Gravitationalwave observatories

Gravitational waves ("G waves") are hypothesized to travel through space at the speed of light much as electromagnetic radiation does. They are a consequence of Einstein's general theory of relativity (GR), but they have never been detected. They arise, in the theory, from an accelerating mass, specifically from an oscillating quadrupole moment of the mass distribution of a material object. The strength of the wave (the strain) is derivable from the second time derivative of the quadrupole moment. This is analogous to an accelerating charge giving rise to a transverse propagating electric field.

The existence of G waves has been confirmed, in a sense, by the observed loss of orbital energy by a binary star system consisting of two neutron stars. One of the two stars is a radio pulsar which allows one to track its orbit precisely, through Doppler shifts of the pulses. The system, the Hulse-Taylor (H-T) pulsar, is found to lose energy at exactly the rate predicted by GR for such a system radiating G waves. Thus we have seen the source of the waves but not the waves themselves.

General usage is that "gravity wave" is an oscillation that occurs in fluids, whereas the cumbersome "gravitational wave" is used for changes in the gravitational field as described by Einstein. Too bad; history rules. Here, I choose to use "G waves".

Here we describe the H-T pulsar, estimate the signal strength expected from the last seconds of coalescence of such a system, describe two types of detectors, bars and interferometers, and finally outline ongoing efforts to obtain sufficient sensitivity to detect the expected signals.

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