To test this prediction, British astrophysicist Arthur Eddington (1882–1944) organized a major scientific expedition in 1919 to observe the sky during a solar eclipse. With the Moon shading the Sun’s bright light, astronomers measured the relative positions of distant stars near the Sun’s position at that time. Then they compared them to those positions measured at night, when the Sun was not in the field of view. The apparent positions were indeed different, and the discrepancies were consistent with the results predicted by Einstein’s theory. This observational confirmation of the general theory of relativity changed the field of physics forever. The discovery made news headlines, and Albert Einstein became an international celebrity.

According to the special theory of relativity, the speed of a beam of light is the same no matter how the light source is moving. It doesn’t matter who observes the light beam, or how the observers are moving. This means that the speed of light is the fastest speed at which anything can travel in the universe.

Furthermore, if the speed of a light beam is constant, that means that other properties of motion must change. Since speed is defined as the distance traveled divided by the elapsed time, this means that the distances and times experienced by any object will change depending on how fast it is moving. When you are moving, you experience time more slowly than someone else who is standing still.

Finally, since mass can be thought of as the amount of resistance that an object has to motion, any object that has mass cannot travel at the speed of light. Only energy in the form of electromagnetic radiation—that is, light—can travel at that speed; and matter is not energy, but can be converted into energy. This is represented by the famous equation E = mc².