Gravitational Waves
Gravitational waves are ripples in the curvature of spacetime that travel at the speed of light, generated by accelerating masses such as orbiting compact objects, and now directly detected, opening a new window on the universe.
Definition
Gravitational waves are propagating, transverse perturbations of the spacetime metric, solutions of the linearized Einstein equations, that carry energy and momentum away from accelerating, non-spherically-symmetric mass distributions and stretch and squeeze the distances between freely falling test masses.
Scope
The area covers the theory of gravitational radiation: the linearized Einstein equations and their wave solutions, the two transverse polarizations and the effect of a passing wave on free masses, the quadrupole formula for emission, the astrophysical sources, and the laser-interferometer and pulsar-timing techniques used to detect waves and read out the properties of their sources.
Sub-topics
Core questions
- How do the Einstein equations predict wave-like solutions, and how fast do they travel?
- What kinds of astrophysical systems emit detectable gravitational waves?
- How can such tiny distortions of spacetime be measured?
- What new astrophysics is revealed by detecting gravitational waves?
Key concepts
- Linearized Einstein equations
- Transverse-traceless gauge
- Two polarizations (plus and cross)
- Quadrupole formula
- Strain
- Multi-messenger astronomy
Key theories
- Linearized gravity and wave solutions
- Expanding the metric around flat spacetime and choosing a suitable gauge reduces the Einstein equations to a wave equation, whose solutions are transverse, traceless gravitational waves with two polarizations traveling at the speed of light.
- Quadrupole formula
- To leading order, the gravitational-wave luminosity of a source is set by the third time derivative of its mass quadrupole moment, so only non-spherical, accelerating mass distributions radiate, and the emission is typically very weak.
Clinical relevance
Gravitational-wave astronomy has become an observational science: detections of merging black holes and neutron stars test general relativity in the strong-field, dynamical regime, measure the masses and spins of compact objects, provide an independent route to the expansion rate of the universe, and, when paired with light, enable multi-messenger studies of cosmic explosions.
History
Einstein predicted gravitational waves in 1916 and long doubted their reality; indirect evidence came from the orbital decay of the Hulse-Taylor binary pulsar in the 1970s, and after decades of detector development the LIGO interferometers made the first direct detection of a black-hole merger in 2015, recognized with the 2017 Nobel Prize.
Debates
- Reality and energy of gravitational waves
- For decades it was disputed whether gravitational waves were physical or pure gauge and whether they carried energy; the sticky-bead argument and the eventual detections settled that they are real and transport energy, though subtleties of gravitational energy localization persist.
Key figures
- Albert Einstein
- Joseph Weber
- Rainer Weiss
- Kip Thorne
- Barry Barish
Related topics
Seminal works
- einstein1916b
- abbott2016
Frequently asked questions
- What does a gravitational wave physically do as it passes?
- It alternately stretches space in one transverse direction while squeezing the perpendicular direction, changing the separation between freely falling masses by a tiny fractional amount; this oscillating strain is what interferometers are built to measure.
- Why are gravitational waves so hard to detect?
- Gravity is extraordinarily weak, so even violent astrophysical events produce strains of order one part in 10^21 at Earth, requiring kilometre-scale interferometers stabilized against every competing source of noise to sense distance changes far smaller than a proton's width.