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I was always under the impression that gravity was a field. How can something be both a field and a wave?
Submitted by Eamon from Australia

Waves are just a certain behavior of a field when it changes in a certain way with respect to location in space and time. Fields can do other things besides wave, like not change with time or change in non-wavy ways. Every physical field has to be able to wave as a result of the finite speed limit (the speed of light) and conservation of energy. There is a nice conceptual summary of the argument in the gravitational waves section of this web site.

Mathematically speaking, a field is a number (or set of numbers) which depends on location in space and time. In physics, the fundamental fields we are interested in yield numbers which are the answers to questions like "If I put a little charge here, how hard and in which direction would it get pushed?" (The charge has to be little so that its own field doesn't disturb the existing field which you are trying to measure.) For example, the answer to the question "How hard (per unit charge) will this little electric charge get pushed, depending where I put it and when?" is the electric field. If you change "electric charge" in the question to "mass," the answer is the gravitational field.

Actually, that last one was for Newton's theory of gravity. Einstein's relativistic theory is conceptually different because different observers may disagree on how hard the push is and which direction it goes, since they can have different ideas of motion. But mathematically you can still phrase it in terms of a field, which turns out to be related to the geometry of spacetime. That field must allow for the possibility of waves, which means it must be possible to have waves in spacetime itself. Those are the gravitational waves that Einstein@Home is trying to detect.

So what's different between waves and the other things fields can do? Obviously they "wave": Some number goes up and down with respect to time and position. What's not so obvious is that this means waves can move energy around (and information and other things) even through empty areas (no masses, charges, etc). They're how distant parts of the universe communicate with each other, which is why they're so important to astronomy.