Light pulses flout sacrosanct speed limit.

Five years ago, a wave of discontent swept away the 55-mile-per-hour U.S. speed limit. Nowadays, some physicists are taking a hard look at the 670-million-miles-per-hour speed limit of light in a vacuum, or c.

Albert Einstein posted this limit in his 1905 theory of special relativity. Although popular lore and some physics textbooks still contend that nothing races faster than c, experiments going back decades have repeatedly shown that light can beat that speed under certain conditions.

A few scientists argue that those experiments hint that Einstein was wrong. Two new experiments reveal dramatic additional evidence of superluminal velocity but make no clear case for repealing Einstein's law, scientists say.

In one study, conducted in Italy, scientists propagated superluminal microwaves through air by bouncing them off a mirror. In the other, led by a New Jersey researcher, a laser pulse approaching a gas-filled cell's entry window materialized at the cell's exit glass before even reaching the cell.

Although superluminal phenomena might someday help speed up computers--an avenue being explored by Raymond Y. Chiao of the University of California, Berkeley--the main excitement around these experiments stems from basic physics implications.

At stake is the idea that a cause must precede an effect. If experimenters found that information can go somewhere faster than c, "you would get into nonsensical types of predictions, like going back in time and shooting your grandmother," explains Peter W. Milonni of Los Alamos (N.M.) National Laboratory.

Gunter Nimtz of the University of Cologne in Germany contends that information can indeed travel faster than c, casting doubts on both causality and special relativity. In 1995, for example, his research team encoded Mozart's 40th symphony in a microwave beam traveling at 4.7 times c to a receiver.

However, Aephraim M. Steinberg of the University of Toronto argues that aside from Nimtz and a few other "vocal dissenters," mainstream physicists agree that such experiments "do not support any idea of causality violation." One challenge, however, is to exactly define information, or a signal.

Experiments dating back to the early 1990s by Nimtz, Steinberg, Chiao, and others have shown superluminal tunneling of optical photons through mirrors (SN: 7/2/94, p. 6) and of microwaves through so-called forbidden zones of waveguides.

The Italian scientists, led by Anedio Ranfagni of the Italian National Research Council in Florence, devised their experiment so that reflected microwaves in open air overlap and interfere as the waves speed away from the mirror. Constructive interference creates a moving pulse along the axis of the apparatus whose speed varies according to the configuration of the experiment. The researchers report in the May 22 PHYSICAL REVIEW LETTERS that within 1.4 meters of the mirror, they clocked such pulses at up to 125 percent of c. Beyond that distance, the effect dies out.

Because electromagnetic waves radiate through air much as they do in a vacuum, Chiao says, the "spectacular work" by the Italians demonstrates that even in a vacuum, light could outpace c.

In the laser experiment by Lijun Wang of NEC Research Institute in Princeton, N.J., and his colleagues, the superluminal pulse, which was preceded by a "pump" pulse to excite the amplifier, has a negative velocity. That means that it "arrives at a distant point `earlier' than it even arrives at the input," explains Steinberg, who is acquainted with the unpublished study but is not a coauthor.

This isn't magic, he says. Rather, amplifiers, like the cell in the experiment, respond to certain frequencies by building a replica of the incoming pulse at the output. In this case, the time a pulse with speed c would take to cross the cell, multiplied by 300, is the head start the outgoing pulse gains over the incoming one.

What's more, any rounded pulse contains a central peak and tapering wings extending far out behind and ahead. The wings contain all the information needed to reconstruct the peak, so as soon as the forward wing of the incoming laser pulse arrives, the cell spits out a full-scale version of the peak.

Although Wang declined to discuss the study, which was submitted to NATURE, some of its results were described May 30 in The New York Times.