If light travels in a straight line , why can't we use it as absolute frame of reference? This thought experiment is done in the absence of external forces and gravity. One person standing inside a box ,sends a photon parallel to the bottom of the box from one end to another end , whether this photon will hit the point at the other side of the box which is at the same height as the point from which the photon is sent or it will hit another point ? ,if this is the case , then by calculating the distanced between the two points and dividing the time it took the photon to hit the other end can we able to calculate the absolute speed of the box ? i know i am wrong some where, but can you help me out ?