Bryan Moss, <email@example.com>, writes:
> Eliezer S. Yudkowsky wrote:
> > Actually, from what I've read, Einstein came up with the General Theory
> > of Relativity completely out of the blue, based on no experimental
> > evidence whatsoever. I find this to be the single most impressive
> > mental feat of all time.
> Previous to Relativity there had been experiments to show that the speed of
> light could be absolute and that time dilation could be necessary to explain
> some inconsistencies in electromagnetism. In fact Relativity was Einstein's
> answer to the apparent time dilation necessary in electromagnetism (there
> had been papers on the concept of relativity but apparently Einstein hadn't
> read them). Later Minkowski created the concept of spacetime. Einstein
> came up with the idea of spacetine curvature while investigating tidal
> gravity. Not exactly 'out of the blue' but still very impressive.
I think what Eliezer means is that there was no experimental evidence showing any problems with Newtonian gravitation. However it is clear that Newtonian gravity is incompatible with special relativity, since it requires instantaneous action at a distance. If you try to patch it up with "lagged gravity" (where you are attracted to where the object was at the speed-of-light time in the past) you get unstable orbits. It was an attempt to come up with a theory of gravity that is consistent with special relativity that led, after enormous difficulty, to general relativity. This then turned out to make some predictions that were slightly different from Newtonian gravity, and to everyone's amazement they were confirmed, turning Einstein into a superstar.