Gravity

Einstein's special theory of relativity applies only to steady, unchanging motion through space-time, not accelerating motion like an object falling toward Earth.

• It troubled Einstein that his theory didn't include gravity, and his battle to incorporate it made symmetry central to his thinking.
• Einstein later understood that gravity is the curvature of space-time itself. Falling objects follow the space-time path carved out for them.
• After general relativity was published, it appeared that energy might not be conserved in strongly curved space-time. But mathematician Emmy Noether proved that the amount of energy (including mass), the amount of electric charge, the amount of momentum, are all associated with a particular symmetry, a change that doesn't change anything.
• Noether showed that the symmetries of general relativity ensure that energy is always conserved.