1. The problem statement, all variables and given/known data Derive the result DeltaT <0 for U> (sqrt(1-v^2/c^2)+1)/v/c)c 2. Relevant equations DeltaT = u/l + (u-v/1-uv/c^2)1/l Where: DeltaT is the time for the tachyon to go and come back. u is the velocity of the tachyon l is the distance that the tachyon goes v is the velocity of the receiving end moving away from the tachyon c is the speed of light 3. The attempt at a solution I tried to substitute values for u(2c),l(100m) and v(0.5c) but it gives me DeltaT = something/0 and I'm trying to get time less than zero, not infinite time. I'm not sure that I need to put values in the equations to proof that DeltaT <0 for U> (sqrt(1-v^2/c^2)+1)/v/c)c .