Radiation (continued)
Gravitational radiation
A gravitational wave, also known as gravitational radiation,
consists of a self-propagating pattern of spacetime curvature.
Gravitational waves travel at the speed of light and are gener-
ated by the acceleration of mass (Fig. 2). German-born U.S.
theoretical physicist Albert Einstein first predicted the existence
of gravitational waves in his general theory of relativity in 1916.
Gravitational waves proved too weak to be directly detected
until 2015, when an experiment called the Laser Interferometer
Gravitational-Wave Observatory (LIGO) detected waves passing
through the Earth. LIGO is sensitive to a strain of one part in
1022, which is on the order of one ten-thousandth of the diam-
eter of a proton. The gravitational waves were generated by the
merger of black holes more than a billion light-years away. LIGO
has since made numerous detections of black hole and neutron
star mergers. Gravitational radiation is extremely weak and is
non-ionizing.
Ionizing versus non-ionizing radiation
An important classification of radiation is ionizing versus
non-ionizing. Ionizing radiation has enough energy per particle
to eject electrons from atoms and break chemical bonds.
Because of this, ionizing radiation can trigger chemical reac-
tions, wear down materials, induce radiation sickness, produce
genetic mutation, and cause cancer. In contrast, non-ionizing
radiation does not have enough energy per particle to directly
cause permanent damage. However, large amounts of non-
ionizing radiation can still indirectly cause damage through
excessive heating or pressure effects. Extreme ultraviolet rays,
x-rays, gamma rays, and most types of particle radiation are
ionizing. Radio waves, infrared waves, visible light, low-frequen-
cy ultraviolet, acoustic radiation, and gravitational waves are
non-ionizing.
Fig. 2: Illustration of gravitational radiation generated by the impending merger of two neutron stars. (Credit: R. Hurt/Caltech-JPL)
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