We know that gravity is real, a force to be reckoned with by skydivers, roller coaster designers, space engineers and sneaker-clad kids in search of the perfect jump shot.

On a larger scale, gravity keeps the moon under Earth's sway and the planets in their celestial ballet around the sun. Without gravity, our Milky Way galaxy would fly apart, stars by the billions unleashed from their collective embrace.

Albert Einstein, who knew a thing or two about physics, theorized that gravity was not the result of some mysterious attraction between two objects. Rather, it is a fundamental consequence of the structure of the cosmos. A smaller mass is drawn toward a larger mass because it travels through space that is warped by the larger object.

Physicists say it is useful to imagine space as if it were a stretched sheet of rubber. Drop an object like our sun onto it and the sheet sags. Drop a smaller object on it, say Earth, and the smaller mass will roll toward the larger mass.

The first observational test of Einstein's General Theory of Relativity was made in 1919 by a solar-eclipse expedition led by British astronomer Arthur Eddington. They photographed stars in the neighborhood of the eclipsed sun and found them to be shifted slightly away from their normal positions relative to each other. The scientists concluded that light waves from the stars were bent by the gravitational mass of the sun as predicted by Einstein's theory.

The theory also predicts that very large masses moving quickly through space should create ripples in the fabric of space, like waves on the surface of a cosmic pond.

Eddington, while a fan of Einstein's theory, was skeptical about the existence of such gravity waves, however. He and others questioned whether they might merely be ghostly artifacts of the equations of Einstein's theory. Eddington reportedly asked whether "gravitational waves propagate at the speed of thought."

During the 1960s, Joseph Weber of the University of Maryland claimed to have found evidence of gravity waves by listening for their "ring" in a large suspended bar of aluminum. But his claims were not replicated by others.

There is reasonably good indirect evidence, however, that gravity waves exist. Einstein's theory predicts that the biggest waves would be caused by huge events such as the slow death spiral of two neutron stars. Astronomers have observed several examples in which pairs of neutron stars, merging in a death spiral, were emitting gravitational energy at just the rate predicted by Einstein.

While some experimenters continue to search for gravity waves with the technique pioneered by Weber, sensitive interferometers such as those operated by the Laser Interferometer Gravitational-Wave Observatory (LIGO) are considered the best bet for eventually finding direct evidence that gravity waves are real.