Gravity Probe-B: Results & General Relativity

[pervect]

I'm also quite curious as to the GP-B results, though I don't have anything riding on it.

I guess april 15 is the magic date, I imagine some people "in the know" already have an idea what will be presented in the way of the preliminary data analysis at the APS conference but of course they aren't talking publically.

As far as your points on precession goes, I do agree with your recent points about the apparent precession of distant stars from an accelerating observer, and I think any remaining issues are semantic.

 

[magnetar]

http://einstein.stanford.edu/

 

[Chris Hillman]

The GP-B press release in plain English?

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"?

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

 

[marcus]

Thanks for the plain English summary! It saves an interested audience member like myself much possible confusion and bother.

 

[rbj]

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. ... 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). ...
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?"
...
1. The preliminary data analysis did confirm the de Sitter precession formula to higher accuracy than ever before.

so, just to be clear, Chris: does the GP-B results, at least so far, confirm the GTR quantitative prediction of the amount of de Sitter precession over the quantitation preditions of the other theories in this "large class of metric gravitation theories"? did they say as much or are they still hedging this?

 

[Chris Hillman]

The "Acceptable Sliver" in PPN Space

Hi, rbj,

does the GP-B results, at least so far, confirm the GTR quantitative prediction of the amount of de Sitter precession over the quantitation preditions of the other theories in this "large class of metric gravitation theories"? did they say as much or are they still hedging this?

Regarding "confirm the GTR quantitative prediction of the amount of de Sitter precession over the quantitative preditions of the other theories", this phraseology seems to miss one of the points I was trying to emphasize. See the review paper by Will on the webpage I cited and look for the diagrams showing regions of "allowed theories" parameterized by the PPN formalism. Observe that in the PPN picture, gtr sits smack dab in the center of a very thin "acceptable sliver" sitting inside a large (but finite dimensional) space of alternative gravitation theories. This sliver represents the theories which are in agreement with current observation/experiment. (Modulo inevitable quibbles over what observations to accept at face value.) However, there are still points in this sliver which are arbitrarily close to the point representing gtr.

The "acceptable sliver" has just been reduced in one dimension, if you like, because GP-B has measured the de Sitter precession to greater accuracy than ever before. Now, gtr still sits in the center of the newly reduced "sliver", but given the fact that limits of accuracy will always be with us, you can never hope to reduce this sliver to a single point! By Occam's razor, gtr is preferred theoretically because it happens to be the simplest of all these theories, which is one reason for paying so much more attention to it than to the other theories in the "acceptable sliver".

As for "hedging", the whole point of my post was to explain why there is no "hedging" from the Stanford team! Rather, they said that they are not yet ready to say anything about the more interesting Lense-Thirring precession. While this is certainly disappointing, they went on to carefully explain why that is the case. They carefully explained why they think they know what they have to do in order to "safely remove" the unanticipated errors in order to finally obtain the desired clean test, and they said they hope to complete this work by December. Science is hard, and sometimes you just have to be patient while people work out some wrinkles.

 

[magnetar]

Gravity probe -B backs general relativity!?
http://physicsweb.org/articles/news/11/4/11/1