APS News

Einstein@Home Scours the Heavens for Gravity Waves

By Richard Harth

An APS-inspired distributed computing program, Einstein@Home, is currently enhancing the search for gravitational radiation with over 70 teraflops of computational power, well outpacing other available computing resources.

Einstein@Home was begun during the World Year of Physics in 2005, when, in commemoration of the 100th anniversary of Einstein’s annus mirabilis, the APS instigated a number of outreach efforts, drawing on the public’s fascination with all things Einstein, including his revolutionary ideas about gravity.

James Riordon, head of media relations for APS, first presented the concept of Einstein@Home to Peter Saulson of Syracuse University, who was then the spokesperson for the Laser Interferometer Gravitational Wave Observatory (LIGO). Following years of strenuous construction, calibration, obsessive tweaking and design innovation, LIGO’s three massive interferometers, (located in Hanford, Washington and Livingston, Louisiana), are now taking data in search of gravitational waves.

As Saulson explains: “Einstein@Home came into being because of the happy confluence of a need and a desire.” The need on the part of the LIGO team was for vast computing power, particularly to aid the search for continuous wave sources not yet detected in radio, x-ray or other emission. The desire was to encourage direct public involvement in challenging frontiers of science.

Einstein@Home gathered momentum when Bruce Allen of the University of Wisconsin-Milwaukee took charge of the project, assembling a multinational team to approach the formidable challenges of code-writing and hardware assembly.

A celestial screensaver on which constellations, known pulsars and supernova remnants appear, activates when a member’s computer is idle. The relative positions of the earthbound Hanford and Livingston observatories and the Geo600 detector (in Hanover, Germany) can be seen, as well as cross-hairs marking the area of sky being searched.

Einstein@Home analyzes data from successive LIGO scientific runs, comparing the interferometers’ data with anticipated gravitational waveforms from a neutron star at each sky location. The results are forwarded to LIGO’s servers and another chunk of data is then downloaded by the screensaver for analysis.

Public enthusiasm for the project has been strong, with some 75,000 current users in 206 countries, donating their idle computer time for the search.

LIGO’s exquisitely sensitive search for gravitational waves is conducted by looking for changes in the path lengths of laser light traveling down the 2 or 4 km interferometer arms. Differences in strain amounting to less than one thousandth the diameter of a proton can be measured as a passing gravitational wave alternately squeezes and stretches the weave of space-time.

LIGO’s pursuit of gravity waves has focused on four primary sources: inspiraling binary systems (of either black holes or neutron stars), stochastic background emissions (from primordial events, including the Big Bang), various “burst” sources, (including gamma ray bursts), and continuous wave sources, specifically, rapidly spinning neutron stars known as pulsars. It is these pulsars—city-sized objects with staggering densities (equal to hundreds of millions of tons per cubic inch), which Einstein@Home is designed to stalk.

Unlike binary coalescences, stochastic noise or burst signals, pulsars are continuous sources which emit gravity waves at twice the star’s rotational frequency. The amplitude of gravitational emission depends critically on the star’s degree of asymmetry, making highly spherical pulsars too faint for detection.

Prime candidates for Einstein@Home are nearby pulsars with significant ellipticity. Such sources should emit detectable gravity radiation which propagates as pure sine waves. Their detection however, entails a complication. The spinning earth and its orbital motion modify the expected sinusoidal gravity wave, due to the Doppler shift. This modification is specific for each location in the sky.

Saulson notes that the resulting frequency and amplitude modulation of the gravity wave presents a two-edged sword for LIGO investigators: “If you solve the problem of how to look for this, it has a wonderful benefit. Once we find something, we’ll be able to know exactly where it is. But in order to get to that point, we have to look not just at every possible sine wave frequency, but at a huge number of possible combinations of amplitude and frequency modulation.” 

Bruce Allen points out that recent revisions in the search algorithm have improved LIGO’s sensitivity by a factor of 5, thereby increasing the number of anticipated sources by a factor of 125. LIGO’s ever-increasing sensitivity has put science within striking range of a first detection. Many express optimism that an enhanced version of LIGO, (slated to begin operation in 2009), may pick out the first delicate signals, almost a decade ahead of a comprehensive overhaul.

By around 2016, advanced LIGO’s dramatically expanded powers of perception promise to revolutionize the field, capturing gravitational waves from a plethora of sources. “There’s lots of reasons to think that we’ll see some signals that will have electromagnetic counterparts and others where we will be on our own,” says Saulson who describes Einstein@Home as “by far the best way we have found to involve the general public in the excitement of our search.”

Interested participants can download the program and join the hunt at Einstein@Home.

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Editor: Alan Chodos
Contributing Editor: Jennifer Ouellette
Staff Writer: Ernie Tretkoff