t'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.
Doppler effects affect us forever
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.
As Doppler himself realized, light is also subject to this effect, meaning it behaves more like a sound wave than a rifle shot. Now, sound waves are rhythmic
disturbances in the air, which is clearly not the case for light waves. This led many researchers to hypothesize a much denser material--a fluid called "Ether"--through which light must propagate. Such a fluid would have to permeate every corner of space, from the interplanetary vacuum to insides of solid objects, including the planets themselves. This was a fairly weird suggestion, since we don't appear to be swimming around in any superdense fluid, but the discrepancy was cleared up by one additional hypothesis: that Ether did not interact with matter. That was weird, too, but at least it fit the facts.
Well, most of them.
One consequence of this theory is that it permits matter to travel faster than light, in exactly the same way that a rifle bullet can travel faster than its own sound waves.
However, while astronomers had managed to locate some very speedy objects up there in the heavens, they could not find any superluminal ("faster than light") ones.
Too, since the Earth was moving around the sun--and therefore through the stationary Ether--at some 18 miles per second, one side of the Earth should be facing into the oncoming Ether, while the opposite side faced away. So the starlight falling on one side should be sleeting in 36 miles per second faster than on the other. (A train moving through stationary air sees exactly this effect with sound waves.)
But in fact, this effect was not observed--there was no difference in the speed of light from one side of the Earth to the other. There was also no difference in the speed of light from a moving object than from a stationary one. The speed of light was the same for all observers, always. No superdense fluid could explain that. Instead, there seemed to be something fundamental in the structure of the universe, that made the speed of light an absolute. As a young Munich patent clerk pointed out in 1905, "E" apparently, for some reason, equaled "mc2." And that changed just about everything.
Einstein, tachyons and FTL, oh my!
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").
Do tachyons exist in the physical universe? If so, their masses would have to be imaginary, meaning a multiple of i, the square root of negative 1. That would be weird, and difficult to measure--no tachyon of any sort has ever been detected. Probably. But there is a subatomic particle--the neutrino--which has caused some scientists to wonder. Neutrinos are produced in great quantity by the nuclear reactions inside our sun, and every other in this star-spangled universe. They travel at or near the speed of light, meaning their mass--if they have one--must be something very close to zero. But it's hard to measure, and sometimes the sun's stormy surface kicks out a burst of neutrinos which we observe several seconds before an obviously related burst of photons. So yeah, the evidence is sparse, but it's tempting to speculate the neutrinos are maybe going a little bit faster than "c."
There are a few other things that can go faster than light, by virtue of not being "things" at all. The spot from a laser pointer is one example--shine it at the wall in front of you and you can make it move around quite rapidly. The farther the wall, the faster (and dimmer) the moving spot; shine it at a target thirty thousand miles away and you can easily move it faster than "c." The individual photons, of course, still move as slowly as ever--it's exactly like waving a firehose around so that the splash of its impact travels faster than the speed of the water through the hose. The splash is a process, not an object, so it isn't constrained by relativity.
Can we send messages faster than light this way? Alas, no. We could certainly shine a gigantic laser pointer at Alpha Centauri, then quickly snap it around to Vega, and anyone looking up at the night sky in those distant solar systems would see the ruby flash. But the only information that hops the gap from AC to V is, "I'll bet those other guys saw that flash, too," which in a mathematical sense is no information at all. The Einsteinian universe turns out to have some sharp restrictions against FTL transmissions.
Fortunately, there's more to our universe than even Einstein suspected. The burgeoning and highly weird field of quantum mechanics offers dazzling hints--and even hard experimental evidence--of faster-than-light phenomena, which we'll discuss here next month!
Wil McCarthy is a rocket guidance engineer, robot designer, science fiction author
and occasional aquanaut. He has contributed to three interplanetary spacecraft, five
communication and weather satellites, a line of landmine-clearing robots, and some
other "really cool stuff" he can't tell us about. His short fiction has graced the
pages of Analog, Asimov's, Science Fiction Age and other major publications, and his novel-length works include Aggressor Six, the New York Times notable Bloom, and The Collapsium.
