The GP-B press release in plain English?
magnetar said:
The gravity probe-b mission had finished collection date for a long time .Why not announce any result ??
Do you think that the results will consistent with "general relativity"?
pervect said:
I'm also quite curious as to the GP-B results, though I don't have anything riding on it.
After more than forty years (!) of development, GP-B was finally launched in April 2004. It recorded data for more than a year, and the Stanford team has been analyzing this data ever since. They will hold a press conference on the 18th but a press release has been made available today. The total cost of the experiment is said to be some 700 million dollars. For comparison, this is less than the cost of a one mile stretch of a proposed six lane traffic tunnel in an American city.
Recall that
geodetic precession or
de Sitter precession of the spin axis of a gyroscope in a quasi-Keplerian orbit around a nonrotating massive object (think of a Schwarzschild object, treated in weak-field gtr) has already been confirmed to good accuracy by previous work. See http://science.nasa.gov/headlines/y2000/geodetic.htm . The goal of GP-B is to measure a much smaller effect, the
frame-dragging precession or
gravitomagnetic precession, or
Lense-Thirring precession, which takes account of rotation effects when the massive object is spinning about its own axis (think of a Kerr object, treated in weak-field gtr). These effects should not be confused with the precession of the periastria (locus of closest approach) of a small object in quasi-Keplerian orbit around a massive object.
Recall also that there is a large class of metric gravitation theories (certain relativistic classical field theories) which behave much like gtr but are more complicated in various senses. Most of these also predict de Sitter and Lense-Thirring type effects, but with magnitudes differing from the gtr prediction, in any given situation--- but possibly not by very much. So the goal of GP-B and other tests can be understood as asking the question: "Do we need to consider more complicated theories than gtr, or does gtr suffice to account for all the gravitational effects we can directly test?" See http://math.ucr.edu/home/baez/RelWWW/HTML/grad.html#tests for background on the subject of testing theories of gravitation.
To forestall possible misunderstanding:
In the (embargoed) press release at
http://einstein.stanford.edu/, the GP-B team says:
1. The preliminary data analysis did confirm the de Sitter precession formula to higher accuracy than ever before.
2. It uncovered two unforeseen sources of error involving classical physics of the experimental equipment--- basically, a telescope in an Earth-orbiting satellite. As I understand it, both new error sources arise from a kind of electrostatic friction in the telescope mounting. Figuring this out took a lot of work. The effect is well known in physics, but when the experiment was originally designed, it was thought that the resulting errors would be self-canceling, but this turns out not to be true.
3. These errors need to be carefully modeled before they can be removed in order to test the Lense-Thirring precession formula.
4. The Stanford team hopes to complete this analysis by Dec 2007.
For a more detailed discussion, try
http://twistedphysics.typepad.com/cocktail_party_physics/2007/04/the_not_so_frie.html