Detection of gravitational radiation

There are several techniques to detect gravitational waves, depending on what the frequency of the waves is. The frequency of the radiation, in turn, depends on the astrophysical source. The most prevalent of these techniques is laser interferometers and we will discuss these at some length during the seminar. This page contains links to several other techniques used and the major international facilities currently operational, or soon to be operational ("soon" in gravitational wave detection could be a cosmological time scale, but I'll limit our tour to instruments expected within the next decade).

I will divide the seminar into two parts. Here are sample LIGO viewgraphs and here are LISA ones. These links will be updated by September 15, 2004.

Laser Interferometers

Interferometers are a sensitive method for measuring relative displacements of mirrors (or the space-time between them) and are part of the measurement scheme for all the major earth-based observatories and for an antenna in space as well. Here are links to their home pages as well as other useful tidbits that I could find.

One of the best - and relatively concise - descriptions of the science of LIGO appear in this article that Daniel Sigg wrote as part of the TASI summer study at Boulder, CO, 1998. This is the only strongly recommended reading for the seminar.

The NASA LISA  and the ESA LISA web sites.


Resonant Mass Detectors

Another technique used for gravitational-wave detection are resonant mass detectors, also called 'bar' detectors (probably because the earliest ones were shaped as bars?). The principle of detection here is that a passing gravitational wave sets the resonant mode of a massive piece of metal vibrating. This vibration can be measured with great precision using a resonant transducer and low-noise amplifiers. These are narrow-band antennas, since the 'bars' have narrow vibrational mode linewidths. IGEC is a network of resonant mass detectors that are doing a coincident search for gravitational radiation. Links to detectors that are participating in this collaboration can be found here.

Doppler Tracking

In the 10 to 100 millihertz band, there have been searches for gravitational waves using Doppler tracking of spacecraft. Here the principle is that the Doppler shifts of radio beams between distant spacecraft and the earth are altered by gravitational waves. Despite the similarity in frequency bands, the strain sensitivity of this technique is presently several orders of magnitude higher than that expected for LISA. Here is a talk on measurement of gravitational waves by Doppler tracking of spacecraft, given by John Armstrong at the CaJAGWR seminar series, Caltech, 2000.

Pulsar Timing

Very low frequency gravitational waves can be measured from pulsar timing. The most famous case is the Nobel-prize-winning Hulse-Taylor pulsar, PSR1913+16. A very readable review of pulsar timing theory and measurement appears here.  And here is a talk on measurement of nanohertz gravitational waves using pulsar timing arrays, given by Donald Backer at the CaJAGWR seminar series at Caltech, 2001.

Cosmic Microwave Background Measurements

Sub-femtohertz frequency gravitational waves can be measured by mapping the intensity and polarization of the CMB. The idea here is that primordial gravitational waves produced by inflation after the Big Bang should cause ripples on the surface of last scattering. These ripples should, in turn, cause red- and blue-shifts in the temperature and polarization of the CMB radiation. Here is a rather fun Scientific American article on the subject.

Prepared by Nergis Mavalvala as a resource page for the 8.971 seminar on Wednesday, Sept. 09, 2004.