Gravitational Waves
Gravitational waves are propagating disturbances of the gravitational field — in the language of general relativity, ripples in the curvature of spacetime — predicted to be radiated by accelerating masses and to travel at the speed of light.
Einstein derived them in 1916 and 1918 from the linearised field equations, obtaining the quadrupole formula for the power radiated by a system whose mass distribution changes shape. Because gravity is extraordinarily weak, the expected effect is minute: even a merger of two stellar-mass black holes at cosmological distance produces a fractional change in the separation of two test masses on Earth of order one part in 1021. The first strong evidence was indirect. Russell Hulse and Joseph Taylor discovered the binary pulsar PSR B1913+16 in 1974 and found its orbital period shrinking at very nearly the rate general relativity predicts for gravitational-wave emission; they shared the 1993 Nobel Prize.
Direct detection was announced on 11 February 2016 by the LIGO collaboration, from a signal recorded on 14 September 2015 and attributed to the merger of two black holes of roughly thirty solar masses. Rainer Weiss, Barry Barish and Kip Thorne received the 2017 Nobel Prize. In August 2017 LIGO and Virgo recorded a signal attributed to a neutron-star merger that was followed within seconds by a gamma-ray burst and subsequently by optical observations — the event most often cited as clinching the case, since it ties the interferometer signal to an independently observed astronomical source. More recently, pulsar timing arrays have reported evidence for a low-frequency gravitational-wave background.
The detectors are kilometre-scale laser interferometers, and what they measure is a differential change in arm length far smaller than a proton. Extracting a signal requires matched filtering against a bank of theoretical waveform templates — which is the technical point at which most of the criticism below is aimed.
Open questions in the standard account
Even inside mainstream relativity, gravitational radiation was contested for four decades. Einstein himself doubted the waves' physical reality in the 1930s, and the question was only settled to most relativists' satisfaction at the 1957 Chapel Hill conference, where Felix Pirani, Hermann Bondi and Richard Feynman argued that the waves must carry energy because they can do work on test particles. The underlying difficulty — that the energy of the gravitational field in general relativity is described by a pseudo-tensor rather than a true tensor, and so has no unambiguous local value — has never been removed; it is instead handled by defining energy globally, for asymptotically flat spacetimes.
On this wiki
Several researchers catalogued here dispute either the theoretical basis of gravitational radiation or the detection claims. Related material is under Category:Relativity and Category:Gravity.
The pseudo-tensor objection. Stephen John Crothers argues that gravitational waves as usually described are inconsistent with general relativity itself: since the field equations are non-linear, solutions cannot be superposed, and since the gravitational field's energy-momentum is a pseudo-tensor it does not satisfy the theory's own requirement that physical quantities be tensors. He develops this in General Relativity – A Theory in Crisis and applies the same reasoning to black holes, the assumed sources of the detected signals. Vyacheslav N Streltsov presses a parallel objection about observability in Black Hole Unobservability in General Relativity.
The detection analysis. Chung Y Lo attacks the theoretical foundation of the LIGO analysis in Einstein's Equivalence Principle and Invalidity of Thorne's Theory for LIGO, arguing that the waveform theory used to model the sources is not valid. Because the events are recovered by template matching, this is a criticism of the whole detection chain and not merely of a coefficient — the point being that a search which can only find what its templates already describe cannot independently confirm the templates.
Alternative accounts of gravitational propagation. Satya Pal Asija relates gravitational to electromagnetic propagation in Electromagnetic Light Waves & Gravitational Waves; Peter C M Hahn considers an unconventional use of the signals in Searching Gravitational Waves for Intelligent Messages. Broader critiques of general relativity from which these positions descend are collected by Raymond H Gallucci and others under Category:Relativity.
It should be said plainly that the binary-pulsar orbital decay and the 2017 multi-messenger event are the two strongest pieces of evidence, and that any critique of gravitational waves has to address them rather than the interferometers alone.