fresnel reflection off the endface had already started on its way back to the origin. Just a thought.

Fourth, was the faster light measured through the core, or through the cladding? In single mode fiber, since the cladding has a lower RI than the core, any light that gets into the cladding (from overfilled launch, for example) will travel faster than that going through the core. From the point of view of the light in the cladding, the light in the core seems to be moving backwards, and if the RI difference between the core and the cladding is great enough, there could be some spectacular chromatic dispersion (waveguide plus material dispersion). That could account for many of the effects described in the article. It all depends on what, exactly, the erbium doping brings to the party.

[PatrickHenry]

/best imitation of a CRIDer I could do)

You're not even trying. Light comes from another dimension, the spiritual plane, where the full glory of light is manifest. It has no mass in this base world of flesh because the light we see isn't at all the full reality of light. You fool! Open your heart. The truth is all around you! It is not yet too late.
</internet idiot mode>