During this century, the search for a theory of quantum gravity has followed one or the other of two courses of investigation. In the first approach called 'Covariant Quantum Gravity', the problem of what to use as a background upon which to hang the gravitational field, was solved by breaking spacetime into two parts. One part would represent a flat spacetime, and a second part would act as the dynamic element of the gravitational field. In other words, the second component could be treated like an ordinary field that only weakly disturbs the background geometry of spacetime.
The conceptual difficulty behind this approach is that we were already being persuaded by experimental evidence that no other 'prior geometry' can be tolerated either by general relativity or careful searches for an immutable background reference frame to the universe. This means that after all the mathematical machinery of quantum gravity has run its course, the background field used in Covariant Quantum Gravity must utterly vanish as a completely undetectable scaffolding to the theory; an even more thorough dissappearing act than performed by virtual particles in QED.
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Gravitational waves are propagating gravitational fields, "ripples" in the curvature of space-time, generated by the motion of massive particles, such as two stars or two black holes orbiting each other. Gravitational waves cause a variable strain of space-time, which result in changes in the distance between points, with the size of the changes proportional to the distance between the points. Gravitational waves can be detected by devices which measure the induced length changes. Waves of different frequencies are caused by different motions of mass, and difference in the phases of the waves allow us to perceive the direction to the source and the shape of the matter that generated them. (Source: NASA-The Laser Interferometer Space Antenna (LISA).)