<P> Special relativity does not prohibit this . It tells us that it is wrong to use Galilean relativity to compute the velocity of one of the particles, as would be measured by an observer traveling alongside the other particle . That is, special relativity gives the right formula for computing such relative velocity . </P> <P> It is instructive to compute the relative velocity of particles moving at v and − v in accelerator frame, which corresponds to the closing speed of 2v> c . Expressing the speeds in units of c, β = v / c: </P> <Dl> <Dd> β rel = β + β 1 + β 2 = 2 β 1 + β 2 ≤ 1 . (\ displaystyle \ beta _ (\ text (rel)) = (\ frac (\ beta + \ beta) (1 + \ beta ^ (2))) = (\ frac (2 \ beta) (1 + \ beta ^ (2))) \ leq 1 .) </Dd> </Dl> <Dd> β rel = β + β 1 + β 2 = 2 β 1 + β 2 ≤ 1 . (\ displaystyle \ beta _ (\ text (rel)) = (\ frac (\ beta + \ beta) (1 + \ beta ^ (2))) = (\ frac (2 \ beta) (1 + \ beta ^ (2))) \ leq 1 .) </Dd>

What is faster than a speed of light