image credit: André Karwath, Creative Commons; image source; second image courtesy of NPL
Einstein's theory of special relativity says that a moving clock, when compared to a stationary clock, runs slow. And general relativity, his theory of gravity, says that the weaker the gravitational field, the faster a clock in that field runs. These predictions were tested in 1971 by flying atomic clocks around the world. Einstein's relativity theories correctly explained what happened.
To learn about a more accurate update to this experiment 25 years later, see Time Flies. Click on the image to see a photo of the atomic clocks in the updated experiment.
Particles break light-speed limit
You may have heard the shocking news that an experiment at an Italian laboratory found the speed of a neutrino beam to be slightly greater than the speed of light. This directly violates special relativity, which predicts that no particle can move faster than the speed of light (please note that the "speed of light" referred to here is the speed of light in a vacuum). To learn more about this experimental result, see Particles break light-speed limit.
Update: It turns out that the neutrinos observed in this experiment were not going faster than light after all. As you can read in this short Science Magazine article, the error in the measurement was due to a loose cable connection.
(This feature was updated on September 21, 2013.)