Thursday, May 05, 2011

Gravity Probe B final results: frame dragging within 20 percent

Stanford's $0.76 billion satellite mission to test Einstein's theory of gravity, Gravity Probe B (Wiki), has announced the final results during the press conference aired by NASA TV a few hours ago. The results will be published in Physical Review Letters:
Gravity Probe B: Final results of a space experiment to test general relativity
If you read the abstract of the paper above, you will see that their measured geodetic effect is -6,602 mas/yr (milli-arcseconds per year), while the prediction of GR is -6,606 mas/yr (an excellent agreement within 0.06 percent or so; however, their official error margin, 18.3 mas/yr, is larger - 0.3% of the figure).

The more difficult measurement of the frame-dragging drift rate gave them -37.2 mas/yr, to be compared with -39.2 mas/yr according to GR. The discrepancy is just 7 percent or so. However, their actual announced error is closer to 19 percent (7.2 mas/yr), so they can barely prove that the frame-dragging effect is nonzero at the 5-sigma confidence level.

The accuracy of the frame-dragging measurement by Gravity Probe B has been a major disappointment; the previous, incomplete results from the probe were not even able to prove at the 5 sigma level that the effect was nonzero. Instead of a 19% error, the plans expected a 1% error; the main source of trouble has been electric polarization of the gyroscopes that contaminated the signal with lots of Coulomb forces whose harmful impact was amplified by irregular patches on the surface of the spheres. Because of that, Gravity Probe B actually turned out to be less accurate than the LAGEOS experiment that measured frame dragging within 10% in 2004.

Nevertheless, if you ask whether the probe has helped to dramatically eliminate doubts that GR is correct once again, the answer is a resounding Yes.

Remarkably, I first learned about the final results from Anthony Watts' climate blog. He is a fellow skeptic and my bookmarks equally contain the "mainstream scientific" blog Real Climate. Despite this equal status, I have learned about dozens of interesting hot news from Anthony Watts - not only about the climate - but I have never learned anything about science from Real Climate. That's not a surprise. The newest article on Real Climate is concerned with the proposition that deniers are deniers and denialism is a common trait.

See also The New York Times, Science Now, and dozens of others at Google News.

1 comment:

  1. If all the money wasted on the Gravity Probe B (I think it was like $800 million), VLBI could've done better with those same funds. For their gravidational lensing test, they got within 0.03% of GR. This Nature article seems interesting, too.