Gravitational waves
Introduction
Gravitational waves are disturbances in the curvature of spacetime that are generated by accelerated masses and propagate as waves outward from their source at the speed of light. They were first proposed by Henri Poincaré in 1905 and subsequently predicted in 1916 by Albert Einstein on the basis of his general theory of relativity. Gravitational waves transport energy as gravitational radiation, a form of radiant energy similar to electromagnetic radiation.
Theory of Gravitational Waves
Gravitational waves are a consequence of the Lorentz invariance of the theory of general relativity, which is a fundamental symmetry of the laws of physics. They are solutions to the Einstein field equations, which describe how matter and energy in the universe interact with spacetime. These waves are transverse, meaning they vibrate perpendicular to the direction of propagation, and they have two independent polarizations.
Detection and Observation
The first indirect evidence for the existence of gravitational waves came from the observation of the binary pulsar PSR B1913+16 by Russell Hulse and Joseph Taylor, for which they were awarded the 1993 Nobel Prize in Physics. The first direct observation of gravitational waves was made on 14 September 2015 by the Laser Interferometer Gravitational-Wave Observatory (LIGO) and was announced on 11 February 2016. Since then, several other gravitational wave events have been observed by LIGO and other detectors around the world.
Sources of Gravitational Waves
The most powerful sources of gravitational waves are cataclysmic events such as supernovae, neutron star collisions, and black hole mergers. However, any mass that is accelerated or deformed in a non-spherically symmetric or non-cylindrically symmetric manner will produce gravitational waves. This includes the rotation of asymmetric celestial bodies, the interaction of stars in binary systems, and even the Big Bang itself.
Effects of Gravitational Waves
Gravitational waves cause a distinctive pattern of strain in spacetime as they pass through an area, causing distances to alternately stretch and compress. This effect is incredibly small and difficult to detect. The strongest gravitational waves detected so far have caused changes in distance on the order of 1 part in 10^21, which is less than the diameter of a proton.
Gravitational Waves and Cosmology
Gravitational waves carry unique information about their dramatic origins and about the nature of gravity that cannot be obtained by traditional astronomical observations. They provide astronomers with a completely new way of observing the most violent events in the universe. This has opened up a new way of studying the universe and has ushered in a new era in observational cosmology.