Speed of light relative to what

Nope. Velocities aren't actually simply additive, as our intuition tells us they should be. You have to use the relativistic velocity addition formula. In the case you mentioned, you would observe the galaxy as receding from you at:


(v1 + v2)/[1 + v1 v2/c^2]
= (.5c +.6c)/[1 + (.5c x .6c)/c^2] = 1.1c/(1 + .5 x .6) = 1.1/1.3 = .846c (where c is the speed of light in vacuum)

If you're interested, there are a lot of treatments of special relativity for the layman.

Interestingly, an observer on the Earth would see the galaxy as going one way at .5c and you as going the other way at .6c, so by definition, he would say that you and the galaxy are separating at 1.1c, but no observer, including you, will ever observe any single material object as travelling at c, the speed of light in vacuum, or higher.

mike1886

As a layman myself I can recommend Brian Greene's book "the Fabric of the Cosmos" if you want to get your head round relativity. Your question "relative to what?" is the fundamental issue explained by Greene as a continuing dialogue between Newton, Einstein and later thinkers.

Brian Greene's book is great. But in case you don't want to read it, i'll save you the trouble. Turns out Einstein discovered that the sped of light is constant from ANY reference point. That is to say, if you chase after a beam of light at say .5c you would expect to see it moving away from you at only .5c since c - .5c = .5c However, this is not the case. The beam would still be moving way at 1c

... which is due to distortions of time and space related to your own velocity, resulting in localized measurement of c as a constant.