Gravitational Wave Frequently Asked Questions

Gravitational waves are an inevitable consequence of Einstein’s theory of gravity, known as General Relativity. If we believe Einstein was right, and we believe that astronomers have a reasonably accurate picture of our local universe, then detectable gravitational waves must exist. However, they have yet to be directly detected, and remain an exciting and elusive target.

1) What is a gravitational wave?

A gravitational wave is a traveling ripple in the fabric of space-time.

2) Is that as weird as it sounds?

Yes, it’s really weird. When a gravitational wave passes through someplace, the space of that place is stretched and squeezed. When space is squeezed, everything in that space is squeezed. So, for example, when a gravitational wave passes through earth, the earth is squeezed in some direction - so for a moment it looks a little less like a round ball and a little more like an oblong egg.

Another picture is by analogy with electromagnetic waves. Electromagnetic waves (such as visible light) are traveling perturbations in the electric and magnetic fields that exists everywhere around us. Gravitational waves are traveling perterbations in the gravitational field that exists everywhere around us.

3) Where do gravitational waves come from?

Gravitational waves come from dynamic, or even violent, events involving very dense objects. These are things such as supernovas, collisions of black holes, and collisions of neutron stars. Measurable gravitational waves do not come from the earth, the sun, or anything else in our solar system.

4) Wait a minute. If gravitational waves squeeze things (even the earth!) shouldn’t it be really obvious when they pass through?

Yes and no. Yes, in principle, it should be simple to tell. In practice, no, it is not easy, because the effects are very small. By “very small”, I really mean in the Douglas Adams sense of “you just won't believe how vastly, hugely mind-bogglingly” small these effects are. A current upper bound on the effect of passing gravitational waves is that a 4 kilometer long track, when squeezed by a gravitational wave, would shrink 10^-18 meters...that's less than the diameter of an atom’s nucleus!

5) The diameter of an atomic nucleus? That’s stupid. Nobody can measure something that small.

It's not stupid, but it is very hard to do. However, projects such as LIGO use instruments that can and do measure such tiny distances. It is really amazing! Using these powerful tools, the first verifiable detection of a gravitational wave could happen anytime in the next few years. That’s one reason why this work is so exciting right now!

6) If no one has detected gravitational waves yet, how do you know they are real?

There is strong evidence for the existence of gravitational waves. The most striking example is the Hulse-Taylor binary pulsar. This system has been observed by radio astronomers for over 30 years, and in that time, the orbit has changed. The change in orbit precisely matches predictions based on expected gravitational wave emission. The theory and experiment agree to better than 1%, a precision rarely seen in astrophysics. You can see a plot of the impressive agreement on page 4 of this paper.

7) Can I learn more about gravitational waves?

Yes. Lots of information is available. If you like, you can start by following some of the links available on the resources page.