# gravitation: The Law of Universal Gravitation

Since the gravitational force is experienced by all matter in the universe, from the largest galaxies down to the smallest particles, it is often called universal gravitation. (Based upon observations of distant supernovas around the turn of the 21st cent., a repulsive force, termed dark energy, that opposes the self-attraction of matter has been proposed to explain the accelerated expansion of the universe.) Sir Isaac Newton was the first to fully recognize that the force holding any object to the earth is the same as the force holding the moon, the planets, and other heavenly bodies in their orbits. According to Newton's law of universal gravitation, the force between any two bodies is directly proportional to the product of their masses (see mass) and inversely proportional to the square of the distance between them. The constant of proportionality in this law is known as the gravitational constant; it is usually represented by the symbol G and has the value 6.670 × 10−11 N-m2/kg2 in the meter-kilogram-second (mks) system of units. Very accurate early measurements of the value of G were made by Henry Cavendish.

Newton's theory of gravitation was long able to explain all observable gravitational phenomena, from the falling of objects on the earth to the motions of the planets. However, as centuries passed, very slight discrepancies were observed between the predictions of Newtonian theory and actual events, most notably in the motions of the planet Mercury. The general theory of relativity proposed in 1916 by Albert Einstein explained these differences and provided a geometric explanation for gravitational phenomena, holding that matter causes a curvature of the space-time framework in its immediate neighborhood.

Analogous to electromagnetic waves, gravity waves were predicted by Einstein's general theory of relativity. A hypothetical particle, given the name graviton, has been suggested as the mediator of the gravitational force; it is analogous to the photon, the particle embodying the quantum properties of electromagnetic waves (see quantum theory). Tantalizing evidence for the existence of gravity waves came from astronomical observations of a binary pulsar designated 1913+16. The rate at which the two neutron stars in the binary rotate around each other changes in a manner that is consistent with the emission of gravity waves. The subsequent search for gravity waves has involved the building of large interferometers sensitive enough to detect the faint waves directly (see interference). The Laser Interferometer Gravitational Wave Observatory (LIGO), supported by the National Science Foundation, consists of two interferometers constructed in the 1990s, one in Hanford, Wash., the other in Livingston, La.; each has two 2.5-mi-long (4-km) arms at a right angle to each other. LIGO begin its work in 2002, but did not detect any gravitational waves until after an upgrade completed in 2015. Since late 2015 (reported beginning in 2016), LIGO several times has detect gravitational waves that resulted from the merging of two black holes. The European Gravitational Observatory's Virgo gravitational wave detector, near Pisa, Italy, became operational in 2017, and later that year it and LIGO detected gravitational waves from another black-hole merger and from a neutron-star merger. Begun by French and Italian scientific research organizations and now including personnel from institutes in other European nations, Virgo has a design similar to LIGO's, with two arms 1.86 mi (3 km) long. The proposed, even more ambitious Laser Interferometer Space Antenna (LISA) was originally a NASA–European Space Agency project but NASA withdrew in 2011 due to a lack of funding.