Ask a Physicist

Have we had any positive detections of gravitational waves other than the detection of their effects?
Submitted by Phillip from Canada

Not yet. We just passed the thirtieth anniversary of the discovery of the first indirect evidence, two neutron stars orbiting each other whose orbit has been shrinking at precisely the rate it should due to recoil from gravitational waves. And nowadays we know of a lot more systems like that one. But we haven't directly detected the waves.

The next question is probably "When do we expect to detect something?" That depends who you ask, and the answers vary so widely that the bookies at Ladbrokes cut off betting on it. There is a small chance we'll see something in the next five years, and a very good chance after that when some upgrades are made to LIGO and the LISA spacecraft (think "LIGO in space") are launched. By that time a lack of detection would be more revolutionary than a detection!

Why the big uncertainty? Basically, the kinds of things that make gravitational waves (at frequencies LIGO can see) are things we don't know much about with other observations. Radio astronomers can tell us all sorts of things about a couple thousand neutron stars in our galaxy, and x-ray and other astronomers can tell us about a few more.

But only a handful of neutron stars are paired up with other neutron stars that are close enough to show the indirect evidence of gravitational waves. Based on our knowledge of the life cycles of stars, we expect that there are a lot more pairs that are not seen with radio or anything else. You can try to leverage that handful that are seen to estimate the population that is invisible to radio but not to LIGO, but there are huge uncertainties. It's like trying to estimate the demographics of the United States from a sample of ten Americans, and they were chosen from a bodybuilders' convention so you know it's not a representative sample.

If you try to guess how many isolated neutron stars are emitting gravitational waves (the ones Einstein@Home is searching for), it's even harder because less is known. The same goes for black holes, supernova explosions, and so on. But if you turn that around, it means when we do detect gravitational waves we'll be learning information that can't be learned any other way, which makes it much more interesting.