Since then, spatial curvature has been tested and confirmed to greater and greater accuracy, giving physicists enormous faith in the general theory of relativity--but not necessarily absolute faith. "The very fact that Einstein was so successful has a tendency to make people shrink from saying something may be not right,'' says C. N. Yang, a Nobel Prize- winning physicist at the State University of New York at Stony Brook. "But science progresses precisely by finding fault with what has already been accepted." And, surprisingly, the curvature of space-time in its various manifestations is the only direct effect of general relativity that has ever been tested. Frame-dragging, which can be considered a distinct phenomenon entirely, if not a completely new kind of force, has never been measured or observed directly.

In 1918, two years after Einstein formulated his theory and the year before the eclipse that confirmed it, two Austrian physicists, Josef Lense and Hans Thirring, calculated that as a natural consequence of Einstein's theory, a massive body spinning in space would drag space and time around with it. To explain precisely why this is so is difficult, but four decades ago, when Leonard Schiff took to thinking about the effect, he suggested a purely intuitive illustration: Imagine Earth immersed in a viscous fluid, he said, like molasses. Spin the planet, and the molasses, depending on just how viscous it is, will be pulled around with it. Any object in the molasses will be pulled around as well. This frame-dragging effect should be most noticeable close to the rotating Earth, and should eventually fade to virtually nothing farther away.

Measuring such an effect is where the perfect gyroscope comes in. Gyroscopes have been around since the early 1850s, when French physicist Jean-Bernard-Léon Foucault invented them to demonstrate that Earth rotated. (Foucault's more famous pendulum served the same purpose.) A gyroscope is little more than a flywheel--a bicycle wheel, for instance--that spins around an axis. But once the wheel is set spinning, the axis of the gyroscope will keep pointing in the same direction as long as no other force comes along to reorient it. This effect depends on a principle of physics known as the conservation of angular momentum, which explains why, for instance, it's easy to sit upright on a bicycle when it's rolling along and the wheels are spinning, and much, much harder to do so when it's at rest.

As Schiff figured it, the way to measure frame-dragging would be to take a perfect gyroscope--one that would continue to spin and point in the same direction effectively forever--and send it into space. This gyroscope could be set spinning with its axis pointing at some distant object whose position in the universe appeared fixed--a star, for instance. If the gyroscope was sufficiently close to perfection and frame-dragging was just a figment of Einstein's imagination, then the gyroscope would go on pointing at its chosen star for eternity. But if frame-dragging existed, the gyroscope would keep pointing in the same direction with respect to local space-time, but that local space-time would be dragging along with Earth. As a result, the axis of the gyroscope would slowly begin to slip off its alignment with the star. Or, as Everitt puts it, "the star is providing a reference to what space is doing out there, and the gyroscope is affected by what space is doing right down here. And if we line up our gyroscope on a distant star, it is not necessarily going to stay lined up on that star."

The effect is like that of a sundial. We cannot feel Earth's rotation, but if we align a sundial with a relatively fixed object like the sun, it will soon become obvious that one of the two is moving. Unlike the rotation of Earth, however, frame-dragging is an infinitesimal phenomenon.

Suppose a perfect gyroscope were put into orbit 400 miles up, where it would be beyond most other Earthly disturbances. As Schiff figured, frame-dragging would skew the gyroscope from its original orientation by some 42 thousandths of an arc second (or seven ten- thousandths of a degree) each year. That's not much. A thousandth of an arc second is the perceived width of a human hair when viewed from ten miles away, which means that 42 milli-arc seconds is the width of a single human hair seen from a quarter mile away.

With this in mind, the Gravity Probe B experiment can be thought of as two coupled pointers that orbit Earth together. On the one hand is a telescope that will set its sights on the distant star and keep pointing at it no matter what. On the other are the perfect gyroscopes, in this case four of them, which will initially be aligned on that distant star as well. As local space drags after Earth, however, the gyroscopes should drag with it, while computers and excruciatingly gentle thrusters on the satellite will keep the telescope pointing at the star. Given time, the misalignment between telescope and gyroscopes should become big enough to measure--if just barely.

That the change in alignment over the course of a year will add up to less than the width of a human hair explains much of what the Stanford scientists have been doing for the past few decades. They have had to develop not only near-perfect gyroscopes but a near-perfect environment in which to spin them, and a near-perfect telescope and guidance system to keep that telescope lined up on a suitably distant star. The Gravity Probe B crew like to boast that their success at creating a "near-zero environment"--no gravitational acceleration (other than perhaps frame- dragging), no atmosphere, no magnetic field, no electric field, no nothing- -will give them a measuring device of extraordinary sensitivity. "There is no substitute for lots of sensitivity," says Everitt, defining the philosophy of the program.

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