LIGO Vocabulary

Absorption: LIGO's mirrors reflect most of the laser light that strikes them and transmit nearly all of the rest. A tiny fraction of the light is absorbed by the atoms in a mirror, heating the mirror changing its shape by small amounts.

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.

Duty factor (or duty cycle): A ratio of the time that an interferometer stays locked at full power compared to the total clock time of the interval, usually reported as a percentage

Earthquake stops: Viton-tipped screws that are threaded into a mirror's suspension cage. The stops point at the edges of the mirror, providing cushions against excessive swinging of the mirror in circumstances such as a nearby earthquake.

Electronic Log: LIGO's official lab books. "If it's not in the E-log, it didn't happen." Both the Hanford and Livingston E-logs are viewable on the Web. Surf to the LIGO Laboratory and click on the Electronic Log links under Observatories. The Detector logs hold the most information

Engineering run: These occur with the interferometers in full science mode but are shake-down runs that follow rounds of commissioning and that precede science runs.

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.

Gate valves: Large valves that partition LIGO's vacuum enclosure into discreet sections, permitting a vent in one portion of the system while the remainder is held at low pressure.

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.

Interferometer labels: H1 is the Hanford 4-km detector, H2 is the Hanford 2-km detector, and L1 is the Livingston 4-km detector.

Light year: One of the common units of astronomical distance, representing the distance that light will travel in a year (roughly 9.5 trillion kilometers). Parsecs are often used instead of light years -- 1 parsec is the equivalent of 3.26 light years.

Mirror labels: LIGO refers to its mirrors as "test masses." The label "ITMX" stands for "Inner Test Mass on the X Arm." This is the inner mirror of the 2.5 mile X-arm Fabry Perot cavity. This mirror rests just outside the beam splitter, 2.5 miles away from its companion ETMX, the end test mass on the X arm.

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.

Optical lever: Part of the suspension subsystem. Optical levers provide angular control of the mirrors. An 'oplev' consists of a small laser that undergoes an angled reflection off a LIGO mirror through a window in a vacuum chamber. The reflected light then strikes a target photodiode that acts as the sensor in a feedback loop.

Pump-down: The process that takes a region of LIGO's beam enclosure from ambient pressure down to the operational pressure of 10-9torr. Turbo pumps remove atmosphere down to about 10-6 torr and ion pumps handle the remainder. Gate valves are used to partition the vacuum enclosure, permitting small portions to be vented.

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.

Radio frequency: LIGO generates megahertz frequencies in order to add RF sidebands to the main laser light. These same frequencies are used to demodulate the light in several instrument subsystems to create error signals for use in control loops.

Science run: A period of time in which the three LIGO interferometers (possibly in concert with the international interferometers) run at high sensitivity in the absence of any intrusive maintenance. A science run's purpose is to gather the maximum volume of data of the highest possible sensitivity.

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 a small amount of light nonetheless 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.