It's not just a good idea, it's the law: 186,287 miles per second. The fact that sound waves travel at a finite speed--roughly 330 meters per second--has been known since ancient times. It's obvious, really, when you stand back a ways and observe the falling of a tree or the clapping of a pair of hands, and the sound arrives noticeably later than the sight itself. The fact that light waves also travel at finite speed is much harder to notice, because that speed is almost a million times faster. But by the end of the Renaissance, astronomers--viewing events much more distant than a few hundred meters--had begun to suspect the truth.
In 1676, the Danish astronomer Olaus Roemer observed that the orbital period of Jupiter's moons appeared to vary with the Earth's seasons. He reasoned that the time difference could be accounted for by motion of the Earth itself--towards Jupiter at one point in its orbit, and away from it at the other. (Imagine standing near a merry-go-round while a particular horse goes round and round, alternately approaching and retreating from you.) Using the crude range-to-Jupiter measurements available at the time, Roemer was able to estimate the speed of light to within an order of magnitude. His best guess was about a third of the actual answer, which isn't too bad under the circumstances. In fact, no significantly better methods for calculating "c" were devised until 173 years later, when French scientist Armande Fizcau placed a light source behind a slotted, spinning disc, and reflected the resulting flashes off a mirror placed eight kilometers away. His measurement was within 5 percent of the value we use today.
Around this time, experimenters began to notice another peculiar property of light: it behaved like a wave. If a rifle bullet travels a thousand meters per second, and a steam locomotive travels ten, then a bullet fired forward from a moving train goes 1010 meters per second, and one fired backward goes 990. Interestingly, though, a sound wave doesn't behave this way. The speed of sound varies with altitude and temperature and barometric pressure--the denser the air, the faster the speed--but for a given set of conditions the speed of sound is just that: the speed at which all sound travels, regardless of source.
The sound of our rifle shot (which is actually slower than the bullet itself) goes the same speed in both the forward and reverse directions, no matter how fast the train is going. Still, in the forward direction the sound waves crowd together as their source moves along behind them, meaning they will arrive more frequently in the ear of a listener, somewhere up ahead of the train. And since frequency is related to pitch, the listener interprets this difference as a higher-pitched sound. In the opposite direction, the sound waves are spread out, and the pitch is lower. This effect was first characterized by Christian Doppler in 1842, and is known today as the Doppler effect.
This equation--Einstein's theory of relativity--tells us that to accelerate any mass to the speed of light requires an infinite amount of energy. The accelerated mass also experiences infinite time dilation, so that (for example) one second elapsing on a spaceship traveling at light speed equals infinity in the outside universe. Clearly these are not mere inconveniences--it's relativistically impossible for any material object to travel at the speed of light.
Another consequence of relativity--or more properly, of the early quantum theory Einstein developed at the same time--is that light, even though it's a wave, can sometimes act as a stream of particles, which we call photons. These can travel at "c"--the speed of light--because they have no mass, which sets a handy precedent: anything massless can travel that fast. But what about faster? Interestingly, the equations are symmetrical; it takes infinite energy to reach the speed of light, but not to exceed it, so while there's no way for a slower-than-light particle to become a faster-than-light one, a particle which starts out faster than light--and stays that way--is permitted by the theory. In 1967, physicist Gerald Feinberg even coined a name for such particles: the Greek word "tachyon" (roughly, "swift thing").
Einstein saw the universe as a marvel of geometry--a sweeping four-dimensional structure whose curves, both cosmic and intimate, defined the force of gravity which held the whole thing together. Distant parts of this universe were isolated from one another by a "locality principle," which held that interaction between distant portions of the 4-D universe must be limited by the speed of light. Since the universe appeared (and still appears) to be expanding at a substantial fraction of that speed, this actually meant that some regions of the universe--on "opposite sides," as it were--could never see or detect each other at all! On a more practical note, it meant that the transmission of information over short distances--say, between neighboring planets or stars--was also limited to "c," the speed of light.
Science fiction has popularized the concept of "wormholes," which essentially are geometric deformations of the universe, like folds in a tablecloth, which bring two distant points into contact, allowing light or particles (or starships, yeah) to "skip over" the intervening distance. In fact, this idea originated with Einstein as well. However, this sort of bending and twisting requires such enormous and finely controlled gravity fields that we literally can't envision any technology that could accomplish it.
Fortunately, we may not need to: quantum mechanics--the study of time and space and matter/energy in the very tiniest of increments--reveals an even stranger universe, where similar shortcuts--free for anyone's use--are quite literally more common than dirt. These aren't wormholes, but more subtle effects like "quantum entanglements" between distant particles, which allow (in fact, force) them to interact instantaneously, regardless of the distance between them.
If you find this concept a bit baffling, don't worry--so do the most brilliant physicists. No one really knows what to make of all this, or where it might lead us someday. But for now we have some very cool findings to kick around.
Pulsar bursts move 'faster than light'
Every physicist is taught that information cannot be transmitted faster than the speed of light. Yet laboratory experiments done over the last 30 years clearly show that some things appear to break this speed limit without upturning Einstein's special theory of relativity. Now, astrophysicists in the US have seen such superluminal speeds in space – which could help us to gain a better understanding of the composition of the regions between stars.
Superluminal speeds are associated with a phenomenon known as anomalous dispersion, whereby the refractive index of a medium (such as an atomic gas) increases with the wavelength of transmitted light. When a light pulse – which is comprised of a group of light waves at a number of different wavelengths – passes through such a medium, its group velocity can be boosted to beyond the velocity of its constituent waves. However, the energy of the pulse still travels at the speed of light, which means that information is transferred in agreement with Einstein's theory.
Now, astrophysicists claim to have witnessed this phenomenon in radio pulses that have travelled from a distant pulsar.
The discovery has been made at the University of Texas at Brownsville, where Frederick Jenet and colleagues have been monitoring a pulsar – a rapidly spinning neutron star – more than 10,000 light years away. As pulsars spin, they emit a rotating beam of radiation that flashes past distant observers at regular intervals like a lighthouse. Because the pulses are modified as they travel through the interstellar medium, astrophysicists can use them to probe the nature of the cosmos.
Several factors are known to affect the pulses. Neutral hydrogen can absorb them, free electrons can scatter them and an additional magnetic field can rotate their polarization. Plasma in the interstellar medium also causes dispersion, which means pulses with longer wavelengths are affected by a smaller refractive index.
Jenet's group thinks that anomalous dispersion should be added to this list. Using the Arecibo Observatory in Puerto Rico, they took radio data of the pulsar PSR B1937+21 at 1420.4 MHz with a 1.5 MHz bandwidth for three days. Oddly, those pulses close to the centre value arrived earlier than would be expected given the pulsar's normal timing, and therefore appeared to have travelled faster than the speed of light.
The cause of the anomalous dispersion for these pulses, according to the Brownsville astrophysicists, is the resonance of neutral hydrogen, which lies at 1420.4 MHz. But like anomalous dispersion seen in the lab, the pulsar's superluminal pulses do not violate causality or relativity because, technically, no information is carried in the pulse. Still, Jenet and colleagues believe that the phenomenon could be used to pick out the properties of clouds of neutral hydrogen in our galaxy.
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."