Introduction to LIGO & Gravitational Waves
One enhancement is to place LIGO’s optical components inside a vacuum. On the superficial level this keeps air currents from disturbing the mirrors (even in a well-isolated and contained system, temperature differences along the arms of the detector could induce winds) but mainly this is to insure that the laser light can travel a straight path in the arms. Slight temperature differences across the arm will cause the light to bend due to temperature dependent index of refraction (a measure of how much light bends as it passes through a medium). Even slight bending of the light in the arms will cause the laser to hit the inside of the approximately 1.2 meter diameter beam tube over its 4,000 meter length. Ultimately, LIGO is the largest sustained ultra-high vacuum in the world (8x the vacuum of space) keeping 300,000 cubic feet (about 8,500 cubic meters) at one-trillionth the pressure of Earth’s atmosphere.
Another measure is to add seismic isolation systems internally and externally to LIGO. Internally, there are tiny magnets attached to the back of each mirror and the positions of these magnets are sensed by the shadows they cast from LED light sources. If the mirrors are moving too much, an electromagnet creates a countering magnetic field to push or pull the magnets and mirror back into position. This method is not only good for countering the motion of the mirrors due to local vibrations, but it is also used to counter the tidal force of the Sun and the Moon as they pull the mirrors towards them just like they pull on the water in the ocean. Externally, there are hydraulic systems that counter the Earth’s surface vibrations (as detected by nearby seismometers) before they can cause vibrations in the internal components of LIGO.