No thing goes faster than light

The observation of a light pulse leaving a gas-filled chamber before it had even arrived sparked a media frenzy, yet the laws of physics have remained intact.

Nothing can travel faster than light. Despite a recent raft of reports in the media, this statement is as true now as it ever was. Nonetheless, experiments over the past 20 years have been forcing us to re-examine what we mean by the word "nothing". In the latest experiment, a group of researchers at the NEC Research Institute in Princeton, US, observed the peak of a laser pulse leave a small cell filled with caesium gas before it had even entered the cell (L J Wang, A Kuzmich and A Dogariu 2000 Nature 406 277). Apparently, the peak of this pulse is simply not the kind of "thing" to which Einstein's famous law applies.

At almost 300 000 km s¹, the cosmic speed limit, c, is one of the most widely known constants in physics. A massive object needs infinite energy to reach c, while massless particles like photons always carry their energy at precisely the speed of light. More importantly, the relativistic notion of simultaneity makes it clear that no information can travel faster than light without throwing all our concepts of cause and effect into disarray. Relativity teaches us that if two spacetime events are separated so that they cannot be connected by any signal travelling at c or less, then different observers will disagree as to which of the two events came first. Since most physicists still believe that cause needs to precede effect, we conclude that no information can be transmitted faster than the speed of light.

Nevertheless, velocities greater than c can be observed. Suppose a lighthouse illuminates a distant shore. The rotating lamp moves quite slowly, but the spot on the opposite shore travels at a far greater velocity. If the shore were far enough away, the spot could even move faster than light. However, this moving spot is not a single "thing". Each point along the coastline receives its own spot of light from the lighthouse, and any information travels from the lighthouse at c, rather than along the path of the moving spot. Such phenomena are described as the "motion of effects", and are not forbidden by relativity.

In optics, the possibility of superluminal velocities was with us throughout the 20th century. The overall velocity (or "group velocity") of an optical pulse passing through a medium is determined by the way the refractive index varies for the different frequencies that make up the pulse. Since the peak of the pulse occurs when all the frequencies add up in phase, the peak can be delayed by a large amount if each component experiences a very different refractive index (see figure 1a).

When the energy of the optical pulse differs from the energy difference between two electronic energy levels in the atoms of the medium (i.e. when the light is far from resonance), the refractive index increases with frequency. This "normal" dispersion reduces the group velocity below c. Roughly speaking, an atom may temporarily absorb a photon, even though the light is not exactly at resonance, and re-emit it some time later, thus slowing down the light.

However, the behaviour of the light pulse is very different closer to the absorption line, where the refractive index decreases with increasing frequency. This behaviour leads to so-called anomalous dispersion in which the sign of the delay changes, which means that the group velocity can exceed c. This problem was treated in a classic analysis by Arnold Sommerfeld and Léon Brillouin, who pointed out that the strong absorption and distortion that occur at the resonant frequency generally make the group velocity a meaningless concept. They demonstrated that neither information nor energy can travel faster than light in this region. Throughout most of the 20th century, this was usually accepted as the last word on superluminal group velocities.

However, the field was revived in 1970 by Geoffrey Garrett and Dean McCumber, then both at Bell Laboratories in the US. They showed that it should be possible to observe an undistorted Gaussian pulse with a group velocity exceeding the speed of light, or even with a negative group velocity, provided the pulse has a narrow bandwidth and the region though which it travels is short. This effect was dramatically confirmed in an experiment by Steven Chu and Stephen Wong, then also at Bell Labs, in 1982 (Phys. Rev. Lett. 48 738).

Although Sommerfeld and Brillouin's conclusion that neither energy nor information travels faster than c remains valid, the group velocity is not entirely meaningless. The smooth Gaussian waveform is reshaped by the absorber, leading to a peak at precisely the time predicted by the group velocity. As for the energy, most of it is absorbed by the medium, and the sensible conclusion is that the transmitted energy comes from the leading edge of the incident pulse, which never travels faster than the speed of light.

Conventional wisdom slowly began to adapt to the idea that superluminal group velocities need not imply that the pulses are extremely distorted, as long as most of the energy in the pulse is absorbed. This absorption makes it possible for the velocity of the energy propagation, like the velocity of the information, to remain less than the speed of light regardless of the superluminal speed of a peak.

Over the past ten years, similar superluminal effects have been studied in connection with quantum-tunnelling experiments. In such experiments, the transmitted energy is once again quite small (R Y Chiao and A M Steinberg 1997 Progress in Optics XXXVII 347).

In contrast, the NEC team creates a region of anomalous dispersion in a nearly transparent medium. Wang and co-workers do this by pumping energy into the caesium vapour to create a kind of optical amplifier. First a laser is used to pump most of the caesium atoms into a particular spin state. Next, two additional pump lasers are used to lend energy to the atoms. These atoms can amplify light from yet another "probe" laser by making an electronic transition in which they absorb "pump" energy and re-emit it into the probe beam. There are two specific frequencies at which such a probe can be amplified in this way. By replacing absorption with amplification, the NEC team can swap the regions of normal and anomalous dispersion (see figure 1b). A region halfway between the two amplification lines appears where there is little loss, amplification or distortion. Here the group velocity becomes negative and nearly constant. Indeed, Wang and co-workers measured a group velocity of c/310. In other words, a pulse travelling a distance, L, is advanced by 310L/c.

The meaning of a negative group velocity is illustrated in figure 2. Within the cell, the peak of the pulse travels backwards relative to the direction it is moving in outside the cell. Long before the incident light pulse reaches the cell, two peaks appear at the far end: one travelling away from the cell at c, the other travelling back towards the entrance. This second pulse travels 300 times more slowly and is timed to meet up with the incident peak. The transmitted pulse travelling at c appears to leave the cell some 60 ns before the incident pulse arrives, enough time for it to travel an additional 20 metres.

