Tachyons, hypothetical sub-atomic particles which travel faster than light, once attracted a lot of publicity yet we do not hear much discussion of them today. Whatever happened to tachyons?
Over a century ago, Albert Einstein anticipated odd things happening on a spaceship travelling at speeds close to that of light (roughly 300 000 km/s) and impossible things happening when travelling at more than the speed of light. It is not necessary here and now to go into why this should be, but these predictions are enough to convince much smarter people than me that the speed of light is a fixed, fundamental speed limit in the Universe and that no material objects can ever attain or exceed this crucial speed.
However some theoretical physicists have gone for a walk on the wild side by speculating on the possible existence of particles of matter which always travel faster than light, avoiding the complications of acceleration past the cosmic speed limit. Physicist Gerald Feinberg even gave them a name, tachyons (from the Greek takhus, meaning “fast”, and the English ”-on”meaning “elementary particle” (yes, really)). There is a history of particle physics predicting the existence of theoretical particles needed to fill gaps in our knowledge which are later discovered to be real, neutrons being the classic example. Could this happen with tachyons?
If they existed tachyons would be really, really bizarre things. For example they would always be moving faster than light, dropping to less than 300 000 km/s would be as impossible for them as exceeding this speed is to us. Stranger still, their mass would be imaginary. “Imaginary” is used in its mathematical sense, meaning a multiple of the square root of -1, whatever that may mean in the real world. Not only that, adding kinetic energy to a tachyon would make it slow down, but it would take infinite energy to drop its velocity down to the speed of light! Conversely, a tachyon shedding energy would continuously accelerate. This leads to a subtle argument against tachyons existing.
Cherenkov radiation (also, and more correctly, known as Čerenkov radiation) emission occurs when charged particles pass through a medium at a speed greater than the velocity of light in that medium. For example, the speed of light in water is about three-quarters of its speed in free space. Beta particles emitted in an operating nuclear reactor whizz though the surrounding water at speeds exceeding this (but still lower than the speed of light in free space), and Cherenkov radiation is produced as a by-product, appearing as an eerie blue glow. Assuming tachyons are electrically charged (and this is an assumption, there is no evidence either way), as they move though space they would continuous lose energy as Cherenkov radiation emission. As tachyons losing energy accelerate, they would get faster still, give off yet more Cherenkov radiation, accelerate more and so on. Basically this feedback loop would mean that by now any tachyons would be travelling at almost infinite velocity and the whole Universe ought to be filled with the blue glow of their passing. Obviously we have never observed anything like this.
(Update: physicist Ethan Siegel argues that tachyons moving through a vacuum would not generate Cherenkov radiation, going on to state that “tachyons don’t emit Čerenkov radiation unless they move slower-than-light in a medium!”)
As a final piece of weirdness, tachyons would propagate backwards in time, being destroyed before flying around space until they were created. This implies signals sent with tachyons could be received before they are transmitted, either opening up a Pandora’s Box of paradoxes (what lazy TV writers call “wibbly-wobbly timey-wimey stuff“) or conclusively proving the whole idea of tachyons as physically real objects to be hopelessly unfounded.
Physicists played around with the concept of tachyons in the 1960s through to the mid-70s, and a few performed experiments to detect tachyons in cosmic ray showers. The idea was that ultra high-energy cosmic ray particles collide with air molecules in the upper atmosphere creating a secondary shower of billions of daughter particles, some of which can be detected with ground-based instruments. Possibly tachyons could be also be created in these high altitude (about 20 km overhead) collisions, and would obviously race ahead of their slower than light siblings. Some of the tachyons could be observed as detection events some 60 microseconds or so ahead of the main secondary shower (which is composed of particles moving at just under the speed of light). In 1973 two Australian researchers, Roger Clay and Philip Crouch, claimed successful detection of a tachyon but no one has ever been able to duplicate this result, and Clay has subsequently concluded “No positive evidence has been found using conventional scintillation detectors for producing a tachyon signal”. Experiments to create tachyons in collisions with particle accelerators at Amhearst College and Brookhaven National Laboratory (both in the USA) also drew blanks.
After the flurry of scientific interest in the early to mid-1970s,the existence of tachyons was looking unlikely, nevertheless tachyons spread rapidly (how else?) to the pages of science fiction. Beams of tachyons were used to make telephone calls between the stars. Mighty starships leapt across the galaxy and still got back home in time for tea thanks to the Tachyon Drive (to non-aficionados of science fiction I should explain that in SF a space propulsion device is always called a ‘drive’). At the press of a button, a Tachyon Drive seems to convert all the matter of a spaceship, its cargo and passengers painlessly and instantaneously into tachyons so they can shoot off into the deep black at superluminal speed and then be transformed back into ordinary matter again at journey’s end. This process seems remarkably easy and trouble-free; presumably it is all done by magic. A less often used alternative Tachyon Drive is the Tachyon Rocket concept (which I remember only from Joe Haldeman‘s 1974 classic The Forever War) where a relatively conventional space vehicle is accelerated to high but subluminal speeds by ejecting an exhaust of tachyons. The most important fictional treatment of tachyons is Gregory Benford’s Timescape (1980) which was not an epic space opera, rather it told two interlinked earthbound stories. In 1998 British scientists attempt to use tachyons to send a message into the past warning of an ongoing environmental catastrophe, while in 1962 an American physicist is puzzled by “noise” in the data from an experiment and he begins to wonder if this could be some kind of received transmission. It is a fine book which was very well-received at the time but is sadly rather forgotten now.
Do tachyons exist? Unlike neutrons, tachyons are not actually required to exist by any physical theory and indeed their existence would raise more problems than it would solve (special relativity would be wrong for a start- which would be kind of awesome). Although it is possible to describe them mathematically it would appear certain that tachyons do not exist in the real Universe. Little has been written about them in scientific publications recently apart from some fevered speculations in 2011 when CERN researchers thought they had observed neutrinos moving faster than light, results later found to be in error. Beyond science, tachyons still show up all the time in streams of technobabble in comics and TV shows. More distressingly, “tachyon healing” is advertised as a New Age lifestyle therapy and even as an alternative medical treatment for real illnesses. I have read some of the rationales and product descriptions on sites advertising this sort of junk and found only scare-mongering nonsense, aimed at extracting money from the unwell or anxious, proving that while space has a speed limit, there are no limits to folly and dishonesty.
(article by Colin Johnston, Science Communicator)