Gravitational wave

Gravitational wave, the transmission of variations in the gravitational field as waves. According to general relativity, the curvature of space-time is determined by the distribution of masses, while the motion of masses is determined by the curvature. In consequence, variations of the gravitational field are transmitted from place to place as waves, just as variations of an electromagnetic field travel as waves. If the masses that are the source of a field change with time, they radiate energy as waves of curvature of the field. Gravitational waves were first directly detected by the Laser Interferometer Gravitational-Wave Observatory (LIGO) in 2015.

Theory and sources

Superficially, there are many similarities between gravity and electromagnetism. For example, Newton’s law for the gravitational force between two point masses and Coulomb’s law for the electric force between two point charges indicate that both forces vary as the inverse square of the separation distance. Yet in Scottish physicist James Clerk Maxwell’s theory for electromagnetism, accelerated charges emit signals (electromagnetic radiation) that travel at the speed of light, whereas in Newton’s theory of gravitation accelerated masses transmit information (action at a distance) that travels at infinite speed. This dichotomy is repaired by Einstein’s theory of gravitation, wherein accelerated masses also produce signals (gravitational waves) that travel only at the speed of light. And, just as electromagnetic waves can make their presence known by the pushing to and fro of electrically charged bodies, so too can gravitational waves be detected by the tugging to and fro of massive bodies. However, because the coupling of gravitational forces to masses is intrinsically much weaker than the coupling of electromagnetic forces to charges, the generation and detection of gravitational radiation are much more difficult than those of electromagnetic radiation. Indeed, it was nearly 100 years after Einstein’s discovery of general relativity in 1916 that there was a direct detection of gravitational waves.

Nevertheless, there were strong grounds for believing that such radiation existed. The most convincing concerned radio-timing observations of a pulsar, PSR 1913+16, located in a binary star system with an orbital period of 7.75 hours. This object, discovered in 1974, has a pulse period of about 59 milliseconds that varies by about one part in 1,000 every 7.75 hours. Interpreted as Doppler shifts, these variations imply orbital velocities on the order of 1/1,000 the speed of light. The non-sinusoidal shape of the velocity curve with time allows a deduction that the orbit is quite noncircular (indeed, it is an ellipse of eccentricity 0.62 whose long axis precesses in space by 4.2° per year). The system is composed of two neutron stars, each having a mass of about 1.4 solar masses, with a semimajor axis separation of only 2.8 solar radii. According to Einstein’s theory of general relativity, such a system ought to be losing orbital energy through the radiation of gravitational waves at a rate that would cause them to spiral together on a timescale of about 3 × 10⁸ years. The observed decrease in the orbital period in the years since the discovery of the binary pulsar does indeed indicate that the two stars are spiraling toward one another at exactly the predicted rate. Gravitational radiation is the only known means by which that could happen. (American physicists Russell Hulse and Joseph H. Taylor, Jr., won the Nobel Prize for Physics in 1993 for their discovery of PSR 1913+16.)

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Physics and Natural Law

The implosion of the core of a massive star to form a neutron star prior to a supernova explosion, if it takes place in a nonspherically symmetric way, ought to provide a powerful burst of gravitational radiation. Simple estimates yield the release of a fraction of the mass-energy deficit, roughly 10⁵³ ergs, with the radiation primarily coming out at wave periods between the vibrational period of the neutron star, approximately 0.3 millisecond, and the gravitational radiation damping time, about 300 milliseconds.

Detectors and observations

Three types of detectors have been designed to look for gravitational radiation, which is very weak. The changes of curvature of space-time would correspond to a dilation in one direction and a contraction at right angles to that direction. One scheme, first tried out about 1960, employed a massive cylinder that might be set in mechanical oscillation by a gravitational signal. The authors of this apparatus argued that signals had been detected, but their claim was not substantiated.

In a second scheme an optical interferometer is set up with freely suspended reflectors at the ends of long paths that are at right angles to each other. Shifts of interference fringes corresponding to an increase in length of one arm and a decrease in the other would indicate the passage of gravitational waves. One such interferometer is the Laser Interferometer Gravitational-Wave Observatory (LIGO), which consists of two interferometers with arm lengths of 4 km (2 miles), one in Hanford, Washington, and the other in Livingston, Louisiana. LIGO was the first observatory to directly detect gravitational waves. On September 14, 2015, it observed two black holes 1.3 billion light-years away, which were 36 and 29 times the mass of the Sun, spiralling inward to form a new black hole of 62 solar masses. The remaining three solar masses were converted into gravitational waves.

A third scheme, the Evolved Laser Interferometer Space Antenna (eLISA), is planned that uses three separate, but not independent, interferometers installed in three spacecraft located at the corners of a triangle with sides of some 5 million km (3 million miles). A mission to test the technology for eLISA, LISA Pathfinder, was launched in 2015.

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