# Theory of relativity/Talk

I think the reason for the word "relativity" should be explained on this page. What is relative, and why?

appears to be done.

Regarding a recent change to this article--I'm no physicist, but I always thought that relativity states that mass increases as velocity approaches the speed of light. Am I wrong on this?

Rest mass is a constant, but mass itself indeed increases in such a way that one can never accelerate an object beyond c.

I'd like very much to hear about the twins paradox. Never been much comfortable with that one.

The basic non-symmetry is that one twin must accelerate to return to the other to compare ages face-to-face. Whilst they are in inertial frames, it holds. Dave McKee

It's a convention, not an absolute. In the way that mass is used currently in physics, it's an invariant between reference frames, i.e. it doesn't change with velocity. Mass = "rest mass", and "relativistic mass" is not used. There is an alternate formulation of relativity that uses the concept of "relativistic mass" because using it lets you keep using some familiar Newtonian mechanics (e.g. F=mr a). But the invariant mass approach turns out to be somewhat easier to generalise into GR, so that's what basically everyone uses now. You'll still find "relativistic mass" in some textbooks (e.g., Feynman's Lectures on Physics) and in a lot of popularisations (I think it's in "A Brief History of Time"), but not in, say, graduate level textbooks and research papers.

For more on this, see [ http://www2.corepower.com:8080/~relfaq/mass.html]. Come to think of it, this should probably be written up in mass. -- DrBob

I remember reading once upon a time in some primary school-level book on relativity that if you approached a black hole you might never experience enterring it due to time dilation -- as you approach it your velocity approaches c, but time dilation reduces subjective time to the point you never enter it. Is this true, or is this just some mangled garbage? -- SJK

That happens according to a frame of reference far from the black hole. in your frame, you are swallowed in a finite time. (from what i remember)--AN