As an amateur theoretical physicist, I know all about the principle that the speed of light is the ultimate speed limit in the universe. I’ll be the first to admit that it’s difficult to wrap my brain around this concept, but I accept that it’s true. Light not only travels really, really fast, it also travels at a constant speed, irrespective of the relative speed of an observer. Furthermore, any bit of matter that is in motion increases in mass as its speed increases, approaching infinite mass as it approaches the speed of light (and requiring, in theory, infinite energy to accelerate it to that speed). Taken together, this information rather strongly suggests that nothing can be made to travel faster than light. The details of the math and physics don’t fully make sense to me, even after reading the works of Einstein and several modern physicists. But then, these folks are professionals in the field whereas I am not; if they say that their long years of research lead them to conclude unhesitatingly that nothing can move faster than light, who am I to disagree?
Faster than a Speeding Photon
But in 1962, a group of physicists (O.M.P. Bilaniuk, V.K. Deshpande, and E.C.G. Sudarshan) published the article “Meta Relativity” in the American Journal of Physics, in which they made the provocative observation that Einstein’s Special Theory of Relativity does not actually prohibit matter from traveling faster than light, only from being accelerated to faster-than-light speeds. This may seem like an irrelevant distinction—and perhaps it is. But suppose there were a particle that came into existence already traveling faster than light. Because it did not have to be accelerated in order to reach that speed, it does not violate Special Relativity. Physicist Gerald Feinberg gave this hypothetical particle the name tachyon in 1967, from a Greek word meaning “speedy.” Later, the term tardyon was coined in order to identify ordinary, slower-than-light particles; these are also sometimes known as bradyons.
The tachyon, if it existed, would have a number of fascinating properties. Unlike ordinary particles, it would have to decrease in mass as it went faster, meaning that the speed of light—at which its mass would be infinite—would be just below its slowest possible speed. Likewise, adding energy to the tachyon would slow it down, rather than speed it up; to slow it all the way down to the speed of light would require infinite energy. For a long time, physicists believed that a tachyon’s mass would have to be an imaginary number—a number with a factor that’s the square root of –1—though more recent formulations of tachyon theory suggest that such a particle could have a real mass. Most intriguingly, a tachyon, if it is to adhere to the principle of relativity, would actually be able to travel backward in time—seemingly making all sorts of trouble for the notion of causality.
The Tachyon at the End of the Rainbow
But of course all this is simply hypothetical. The fact that a tachyon travels faster than light is a bit like the fact that a leprechaun will give you a pot of gold if you catch it—cool, but sort of meaningless if the thing in question doesn’t actually exist. Even though the existence of tachyons would not contradict the laws of physics, that doesn’t mean there is such a thing. But unlike leprechauns, tachyons can be tested for scientifically.
Electrically charged tachyons in a vacuum would, if they existed, produce a type of energy known as Čerenkov radiation; this can be measured experimentally. Despite numerous tests, no Čerenkov radiation has ever been observed in a vacuum, which suggests at the very least that no charged tachyons exist—and if tachyons of any sort exist, we would expect some of them to be charged, making the existence of any seem improbable. However, there are other ways one might possibly detect a tachyon, even one without a charge. And a few anomalous experimental results over the past couple of decades that could conceivably be attributed to tachyons have kept some measure of hope alive. Annoyingly, though, while it may someday be scientifically possible to prove the existence of tachyons, it will never be possible to definitively disprove their existence. So the jury may be out on this one indefinitely.
Meanwhile, some unscrupulous makers of “New Age” books and products have capitalized on the notion of tachyons as a marketing gimmick, claiming that their existence is not merely a certainty, but somehow a way for you to achieve greater health and well-being. You can even purchase “tachyonized” herbal supplements, garments, toys, jewelry, and so on. Unfortunately, this term is scientifically meaningless. No one even has proof that tachyons exist, and if they did exist, it’s not clear that they would have any use at all in normal time and space. One certainly cannot generate or attract tachyons artificially, except as a plot device in science-fiction stories about time travel or faster-than-light starships. Still, those ads are awfully appealing. If I can find a deal on a box of tachyonized breakfast cereal with a free leprechaun, I think that would be a good investment.
Note: This is an updated version of an article that originally appeared on Interesting Thing of the Day on October 29, 2004.