# Simple Question-I hope-difference between SP and GP

1. Sep 11, 2009

### rasp

What defines the local frame of reference in SP in which the speed of light is constant C, vs the cosmological frame of reference of GP in which the speed of light can be different than C? My guess is that the locaL FRAME SHARES A COMMON GRAVITATIONAL FORCE, BUT THIS MAY BE TOO MUCH OF A SIMPLIFICATION.

2. Sep 11, 2009

### JesseM

What does the "P" in "SP" and "GP" stand for? I take it you're talking about special relativity and general relativity? In special relativity spacetime is flat everywhere, so a frame doesn't have to be local for the speed of light to be c, it just has to be "inertial", meaning that any observer at a fixed position coordinate in a given frame is not physically accelerating (they feel no G-forces). An inertial frame is usually thought of as being constructed out of a grid of rigid rulers and clocks which are all at rest with respect to one another and moving inertially. In general relativity spacetime is curved so long rulers can't really remain "rigid", but if you zoom in on a small region of spacetime curvature effects (tidal forces) can become negligible, so this is where you can have a "locally inertial" frame, in a small region of spacetime where tidal forces are not detectable. In such a region, a locally inertial frame can be thought of as being constructed out of rulers and clocks which have no non-gravitational forces acting on them, meaning they are in freefall. For more on this, please read http://www.aei.mpg.de/einsteinOnline/en/spotlights/equivalence_principle/index.html [Broken].

Last edited by a moderator: May 4, 2017
3. Sep 13, 2009

### rasp

Thanks JesseM. I appreciated the reference to Einstein online...seems to be a good level for me to get started with.