Astrophysics (index)

Gravitational Wave

(ripples in the curvature of spacetime propagating as a wave)

A Gravitational Wave (GW) is a phenomenon of General Relativity, which models Gravity as curves in spacetime (Curvature), specifically of waves in spacetime propagating from disturbances. Gravitational Wave Strain is essentially the amplitude of the wave.

A similar-sounding term, Gravity Wave is entirely different: a wave on a boundary surface of a fluid using gravity (buoyancy) as its restoring force. An ocean wave is an example.

The first indirect evidence of gravitational waves was the observation of the decays of orbits of binary systems that matched the predictions of the effects of gravitational waves. All orbits do the same, but the decay is typically insignificant and non-detectable. Heavy objects (e.g., Compact Objects) in small orbits could produce waves sufficiently intense to detect directly, and a final collision between objects would be the most detectable.

Clear detection of well-explained gravitational waves must detect a signal beyond than the expected background of extremely slight waves from ongoing phenomena. For example, the expected waves of the final fall of Black Hole binaries as they merge, would have to show above the plethora of lesser waves from black hole binaries not yet so close together or more distant.

Among the efforts to detect gravitational waves are space missions LISA, and New Gravitational Wave Observatory, Earth Michelson Interferometers such as LIGO (which has spotted the waves of black hole mergers) and European Gravitational Observatory, and Earth Pulsar-timing analysis efforts, including NANOGrav, Parkes Pulsar Timing Array, International Pulsar Timing Array, and European Pulsar Timing Array. Detecting GWs in pulsar timing histories requires considerable data storage and processing. Software for analysis includes:

  • Tempo
  • Tempo2

The signal from a gravitational wave detector of a compact object merger would be increasing Frequency, and is referred to as a Chirp. Gravitational waves from Supermassive Black Hole mergers (within Galaxy Mergers) would have frequencies in the range of 10-9Hz (about a cycle per year) to 10-6Hz (about a cycle per day) and are a target of Pulsar Timing Arrays.

The formation of GWs requires a type of assymmetry in the motion of masses: for example, a perfectly symmetric Supernova (matter ejected uniformly in all directions) will not trigger waves. An orbit does, which is why the observation of a decaying orbit (involving objects massive enough to detect such a decay) was taken as evidence of GWs. The Quadrupole Moment of Mass produces the waves, called Gravitational Quadrupole Radiation. Albert Einstein, in his development of general Relativity, derived the Quadrupole Formula which describes the waves produced by a reconfiguration of mass.

GWGW150914Gravitational Wavegeneral prefix for GWs.

Referenced by:
Black Hole Binary (BHB)
Birkhoff's Theorem
Coherent Light
Compact Object
European Gravitational Observatory (EGO)
European Pulsar Timing Array (EPTA)
Final Parsec Problem
Gravity Wave
Gravitational-Wave Memory
Gravitational Wave Strain
International Pulsar Timing Array (IPTA)
Michelson Interferometer
New Gravitational Wave Observatory (NGO)
Nanohertz Gravitational Waves
Parkes Pulsar Timing Array (PPTA)
Pulsar Timing Array (PTA)
Pulsar (PSR)
Strong-Field Gravity
Transient Astronomy