Tachyon leaves a bar

A tachyon leaves the bar.

The bartender says
- We don't serve tachyons here!!

A tachyon enters a bar.











Background

Tachyon is the hypothetical particle that moves faster than the speed of light. The term "tachyon" comes from the Greek word "ταχύς" which means fast or rapid. We underline that this particle is hypothetical and was introduced to describe states with negative mass squared.

According to Special Relativity theory of Albert Einstein, there is an upper bound in the speed in nature and no particle can move faster than this value. This bound is the speed of light, i.e. the speed that light travels through space-time. Particles with no mass travel at the speed of light and massive particles travel with lower speeds. The heaviest a particle is the lower its maximum speed is.

However tachyon has negative mass and this would imply that it could travel faster than the speed of light. But, what are the implications of this fact? According to special relativity any information that could travel faster than the speed of light violates the principal of causality. The latter refers to the statement that every effect has a cause and they are connected in a causal relation. In other words the effect presupposes the cause.

To understand this better let us imagine the following thought experiment. Alice and Bob are two very good friends. They have agreed that every time Bob thinks of Alice he should send her a message and then Alice is happy. So the event Alice is happy presupposes Bob has sent a message. So the cause is the message due to Bob. This is all true because no message (information) can be sent faster than the speed of light. However let us consider that the information "Bob thinks Alice" travel faster than light (faster than the message), then Alice would be happy without Bob sending her a message, in other words without any cause. Simply, Alice would be happy without receiving any statement of love from Bob.
(Comment for the experts: Bob and Alice have a more complicated quantum love-life; they appear to be entangled and their love is a "spooky action at a distance").

Therefore breaking causality would lead to paradoxes like going back on time and killing your own grandfather. (Then who would have given birth to your father and so on?). In other words traveling with speed greater than the speed of light implies that events have no causal connection and thus a tachyon could first leave the bar, then be insulted by the barman and finally go into the bar. QED.