Albert Einstein first published his theory of special relativity in 1905; his work on general relativity followed in 1915. By 1923, Premier Productions made this silent film to explain the most salient bits. Although it's entirely silent (and of course quite dated), it's a thoroughly lucid way to understand Einstein's most important work.
My favorite stunt is around 1:45 when the filmmakers strap two pistols to a wheel, set it spinning madly, then fire the pistols simultaneously. What could possibly go wrong? Incidentally, the filmmakers used animation rather than real slow-motion photography to show the relative speeds of light and bullets in this bit. Again, 1923. The other excellent part concerns the relativity test conducted during the eclipse of 1919. Enjoy!