Alternative theories being tested by Gravity probe B

marcus

Quote by SpaceTiger

And when the next prediction fails, will we see another modification/generalization? Come on people, don't you think your time would be better spent working on something new?

Nick SpaceTiger, good to see you back!
Pervect said you were busy with PhD thesis and he was urged to step in and fill the gap. Does this mean that the thesis is done now?
or pending approval? In any case congratulations on forthcoming PhD

SpaceTiger

Quote by marcus

Nick SpaceTiger, good to see you back!
Pervect said you were busy with PhD thesis and he was urged to step in and fill the gap. Does this mean that the thesis is done now?
or pending approval? In any case congratulations on forthcoming PhD

Hi marcus! I'm afraid my thesis is still a work in progress and mentor duties will be primarily carried out by pervect and Janus, but I'll still be stopping by from time to time. Thanks.

JonathanK

To SpaceTiger: Hi, what makes the calculation so strong is that it is neither a modification nor a generalisation, as you seem to be saying. The theory hasn't been changed - instead the calculation works straight from the already published conceptual picture, and proves in an unambiguous way what the prediction should have been. The theory is still under development, but not in this area - the numerical value of the curvature component doesn't go into the calculation, it comes out of it. Jonathan

SpaceTiger

Quote by JonathanK

To SpaceTiger: Hi, what makes the calculation so strong is that it is neither a modification nor a generalisation, as you seem to be saying. The theory hasn't been changed - instead the calculation works straight from the already published conceptual picture, and proves in an unambiguous way what the prediction should have been.

So what makes the calculation so strong is the fact that you messed up in the first one?

JonathanK

Hi, I messed up in the first one for the following reasons!

In the first paper I pointed out a number of ways in which PSG mimics GR, but unfortunately missed one way. Having missed it, I wrote "There is no space curvature in PSG, so it should not mimic GR to the extent of causing a straight line parallel transported through a gravitational field to change its orientation. This removes the curvature component, which is two thirds of the predicted geodetic effect from GR.."

At that point I thought what some think - that only curvature will produce the geodetic effect. After the effect was measured I did a calculation using two points on the spin axis of one of the gyros on GP-B - the centre, and one end, at the surface of the sphere. Worked out their positions after half an orbit, with very slightly different speeds (due to different heights in a Planck scale refractive medium that affects matter). It's a thing that I'd tried last year but never completed - out came the curvature component of the geodetic effect, it gave 4.4 arcsecs/yr. Sad I'd missed it, but happy that it vindicates the theory, as I think it does. But I await the views of others, and would appreciate any thoughts you might have.

JonathanK

Hello,

Well it seems that if there was a problem with the calculation it would have been pointed out - Garth and probably SpaceTiger have read the paper, hopefully others. It's a very simple idea - matter is slowed in its motion through the field by the same factor as light is slowed (and for that matter as time is slowed), which is sqrt [1 - (2GM/rc^2)]. When one applies that to local matter within an orbiting object, one finds the object turns - the calculation shows this is exactly as in the geodetic effect, both numerically and in terms of direction. I'm all for GSCC, glad it's back on the list - experiment can help shape theories. For PSG it has helped shape the interpretation rather than the theory, and I'll take the lack of criticism as positive. The wider question this has bearing on is what possible interpretations for the geodetic effect do we have, now that it has been measured directly. Is curvature the only one? Thanks, best wishes Jonathan

Garth

Latest June Mission Update from the GP-B team.

Nothing new to report on the results.

This GP-B update is essentially a revised and expanded version of the GP-B press release that we posted in April that tells a more readable and complete story about the goals of GP-B, what we've accomplished to date, the surprises we've encountered and how we're addressing them, and the steps we are taking towards a final results announcement at the end of the year.

