Astronomy Principles and Practice Fourth Edition.pdf

398 High energy instruments and other detectors
Figure 23.15. A simple layout for a possible gravity wave detector.
theory of general relativity. An estimate of the expected flux can be made by taking the simple case of
a stellar binary system. For such a source, the energy L G released may be expressed as
L G ≈ (2 × 10 −63 )
M 2 1 M2 2
(1 + 30e 2 )
(M 1 + M 2 ) 2/3 P 10/3
where M 1 , M 2 are the component masses, P is the orbital period and e is the orbital eccentricity. In a
typical dwarf nova system, M 1 = M 2 = 1·5 × 10 30 kg, P = 10 4 s and the simple case of a circular
orbit gives e = 0. Substituting for these values gives
L G = 2 × 10 24 W.
If the system is at a typical stellar distance, say 250 pc, the flux arriving at the Earth is
F G = 3 × 10 −15 Wm −2 .
When compared with other radiations received from celestial objects, this is by no means small.
For example, limiting faintness for detection of a radio source is approximately 10 −29 Wm −2 Hz −1 .
However, the means of detection is very different from that for radio waves.
The progress of gravitational waves through a medium can only be detected by its effect on the
mass centres in the body. In other words, it is the deformity or strain within the detector that needs to
be measured as the gravity waves pass through it. Translating the gravitational flux to the anticipated
amount of movement within a test mass detector suggests that the possible changes in length are
exceedingly small, that is of the order of 10 −12 times the diameter of a hydrogen atom.
The challenge of the detection of gravitational radiation was taken up by Weber in the United
States and, in 1969, claims were made that gravity waves were being observed from the centre of the
Galaxy. However, the detection was not confirmed by other workers and since that time, the sensitivity
of various detector systems has increased by very large factors—still without success. A possible
working arrangement for a gravity way detector is depicted in figure 23.15.
Currently, there are several experimental search groups pioneering systems of gravitational
wave detectors. These include instruments such as GEO600 (Germany/UK), LIGO (USA), VIRGO
(Italy/France) and TAMA300 (Japan). Their arrangements are based on a pair of monolithic masses