A gravitational wave detector makes use of cutting edge technology to become very sensitive to the small disturbances caused by passing gravitational waves. The location of the detector is equally important; even if we can make the best mirrors and lasers, we can never escape environmental noise, such as, for example, the small vibrations of the Earth called `seismic noise' or variations in the temperature. These can be limited however by placing the detectors somewhere as isolated as possible. Also, if the environment is well understood, we might be able counter the effects of unwanted noise using data processing methods.

GEO 600 detector
GEO 600 detector site. Image credits: Albert Einstein Institute Hannover

Seismic noise comes from the motion of the Earth, either below or on the surface. Such motions are created by nature itself, for example from waves crashing into the shore or distant earthquakes. In addition, the activity of human beings and machines causes vibrations in the ground that a gravitational wave detector can 'feel'.

The mirror suspension systems are designed to isolate the mirrors from external motion as much as possible, but some amount of noise always remains. Interferometers should be built in areas where the seismic activity is small and well understood.

Another method for isolation the optical system from environmental noise is to put all the mirrors and laser beams inside a vacuum system. A vacuum is actually empty space with absolutely nothing inside, not even air. Sound vibrations can only travel through a medium, such as air or solid rock, so less air inside the detector arms means less noise affecting your laser beam. Another good thing about having a vacuum is that there are less air molecules for the laser light to bounce off and scatter in different directions, so less light is wasted.

LIGO vacuum system

Part of the LIGO vacuum system. Image credits: LIGO Laboratory

In order to keep a vacuum one has to build metallic vessels and tubes and use special pumps to extract the air inside. The vacuum system must be very tough or the pressure from the air outside would crush it.

From the point of environmental noise it would be a very good idea to build a detector underground, for example in an old mine. Underground caverns typically offer long-term stability, and there are less disturbances from the outside world the deeper you go. It would greatly improve the detector's sensitivity, but it costs more and more money the further down you go.

In summary: The location of real gravitational wave detectors must chosen carefully, and the best locations will have the most stable, low noise and predictable environment.