where
is the change in separation of two masses a distance
L
apart; for the strongest allowed component of gravitational
radiation the value of
h
is proportional to the third time derivative of the quadrupole
moment of the source of the radiation and inversely proportional
to the distance to the source. The radiation field itself is
quadrupole in nature and this shows up in the pattern of the
interaction of the waves with matter.
The problem for the experimental physicist is that the
predicted magnitudes of the amplitudes or strains in space in the
vicinity of the earth caused by gravitational waves even from the
most violent astrophysical events are extremely small, of the
order of
or lower [55
]. Indeed current theoretical models on the event rate and
strength of such events suggest that in order to detect a few
events per year - from coalescing neutron star binary systems for
example - an amplitude sensitivity close to
over timescales as short as a millisecond is required. If the
Fourier transform of a likely signal is considered it is found
that the energy of the signal is distributed over a frequency
range or bandwidth which is approximately equal to 1/timescale.
For timescales of a millisecond the bandwidth is approximately
1000 Hz, and in this case the spectral density of the
amplitude sensitivity is obtained by dividing
by the square root of 1000. Thus detector noise levels must have
an amplitude spectral density lower than
over the frequency range of the signal. Signal strengths at the
earth, integrated over appropriate time intervals, for a number
of sources are shown in Fig.
1
.
The weakness of the signal means that limiting noise sources
like the thermal motion of molecules in the detector (thermal
noise), seismic or other mechanical disturbances, and noise
associated with the detector readout, whether electronic or
optical, must be reduced to a very low level. For signals above
10 Hz ground based experiments are possible, but for
lower frequencies where local fluctuating gravitational gradients
and seismic noise on earth become a problem, it is best to
consider developing detectors for operation in space [26].
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Gravitational Wave Detection by Interferometry (Ground
and Space)
Sheila Rowan and Jim Hough http://www.livingreviews.org/lrr-2000-3 © Max-Planck-Gesellschaft. ISSN 1433-8351 Problems/Comments to livrev@aei-potsdam.mpg.de |