Ask a Physicist

What will physicists do if there is still no sign of gravity waves when advanced LIGO is operational? Will they abandon the search or modify the theory?
Submitted by Baharudin from Malaysia

If advanced LIGO detects nothing after a year or two, that will be even more revolutionary than if it does. But I wouldn't bet on it.

One reason is that we have a lot of indirect evidence for gravitational waves. I talked a little about this here, but let's have some more detail:

The most famous example is the Hulse-Taylor pulsar. It's a binary - two neutron stars orbiting each other. We see a periodic radio signal coming from one of them because there's a beam which sweeps across the Earth like a searchlight as the star rotates. That signal is like a GPS clock that lets us track the motions of the stars, and it tells us that they are spiraling in towards each other - the orbit is shrinking very slowly. Gravitational wave emission should shrink the orbit at precisely the rate that's observed, so it's a stunning confirmation of Einstein's theory. That's why Hulse and Taylor got a Nobel Prize in 1993.

Since then, we've found a lot more binaries like this with better radio searches. LIGO can't detect these particular binaries - it can only detect the last few minutes of inspiral before merger - but we know there are more that we haven't seen in radio. For example, many pulsar beams aren't pointed toward us. But the gravitational waves come out in all directions, so these count for estimating how many inspirals LIGO will detect per year. Putting together everything we infer from observations (like the rate of supernova explosions which turn normal stars to neutron stars) and calculations from astrophysics (like how often a supernova breaks up a binary), we get an estimate of a few mergers per hundred thousand years per galaxy. Since there are a million galaxies within range of advanced LIGO, we expect dozens of mergers per year. There are some uncertainties in this number, but it's very hard to push the rate below one per year.

So if advanced LIGO doesn't find a merger after a year, we wouldn't decide to change relativity theory, but we would start taking a hard look at our understanding of the stellar life cycle. And LIGO would keep going, because even without a signal you can set upper limits - a topic I'll come back to in part 2.