When NASA's Gravity Probe B (GP-B) satellite launches on April 17th, it will begin a rigorous test of Einstein's general theory of relativity. The result will either support or challenge one of the fundamental tenets of modern physics. This seminal test would not be possible without a key contribution by scientists at the Harvard-Smithsonian Center for Astrophysics (CfA). Their ground-based measurements will be combined with those from the satellite to determine precisely how the Earth's mass and rotation affect the fabric of space and time.
"This NASA mission is the culmination of 45 years of work at Stanford University. Here at the CfA, we are providing a key piece of astronomical information that Stanford needs to complete its test of Einstein's theory with the full intended accuracy and reliability. The satellite measurements have to be adjusted for at least one very significant astronomical effect. We are using radio telescopes to measure the required adjustment," says Smithsonian radio astronomer Michael Ratner (CfA), who works with CfA Director Irwin Shapiro on the project.
At the heart of the GP-B experiment are four gyroscopes, each holding a precisely fabricated spinning mass the size of a ping-pong ball. They are kept nearly free from disturbance, so that they provide an almost perfect space-time reference system. By measuring, very precisely, tiny changes in the direction of spin of those four gyroscopes, physicists will calculate how space and time are warped by the presence of the Earth, and how the Earth's rotation drags space-time around with it. These effects, though small for the Earth, have far-reaching implications for everything from the nature of black holes to the structure and evolution of the universe.
"Our understanding of cosmology is based on the interpretation of astrophysical data and assumes that general relativity is correct. If general relativity were found to be substantially wrong, it would have a profound effect on our description of the cosmos and its history. Even a small discrepancy found in a local measurement, like the one to be made by GP-B, could strongly affect our understanding of the universe," says CfA Associate Director Robert Reasenberg, who serves on the GP-B Science Advisory Committee and has worked on tests of general relativity for three decades.
According to general relativity, we live in a four-dimensional universe. Space and time are interwoven and inseparable. The presence of any massive object like the Earth will bend the space-time fabric, essentially warping space.
One prediction of the theory of general relativity is that starlight will be bent by gravity. That effect was first measured in 1919 during a total solar eclipse. Other predictions are not so easy to confirm.
One of the more unusual predictions of general relativity is the phenomenon of frame dragging, in which the rotation of a massive object like the Earth causes a twisting of the local space-time fabric. A visible result of this effect is that the direction of the spin axis of each gyroscope on the spacecraft turns very slowly toward the east.
Each spin axis also will appear to turn at a faster rate in the north-south direction, as a result of the spacecraft's polar orbit through the warped space around the Earth. Gravity Probe B will measure both effects during its approximately 1-1/2 year mission.
In order for GP-B to measure any "twist" or curvature of local space-time, it must use gyroscopes that are nearly perfect, which will not wobble or drift while spinning. Using ultrastable gyroscopes ensures that any angular change to the gyroscope's spin axis is due to relativistic effects.
Gravity Probe B carries a telescope that focuses on a guide star in order to provide a reference point for measuring tiny deflections in the gyroscopes' spin axes. The whole spacecraft is continually kept aligned to this star. Yet the star shifts its apparent position as both it and the Sun independently orbit the center of the Milky Way. As seen from the GP-B spacecraft, the apparent position of the guide star also is affected by the spacecraft's orbit around the Earth and the Earth's orbit around the Sun.
To compensate for those effects, Shapiro and his colleagues have monitored the GP-B guide star for the past 7 years using a variety of radio telescopes. That monitoring will continue through the lifetime of Gravity Probe B. Only after all of the spacecraft data have been collected will the calculations be made that will test Einstein's theory.
"If the predictions of general relativity are confirmed, I personally would feel a sense of satisfaction that not only Einstein's historic theory of gravity, but also the many years of work on the GP-B project, had all succeeded spectacularly. Conversely, if the Gravity Probe B results are inconsistent with Einstein's theory, I would be excited about the prospect of what might come next. It certainly would motivate a fresh look at the foundations of physics," says Ratner.
He adds, "No matter what the outcome, the Gravity Probe B program will stand as a testament to the determination of scientists to subject even their favorite theories to empirical tests. It is this process of testing that gives science its ongoing validity, even while driving science unpredictably onward."
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