In this chapter we discuss the main noise sources which limit
the sensitivity of interferometric gravitational wave detectors.
Fundamentally it should be possible to build systems using laser
interferometry to monitor strains in space which are limited only
by the Heisenberg Uncertainty Principle; however there are other
practical issues which must be taken into account. Fluctuating
gravitational gradients pose one limitation to the interferometer
sensitivity achievable at low frequencies, and it is the level of
noise from this source which dictates that experiments to look
for sub-Hz gravitational wave signals have to be carried out in
space [94,
86,
12,
53]. In general, for ground based detectors the most important
limitations to sensitivity result from the effects of seismic and
other ground-borne mechanical noise, thermal noise associated
with the test masses and their suspensions and shot noise in the
photocurrent from the photodiode which detects the interference
pattern. The significance of each of these sources will be
briefly reviewed.