Relativity can be described using space-time diagrams . ... Thus two observers moving with respect to each other can still agree on the time, and thus the distance between two objects, which is the difference in their positions measured at equal times, can be defined. ... If it did, then the rate of a "bouncing photon clock" moving with respect to the ether . ... For (much) more online information about gravity and space-time, see John Baez's General Relativity Tutorial or the Relativity FAQ . ...