In September 2015, gravitational waves were detected for the first time. Generated by the collision of two black holes 30 times as massive as the Sun, for a brief moment these waves transmitted more power than all the stars in the Universe combined – yet their only measured effect was to jiggle pairs of mirrors, separated over several kilometres, by less than the diameter of a proton.
What are these elusive phenomena, emitted by some of the strangest objects in the Universe and detected with our most sensitive instruments?
Gravitational radiation, or gravitational waves, are often described as "ripples in the fabric of spacetime", and depicted using rubber-membrane images like the one shown here. This description is poetic, and fits with the gravity-as-curvature interpretation of Einstein's General Theory of Relativity. However, such imagery is not necessarily the best approach to understanding the mechanics and properties of gravitational waves.
Gravitational radiation is not unique to General Relativity, but is a necessary consequence of any theory of gravity that obeys causality (changes in a field are felt at a distance 𝑟 after a time no less than 𝑟/𝑐 where 𝑐 is the speed of light) and conservation of energy (the propagating part of the field diminishes as 1/𝑟).
This is the case for other fields besides gravity: gravitational radiation is analogous to electromagnetic radiation, a more familiar phenomenon that covers everything from radio waves, through visible light, to x-rays and gamma rays. The following pages begin with a discussion of how electromagnetic radiation arises from the principle of causality, and how a similar underlying mechanism leads to gravitational radiation. I will then explain how gravity differs from electromagnetism, and how this affects the generation and detection of gravitational waves.
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