1. GP-B SUCCEEDED IN COLLECTING THE DATA TO TEST EINSTEIN'S PREDICTIONS ABOUT GRAVITY

2. THE EFFECTS OF GENERAL RELATIVITY ARE CLEARLY VISIBLE

3. FIRST PEEK AT RESULTS PRESENTED AT APRIL APS MEETING; FINAL RESULTS IN DECEMBER
(geodetic precession = GR to 1% - however present estimate ("hint") of result ~30 mas too high but error is still ~100 mas)

4. THE TWO SURPRISES & THEIR IMPACT ON THE EXPERIMENT

i. Shortly after the gyros were spun up in August 2004, we discovered that the polhode motion of the gyro rotors, which was expected to exhibit a constant pattern throughout the experimental period, was changing over time, significantly complicating the calibration of the gyroscope readout angles.
ii. During the post-experiment instrument calibration testing in August-September 2005, the spin axes of the gyroscopes were found to be affected by certain class of small classical torques, known as "misalignment torques," whose effects must be rigorously separated from the relativity measurements. (Due to electrostatic patches on the rotors and their housing.)

(Italics my text)

5. NEXT STEPS-MOVING TOWARDS A FINAL RESULT
i. Fine calibration of the gyroscope/telescope scale factor
ii. Refining the analysis of the misalignment torques
iii. Extremely precise VLBI measurement data will be substituted for the current 1997 Hipparcos [star] Catalogue values for the proper motion of IM Pegasi.

We continue to wait and see!

Garth

Chronos

But these minor corrections are just that, Garth, minor. I fail to see how they can affect the conclusions. The results are GR concordant and will remain very much so, IMO.

Garth

I was just reporting what the GP-B team said at the conference, in the mist of the errors presently being reduced they saw just the hint of a non-GR signal. See APS talk by Francis Everitt page 21, you will see the successive error ellipses honing down on a value about 30 mas away from the GR prediction.

Perhaps it was wishful thinking on their part - we shall wait and see!

Garth

wolram

Does every one consider this test (clean) it seems to me like trying to make some sense out of white noise, i mean it is not like a some number registers on a dial, if the test was repeated with the same results it would be more
convincing.

Or is this the one test that has been done with so much care that no other conclusions are probable.

Garth

They have a problem, which they freely admit. The presence of this noise almost ruined the experiment, however it is not white noise and the spurious signal can be extracted objectively.

The two unexpected effects are linked. First the electrostatic patches have put an extra torque on the gyro rotors that perturbs them, secondly the energy used up in this perturbation gradually reduces the polhode motion (wobble) making that more difficult to model.

However once they have accurately modelled the geometry of the patch effects, which are different for each of the four gyro rotors, they can subtract that from the precession signals and accurately model the polhode motion. It is this that they are completing at present.

You can read all about it by following the links on their website.

Garth

Garth

Just to make clear what the present situation is:

(mas = milliarcsec)
The geodetic N-S precession is predicted by GR to be - 6606 mas/yr, however there is a solar geodetic precession N-S component of + 7 mas/yr and the proper motion of IM Pegasi +28 mas/yr to take into account, resulting in a net expected N-S precession of -6571 [itex]\pm[/itex]1 mas/yr.

The frame-dragging E-W precession is predicted by GR to be -39 mas/yr, the solar geodetic precession E-W component of -16 mas/yr and the proper motion of IM Pegasi -20 mas/yr to include, resulting in a net expected N-S precession of -75 [itex]\pm[/itex]1 mas/yr.

From pages 20 and 21 of Francis Everitt's April APS talk, we find: A series of error ellipses on the N-S v E-W precession plot with centres respectively at (-6584 [itex]\pm[/itex]60, -83 [itex]\pm[/itex]22 mas/yr) June 2006, (-6597 [itex]\pm[/itex]17, -92 [itex]\pm[/itex]15 mas/yr) December 2006, (-6595 [itex]\pm[/itex]12, -98 [itex]\pm[/itex]7 mas/yr) March 2007 and (-6603 [itex]\pm[/itex]8, -98 [itex]\pm[/itex]7 mas/yr) March 2007.

