What LIGO didn’t see is historic – Hobart and William Smith Colleges \
The HWS Update

What LIGO didn’t see is historic

The LIGO project, funded by the National Science Foundation and based at the California Institute of Technology in Pasadena, recently made news for what wasn't seen.

An analysis by the international LIGO (Laser Interferometer Gravitational-Wave Observatory) Scientific Collaboration has excluded a previously leading explanation for the origin of an intense gamma ray burst that occurred last winter. Gamma ray bursts are among the most violent and energetic events in the universe, and scientists have only recently begun to understand their origins.

Scientists at Cal Tech and Massachusetts Institute of Technology are working to detect cosmic gravitational waves and develop observations of them as an astronomical tool. Research is carried out by the LIGO Scientific Collaboration, a group of 580 scientists at universities around the United States and in 11 foreign countries.

At Hobart and William Smith Colleges, the LIGO research effort is led by Assistant Professor of Physics Steven Penn, who investigates thermal noise in the LIGO test mass mirrors. The advances in mirror design developed by Penn and others will allow Advanced LIGO to detect fluctuations in the 4 km detector as small as 10-19 m, or one ten-thousandth the diameter of a proton.

The LIGO Scientific Collaboration interferometer network includes the GEO600 interferometer in Hannover, Germany, designed and operated by scientists from the Max Planck Institute for Gravitational Physics and partners in the United Kingdom.

Each of the L-shaped LIGO interferometers (two in Hanford and one in Livingston, La.) uses a laser split into two beams that travel back and forth down long arms in evacuated tubes. These beams monitor the distance between precisely figured mirrors, and according to Einstein's 1916 theory of general relativity, the space between the mirrors changes very slightly when a gravitational wave — a distortion in space-time produced by massive accelerating objects that propagates outward through the universe — passes by.

The interferometer is constructed so it can detect a change of less than one-thousandth the diameter of an atomic nucleus in the lengths of the arms relative to each other.

On Feb. 1, 2007, the Konus-Wind, INTEGRAL, MESSENGER, and SWIFT gamma ray satellites measured a short, intense outburst of energetic gamma rays originating in the direction of M31, the Andromeda galaxy, about 2.5 million light years away. The majority of such gamma ray bursts are thought to emanate from the merger of two massive, compact objects, such as neutron stars or black hole systems.

During this particular blast of gamma rays, known as GRB070201, the interferometers at Hanford were in collecting data and did not measure any gravitational waves in the aftermath of the burst.

That non-detection was itself significant.

Such a monumental cosmic event occurring in a nearby galaxy (such as the Andromeda galaxy) should have generated gravitational waves that would be easily measured by LIGO's ultra-sensitive detectors. The absence of a gravitational wave signal meant GRB070201 could not have originated in this way in Andromeda. Other causes are now the most likely contenders.

LIGO's contribution to the study of GRB070201 marks a milestone for the project, says Caltech's Jay Marx, LIGO's executive director. “LIGO is now producing significant scientific results. The non-detection of a signal from GRB070201 [is] an important step towards a very productive synergy between gravitational wave and other astronomical communities that will contribute to our understanding of the most energetic events in the cosmos.”

David Reitze, professor of physics at the University of Florida and spokesman for the LIGO Collaboration, said “This is the first time that the field of gravitational wave physics has made a significant contribution to the gamma ray astronomical community, by searching for GRBs in a way that electromagnetic observations cannot.”

Up until now, Reitze says, astronomers studying GRBs relied solely on data from telescopes conducting visible, infrared, radio, x-ray, and gamma ray observations. Gravitational waves offer a new window into the nature of these events.

“We are still baffled by short GRBs. The LIGO observation gives a tantalizing hint that some short GRBs are caused by soft gamma repeaters. It is an important step forward,” says Neil Gehrels, the lead scientist of the SWIFT mission at NASA's Goddard Space Flight Center.

The next major construction milestone for LIGO will be the beginning of the Advanced LIGO Project, which is expected to start in 2008. When completed Advanced LIGO will be 10 times more sensitive than the existing project. The increased sensitivity will allow scientists to detect cataclysmic events such as black-hole and neutron-star collisions at significantly greater distances.

Penn, a member of the HWS faculty since 2002, holds a bachelor's degree and his Ph.D. from Massachusetts Institute of Technology. He has received grants from NSF to fund the development of advanced instrumentation and detector characterization for the LIGO observatories that measure miniscule distortions in space-time.