>>12061953In special relativity, space-time is a 4D vector space, with each point being an 'event'. There are time-like events, which can be thought of as events reachable at speeds less then c, light-like, into which one has to be at c, and space-like. It is interesting to note that time-like events always remain time-like in a Lorentz transformation, and similarly with light-like and space-like, and that it is also known that you can't accelerate and surpass c if you have mass. Particles with mass m travel at less then c, with mass 0 travel at c, and an imaginary mass travel greater then c. The issue is that while time-like events preserve casuality, space-like ones do not, and this can lead to situations where in a different frame, the reception of a tachyonic particle affects its transmission. The non-existence of such particles is not based on any physics principle but on logical ones.
In general relativity, I am wary of referring to 'speed' in many senses, but there is '''theoretically''' nothing to prevent something like an alcubierre drive, which would apparently go faster then c. The actual requirements for such a device are quite unrealistic though.