It was this last reading for the geodetic precession that Francis Everitt reported at his April APS talk. If we also include that 'glimpse' of the E-W precession as well we have net values of:
(-6603 [itex]\pm[/itex]8, -98 [itex]\pm[/itex]7 mas/yr)
whereas GR predicts:
(-6571 [itex]\pm[/itex]1, -75 [itex]\pm[/itex]1 mas/yr).

In other words the actual readings are larger than GR predicts by 32 mas/yr in geodetic precession and 23 mas/yr in frame-dragging precession.

However, they reported an overall error, which is still being reduced, caused by residual gyro-to-gyro inconsistencies due to incomplete modeling of ~ [itex]\pm[/itex]100 mas/yr.

This renders the present geodetic 'glimpse' as being consistent with GR to within about 1[itex]\frac{1}{2}[/itex]%, whereas the frame-dragging precession is at present swamped by noise.

The running now stands:

Einstein's General Relativity(GR)
Brans-Dicke theory (BD)
Moffat's Nonsymmetric Gravitational Theory (NGT)
Stanley Robertson's Newtonian Gravity Theory (NG),
F. Henry-Couannier's Dark Gravity Theory (DG).
Alexander and Yunes' prediction for the Chern-Simons gravity theory (CS).
Kris Krogh's Wave Gravity Theory (WG)
Hongya Liu & J. M. Overduin prediction of the Kaluza-Klein gravity theory (KK).

The predictions are now:

GPB Geodetic gross precession (North-South)

GR = -6606 mas/yr.
BD = -[itex](3\omega + 4)/(3\omega + 6)[/itex] 6.606 arcsec/yr. where now [itex]\omega[/itex] >60.
NGT = -6606 - a small [itex]\sigma[/itex] correction mas/yr.
NG = -6606 mas/yr.
DG = -6606 mas/yr.
CS = -6606 mas/yr.
WG = -6606 mas/yr.
KK = -(1 + b/6 - 3b² + ...) 6606 mas/yr. where 0 < b < 0.07.

We await the GPB gravitomagnetic frame dragging gross precession (East-West) result.

GR = -39 mas/yr.
BD = -[itex](2\omega + 3)/(2\omega + 4)[/itex] 39 mas/yr.
GSCC = -39 mas/yr.
NGT = -39 mas/yr.
NG = -39 mas/yr.
DG = 0 mas/yr.
CS = -39 mas/yr. + CS correction
WG = 0 mas/yr.
KK = -39 mas/yr.

Note: in the above list only the Kaluza-Klein gravity theory (KK) is able to produce a geodetic precession larger (negatively) than GR if 0 < b < [itex]\frac{1}{18}[/itex], (by a maximum of 11.5 mas when b = [itex]\frac{1}{72}[/itex]), but it cannot produce a frame-dragging precession larger than GR.

Garth

JonathanK

About the apparent extra 32 mas/yr - there's a '96 paper by Kenneth Nordtvedt, which shows that the analysis of the lunar laser ranging data done in the '80s also gave a precession slightly greater than the geodetic precession. Nordtvedt's explanation probably wouldn't cover both anomalies, but if he's wrong… I haven't got time to look into it, but Garth, you might want to see if in relation to context the two are equivalent. It would then be a new anomaly.

The paper is "On the 'geodetic precession' of the lunar orbit", Class. Quantum Grav. 13 (1996) 1317-1321.

J.

Garth

Quote by JonathanK

About the apparent extra 32 mas/yr - there's a '96 paper by Kenneth Nordtvedt, which shows that the analysis of the lunar laser ranging data done in the '80s also gave a precession slightly greater than the geodetic precession. Nordtvedt's explanation probably wouldn't cover both anomalies, but if he's wrong… I haven't got time to look into it, but Garth, you might want to see if in relation to context the two are equivalent. It would then be a new anomaly.

The paper is "On the 'geodetic precession' of the lunar orbit", Class. Quantum Grav. 13 (1996) 1317-1321.

J.

