Ask a Physicist

In order to detect gravitational waves from a pulsar, wouldn't that pulsar have to precess about its axis since symmetrical configurations would not produce gravity waves? In other words, doesn't the source have to have a quadrupole moment, where the strongest sources would have a quadrupole moment Q~M x L^2, where M is the mass of the source and L is its size?
Submitted by Robert from Sunnyvale, California

The original e-mail had two questions, so I'll answer one now and the other later.

Precessing stars do radiate gravitational waves, but they're not the only ones. Any asymmetry about the rotation axis will do.

For those who haven't heard the word, "precession" is when an object with a nonspherical shape wobbles around its rotation axis. Think of passing an American football: When thrown well, it spins steadily around its long axis; when thrown badly, the spin axis doesn't quite coincide with the long axis and it wobbles around. Stars spin around or nearly around a short axis rather than a long axis, but they can wobble just the same.

Precession probably won't be detected by LIGO, though. It's pretty rare in known neutron stars - basically they need to be hit by something and knocked off-axis, which doesn't happen often in space, and they're precessing at frequencies too low for LIGO. High-frequency ones can get hit too, but interactions with the solid crust of the neutron star stop the resulting precession much more quickly than for low-frequency ones.

Since neutron stars have solid crusts and are born in violent supernova explosions, they probably will have mountains. Like precession, mountains produce some quadrupole asymmetry, so they will radiate. The quadrupoles are expected to be a lot less than the mass times the radius squared, but it turns out that the biggest mountains still could be detected by LIGO. (Partly this is because there's no restriction on frequency.)

Fluid waves (below the crust) can also radiate, and some kinds called "r-modes" might even feed on their own radiation. This is more speculative than mountains (it's a story in itself), but if we detect it we verify one of the stranger predictions of relativity and learn more about the properties of exotic matter deep down in neutron stars.