Gravitational-Wave Antenna Overview

Why did we try to detect gravitational waves (GWs)?

In his 1916 General Theory of Relativity, Albert Einstein predicted their existence as cosmic ripples that traveled at the speed of light, carrying with them information about their origins as well as clues to the nature of gravity itself.

What is the purpose of this event?

This is an international recognition of a monumental achievement: the confirmation of Einstein's prediction of 104 years ago.

When were gravitational waves first detected?

On 14 September 2015, using the two LIGO antennas, and many have been detected since that time.

What caused the gravitational waves that have been detected?

The collisions of binary neutron stars or of black holes that occurred anywhere from hundreds of millions to nearly 3 billion years ago, and GWs from these events travel through the Universe virtually unimpeded.

How were these gravitational waves detected?

Initially using the two LIGO antennas, and then the LIGO and Virgo antennas together.  Each antenna consists of an L-shaped laser interferometer with kilometer-scale arms, and GWs produce minute changes in the arm lengths as small as one-thousandth of a proton's diameter.

How do we know the source of these gravitational waves?

Gravitational signals are received by each antenna at slightly different times.  The concurrent operation of the three antennas (and in the near future four and five antennas) permits through “triangulation” the precise source of these signals, and the antennas thus operate as a “single antenna.”   As such, optimal GW detection requires the global coordination that started with LIGO and Virgo in 2007, and which Japan's KAGRA joined in 2020.

Why was the detection of GWs from a neutron star merger the "Dawn of  Multi-Messenger Astronomy"?

  • On 14 August 2017, the 3 antennas working together determined with sufficient angular precision the location of a binary neutron star merger.  This triggered a follow-up campaign of nearly 70 observatories around the world and in orbit to detect electromagnetic "messengers" ranging across the spectrum from x-ray and gamma-ray bursts to radio waves.
  • The concomitant observation of both electromagnetic and gravitational waves from this neutron star merger, with each emitting at a different stage of the explosion process, permitted the study of the time evolution of a violent cosmic event that had occurred 130 million light-years ago.  Lasting for many months, this observation gave precious information about the evolutionary processes of the neutron star merger event.

What is the impact of GW detection on other scientific disciplines?

  • Astrophysics: GW detection is providing a new cartography of the Universe, which when compared with the astrophysical cartographies as based on electromagnetic messengers is revolutionizing our ideas about the structure and evolution of the formation of stars and galaxies.
  • Cosmology: GW detection is also giving access to new cosmological measurements of the Universe expansion parameters such as the Hubble Constant.  By also allowing us to probe for dark matter candidates, GW detection addresses a key mystery of Cosmology: the nature of dark matter and energy.
  • Particle and Nuclear Physics: GW detection also probes the equations describing the state and surface characteristics of neutron stars, where atomic nuclei and eventually quarks are condensed in unprecedented densities - thus permitting the study of quark-nuclei phase transitions and/or the presence of exotic particles.
  • Gravitation: The study of GW speed and polarization, as well as the waveforms of the merging neutron stars or black holes, provides stringent tests of Einstein's 1916 General Theory of Relativity.