Ask a Physicist

Would so-called solid quark stars that undergo "glitches" produce gravity waves due to the changing moment of inertia? Could detection of this be used to prove or rule out the existence of strange quark stars?
Submitted by Robert from Sunnyvale, California

A "glitch" is what radio astronomers call it when they see a pulsar's frequency suddenly jump up a little bit. Most of the time the frequency goes down steadily (and very slowly) because the pulsar is radiating energy and angular momentum in electromagnetic waves (radio, light, etc) and gravitational waves. Glitches can't happen to an all-liquid star, so the observations tell us that part of the star is solid. Basically, something must have cracked and the structure changed abruptly. That something could be the solid part itself, or various interactions between the solid and liquid parts.

Neutron stars should be mostly liquid with a thin solid crust on top. But some pulsars might be dense enough to be made of quarks mingling freely rather than confined into the groups of three we call neutrons. The properties of quark stars are harder to predict than those of neutron stars (which are already tricky). Quark stars are thought to be mostly gas under a thin solid crust, but it's possible they're completely solid.

After the abrupt disturbance of a glitch, the whole star should ring like a bell, and the motion of the matter will produce gravitational waves. The exact kind of ringing could reveal in principle the structure of the star, like what fraction is solid and what it's made of. Unfortunately these signals are too weak to be detected with the present generation of instruments or even the next generation. Also they don't last very long, which hurts the detectability of already weak signals.

But Einstein@Home might detect what comes before the glitch. What cracked might have been a lump in the solid part of the star. If so, it would make gravitational waves as it spins around, day after day. The long duration helps us detect that signal, although we need a lot of computing power to take advantage of it. That's why we ask you folks to join Einstein@Home. That long-lived signal might indicate if the source is a neutron star or something else, since the waves from solid quark stars could be a lot stronger than from neutron stars.