Gravitational Waves - The Sound of the Dark Universe

son interferometer. For a gravitational
wave detector, the mirrors
are the test masses, so that any
change of the distance between
them due to a gravitational wave
changes the length of the light
path, which in turn is measured by
interference with the other beam.
For instance, as the wavelength of
red light is in the order of 0.6 µm,
displacements of less than say 0.1
µm can be resolved.
Yet, this value is still a long way off
from resolving the tiny shifts caused
by gravitational waves. In order to
enhance a Michelson interferometer’s
capabilities, interferometers
nowadays make use of an invention
by Charles Fabry 16 and Alfred
Perot 17 who added further optical
elements constituting Fabry-Perot
cavities (not shown in Fig. 5). The
idea is to allow the two beams between
the beam splitter and the respective
mirrors to pass several hundred
times back and forth instead
of just once. This multiplies the effective
displacement of the mirrors
by the same factor thereby greatly
enhancing the sensitivity of the
system.
Yet, with increasing sensitivity,
spurious noise tends to appear that
can mask the signal caused by gravitational
waves. For instance, the
light produced by a laser consists
of a flow of photons. However,
they come at random, just like the
droplets of a rainfall. This leads to
high-frequency noise in the output
of the detector. In addition, for
sufficiently high laser power, the
photons reflected by the mirrors
Fig 5: The basic configuration of a Michelson Interferometer. A light source
(today mostly lasers) emits light in the direction of the beam splitter. This element
produces two beams, of which one travels ahead in the same direction as before,
while the second is reflected by 90°. Both beams travel towards the mirrors, which
reflect them back towards the beam splitter, which acts on the beams again. Finally,
both beams travel towards the detector where they interfere. The test masses’ surfaces
act as mirrors, so that any displacement of the test cubes causes a change in
the path lengths of the beams. The difference between the two optical paths makes
the interference pattern change and the detector deliver an output.
on the test masses transfer a random
momentum, which tends to
disguise a signal at lower frequencies.
External effects, such as seismic
turbulences and other forms of
environmental vibrations acting on
the test masses, are sources of detector
noise. In order to suppress
this type of noise, two (or more)
widely spaced sites are used. The
stochastic environmental noise
will be different at each site so that
appropriate algorithms are able to
suppress it to some extent. On the
other hand, any gravitational wave
signal will be the same at all sites
(aside from an eventual time shift).
All these – and many other – effects
must be taken into account
before an observatory becomes
able to monitor gravitational
waves. As an engineer said: The
16
Maurice Paul Auguste Charles Fabry, 1867, Marseille – 1945, Paris, French physicist.
17
Jean-Baptiste Alfred Perot, 1863, Metz, France – 1925, Paris, French physicist.
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