Claiming that something can move faster than light is a good conversation-stopper in physics. People edge away from you in cocktail parties; friends never return phone calls. You just don't mess with Albert Einstein. So when I saw a press conference at the American Astronomical meeting this past January on faster-than-light phenomena in the cosmos, my first reaction was to say, Terribly sorry, but I really have to go now. Astrophysicists have been speaking of FTL motion for years, but it was always just a trick of the light that lent the impression of warp speed, a technicality of wave motion, or an exotic consequence of the expansion of the universe. These researchers were claiming a very different sort of trick. Dubious though I was, I put their press release in my "needs more thought" folder and today finally got around to taking a closer look. And what I've found is utterly fascinating.
The researchers, John Singleton and Andrea Schmidt of Los Alamos and their colleagues, have built a sort of wire in which an electric pulse can outpace light. They get away with it because the pulse is not a causal process. It does not ripple down the line because charged particles are bumping into each other, a process that is subject to Einstein's speed limit. Instead, an external controller drives the particles and can synchronize them to make a pulse pass through the wire at whatever speed you want. The particles are like dominos in a row. A causal process is the usual domino effect in which each domino knocks down the next; the dominos move at their own speed, determined by their size and spacing. An acausal process is if you knocked down all the dominos with your hand; the dominos move however fast you can make them. The photo above shows an early version of the contraption; the wire is the white arc on the right, and the controllers are the circuit boards on the left.
This method of breaching the speed barrier might seem like cheating -- after all, no material object is breaching the light barrier. But electromagnetically it doesn't matter. Whatever the origin of the pulse in a wire, it involves the motion of electric charge and emits electromagnetic radiation. The radiation propagates outward at the speed of light, but is forever shaped by the speed of whatever generated it. When Singleton, Schmidt, and the rest of their team generate slower-than-light pulses using their technique, the resulting radiation looks just like the radiation created by ordinary causal pulses. For faster-than-light pulses, the radiation looks just like the radiation that would be created if charged particles really could exceed the speed of light. In other words, it looks pretty weird.
Not only is the radiation tightly focused in space, it is tightly focused in time -- a pulse that originally takes, say, 10 seconds to generate might be squeezed into 1 millisecond as all the electromagnetic wavefronts get jammed together. The temporal focusing causes the radiation to spread out over a wide swath of the electromagnetic spectrum. In addition, the focusing provides a degree of amplification, causing the intensity of the radiation to diminish not with the inverse square of the distance but with the inverse distance.
This focusing could be very useful for transmitting radio waves with a minimum of power, but Singleton and Schmidt's main interest is applying the idea to astrophysics -- in particular, to pulsars. Astrophysicists think these objects are hyperdense neutron stars that generate radio pulses as they spin, much like a lighthouse. But they have struggled to explain why the radio pulses are so sharp and why they appear over such a broad range of the spectrum. Singleton and Schmidt, building on work in the 1980s by Houshang Ardavan of Cambridge University, argue that these properties are natural consequences of FTL electric currents driven by the neutron star's magnetic field. For simple geometric reasons, beyond a certain distance from the star, the magnetic field sweeps through the atmosphere at faster than light.
The researchers are now applying their model to another mystery of astrophysics, gamma-ray bursts. Astrophysicists typically estimate the intrinsic power generation of these bursts by assuming the inverse-square law, and the values they get are off the charts. But if FTL effects are involved, the inverse-square law might be overestimating the power and astronomers should really be using a simple inverse law.
Singleton says the basic principle of FTL currents goes back to work by English physicist Oliver Heaviside and German physicist Arnold Sommerfeldt in the 1890s, but was forgotten because Einstein's theories dissuaded physicists from thinking about FTL phenomena, even those that evaded the theories' strictures. I've only just touched on this engrossing physics and I recommend you read the team's papers, beginning with this one. "People just don't think about things moving faster than the speed of light," Singleton says. "This is a completely wide open and unexplored field."