That opening phrase to the 1977 movie blockbuster Star Wars has become an iconic quotation that is truer than the science fiction film it opened. In astronomy, the distances involved are so vast that time and distance become irrevocably linked in what Einstein called "space-time." One of our units of astronomical distance, the "light-year," combines both time and distance in to a single unit. The observation of distant astronomical objects is, in fact, the only real form of time-travel known to us. Despite the speed of light being fantastically high (and according to Einstein's theory the universe's speed limit, ~ 300,000 m/sec), it is finite. When the distance between us and one of these objects (a galaxy in this case) is so vast that light, even at its incredibly high velocity, takes a long, long time to reach us, we are looking back into the past--in essence, traveling backward in time. Thus the Star Wars quotation "gets it right" in that a galaxy that is far, far away appears to us as it was a long time ago. The nearest major galaxy to our own, the Andromeda Galaxy, is so far away that the light we see from it has traveled 2.5 million years to get here, and we are seeing it not as it appears "today," but as it appeared 2.5 million years ago. |
