Focus: Faster than a Speeding Light Wave
The starship Enterprise routinely flies faster than light, but of course, nothing really goes that fast. Well, almost nothing. Physicists have been concocting light pulses that do travel faster than c (the speed of light in a vacuum) for almost two decades, although none of the experiments could be used to send information that fast, according to most physicists. The latest demonstration, described in the 22 May PRL, may be the most dramatic, as it dispenses with some of the complexities of most other experiments: The light pulses travel through free space–not a highly absorbing material–and their superluminal (faster than light) feat covers a distance of 30 wavelengths, much farther than in any previous work.
According to relativity, a faster-than-light signal would allow an observer traveling in some fast moving reference frame to see the signal arrive before it was sent–in other words, she could predict the future. That would violate the sacred causality principle, which says that “causes” always precede “effects.” Most physics students learn that for light waves in some media, the “phase velocity”–the speed at which a sine wave travels–can exceed c, since you can’t send a signal with a constant amplitude wave that extends forever into the future and the past. But teachers often say that the “group velocity”–the speed of a wave pulse, like a single traveling “wiggle” on a stretched rope–cannot be larger than c because it represents the speed of a physical signal.
Researchers have now measured many group velocities higher than c. “It’s just not true what they say in the textbooks,” says Raymond Chiao of the University of California at Berkeley. For example, a Gaussian shaped light pulse can travel faster than c through some highly absorbing materials. The explanation is that the central piece of the pulse is attenuated more than the earliest piece. Although the pulse shape is unchanged, it comes out smaller, and the “leading edge” of the input pulse is transformed to become the peak of the emerging pulse, a process called “reshaping.” So no part of the pulse is actually transmitted faster than c, says Chiao.
In the new experiments, led by Anedio Ranfagni of the Italian National Research Council in Firenze, the setup looks innocent enough: The team sent microwaves (3.5 cm wavelength) through a narrow, ring-shaped opening onto a large and nearby focusing mirror, which collimated the waves into a beam propagating back from the mirror, beyond and behind the source. They “modulated” the microwaves with rectangular pulses (sharp “amplify” and “attenuate” commands, in rapid succession) and detected the pulses at positions between 30 and 140 cm from the source, along the beam axis. The slope of their plot of arrival times vs. distance led to an apparent propagation speed of 5 to 7% above c, although beyond about 1 m, the speed approached c, all of which agreed with previous predictions.
Aephraim Steinberg of the University of Toronto explains that such a beam is unusual. The light detected at each point along the beam has actually arrived via an “off-axis detour” to some part of the large mirror, rather than having traveled down the beam. So the light pulse detected at 60 cm simply took less of a detour than the light detected at 30 cm from the source, says Steinberg. “The wave at 60 cm was already en route along a sort of ‘shortcut’ while you were detecting the wave at 30 cm,” he says. The pulses are reshaped by the slit and mirror in this case, rather than by the special properties of a medium through which the light propagates.
Ranfagni doesn’t accept Steinberg’s interpretation and states in the paper that “a shadow of a doubt” remains regarding the existence of superluminal signals. Despite some shortcomings in the quality and quantity of data, Steinberg believes the new results are correct and important because “it’s much harder to see where the loophole is” that saves causality. He says that as physicists come up with ever more surprising examples of faster-than-light propagation, they hope to approach a “universal understanding” of light propagation in all circumstances.