In 2011 physicists at CERN made one of the great “huge if true” claims of their field, reporting that neutrinos had been observed to get from Switzerland to Italy faster than light. Although subsequently disproven, this wasn’t the first, nor the last, time these poorly understood particles were claimed to break the great cosmic speed limit.
Discussions of these claims often used a shorthand that faster-than-light travel would “make neutrinos tachyons”. So, what is a tachyon, and do they exist, even if not in the form of neutrinos?
The familiar universe consists of massless particles like photons, that can travel at the speed of light in a vacuum, and those with mass that always travel more slowly. An essential feature of Einstein’s Theory of Special Relativity is that for objects with mass, as speed increases so does mass. Under everyday conditions the increase is so small as to be unnoticeable, but particles accelerated to close to light speed can gain many times their mass at rest.
According to Einstein, anything with mass traveling at the speed of light would be infinitely massive, which is impossible. Faster-than-light travel therefore requires the tricky feat of accelerating to a greater speed without ever moving at exactly light speed.
However, in the 1960s physicists started considering the possibility of particles that always travel faster than the speed of light. For them, the obstacle would be slowing down to mere light speed and eventually below it. In 1967, Gerald Feinberg coined the name tachyon.
Putting a name to it led some non-physicists to assume tachyons had been found, or at least evidence for their existence identified. This, despite the now-debunked Alp-crossing claims, is not the case.
However, lacking proof of tachyons’ existence hasn’t stopped physicists devoting a lot of thought to trying to work out what they would be like if they did. It’s a story familiar from wormholes and the multiverse.
Crucially, if tachyons exist they not only can travel faster than the speed of light, but always do. It would be as impossible for them to slow down to speeds below the light barrier as it is for us to reach speeds above it. Deeply counter-intuitively, they must also lose energy as they speed up – slowing down towards the speed of light requires them to gain energy. To get to speeds just slightly above light speed would be immensely energy-intensive.
Tachyons were also envisaged as having “imaginary mass”. It might sound like anyone talking about this is playing a not-very-convincing joke, but imaginary mass is actually a much more widespread concept in physics than tachyons. Fields are known to have imaginary mass, the question is whether particles can too. The answer is doubtful, leaning towards no, but if imaginary mass particles do exist somehow, then they would move faster than light and therefore be tachyons.
More recent explorations of the possibility of tachyons involve them having real mass. Either way, most physicists put them into the “fun to think about, but unlikely to exist” category. Nevertheless, there remain a committed minority of tachyon-believers, and now and then papers get published claiming to provide evidence.
Thousands of papers are published each year that include the word tachyon, and while many of them are describing “tachyon-free” models, others are taking the possibility seriously. Here’s one, for example, exploring the possibility of wormholes supported by matter made of tachyons.
It's possible tachyons exist, but we have no capacity to detect or interact with them. This leads to philosophical questions along the lines of the famous, “If a tree falls in a forest, does it make a sound if there’s no one around to hear it?”
The more interesting possibility is for tachyons to exist and humans to eventually gain the capacity not merely to interact with, but control them. Ultimately that might lead to moving faster than light, or potentially even time travel. These ideas make for great (and terrible) science fiction, but they’re wildly speculative.
The idea of being able to harness tachyons to send messages at faster-than-light speeds is a little closer to the realm of reality. It’s where people’s minds went between the claim of neutrinos outpacing light and its debunking. However, even this would contradict causality and run into the Grandfather Paradox that almost all visions of time travel encounter.
The prospects for tachyons look bleak, but given all the ways physics has surprised everyone, including its most knowledgeable practitioners, over the last century, it might not be safe to dismiss them entirely.
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