We calculate an object's momentum by multiplying its mass by how fast (and in what direction) it moves. Momentum cannot be created or destroyed, whether for a single object or a system of objects that interact with one another, unless another force comes into play. We can apply this tenet equally well to understanding the physics of billiards, collisions between asteroids, or the motions of a hundred billion stars in our Milky Way. Without force the total momentum of any system never goes up or down. Once begun, motion continues forever. This is the reason constant motion is the natural state of anything in the universe. This is the reason everything moves.

But is everything in the universe really moving? The newspaper on your coffee table, the tree in your yard, or the building you live in--don't they all stand still around you as you sit reading this book? In fact, they are still--to you. It's all relative.

Galileo and Newton both realized that the measurement of motion depends completely on your frame of reference. Suppose you see a unicyclist ride past at a certain speed. To him you move backward at that same speed, even though you think you're standing still on the sidewalk. If he is juggling at the same time, he sees the balls bob straight up and down from his hands. However, you see the balls move along forward arcs in space as he pedals past. Both viewpoints, or "frames of reference," are equally valid.

Similarly, let's say two cars approach each other on the road, each moving 50 miles per hour. If you stand on the sidewalk, in Earth's frame of reference, you see each car doing 50. But the driver of each vehicle sees the other car zoom toward him at 100 miles per hour. Velocities simply add together in the world of classical relativity as elucidated by Galileo and Newton. That's all well and good unless the cars somehow accelerate to millions of miles per hour. Then, this kind of relativity would fail. When we deal with the realm of superhigh speed, relativity takes on a special form.

In the late nineteenth century, Newton's laws of motion began to break down for objects that move very fast. The American physicists Albert Michelson and Edward Morley tried to add the speed of Earth's revolution around the Sun to the speed of a light beam using a sensitive light-measuring device called an interferometer. They were searching for signs of the "ether," an invisible and unmoving substance believed by physicists of the day to pervade the universe and carry waves of light. To their great surprise, the combined speed of Earth's motion and the ray of light was always exactly the same as that of light alone. Light did not seem to follow the known rules of Newtonian motion.

This puzzle lasted for nearly two decades. Then, in 1905 a theory that explained the startling result arose from the mind of 26-year-old Albert Einstein, a German physicist who worked by day as a patent officer in Switzerland. His mathematical treatise, innocently titled "On the Electrodynamics of Moving Bodies," presented a revolutionary idea that would become known as the special theory of relativity. Einstein asserted that the speed of light--186,282 miles per second--remains constant and can never be exceeded. Further, he said, speed is independent of how quickly an observer might move. Passengers on a spaceship traveling at 186,281 miles per second would still measure their headlight beams streaming away at the full speed of light. Observers on the ground would see the beams moving at exactly the same speed.