Testing Relativity
Published on Apr 7, 2007 at 11:17 pm.
1 Comment.
Filed under physics.
In 1915, Albert Einstein set forth his general theory of relativity. Like the special theory, general relativity shows that time and space are relative rather than absolute quantities. Special theory has been tested many times. General theory, though, is a bit tougher to test in full. Among the predictions of general relativity are that space is curved by gravity. One of the first attempts to test general relativity came only a few years after Einstein’s theory was posted. In 1919, Sir Arthur Eddington set forth to measure the effects of gravitationally warped space by observing stars the Sun in the sky during a solar eclipse. The effect of the Sun’s gravity would shift the apparent position of the stars slightly. He confidently announced that his findings confirmed the predictions of general relativity. However, later analysis of his results show that his observations were at the very limit of his ability to detect, and such tentative findings today would probably not be accepted for publication. But, more recent observations of gravitational lenses and distortions of light due to gravity do indeed occur. Other predictions of general relativity have also been tested. Each test was in agreement with theory. However, most of these tests have involved electromagnetic waves.
April 20, 2004, though, a Boeing Delta rocket lifted off from Vandenberg Air Force Base with a highly sophisticated satellite designed to use mechanisms to test general relativity. This is the Gravity Probe B satellite. The Gravity Probe B satellite used extremely accurate gyroscopes to measure the distortion of space by Earth’s gravity.
There are two main experiments. One is to monitor the precession of the gyroscopes due to the warping of space by Earth. This curvature of space causes the gyroscopes to precess by about 6.6 arcseconds per year, well within the ability of the satellite to measure. The curvature of space has been determined before by observations of the gravitational bending of light. It has also been determined through the precession of Mercury’s orbit, which is slightly more than that predicted in Newtonian physics, but is in keeping with expectations from general relativity. Even the tiny effect that Earth has in warping space has been measured as far back as 1988. But, Gravity Probe B was designed to measure this effect in far higher precision than ever done before. This will show whether the effect is actually the size predicted by general relativity.
Another experiment that Gravity Probe B has worked on was an attempt to measure the effect of Earth’s frame dragging. This occurs when a body rotates and drags space along with it (for lack of a better way of describing it easily). In essense, it is shorter one way around the Earth than the other. Frame dragging is a big deal near a rotating black hole or neutron star, where the gravitational fields are far stronger. But, it also exists around Earth, though it is such a small effect that it has never been measured before. The effect on the gyroscopes should be only about 0.041 arcseconds per year. This is a tiny effect, but it should be within the spacecraft’s ability to measure.
Gravity probe B operated for a bit over a year before running out of liquid helium (needed for the superconducting magnets and magnetic shields). Data collected by the satellite have been carefully studied. All systematic errors need to be accounted for, and all noise and other interference needs to be measured and accounted for. Well, the project scientists are about to release their first findings in about a week. It should be interesting to see what the results are.
I have spoken with someone on the project, and he assured me that there are results, but he wouldn’t say what they are. We’ll just have to wait!
-Astroprof
Image courtesy of Wikimedia
Astroprof’s Page » Einstein was right on April 15, 2007 at 12:46 am: 1
[…] Well, it looks like Einstein was right. The initial results of the Gravity Probe B mission have been released, and the geodetic precession predicted under Einstein’s general theory of relativity has been measured to better than 1% accuracy. The gyroscopes precessed just as predicted. This is the first non-electromagnetic test of general relativity, and it worked fine. The frame dragging experiment that the satellite was to measure is still being evaluated. The frame dragging data was buried by a tiny torque acting on the gyroscopes. However, this is a systematic error, and it is believed that it can be modeled and the effect of the torque removed from the data. Hopefully, what will be left will be the effect of frame dragging. […]