To make the test a little more interesting, frame-dragging and general relativity have cosmological implications ranging from the cataclysmic to the borderline metaphysical. On the cataclysmic side, for instance, are quasars. Take ngc 6251, which for the past 3 million years has been disgorging two monstrous jets of gas in opposite directions. The astrophysical engine at work here is generating the power of 1 trillion suns. And the prime candidate for such a generator is a supermassive black hole. But then there's another condition that the engine must fulfill: in NGC 6251, the jets of gas head straight outward, which means the source of these jets has remained pointed, unwavering, in the exact same direction for 3 million years.
"These compact objects must hold jet direction constant for times as long as 10 million years," says Kip Thorne, a gravity theorist at Caltech. "The only way a black hole can do this is by the gyroscopic action of its spin." In the most likely scenario, the black hole communicates the direction of its spin to the jet via frame-dragging. The frame-dragging pulls particles from near the black hole in a spiral toward the hole's rotational poles, then hurls them straight out in spectacular jets. None of this was known three decades ago, when Gravity Probe B was conceived. The experiment, says Thorne, has gone from one searching for a side effect of general relativity so infinitesimal "as to be interesting in principle but not in practice" to "a crucial test of a possible mechanism of the most violent explosions in our universe."
Then there's the near metaphysical implications of frame- dragging, which have to do with the concept of inertia, or a body's natural resistance to acceleration. (As Newton put it, a body in motion will remain in motion and a body at rest will remain at rest unless acted upon by an external force.) In particular, Gravity Probe B will test the proposition that the inertia of a massive body--Earth, for instance, or anything or anyone on it--is a consequence of the gravitational interaction, the frame- dragging, from all the other mass in the universe. In other words, says Everitt, "the stars out there create our inertia here." This concept is known as Mach's principle, after the great Austrian physicist Ernst Mach, who first proposed that the local properties of matter might originate in the properties of distant bodies.
Although Mach's principle has an astrological ring to it, it can be derived from a simple thought experiment. Imagine you are inside a massive spherical shell that is rotating around you in space--which, after all, is one way of looking at our actual situation. (You can think of Earth as rotating, but you can just as easily think of Earth as standing still while the rest of the universe rotates around it.) Now, with frame- dragging, the spherical shell very slightly pulls space and time around with it, which means you, too, will rotate slightly along with it. That's simple enough. Double the mass of the shell, and you double the frame- dragging effect. Now, consider that sphere getting more and more massive while simultaneously shrinking down around you until it's so dense, and the frame-dragging is so great, that as it rotates, you and your immediate universe will rotate right along with it. Now, the universe might conceivably be that massive; all the stars and galaxies and clusters of galaxies in the universe might really add up to so much mass that they serve the purpose of this supermassive rotating sphere. If so, we're all locked inside, with our inertia coming to us from the frame-dragging of all the distant stars.
To most physicists, inertia is a product of the masses of elementary particles, and those masses are determined by an esoteric concept known as the Higgs mechanism, which has nothing to do with gravity or general relativity. Since one of the goals of theoretical physicists is to find a single theory--known colloquially as a theory of everything--that uses a single, consistent set of mathematical equations to explain both the microscopic world of quantum physics and the macroscopic world of gravity, one way to go about connecting those two worlds would be to find out where inertia comes from. Does it come from the macroscopic world, the Einsteinian frame-dragging of the universe at large, or from the microscopic world of quantum physics? Since a potential theory of everything should resolve Mach's principle along with gravity and the quantum forces, an experiment, like Gravity Probe B, that can provide some insight may be helpful.
That does not mean, however, that everyone in the scientific community believes Gravity Probe B is worth the investment, which will eventually add up to more than half a billion dollars. Over the long years of the project, Everitt has become famous for working the congressmen and bureaucrats in Washington to keep his funding going, and some of his colleagues have accused him of selling his science to Washington before he sold it to them. In the early 1990s, a vocal contingent of astrophysicists and cosmologists contended that Gravity Probe B was way too expensive to be continued. They argued that the probe was effectively a physics project, but the money for it was coming from their astrophysical NASA budget. That budget, they pointed out, was shrinking with every passing year, and half a billion dollars constituted a disproportionate share for a project that would very likely only confirm something that everyone already believed. And even if Gravity Probe B were to find that the size of the frame- dragging effect differs significantly from the prediction of general relativity, no one except Everitt and his colleagues would believe it anyway. The investment in Gravity Probe B would immediately require that somebody spend another half billion dollars to build Gravity Probe C to confirm the discovery.
