Tachyon particles are theoretical sub-atomic objects that, if they existed, would travel faster than the speed of light.
So, who came up with this, seemingly, crazy idea? It was physicist Gerald Feinberg who, in 1967, produced a scientific paper called "Possibility of Faster-Than-Light Particles" and it was he who coined the term "tachyon", which simply means "fast particle".
While everyone agrees with Einstein's Theory of General Relativity, which states that nothing can exceed the speed of light, Feinberg pointed out that this only applied to things that are moving at less than this speed to start with. In other words, if anything has to accelerate to light speed, it cannot pass that barrier.
However, if a particle is already at super-light speeds, then this does not violate Einstein's theory. Hence the tachyon particle.
You may be thinking by now, what's the point of theorising these weird particles? Well, I imagine it's because, if a theoretical physicist proves that a particle can exist, without flouting existing laws, then he assumes that it probably does.
There's worse to come...
Here's the first little gem: a tachyon has imaginary mass. Yes, that's right. But don't let the word "imaginary" put you off. It's related to the concept of "imaginary numbers" and the use of the word can be a little confusing.
An imaginary number is something like the square root of minus one. It can't be written as a fraction and is therefore designated "imaginary". It could just as easily be called "weird". In fact the square root of minus one is used in advanced mathematics and represented by i. Used in the case of a tachyon, it means that the mass of the particle only exists if it is travelling at super-light speeds. At light speed the particle's mass would be zero, so that is the lower limit of its speed. This is in contrast to slower, "normal" particles, which cannot go faster than this limit.
The next characteristic of these strange little objects is the fact that the faster they go, the less energy they have. This is the equivalent to a cyclist, for example, gaining speed as he pedals less! Tachyon particles are thought to lose energy in a form called "Cherenkov radiation", which is also seen in nuclear reactors. This has led to the intriguing situation that the particle will eventually have zero energy, at which point it will be travelling at infinite speed.
One way of considering this faster-than-light speed is that it manifests as travelling backwards in time, leading to the next preposterous suggestion.
This is the name given to a thought experiment first proposed by Albert Einstein in 1907 and is also known as Tolman's Paradox. It suggested that tachyon particles could be used to send information into the past. This would violate causality, in that an effect would happen before its cause.
Imagine two people, A and B, both with an antitelephone. A sends a message at 3pm, which B receives at 2pm. B has had instructions to reply to A, telling him not to send the original message (A has to comply). A then receives this message at 1pm and dutifully does not send the 3pm message. So we have the situation that the original 3pm message couldn't be sent only if it sent!
It's a good job tachyon particles are only theoretical.