Six years after publishing the Special Theory of Relativity, Einstein took a critical step toward a General Theory in his 1911 paper On The Influence Of Gravitation On The Propagation Of Light. Galileo's observation that all object's fall with the same acceleration coupled with Einstein's Principle of Relativity lead to the conclusion that light rays bend in a gravitational field and predict the possibility of black holes.
Relativity playlist: http://www.youtube.com/playlist?list=PLF56602BAC693237E
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