The Grand Design

by Stephen Hawking

Einstein was twenty-six in 1905 when he published his paper “Zur Elektrodynamik bewegter Körper” (“On the Electrodynamics of Moving Bodies”). In it he made the simple assumption that the laws of physics and in particular the speed of light should appear to be the same to all uniformly moving observers. This idea, it turns out, demands a revolution in our concept of space and time. To see why, imagine two events that take place at the same spot but at different times, in a jet aircraft. To an observer on the jet there will be zero distance between those two events. But to a second observer on the ground the events will be separated by the distance the jet has traveled in the time between the events. This shows that two observers who are moving relative to each other will not agree on the distance between two events. Now suppose the two observers observe a pulse of light traveling from the tail of the aircraft to its nose. Just as in the above example, they will not agree on the distance the light has traveled from its emission at the plane’s tail to its reception at the nose. Since speed is distance traveled divided by the time taken, this means that if they agree on the speed at which the pulse travels—the speed of light—they will not agree on the time interval between the emission and the reception.