With One Data Set Analyzed, Einstein@Home Forges Ahead in Search for Gravitational Waves

By Ernie Tretkoff

The Einstein@home distributed computing effort, which searches for gravitational waves, recently finished analyzing its first set of scientific data. Over 60,000 individuals from over 150 countries are now running Einstein@home on their computers, and more people are joining every day.

Begun earlier this year as part of the World Year of Physics, Einstein@home is a distributed computing project, which relies on volunteers’ donated computing time. While a user’s computer is idle, the Einstein@home program searches for gravitational waves in data from LIGO, the Laser Interferometer Gravitational Wave Observatory, which consists of two facilities, one located in Livingston, Louisiana and the other in Hanford, Washington.

In August Einstein@home finished sifting through data from LIGO’s science run S3, which ran from late 2003 to early 2004. Einstein@home is now searching through data from the more sensitive science run S4. In late 2005, LIGO will begin a long data-taking run, S5, at its design sensitivity, and Einstein@home will analyze that data when it becomes available.

Gravitational waves, often described as a sort of “ripple in spacetime,” are predicted by Einstein’s theory of general relativity, but have not been directly detected. The LIGO observatories each consist of a pair of perpendicular laser beams. A passing gravitational wave would change the path lengths of the laser beams slightly, and the detectors would note this tiny change. Gravitational waves are expected to come from a variety of sources, including the inspiral of binary black holes, supernovas, and an overall background. Einstein@home specifically looks for signals from pulsars in LIGO data. Though the gravitational waves from pulsars would be weak, it is possible that these signals could be detected by LIGO.

Einstein@home analyzes the data by looking at each point in the sky and comparing the data from LIGO with the predicted gravitational wave pattern from a pulsar at the given point. Sorting through the data in this manner is computationally expensive, and could not be done with as much sensitivity without the help of volunteer Einstein@home users.

More users enable the project to detect weaker signals from more distant pulsars. “The distance that we can see grows with the number of computers,” explained Bruce Allen of the University of Wisconsin-Milwaukee, who leads the Einstein@Home project.

Any possible signal a computer finds needs to be verified by other computers, so any detection would be made collectively, not just by an individual. “It’s important to keep in mind that Einstein@home works as much by rejecting things that aren’t signals as by detecting signals,” says Allen, “This is really a collective effort.”

Some users have gotten very excited about the project. A few have even purchased used computers from Ebay to dedicate to the project, says Allen. Users receive credits for the work their computers have done, but the credits are just for fun and motivation.

Although Einstein@Home did not find any sources of gravitational waves in the data from the S3 science run, this is not surprising, says Allen, since that data is not sensitive enough to guarantee that Einstein@home would see any pulsars. “This depends on luck: the location, shape, orientation and spin rate of the nearest pulsars to the Earth,” states a progress report posted by Allen on the Einstein@home message boards.

“Our future analysis is going to be of data that’s substantially more sensitive,” says Allen, though it is still not certain that Einstein@home would detect gravitational waves from pulsars. In addition, says Allen, “We’re close to putting into place an improved method of analysis.”

Research and development work is underway to upgrade the instruments for Advanced LIGO, which is scheduled to be completed early next decade. At that point the instruments will be sensitive enough that failure to detect gravitational waves would challenge Einstein’s theory.

Einstein@home is available for Windows, Linux, and Mac operating systems. The program provides a screensaver that depicts the celestial sphere with the major constellations outlined. A moving marker on the screensaver indicates the portion of sky being searched. The Einstein@Home website, which includes more information and instructions on how to sign up, is located at http://einstein.phys.uwm.edu/.