LIGO Vocabulary

Astrophysical events: As used in LIGO, this term refers to events in the universe that are expected to produce gravitational waves. Examples could be the coalescence of pairs of compact binary objects (neutron star/neutron star, neutron star/black hole, black hole/black hole), a supernova core collapse or the boiling of a neutron star. Continuous processes such as the rapid spinning of a neutron star might also produce gravitational waves.

Commissioning: The process of improving the instrument systems and computing infrastructure of the LIGO interferometers. Commissioning takes place between science runs.

Control systems: Also called control loops. These are feedback and control circuits that restrain the interferometer optics by acquiring sensor voltages that indicate the optics’ movement. The system produces a set of forces in response to the input voltage that counteract the movements of a mirror and cause the mirror to sit still.

Dark port: Also called the antisymmetric port. This is the location adjacent to the beam splitter where the laser interference pattern is measured on photodiodes, and is where a gravitational wave signal will be recorded.

Displacement: Movement from one position to another. LIGO typically reports displacements in meters or microns (1 micron = 10-6 meter). Although we usually think of displacement as an object changing its position in fixed space, the premise of LIGO is to measure tiny expansions and contractions in space (spacetime) itself through displacements of the mirrors. To say that LIGO’s displacement sensitivity goal in the gravitational wave band is 10-18 m is to say that the instruments should detect a mirror movement of 10-18 m that is recognizable above the noise floor.

Frequency spectrum: A plot of an amplitude or power as a function of frequency. Such spectra might display amplitude in vertical units of strain or displacement.

Gravitational wave signal: A gravitational wave will change the spacetime composition of one interferometer arm relative to the other. This will result in a change in the instrument’s interference pattern which will register as a voltage fluctuation at the photodetector.

Lock: The condition in which light continues to resonate at high power in the cavities of the interferometer. Disruptions to the optics can misdirect the beam, destroying the resonant condition and rendering the instrument insensitive (“loss of lock”). Lock is threatened by mirror movements of as small as one atomic diameter (against a cavity length of up to 4 km).

Noise floor: LIGO defines noise as any factor other than a gravitational wave that sends a fluctuating signal to the interferometer’s photodetector. The noise floor of the instrument is the point at which a gravitational wave is undetectable because it doesn’t rise above the other fluctuating contributions to the photodetector signal. This level varies with frequency, so the noise floor is usually given as a frequency spectrum.

Radiation pressure: When an electromagnetic wave (light wave) strikes a charged particle, the light’s electric field accelerates the charge in a direction that is transverse to the light’s propagation. Acceleration of the charge creates a magnetic field that interacts with the light’s magnetic field and forces the particle forward in the propagation direction. Radiation pressure from the sun creates comet tails by pushing vaporized material out of the comet’s head. In LIGO radiation pressure pushes on the mirrors, an effect that complicates the control of the mirrors.

Strain sensitivity: Strain sensitivity is determined by the smallest strains that an interferometer can measure across a range of frequencies. Strain is the instrument’s ability to detect a space change within an arm in comparison to the total space (length) of the arm (see displacement).

Saturation: The point at which a signal’s voltage becomes large enough that the system’s electronics can no longer distinguish it from any higher voltage.

Shot noise: LIGO interferometers are tuned to place a dark interference fringe on the photodetectors, but by design small amount of light always travels into the dark port. Rather than a smooth stream, this light is a bumpy collection of photons that strike the photodetectors like rain striking a tin roof. This random behavior will set the noise floor in a fully commissioned first-generation interferometer at frequencies above ~200 Hz.