Newton was the first person to make the connection between the motion of the moon around the earth and the way that normal objects fall straight down. He realized that if something was falling toward the earth, but was going fast enough, it would never hit the ground; it would just keep going around and around. His theory of gravity says that anything in the universe that has mass also has gravity and is tugging on everything else that has mass. Objects that are farther away pull on each other with less force. Right now, the Andromeda galaxy is pulling on every atom in your body, but it is so far away that you can't tell.

Newton's law of gravity works very well for everyday life, but it doesn't really explain why things are attracted to each other, and it doesn't explain why we see photons of light (which have no mass) being affected by the gravity of large galaxy clusters, or what a black hole is. That's where Einstein comes in.

Einstein realized that if you lock yourself in a box and it is sitting on the surface of a planet, you will feel the force of gravity pulling you down. Now, if you put the box in deep space and accelerate it with a rocket, you would not be able to tell the difference between the force of acceleration of the rocket and the force of gravity from the planet! Based on this thought experiment, he reasoned that gravity must be some type of acceleration.

Einstein's theory of gravity, called general relativity, describes the universe as a combination of space and time (or "spacetime") that can be pictured as sort of a stretchy rubber sheet. Everything with mass causes the sheet to curve: the more mass, the more the sheet curves. Now, if something tries to travel across the curved surface, the shortest path it can take will be a line that curves toward objects with mass. It will be as if things are attracted to objects with mass, but in reality they will just be trying to follow the shortest path on the curved sheet. Planets orbit the sun because the sun makes a large depression in the curved surface and they are going too fast to fall into the middle but not fast enough to escape the "gravity well". You and I are pulled to the surface of the earth because we are stuck at the bottom of earth's gravity well. Even light is forced to follow the shortest path possible, so it can be curved by gravity even though it doesn't have mass.

Of course, general relativity also has limitations. It works well on very large scales, but in the center of things like black holes, where gravity is very strong but distances are extremely small, we don't yet know how to combine general relativity and quantum mechanics. This is one of the goals of string theory, but it has yet to come up with any testable predictions, so nobody knows whether it is true or not. Stay tuned: Uniting general relativity and quantum mechanics is one of the "holy grails" of physics, and there are lots of scientists working on it.