Light’s Most Exotic Trick Yet: So Fast it Goes … Backwards?

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.

About the University of Rochester

The University of Rochester ( is one of the nation’s leading private universities. Located in Rochester, N.Y., the University’s environment gives students exceptional opportunities for interdisciplinary study and close collaboration with faculty. Its College of Arts, Sciences, and Engineering is complemented by the Eastman School of Music, Simon School of Business, Warner School of Education, Laboratory for Laser Energetics, and Schools of Medicine and Nursing.

From University of Rochester

21 thoughts on “Light’s Most Exotic Trick Yet: So Fast it Goes … Backwards?

  1. Sorry…the explanation is MUCH more simple. Once you exceed the speed of light (if you indeed could, which you can’t) what you are REALLY observing is time moving backwards, hence the light’s arrival BEFORE it left.

    Any theorioes involving infinities are non-renormalizable, and mathematically useless to make predictions or in the assessment of “observations.” Negative time IS CERTAINLY permitted and is an integral part of both newtonian and relativistic explanations of forces and fields.

  2. This is awsome… My teacher was on one of his rants about you can’t have a negative speed. So I raised my hand and simply told him that you can. I gave him a simple description and he didn’t want to belive it. So looks like somebody will have to make a major math edit…

  3. There are some effects which are first very unbelievable, but you have to look at the circumstances. The preception that somethink can be faster than the speed of light, is nothing new. But this cause in tunnelng and just for a short time. In case of that, light can not travel for a long time with a speed higher than the speed of light.

    So Einstein just make a statement with the theory of relativity about light in the big (long time, long distance)

  4. The light in the fiber is only traveling *relatively* faster than the reference pulse.
    The relevance is a factor of the medium itself, as well as the spacial reference of the observers. Observers is a casual way to describe the measuring technique used to ‘replay’ the *relative* speeds of both light pulses.

    One cannot determine that light is ‘traveling faster than normal light’ unless one actually IS a pulse of light. Let alone, the fact that the reference light pulse would have to exist in the SAME spacial coordinates as well (not parallel on the same plane, and not fired back to back alone the same plane/path, either!).

    Of course, that is where the funky bit gets funkier… if you are trying to measure a DIFFERENCE in speed of two light pulses that are occupying the same space, well, it’s impossible to do if the pulses actually ARE travelling at different speeds…b/c they cannot then occupy the same physical space except for the insant of conception.

    Never mind the ‘informational’ side of light in the above example…because you would just have a double-amplitude single pulse at that point, and unless the waveform itself gets distorted, you’ll never be able to have to pulses of light, occupying the same space, and travelling at different speeds.

    So, Einstein is not yet proven ‘wrong’; any more than a circus mirror would prove him wrong. ‘sall relative, remember?

  5. One of my personal theories is the theory of infinity and how it applies to light. (This gets a bit thick)

    Firstly, I thought of the hyperbola, with the graph extending to the two infinities along each of the two asymptotes. Now if you take a rotated rectangular hyperbola so that the asymptotes lie on the X and Y axes, you get an equation along the lines of Y = K / X, where K is a constant.

    This equation should be smooth and continuous based purely on the observation that y = f(x), where f(x) is the sum of terms that consist of k1 * x^k2, where k1 and k2 represent a range of different constants. This is not the case though, since X cannot equal 0. What I proposed was that the graph joins up at positive and negative infinity, and that therefore positive and negative infinity are the same. Since infinity is the reciprocal of 0, which has no sign, it stands to reason that infinity also has no sign.

    With this in mind, the speed of light has always been unobtainable, due to the increasing force required to increase the speed of the object. When extrapolated, it has been theorised that an infinite force is required to push an object to the speed of light. I would then hazard a guess (with my little knowledge) that the speed of light is the definition of infinity in our universe in terms of velocity. While speeds greater than the speed of light are possible to put down on paper, infinity is, in reality, much closer.

    Enter my theory. If you manage to push past the speed of light (infinity), then it will be travelling backwards at the same speed (negative infinity). If you travel close the speed of light, you may find there is a component of your motion (I’m really just guessing) going backwards faster than the speed of light which is inversely proportional to the forward velocity.

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