LIGO’s primary interferometer optics include exquisite mirrors called "test-masses". The term "test mass" comes from relativity theory, where it is used to describe a mass free from forces and able to follow distortions of space-time. In addition, LIGO's instrument includes laser beam splitters, a power-recycling mirror, and a signal recycling mirror. The mirror technology in LIGO’s detectors represents the accomplishments of a decades-long global collaboration.
LIGO’s test-masses are made of fused silica, cast from ultra-pure material with low OH (hydroxide) content to minimize infrared (IR) absorption. Since LIGO’s laser is an IR laser, its optics must not absorb IR radiation. Doing so would result in heating and in significant changes to the shape of the mirrors, thus critically affecting LIGO’s ability to make the precise measurements it needs to make. LIGO’s main mirrors only absorb one out of every 3.3 million photons, the rest being reflected (or transmitted). Some heating does occur, but an auxiliary system using a CO2 laser heats the mirrors precisely enough to counteract the shape-changes that occur as a result of heating from LIGO’s main laser.
LIGO’s optics are also hefty. The input and end test-masses each weigh 40 kg (88 lbs.) giving them large inertial masses that intrinsically provide some level of vibration resistance thanks to the law of Inertia (although much more is required. See Vibration Isolation). The image at right shows two of LIGO's test-masses before they were installed in the interferometer.
The test-masses are coated with dozens of layers of optical coatings and polished to nanometer smoothness, making them incredible mirrors. Again, this level of precision is required to ensure that LIGO’s laser encounters a clean, stable, and perfectly tooled reflective surface enabling it to follow a clean path as it travels through the interferometer, making about 280 reflections before reaching the photodetector. Without these precautions, detecting a gravitational wave would be impossible.