# Quick overview of relativity

1. Sep 28, 2004

### TheShapeOfTime

Can anyone give me a quick overview of relativity, general relativity and special relativity? What does general and special relativity prove?

2. Sep 28, 2004

### Staff: Mentor

SR has two postulates:
-The laws of the universe are the same regardless of your intertial frame of reference.
-The speed of light is constant.

GR is Einstein's gravity, modeling gravity as a curvature of space-time.

The implications of both are pretty broad.

3. Sep 29, 2004

### pervect

Staff Emeritus
Special relativity can be understood as the invariance of the Lorentz interval for all observers. Special relativity suggests that as the only quantity that is invariant for all observers, the Lorentz interval deserves further study. In special relativity, distance/space, is not invariant, differently moving observers measure objects as having different lengths. Time is also not an invariant quantity, clocks moving along different paths will not agree when they meet up. Only the Lorentz interval is invariant.

General relativity probes the geometry of the Lorentz interval, and finds that it is not Euclidian. Space-time is a non-Euclidian manifold, it can be regarded as being "curved", much as the surface of the Earth.

4. Sep 29, 2004

### chroot

Staff Emeritus
And I can't help mentioning that neither theory proves anything -- they are models which can use to predict what an experiment will do. If a model accurately explains all known experiments (SR & GR do), then a model is considered valid. It is always possible, however, that there will one day be an experiment which is not adequately explained by the theories, and the theories will be shown to be wrong in at least some situations.

We already know GR must be "wrong" in the subatomic domain, because its results are incompatible with those of quantum mechanics. Theories like string theory and loop quantum gravity aim to merge quantum mechanics and general relativity into a single theory with greater predictive power than either theory alone.

- Warren

5. Sep 29, 2004

Staff Emeritus
Or we counld say we already know quantum theory is incorrect because its results are incompatible with GR!

6. Sep 29, 2004

### TheShapeOfTime

Thanks for all your relies! I think I'm beginning to understand it better now.

7. Sep 29, 2004

### blue_sky

Great. When you will start to think that you dont understand it anymore that means you have started to understand a bit of GR.

blue