We've indirectly measured gravitational waves before. Folks won the 1993 Nobel prize in physics for the observations. The "Hulse-Taylor" binary star system is a

We've indirectly measured gravitational waves before. Folks won the 1993 Nobel prize in physics for the observations. The "Hulse-Taylor" binary star system is a pair of neutron stars orbiting very close to each other (the pair of neutron stars orbiting each other would fit inside of our own Sun). Because the stars are pulsars it's possible to measure the orbit with extremely high precision over time. One prediction of the theory of gravitational waves is that the emission of gravitational waves actually removes small amounts of energy from an orbital system over time (like a drag force). But normally these amounts are so small as to be inconsequential even over extremely long periods, but with very dense, very massive, very closely orbiting objects the amount is much higher. And the result is that it causes the orbits to degrade over time, as the stars slowly spiral toward each other. This is a very consistent effect. Here's a graph showing the prediction of the existence of gravitational waves relative to the observations of the change of period of the Hulse-Taylor binary: http://en.wikipedia.org/wiki/File:PSR_B1913%2B16_period_shift_graph.svg http://en.wikipedia.org/wiki/File:PSR_B1913%2B16_period_shif... The tiny horizontal lines on the red dots are the error bars of the observations. You can see how perfectly the observations line up with theory. Also, it's likely that gravitational waves will be observed directly within the next several years. There have been experiments attempting to do so but none of them have been sensitive enough to detect the most likely of the strongest signals of gravitational radiation. The improvements to those experiments that will come online within the next decade or so should be able to do so however.