Gravity Probe B: Relatively Important?

You may have read Wednesday's news: Albert Einstein, Time magazine’s “Man of the Twentieth Century,” indeed did not goof up when he put forth the theory of general relativity.

NASA’s $750 million satellite Gravity Probe B proved that time and space do curve near massive objects like Earth, and also that space and time are dragged along a tiny trace by Earth’s rotation.

But wait a minute! Didn’t we already know this? To high precision?

The truth is, we did. The LAGEOS satellites, lunar ranging, the Cassini mission’s radio experiment, and binary pulsars, to name just a few, have all verified general relativity — including these two particular predictions — sometimes to much higher accuracy than Gravity Probe B (Sky & Telescope, July 2005 issue, page 33).

At least Gravity Probe B will go down in history for being the only experiment to prove Einstein right in one particular way so close his home planet.

GP-B was the child of five decades of lobbying, planning, and execution. It was conceived in 1959 and launched in 2004 to test two predictions of general relativity. The first, the “geodetic effect,” describes the dent that Earth causes in the fabric of spacetime because of its mass. For a two-dimensional analogy, think of a bowling ball sitting on a trampoline.

The second is “frame-dragging,” which Joseph Lense and Hans Thirring proposed in 1918 using Einstein’s theory. This is the amount by which Earth twists spacetime as it spins on its axis. “Imagine the Earth as if it were immersed in honey,” says Francis Everitt, GP-B’s principal investigator at Stanford University. “As the planet rotates, the honey around it would swirl, and it’s the same with space and time.”

Frame dragging results from a general-relativistic effect called gravitomagnetism — so named because it’s analogous to the way magnetism was revealed to be merely the special-relativity transformation of an electrostatic field that moves. So, just as a moving electron creates a magnetic field, a moving mass generates a gravitomagnetic field. The field exerts a sideways force, similar to the force a magnetic field exerts on a charged particle, on any mass moving though it, causing the object’s path to deflect.

This was heady stuff in 1959. But by the time of GP-B’s much-delayed launch, other confirmations of these effects were dissipating much of its rationale.

A Long-Sought Goal

The experiment’s basic design was born in 1920, when physicists J. A. Schouten and Arthur S. Eddington suggested the use of gyroscopes to test general relativity. In 1959 MIT physicist Donald Pugh suggested using a satellite, whose motion would be compensated for atmospheric drag, to provide a perfect, zero-g inertial reference frame in which to hold the gyroscopes.

Orbiting Earth in a special housing at a height of 400 miles, GP-B’s gyroscopes consist of four spheres the size of Ping-Pong balls coated with niobium. They were cited in the Guinness World Records as the most perfectly spherical objects ever made. The housing was set to maintain a perfect lock on a guide star (IM Pegasi). If Newtonian physics were all that applied, the gyroscopes would never change orientation and stay pointed at the star. If Einstein were right, the two effects would cause the gyroscopes to drift by a tiny angle, measured in arcseconds, over the course of the mission.

But GP-B’s confirmation of the geodetic effect to a precision of 0.3% (6,601.8 ± 18.3 milliarcseconds) is not exactly stunning. Astronomers had already done this to 0.002% accuracy, 150 times better, by measuring the time delay in radio signals from the Cassini spacecraft as they passed through the gravitational field of the Sun.

“I am pleased but not impressed by the geodetic-precession part of the result,” said Michael Kramer, a member of an international team studying binary pulsars and a contributor to S&T on the subject (August 2010 issue). “However, for the frame dragging [part], I still think it is a significant advance as it really shows a direct measurement of the effect on a spinning top, which is really quite nice.”

But GP-B confirmed frame-dragging to only 20% accuracy (37.2 ± 7.2 milliarcseconds). Kramer called this “a very nice stone in the big mosaic to understand gravity,” while explaining that experiments on binary pulsars are aiming to calculate frame-dragging more precisely than GP-B.

And here, lunar-ranging experiments are way ahead. The Apollo astronauts left retro-reflector mirrors on the Moon, and laser ranging from Earth can now track their positions to millimeters. At that level of precision, the Moon’s motion in orbit has confirmed gravitomagnetism, the source of frame-dragging, to 0.15%, or 130 times better than GP-B.

“I won’t say there is no value in testing physics in a novel way,” says Tom Murphy (University of California in San Diego), a member of the lunar-ranging project, “but any discrepancy would have been incredibly jarring.”

Other physicists have said that if GP-B produced any other result than it did, they would probably just assume that GP-B’s engineers had pushed their technology too far.

Costs and Benefits

So, was it worth $750 million?

NASA considered terminating the GP-B program several times during its protracted development. As recently as 2008, a panel of 15 scientists ranked GP-B last in a review of which space-science missions should receive funding. But a very perseverant team led by Everitt lobbied Congress hard to see the mission through to completion, bypassing normal channels.

Speaking at a NASA press conference on Wednesday, Rex Geveden, GP-B’s program manager at the time of its launch (now the president of Teledyne Brown Engineering), said “Gravity Probe B is about science, technology and a triumph of the human spirit in the end.” The press-conference panel also boasted of GP-B’s educational value. The lengthy mission served as a training ground for 100 doctoral students, 15 other graduate students, 350 undergraduates, and more than four dozen high-school students.

Those who have long been following the mission heard a note of defensiveness here. Every space mission involves students; rarely does NASA have to cast that far to help justify it.

Even so, one can’t help but wonder what Einstein might have said if he were here to see the results. We can only look back to 1919, when shortly after the solar eclipse that provided general relativity’s very first (though weak) confirmation, he was asked what he might have felt if his prediction had been wrong. “I would feel sorry for the dear Lord,” he said. “The theory is correct.”