Tests of general relativity: Encyclopedia II - Tests of general relativity - Strong field tests

Tests of general relativity - Strong field tests

Observations of binary pulsars have all demonstrated substantial periapsis precessions that cannot be accounted for classically but can be accounted for by using general relativity. For example, the Hulse-Taylor binary pulsar PSR B1913+16, has an observed precession of over 4^o of arc per year. This precession has been used to compute the masses of the components. A binary pulsar discovered in 2003, J0737-3039, has a perihelion precession of 16.88^o.

Similarly to the way in which atoms and molecules emit electromagnetic radiation, a gravitating mass that is in quadrupole type or higher order vibration, or is asymmetric and in rotation, can emit gravitational waves. Two mutually orbiting bodies can also do so. These gravitational waves are predicted to travel at the speed of light. In general relativity, a perfectly spherical star (in vacuum) that expands or contracts while remaining perfectly spherical cannot emit any gravitational waves, as Birkhoff's theory says that the geometry remains the same exterior to the star. More generally, a rotating system will only emit gravitational waves if it lacks the axial symmetry with respect to the axis of rotation. For example, planets orbiting the Sun constantly lose their energy via gravitational radiation, but this effect is so tiny that it is unlikely it will be observed in the near future. Gravitational waves originating from a binary system of a neutron star and a pulsar outside our solar system have been indirectly detected, for which Hulse and Taylor won the Nobel prize. The stars orbit only approximately according to Kepler's Laws, – over time they gradually spiral towards each other, demonstrating an energy loss in agreement with general relativity. Thus, although the waves have not been detected, their effect is necessary to explain the orbits.

The laser interferometer gravitational-wave observatory (LIGO) is an experiment designed to detect gravitational waves. It may lack the sensitivity to detect gravitational waves of astrophysical origin, but the planned laser interferometer space antenna (LISA) is expected to directly detect gravitational waves.

Adapted from the Wikipedia article "Strong field tests", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki