Expert: Courtney Seligman - 10/20/2008

Question
What direction are you traveling in a space vacuum?

Answer
If you mean, how can you tell what direction you are moving in a completely empty Universe, the answer is, you can't. All motion is "relative", meaning that you can only measure motion relative to another object; so if there aren't any other objects, you can't measure your motion, and in fact, the concept of motion has no meaning. (If you were to throw something out into the otherwise empty space, you could measure your relative motion; you would both be moving directly away from each other, but there would be no other way to define direction.)

If, however, you mean, how can you measure your motion in locally empty space, relative to the (very) distant stars, then the answer is, you would use a spectrograph to determine the Doppler shift caused by your motion relative to the stars. An average of the results would yield your speed and direction relative to the stars used to create the average. However, if they had a net motion (e.g., like the motion of most nearby stars around the center of our Galaxy), then you would not be able to measure that portion of your motion without making observations of objects, such as distant galaxies, that are not connected to the local system of stars, and do not share their common motion.

Courtney Seligman
Professor of Astronomy
Long Beach City College

P.S. Sorry for the slight delay in answering your question. It arrived after I left for work, so I didn't get a chance to pick it up until now.