Ask a Physicist

I understand that the gravitational waves scientists believe we can detect are from extremely massive events in space such as binary systems, pulsars, and supernovae. Is Einstein@Home searching for gravitational waves from a particular one of these source types?

The LIGO Science Collaboration is implementing searches for all those sources and more, but the one Einstein@Home is running is the "all-sky pulsar search" as you will see at the top of every WU page. This is not a search for the known pulsars whose locations you see on the screensaver. This is a systematic search of the sky, one location at a time, for any periodic gravitational wave coming from that location. It has to be done for each location because the frequency shifts (Doppler shifts) due to the Earth's motion are different for different sky locations. There are additional Doppler shifts for pulsars in binaries due to the orbital motion, but the current Einstein@Home application is only looking for isolated pulsars.

Most of your CPU cycles are going into something based on Fourier transforms. A Fourier transform is a way of looking at a time series as a sum of sine waves at different frequencies. Pulsar signals should be nearly sinusoidal after the Doppler shifts are taken out, so Fourier transforms pick them up pretty easily. Fourier transforms are numerically pretty efficient, but there are an awful lot of them to do in an all-sky search. That is why Einstein@Home is doing this particular search and not for example the searches for known pulsars, which can be done pretty quickly on a single computer.

Later on Einstein@Home might do some other searches, but the consensus was that this was the best fit (at least for now) because (1) it is the most expensive in CPU cycles, and thus the best use of the enormous power you all are donating, and (2) people would probably be more excited about going after something brand spanking new than a pulsar that's been seen in radio, x-rays, etc for years. I think also that (3) we know where the radio pulsars etc are, but a previously unknown one (whose radio beam isn't pointed towards Earth) might happen to be much closer and thus a much stronger source of gravitational waves. We set our long-term goals by the sources we already know, but we cross our fingers and hope for a pleasant surprise.