You may have learned about the theory of relativity—or at least seen E = mc2 scribbled on a chalkboard somewhere—but nothing compares to hearing Albert Einstein explain it in his own words. In this historic footage, you can watch the theoretical physicist describe the concept that made him famous. First published in 1905, Einstein's theory of special relativity had an enormous impact on how we study the universe. In simplified terms, special relativity states that mass and energy are the same thing in different forms, and one can be converted to the other—or vice versa—under the right circumstances. When Einstein died in 1955, he left behind a legacy that continues to shape the world of science world.