I'm having trouble working this problem. I don't know where to begin. Here's the problem: Let P and Q be any events, P causing Q via propagation of some signal. Let the speed of the signal, call it U, be greater than the speed of light c. In S, the time seperationg is given by delta_t and the spatial seperation is given by delta_x, with both events occurring on the x-axis. In an intertial frame that has velocity v(less than c) along the x-axis and observe these same two events. Show that if v is chosen such that c^2/U < v < c, then in this fram causality is violated, that is the order of the events is reversed, Q occuring before P.