# Alternative theories being tested by Gravity probe B

PF Gold
P: 3,274
 Quote by Polestar101 But even attempting to use the diurnal and annual aberrations as the main calibration tools shows the experimenters did not plan on accounting for any solar system motion relative to the guide star – meaning they essentially used a static solar system model.
The guide star IM Pegasi, chosen because it was also a radio star that could be tracked by the VLBI radio facility, was itself referenced to a distant quasar 3C454.3, which is at a distance of 12 billion light-years.

As the measurements were angular in nature the quasar's Proper Motion across the sky is negligible (see my post 323) and the solar system is therefore effectively 'static' relative to this distant reference point (see my post 329).

The suitability of quasar 3C454.3 as a stable reference point was itself carefully examined as you can see here: The “Core” of the Quasar 3C 454.3 as the Extragalactic Reference for the Proper Motion of the Gravity Probe B Guide Star, its conclusion:
 The core of 3C 454.3 provides a sufficiently stable reference with which to measure the proper motion of the Gravity Probe B guide star, IM Pegasi, relative to the distant universe.
I hope this helps,
Garth
PF Gold
P: 3,274
 Quote by jumpjack Is it possible to explain in a simple way how to calculate the intensity of gravity effect detected by probe-b? I mean, I know the formula F=GMm/r^2 for "standard" gravity, but how can I calculate the force generated by a rotating body? I guess I need to know the distance of the test-body from the main-body, and the length of test-body (to calculate the force which makes it "rotate"), but which is the formula? And can the Moon generate a similar force by rotating around the Earth? It's another kind of "mass current", I think.
jumpjack you may find this page from the GP-B website interesting: Spacetime & Spin.

There is no 'force' generated by a rotating body, but it warps and twists the space-time continuum, which introduces a rotation relative to the background continuum generated by a non-rotating body. The formulae for the geodetic and frame-dragging precessions are difficult to calculate but can be found in Misner, Thorne and Wheeler's book 'Gravitation' on page 1119 if you are interested.

I hope this helps,
Garth
P: 201
 Quote by Garth jumpjack you may find this page from the GP-B website interesting: Spacetime & Spin. There is no 'force' generated by a rotating body, but it warps and twists the space-time continuum, which introduces a rotation relative to the background continuum generated by a non-rotating body. The formulae for the geodetic and frame-dragging precessions are difficult to calculate but can be found in Misner, Thorne and Wheeler's book 'Gravitation' on page 1119 if you are interested. I hope this helps, Garth
And what about the moon moving around the earth? Should it also cause the same effect?

I also found this cool picture:
 P: 29 The moon also has a geodetic precession, which showed up in the results from 25 years of laser ranging. NB, the formula for the geodetic effect in the picture posted above is now not the only formula for it, the other one gives almost identical numbers, so curvature is not the only interpretation that explains it.
P: 201
 Quote by JonathanK The moon also has a geodetic precession, which showed up in the results from 25 years of laser ranging. NB, the formula for the geodetic effect in the picture posted above is now not the only formula for it, the other one gives almost identical numbers, so curvature is not the only interpretation that explains it.
I was not talking about precessione OF the moon but CAUSED BY the moon.
 P: 29 The moon would have the same effect, anything orbiting it would change angle slightly over a long period of time, and over a longer period would rotate.
P: 201
 Quote by JonathanK The moon would have the same effect, anything orbiting it would change angle slightly over a long period of time, and over a longer period would rotate.
Oh God, why can't I explain my thought?!?
I'm talking about moon rotating AROUND earth, not around itself: would it cause any precession effect on earth surface?
 P: 29 yes but much harder to measure, hence GP-B. The earth and moon rotate about a common centre of gravity, and in that frame both precess slightly.
 Astronomy Sci Advisor PF Gold P: 23,274 A paper about GP-B came out today. I'll get the link
 P: 2 Well, it seems that GP-B experimental results face quite a lot challenges even in China's scientists community. I have just read an excellent essay from J. Beijing University of Technology 10/2011 issue that posted a serious challenge: A Question on Gravity Probe B Experiment Results Dr. Hao Shi Abstract The final results of the Gravity Probe B (GP-B) experiment for testing general relativity (GR) theory published on May 4, 2011 by NASA is somewhat controversial. Since GP-B scientific sensors have symmetric property about the satellite spin axis, we thus believe the measurement errors of both the geodetic effect and the frame-dragging effect should be close to each other. However, in the published results, the former is 2.5 times of the latter, which has not been explained by GP-B final report and thus shows that some physics is probably still missing or inadequately addressed in processing experimental data. Quoted from J. of Beijing University of Technology 2011-10
P: 2
 Quote by jumpjack Oh God, why can't I explain my thought?!? I'm talking about moon rotating AROUND earth, not around itself: would it cause any precession effect on earth surface?
Of course, but the question is how can we detect such a small angle from far away? Maybe we can send a laser torch there and then measures the angle with a telescope on earth. Actually that experiment may be even cheaper then GP-B. Then data processing would be much more complex since we probably have to include sun and other planets in our picture since the I and M of the moon are much larger than GP-B gyroscope's. I guess someone must have proposed the idea yet to be funded.
 P: 1 Garth, Thanks for maintaining this thread and for your clear explanations. I hope that this question is not too simplistic (if so, please let me know where to ask): Why is the axis of rotation of the gyroscope more or less fixed to begin with? I mean, relative to what? Is there an absolute not-rotating frame of reference in the universe that all gyroscopes feel? What is the local agent that connects an area of space to that absolute non-rotation? In your post #76 (which is part of a side discussion about a possible sun companion) you said "The orbit of the satellite around the Earth, the Earth around the Sun, the Sun around the COM [Center of Mass] of its supposed binary system, and that COM around the galaxy would not directly affect the pointing of the gyroscopes. They are in free-fall" Is "free-fall" a reference to this absolute non-rotating frame of reference? thanks and regards, Arturo

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