Scientists will soon launch a rocket to test unproven portions of Albert Einstein's theory of gravity
WASHINGTON - Nearly nine decades after Albert Einstein developed a radical new view on gravity, physicists are preparing a satellite- borne experiment, to be launched Monday, meant to verify two important predictions of that theory.
Einstein's general theory of relativity was published in 1916 and went beyond the classic physics of Isaac Newton in describing the gravitational forces at play between bodies large and small.
In Einstein's theory, gravity is not the result of some mysterious attraction between two objects but rather is a consequence of the fundamental structure of the cosmos.
A smaller mass is drawn toward a larger one because the larger one creates a bigger warp in the fabric of spacetime, the three dimensions of space and the fourth dimension of time treated as a single construct.
That fabric can be represented as a taut rubber sheet. Put a large object, such as the sun, on it and the sheet will sag deeply. Less massive objects, such as Earth, will tend to roll toward the larger mass.
As early as 1919, astronomers verified a key prediction of Einstein's theory by recording the bending of light from a distant star as it passed close to the massive presence of the sun. Another experiment in 1976, called Gravity Probe A, verified that the flow of time is slowed near a large body. A rocket carried an extremely precise atomic clock to an altitude of about 6,200 miles. As the Earth's pull weakened, the instrument showed that time speeded up almost imperceptibly - by about 1 part in 10 billion compared to the rate on Earth, according to Kip Thorne, a cosmologist at the California Institute of Technology.
The new satellite experiment, to be launched at 1:01 p.m. Monday aboard a Delta II rocket from Vandenberg Air Force Base in California, is called Gravity Probe B.
The warping of space
If all goes well, the experiment will verify not only the warping of space by the Earth's mass but also whether and how the rotating Earth drags space and time around with it, twisting it a bit like cotton candy spun around a stick.
That phenomenon is called frame dragging. At a recent NASA briefing on the upcoming experiment, Thorne said the Gravity Probe B will be the first instrument to directly measure frame dragging.
"We've never seen the dragging of space into motion by a sphere," Thorne said. It is the last major prediction of Einstein's theory that has not been experimentally verified, he said, though scientists have reported indirect evidence by precisely measuring shifts in the orbits of two laser-reflecting satellites in Earth orbit.
Major aspects of modern cosmology depend on the integrity of Einstein's theory, Thorne said, including the use of frame dragging in descriptions of the behavior of black holes, super-dense objects in space whose gravity is so intense that not even light can escape.
The principle behind the Gravity Probe B instrument is straightforward. Free of disturbing forces, a gyroscope pointed at a star should stay aligned indefinitely, according to Newtonian physics. But Einstein's theory says the spin axis and orientation of the gyroscope should change ever so slightly due to the warping and dragging effects of Earth's gravitational mass on local spacetime.
Perfectly round objects
The Gravity Probe B satellite has four small gyroscopes. The spheres, each about the size of a pingpong ball, are the most perfectly rounded objects ever manufactured, according to NASA. The quartz spheres are coated with niobium metal, which becomes superconducting when cooled by liquid helium to about 450 degrees below zero Fahrenheit. A spinning superconductor acts like a weak magnet, serving as a pointer for the spin axis of the gyroscope. Each gyroscope is more than a million times more accurate than the best inertial navigation gyroscopes used in aerospace applications, according to Francis Everitt, a Stanford University physicist and principal investigator for the experiment. Everitt has spent most of his career shepherding the oft-delayed, $700-million Gravity Probe B project to fruition.
Once the satellite is in orbit 400 miles above the Earth, the gyros will point a telescope at a guide star called IM Pegasi. They will remain locked on that star for about 13 months. Sensors will monitor the gyroscopes for evidence, as Einstein predicts, that they start to slowly point away from the guide star due to warping and dragging effects of Earth's mass.
"Gravity Probe B is one of the few space missions NASA has
conducted with relevance to fundamental physics," said a 1995
review of the project by the Space Studies Board of the National
Research Council. "If successful, it would assuredly join
the ranks of the classical experiments of physics. By the same
token, a confirmed result in disagreement with General Relativity
would be revolutionary."