The story of Einstein@Home [1] centers on the search for gravitational waves from ultra-dense, rapidly-spinning stars, which we will refer to as pulsars. Pulsars form when normal stars collapse under their own gravity. They contain the Sun's mass in a sphere only 10 km in radius whose area is smaller than an average city, corresponding to more than ten trillion times the density of lead 2.1. They spin at frequencies of hundreds of times per second, up to 38500 RPM, or several times as fast as a turbo-charged race car engine.
According to Einstein's General Theory of Relativity [2,3], a pulsar that is not perfectly round acts as a gravitational-wave generator, stirring up ripples in the fabric of space-time. These ripples, termed gravitational waves, move outward from the star in all directions at the speed of light, eventually reaching the Earth. A handful of international projects seek to measure their passage.
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Two of these projects, one in the United States (LIGO), and a British and German collaboration (GEO600), are operating several detectors under the auspices of the LIGO Scientific Collaboration (LSC) [5,6,7,8]. The detectors are designed to capture gravitational waves from one of the four principle types of sources: the inspiral of binary black holes or neutron stars, supernova explosions, an overall (stochastic) background, and pulsars. Einstein@Home is designed to search for waves from this last type of source: pulsars. Of course, these projects and Einstein@Home could also discover something new and unexpected!
LIGO and GEO600 use detectors, known as interferometers, that act as huge surveying devices. Rather than marking distances along the Earth's surface, as do conventional surveying tools, the interferometers mark space and time using laser light, which travels along their kilometer-scale perpendicular arms. Within the instruments a semi-transparent mirror splits a laser beam in half. Each half races along one arm of the interferometer. An interference pattern, which is a series of light and dark bands called fringes, forms at the junction of the arms where the beams recombine following their reflection from mirrors at the ends of the paths. Gravitational waves will alternately stretch and shrink the space along the arms by tiny amounts, carrying the suspended mirrors along for the ride, and causing the round-trip light travel time down the arms to oscillate at the frequency of the wave. This will imprint corresponding tiny oscillations in the position of the fringes that can be measured as small changes in the brightness of the light at a fixed location within the interference pattern (see the figure below).
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Between 2002 and 2005, during breaks in instrument commissioning, several runs of data taking, called Science Runs, were completed. At the current time (September 2005) the LIGO instruments are the most sensitive gravitational-wave detectors on the planet, so Einstein@Home is concentrating on analyzing their data. Commissioning will finish in 2005 and 2006, and a long observational run will begin. However, the numbers of sources and their distances from us are uncertain, and in their first few years of operation it is quite possible that the LIGO and GEO instruments may not detect anything. But that will not be the end of the story! Research and development work is underway to upgrade the instruments early in the next decade. After the upgrade, the instruments will be sufficiently sensitive that failure to detect gravitational waves from some of these sources is so unlikely that it could challenge Einstein's prediction of the existence of gravitational waves!
Einstein@Home participants worldwide run computer programs that draw faint gravitational signals out of the background noise that also imprints on the interference fringes. Some of the background noise is caused by vibrations of the atoms in the mirrors and effects from the quantum nature of light. The electronic measurement of the light at the normal location of one fringe can also be played through a speaker; random noise creates a nondescript "rumble and hiss" from which the ripples of space-time must be extracted. This file (.au format 126kB) plays a sample of data from the LIGO detector [9]. A similar sound file for the GEO600 detector is also available from here (.mp3 format 187kB)[10]. Einstein@Home compares the predicted gravitational wave pattern that would be produced by a pulsar to ten-hour-long segments of interferometer data. Episodes of strong agreement between the predicted pattern and the data represent gravitational wave candidates which then face further scrutiny. The computing requirements to identify pulsar candidates are enormous, requiring billions of calculations that must be performed for large numbers of distinct sky positions. The capacity to perform this search on months of data far outstrips the considerable computing resources of the LSC. Einstein@Home functions as a massive worldwide supercomputer to provide this capacity. In late summer 2005 Einstein@Home participants finished analyzing data from the S3 Science Run, which started at the end of 2003 and finished at the beginning of 2004. They are now starting work on the more sensitive (less noisy!) data from the 2005 S4 Science Run.
LIGO and GEO greatly appreciate the help of the tens of thousands of Einstein@Home participants who are now running the software. Our hope is to expand the program to a much wider community over the coming years.
To go beyond this brief overview, please read the rest of the pages provided here. These provide a more detailed description of Einstein@Home, the pulsar analysis program, and some of what we have learned so far. And please keep using Einstein@Home!
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Einstein@Home S3 Analysis Summary |
Last Revised: 2005.09.11 16:22:17 UTC |
Copyright © 2005 Bruce Allen for the LIGO Scientific Collaboration
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Document version: 1.97 |