- 1915 – Albert Einstein publishes the theory of general relativity.
- 1916 – Albert Einstein predicts the existence gravitational waves as a consequence of the theory of general relativity.[16][17]
- 1957 – Richard Feynman and Hermann Bondi predict that if gravitational waves exist, they are theoretically detectable by a proposed sticky bead argument.[18]
- 1962 – M. E. Gertsenshtein and V. I. Pustovoit publish the first paper describing the principles for using interferometers to detect very long wavelength gravitational waves.[19]
- 1968 – Joseph Weber reports that gravitational waves have been detected, but the report's findings prove to be false. Rainer Weiss analyzes Weber's method and conceives LIGO.[18]
- late 1970s – Indirect confirmation of the existence of gravitational waves comes from observations of a pair of pulsars that move into tighter and faster orbits with an acceleration as expected if they are losing energy by emitting gravitational waves.[20]
- 1984 – Kip Thorne, Ronald Drever, and Rainier Weiss form a steering committee after the NSF asks MIT and Caltech to join forces to lead a LIGO project.[18][21]
- 1994 – LIGO Laboratory Director Barry Barish and his team create the LIGO study, project plan, and budget, receive long-withheld NSF funding, and go-ahead for construction. Barish is appointed Principal Investigator and LIGO, with a budget of US$395 million, becomes the largest overall funded NSF project in history.[21]
- 1997 – The LIGO Scientific Collaboration (LSC) and the Gravitational Wave International Committee (GWIC) are formed.[22][23]
- 2002 – LIGO begins the search for gravitational waves[24]
- 2004 – Advanced LIGO upgrade is approved by the National Science Board.[25]
- 17 March 2014 – Astronomers at the Harvard–Smithsonian Center for Astrophysics erroneously claim that they have detected and produced "the first direct image of gravitational waves" in the cosmic microwave background.[9][10][26]
- 2015 – Advance LIGO begins operation.
- 11 February 2016 – The LIGO Scientific Collaboration announce that they detected gravitational waves on 14 September 2015 from a merger of two black holes about 400 megaparsecs (1.3 billion light years) from Earth.[6][7][8] The merger event is named GW150914.[27]
Effects of passing[edit]
Gravitational waves are constantly passing Earth; however, even the strongest have a miniscule effect and their sources are generally at a great distance. For example, the waves given off by the cataclysmic final merger of GW150914 reached Earth after travelling over a billion lightyears, as a ripple in spacetime that changed the length of a 4-km LIGO arm by a ten thousandth of the width of a proton, proportionally equivalent to changing the distance to the nearest star outside the Solar System by one hair's width.[28] This tiny effect from even extreme gravitational waves makes them completely undetectable on Earth, by any means other than the most sophisticated detectors.
The effects of a passing gravitational wave, in an extremely exaggerated form, can be visualized by imagining a perfectly flat region of spacetime with a group of motionless test particles lying in a plane (e.g., the surface of a computer screen). As a gravitational wave passes through the particles along a line perpendicular to the plane of the particles (i.e. following the observer's line of vision into the screen), the particles will follow the distortion in spacetime, oscillating in a "cruciform" manner, as shown in the animations. The area enclosed by the test particles does not change and there is no motion along the direction of propagation.[citation needed]
The oscillations depicted in the animation are exaggerated for the purpose of discussion — in reality a gravitational wave has a very small amplitude (as formulated in linearized gravity). However, they help illustrate the kind of oscillations associated with gravitational waves as produced, for example, by a pair of masses in a circular orbit. In this case the amplitude of the gravitational wave is constant, but its plane of polarization changes or rotates at twice the orbital rate and so the time-varying gravitational wave size (or 'periodic spacetime strain') exhibits a variation as shown in the animation.[29] If the orbit of the masses is elliptical then the gravitational wave's amplitude also varies with time according to Einstein's quadrupole formula.[30]
As with other waves, there are a number of characteristics used to describe a gravitational wave:
- Amplitude: Usually denoted h, this is the size of the wave — the fraction of stretching or squeezing in the animation. The amplitude shown here is roughly h = 0.5 (or 50%). Gravitational waves passing through the Earth are many sextillion times weaker than this — h ≈ 10−20.
- Frequency: Usually denoted f, this is the frequency with which the wave oscillates (1 divided by the amount of time between two successive maximum stretches or squeezes)
- Wavelength: Usually denoted λ, this is the distance along the wave between points of maximum stretch or squeeze.
- Speed: This is the speed at which a point on the wave (for example, a point of maximum stretch or squeeze) travels. For gravitational waves with small amplitudes, this wave speed is equal to the speed of light (c).
In the above example, it is assumed that the wave is linearly polarized with a "plus" polarization, written h+. Polarization of a gravitational wave is just like polarization of a light wave except that the polarizations of a gravitational wave are at 45 degrees, as opposed to 90 degrees. In particular, in a "cross"-polarized gravitational wave, h×, the effect on the test particles would be basically the same, but rotated by 45 degrees, as shown in the second animation. Just as with light polarization, the polarizations of gravitational waves may also be expressed in terms of circularly polarized waves. Gravitational waves are polarized because of the nature of their sources.
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