 A more common variation of this thought experiment is to send back the signal to the sender (a similar one was given by David Bohm).Suppose Alice (A) is on a spacecraft moving away from the Earth in the positive x-direction with a speed formula_6, and she wants to communicate with Bob (B) back home.Assume both of them have a device that is capable of transmitting and receiving faster-than-light signals at a speed of formula_3formula_8 with formula_9.