He had published the "special theory" of relativity (SR) ten years before. It is SR that incorporates the principle that the speed of light is the same for all observers, and this led in turn to the intriuguing idea of time dilation. This is sometimes explained in terms of a thought experiment. Imagine you are travelling at the speed of light -- riding on a photon, so to speak. You are getting farther and farther away from a clock which, in your 'inertial framework,' is stationary. You won't see any movement of the hands of that clock because no new information from that clock can reach you -- you're going as fast as the fastest speed at which information can travel, by hypothesis. So the clock is frozen.

If there are any particles that can travel faster than protons, as scientists at CERN now seem to be saying, then Einstein's view on time dilation needs revision. For if so, then (to revert to our thought experiment) you might be able to receive information about subsequent movements of the clock's hands while riding on a photon -- if the new information is conveyed by a stream of neutrinos rather than by other photons.

Anyway: ten years later, on this date, Einstein published a follow up work, on general relativity, which explained gravity as a consequence of the curvature of space-time. This GR centers on the following equation:

**R**.

_{uv}- (1/2) g_{uv}R = (8 Pi G/c^{4}) T_{uv}I won't pretend to 'get it,' but I'm told that if R

_{uv }is taken as equaling 0 you have flat space-time, roughly the sort of thing you learned about in high school geometry. The

**T**represents the distribution of matter and energy, so the equation as a whole shows the relationship between matter-energy on the one hand and space-time on the other. A theory can't get much more "general" than that!

_{uv }
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