What is shocking is that such an effect has been observed for the first time without a great deal of attenuation, amplification or distortion of the pulse. It appears as though energy has, in fact, travelled faster than light.

Of course, this is not the case. The effect observed at NEC only works in the presence of an amplifying medium, i.e. a medium that stores energy. In this case the energy is stored in the pump-laser beams. The caesium atoms are prepared in a state that allows them to transfer energy from these beams to the signal beam. The faster-than-light propagation occurs because the pump beams preferentially amplify the leading edge of the incident pulse, lending power to the signal and being repaid by absorbing some of the energy in its trailing edge. (It is important to note that even the dramatic 60 ns advance is only one fiftieth of the width of the pulse.) This is exactly analogous to the intuitive explanation of normal dispersion, except that in this case the atoms temporarily amplify the light pulse rather than absorb it.

A fascinating suggestion is that this experiment might work even for a pulse composed of only a single photon. However, there has been a good deal of controversy over how to discuss the information transmitted through such a system by a single-photon pulse, and many subtle issues remain.

Although relativity emerges unscathed from these experiments, our understanding of exactly which velocities are limited (or not) by c continues to evolve. And even though neither energy nor information is transmitted faster than light in experiments like the one at the NEC, it has already been proposed that the effects may one day be useful in compensating propagation delays in electronic systems.

For the time being, physicists will kept be busy trying to clarify their intuition about relativity and learning how to accurately describe the information carried in real optical or electronic pulses.

Author
Aephraim M Steinberg is in the Department of Physics, University of Toronto, Canada

"Try 32 ft/sec/sec. "

Damn! I had a feeling it was 32 ft./sec/sec. but I went by my failing memory. Mea culpa.

Yes, you see a red or blue shift, but the light still appears to be going at 3X10⁸ m/s, no matter how fast you are going. Since light always appears to go at this speed, traveling at high velocity causes distances and time to distort.

"Wow, you live under 4.125 Gs? "

Yeah, I live on Jupiter and use a really, really high speed internet connection.

I kind of remember these two classics:

If you remember them you should rember their source. I don't remember either.

It has been known for a long time that the crack of a whip is caused by its tip exceeding the speed of sound -- propelled by a human hand.

And long before airplanes could survive exceeding the speed of sound, German V2s were pounding London at speeds greater than sound.

The God of the Bible trancends time and space. In fact, he set the speed of light. If light traveled faster our planet would burn up and any slower and we would be a ball of ice. That also applies to every star in the universe. If light was any faster or slower the physical universe would cease to exist. And yet this is only one of at least 35 finely tuned parameters that ALL must meet specific measurements or life is not possible anywhere in the universe.

Thanks for the reply. I've read that Isaac Newton used similar reasoning to come to the same conclusion. It's one subject that college physics texts don't even mention, unfortunately.

"I hate to tell you this, but it appears we may be married to the same woman."

OMG!

Hey wait a minute, I don't think that's possible but I may have an explanation, it's a DNA thing you know RNA + DNA+ female = Guilt trip, yea baby that's the ticket, genetics!

"No thing goes faster than light"

Not true. Ever see Jesse Jackson heading toward a TV camera?

Yea but Chuckie Schummer gives him a race for his money.

...it'd orbit about where the sun was 8 minutes ago

But we see it where it was 8 minutes ago.

If nothing goes faster than light then what is the purpose of a worm hole?

A worm hole supposedly connects two different regions of space-time. In this case a particle travelling through a worm hole would be experiencing a topological effect. Technically particles would still be travelling less than the speed of light, since a worm hole is considered a topological artifact.

Of course, if no one else was aware of your worm hole you could freak them out by one day showing up on the other side of the universe.

If light traveled faster our planet would burn up and any slower and we would be a ball of ice.

You need a better physics book.

I'm no physics guy, but even if the particle travels thru the actual worm hole at a speed less than light, does not the theory of relativity mean that the actual speed of that particle is greater than light? *I'll say this again, the only physics I know is from my college class which was several years ago. LOL* Forgive my ignorance.

They are a conceptual convenience, but only insofar as they allow easy tracking of the integrals needed for the complete calculation. The diagrams and the integrals do represent reality however.

Of course.

The diagrams and the integrals do represent reality however. The evidence is in the accuracy and precision obtained from the calculations in their description of reality.

The calculations are part of a theoretical description. Integrals are mathematical entities, with no necessary link to any physical phenomena. The important aspect of this is the theory used in desribing the phenomena.

You could use a purely numerical approach in calculating these effects without ever utilizing a Feynman diagram or an integral.

Titus 3:9-11 -- "But avoid foolish disputes, genealogies, contentions, and strivings about the law; for they are unprofitable and useless. Reject a divisive man after the first and second admonition, knowing that such a person is warped and sinning, being self-condemned. "

Prefaced by a mystery bounded by a riddle surrounded by an enigma... Wisdom!

Thanks, Mark. Amongst other things, Titus' observations can surely save the bandwidth round these parts.

"Nothing can travel faster than the speed of sound." "A nuclear explosion inside our atmosphere will undoubtedly cause an uncontrollable chain reaction that will ignite the atmoshpere and incinerate the Earth." I'm with ya blood. No one can concieve it or reproduce it, therefore it can't be. What will tomorrow bring? Nothing? Have we seen it all? I don't think so. Physical Law's were made to be broken. Blackbird.

There's more.

A human being riding on a train moving more than 30 miles per hour will asphyxiate.

A self-powered heavier than air flying machine has never flown.
-Scientific American 3 years after Kitty Hawk.

If you were not aware of a worm hole, it would *seem* like particles could travel faster than the speed of light, but technically they would still be travelling less than or equal to the speed of light.

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