The Search for Gravitational Waves

The Department's involvement in the search for Gravitational Waves began over 30 years ago. In the 1970's Prof. D. Douglass constructed a "Weber" type antenna (massive aluminum cylinder with very high q-factor).  Included faculty Prof. W. Johnson and (then) graduate student M. Bocko who helped carried out the search for GW's at the then prevailing sensitivity levels.  Presently, laser interferometers have surpassed the sensitivity of bar-antennas.

The LIGO (Laser Interferometer Gravitational Observatory) laboratory is funded by the NSF and operates three large interferometers at two intentionally separated sites. One site is at the Hanford reservation near Richland WA, and the other at Livingston parish in Louisiana.

These instruments are Michelson interferometers with two orthogonal arms, each arm is 4 km long. The arms are Fabry-Perot cavities such that the effective length of the arms reaches 600 km.  An aerial view of the interferometer enclosure at the two sites are shown in the photo of the Livingston site and the Hanford site.. Since November 2005, the LIGO interferometers have been accumulating data continuously operating at the designed  sensitivity.

                                            S5 run sensitivity

The analysis of the data from the two interferometers is the responsibility of the LIGO Scientific Collaboration (LSC). Rochester has been a part of the LSC since 2003. The group is headed by Prof. A. Melissinos and includes graduate students Stefanos Giampanis, Tobin Fricke, and Chad Forrest.  A recent graduate member included William Butler (Ph.D. 2004). The signals sought by the LSC can be grouped as follows:

(a) Coalescence of binary systems, be it neutron stars or black holes. When the two bodies in-spiral towards coalescence, the emitted GW has a characteristic frequency chirp which can be recognized with confidence.

(b) Short bursts of gravitational radiation from supernova collapse and similar catastrophic events (for instance gamma ray burst events). Such events are identified by their simultaneous (time coincident) detection in several detectors.

(c) GW's emitted at discrete frequencies as expected from rotating stars (possibly pulsars) with significant quadrupole mass distribution. These extremely narrow lines will exhibit a well defined Doppler shift due to the motion of the earth through the galaxy, a unique signature.

(d) Finally, a stochastic signal of GW's left over from the big-bang or from the previous evolution of the universe should be present.

So far, the LSC has placed significant limits on all of the above mentioned phenomena. Following the present data run, which should end this Fall, the interferometers will be upgraded, and the collaboration will be expanded to include a large interferometer in Pisa, Italy. A proposal has also been submitted to the NSF for the construction of "Advanced LIGO" aimed at improving the sensitivity of the present instruments by a factor of ten. Thus, the rate of gravitational events will increase by a factor of 1000, since the rate is proportional to the volume reached by the detectors. This will certainly lead to the detection of GW's and to the birth of Gravitational Wave Astronomy.