1. The problem statement, all variables and given/known data In a given inertial frame S, two particles are shot out from a point in orthogonal directions with equal speeds v. At what rate does the distance between the particles increase in S? 2. Relevant equations 3. The attempt at a solution Ok so i want to write the trajectories of the two particles in terms of 4 displacements. Then find the difference. Then find the rate of change So X1 = (ct, x1, y1, z1) X2 = (ct, x2, y2, z2) The difference is (0,x1 - x2, y1 - y2, z1-z2) The rate of chance is (0, dx1/dt - dx2/dt, ...) But how do i simplify this using the fact they're orthogonal..? Should i not solve in such a general way. better to pick two 4-displacement vectors obviously orthogonal. How would i do that? I see that something like (ct, 0, 0, 0) is orthogonal to (0, vt, 0, 0) But these aren't the trajectories of two moving bodies in the inertial frame. Actually, the second one doesn't even have a time entry. Is this allowed?