Cosmic explosion, but no gravitational waves
Jan 16, 2008 24 comments
Physicists searching for gravitational waves with the LIGO detector in the US have released their first major scientific result. Oddly enough, however, it stems from having detected no gravitational waves at all.
Instead of heralding the much-anticipated first direct detection of these tiny ripples in space–time, the team announced that gravitational waves did not appear to emanate from the source of a gamma-ray burst detected last year. The LIGO team has used this apparent absence of gravitational waves to gain further insight into the origins of the dramatic astrophysical events that produce intense bursts of gamma rays.
“I wish that the first major announcement were a detection of gravitational waves, but this is not the primary goal of our field,” Kip Thorne of Caltech told physicsworld.com. Thorne, who is a long-time member of the LIGO team, also said: "As I see it, that goal is to open up the gravitational wave window onto the universe so that we can explore poorly understood processes. The LIGO non-observation is in that spirit."
Disturbances in space–time
Gravitational waves are predicted by Einstein’s general theory of relativity, in which gravity arises from the curvature of space–time. The waves are oscillations of space–time that are produced when a mass accelerates. However, despite strong indirect evidence for their existence — in particular from measurements of the rate at which neutron stars in binary systems lose energy and spiral towards one other (a result that earned Russell Hulse and Joe Taylor the 1993 Nobel Prize for Physics) — there is no direct proof. This is partly because their amplitude is so small, with even the most violent astrophysical events disturbing space–time by less than one part in 1022.
LIGO (the Laser Interferometer Gravitational-wave Observatory) is the largest of several facilities designed to detect such disturbances. It comprises two giant interferometers, one located at Hanford, Washington state, and the other at Livingston in Louisiana. By bouncing a laser off mirrors located at the ends of two 4 km-long arms at right angles to one another, any changes in the relative lengths of the arms caused by the passage of a gravity wave would produce a characteristic interference pattern.
Crucially, LIGO's Hanford interferometer was in "science mode" on 1 February last year, when several space telescopes registered a short burst of gamma rays in the direction of the nearby Andromeda galaxy.
First glimpsed 40 years ago, gamma-ray bursts (GRBs) are among the most energetic and mysterious events in the universe. They come in two broad types: "long", lasting between 2 s and a few minutes; and "short", lasting from a few milliseconds to 2 s. In 2003 researchers successfully traced the former to supernovae, but astrophysicists are only beginning to understand the origins of short GRBs.
Colliding black holes
The leading candidate for the majority of short GRBs is the merger of two ultra-dense objects such as neutron stars or black holes — events that should also produce a burst of gravitational waves. However, at a conference on GRBs held in Santa Fe last November, the LIGO team announced that its interferometers had detected no such signature at the time when "GRB070201" went off.
Either the source was not a coalescing binary or there is some exotic situation where the gravitational waves disappear into another dimension Jim Hough, Glasgow University
"We know that coalescing binary have to produce gravitational waves," says Jim Hough of Glasgow University , who is principle investigator for the UK of the GEO600 gravitational wave detector based in Hannover, Germany. "Therefore, either the source was not a coalescing binary or there is some exotic situation where the gravitational waves disappear into another dimension. The latter seems unlikely, but would be very exciting of course!"
Other causes for the event, such as a "soft gamma ray repeater" (SGR) or a binary merger from much further away, are now the most likely contenders. However, Stan Woosley of the University of California at Santa Cruz — who was one of the first to link long-lived GRBs with supernovae — points out that the merger of neutron stars is excluded only to the 90% level, which is not as tight as astrophysicists would like. "If the event was indeed in Andromeda, it was likely a SGR. The likelihood of two neutron stars merging in this nearby galaxy while we happen to be watching is perhaps one in a million years," he says. "However, the result is a technological tour de force which illustrates the potential of coordinated gravity wave and gamma-ray observations."
The result has recently been accepted for publication in the Astrophysical Journal.
About the author
Matthew Chalmers is a science journalist based in the UK