Albert Einstein published two papers between 1905 and 1915 theorizing about the relationships between space, time, mass, and energy. His two theories were called general relativity and special relativity. To prove Einstein's theories, physicist Joseph C. Hafele and astronomer Richard E. Keating set up a simple experiment with commercial airliners in the '70s. When the two men concluded the experiment, Einstein was proven right. The only type of clock that was suitable was an atomic clock, and Hafele and Keating took four of them on the flights.