Relativity of simultaneity

Relativity of simultaneity means that events that are considered to be simultaneous in one reference frame are not simultaneous in another reference frame moving with respect to the first. For example imagine there were volcanoes located on Mars and Venus. We on earth may see these two volcanoes on the different planets erupt and conclude (after appropriate corrections for light travel) that the eruptions were simultaneous; we could calculate the Greenwich Mean Time at which the eruptions occurred. The inhabitants of a distant galaxy, travelling away from ours at a great speed, may one day observe these same eruptions and find that they were not simultaneous, that one occurred before the other. The concept of “simultaneous” or “simultaneity” is not an absolute, but a relative property – it depends on one’s frame of reference.