Relevant papers are:
Kenneth Nordtvedt, Classical and Quantum Gravity, Volume 13, Issue 6, pp. 1317-1321 (1996)
On the `geodetic' precession of the lunar orbit

The Moon's dynamics in the geodetically rotating inertial frame is intrinsically different from its dynamics in a globally fixed frame, possessing different ratios of lunar motion to solar motion, and of tidal strength to squared solar motion. The correct prescription is that the Moon's orbit in globally fixed coordinates equals a dynamically different lunar orbit determined in the geodetically rotating inertial frame plus additional precession of that orbit as it accompanies the local frame.

and

Kenneth Nordtvedt, Icarus Volume 114, Issue 1, March 1995, Pages 51-62 The Relativistic Orbit Observables in Lunar Laser Ranging

Relativistic corrections to the perigee precession rate are found which include not only 4% enhancement of the well-discussed de Sitter "geodetic precession" term, but also a direct contribution from the solar tidal acceleration which is 10% as large.

Nordtvedt's explanation for the excess precession is solar tidal action on the Moon in orbit around the Earth, which does not apply to the GP-B gyros.

I make two further comments:

1. The analysis of the Moon's orbit is very complicated having to take into account tidal reactions as well as planetary perturbations and other effects. When trying to test for something like the geodetic precession how sure are we that all other effects have been correctly accounted for?

2. All measurements of orbits, geodesics, through empty space cannot reduce a degeneracy between GR and a conformal gravity theory in which the action reduces to that of canonical GR in vacuo.

There is therefore the possibility that the geodetic precession may be equal to GR in such observations as these above yet different from GR for the GP-B gyros. It is this point that has not been taken into account by Kenneth Nordtvedt when he said the experiment was worth doing when it was first proposed in the 1950's and begun in the 1960's, but now it is a foregone conclusion.

If the final results of the two precessions are exactly equal to GR then we can rule out such conformally equivalent gravitational theories.

However if the final GP-B results are different from the GR predictions, while other tests of the geodetic and frame-dragging precessions measuring the orbits of planetary bodies, binary pulsars, or satellites, are equal to the GR predictions, then that would indicate a conformally equivalent non-minimally connected scalar field is at work.

We wait to see.

Garth

JonathanK

I meant look at the numbers at a shortcut (this can help!). Then only look further if the two discrepancies come out roughly the same, when allowing for orbit differences. But it might be worth waiting for all possible sources of error to be removed from the GP-B data first.

Garth

This paper on today's physics arXiv claims to correct the GR prediction for the frame-dragging precession: Correction of the Frame Dragging Formula due to the Precession of the Coordinate Axes near a Rotating Body.

The total precession gives the value with respect to chosen coordinate axes, while the precession of the coordinate axes with respect to the distant stars is usually disregarded. We address this problem in two different ways. Firstly, we consider the participation of each particle in the total precession observed from the corresponding particle. Secondly, we find the precession of the coordinate axes and its subtraction from the total precession. Both methods yield the well known value for the geodetic precession and 75% of the well known value for frame dragging effect.

So we obtained the same value for the geodetic precession as predicted by the General Relativity and 75% of the general-relativistic value for the frame dragging effect. In the case of the Gravity Probe B experiment this yields 0.030 milli arc-seconds per year.

Personally I don't believe it; the spin axes of the gyroscopes are Fermi-Walker transported along their geodesics but the basis vectors of the comoving frame are not. They are tied by a pure boost to the PPN coordinate grid, which in turn is expected to be tied to an inertial frame fixed relative to the distant quasars.

Furthermore, the present 'glimpses' of the GP-B frame-dragging result, if anything, are greater than the GR prediction!

Garth

wolram

Will there ever be closure to this test? i do not mean the scientific impossibility of proof beyond doubt, just doubts have been quashed
to a CL that is acceptable.

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Chronos Recognitions: Gold Member Science Advisor	But these minor corrections are just that, Garth, minor. I fail to see how they can affect the conclusions. The results are GR concordant and will remain very much so, IMO.

Garth Recognitions: Gold Member Science Advisor	I was just reporting what the GP-B team said at the conference, in the mist of the errors presently being reduced they saw just the hint of a non-GR signal. See APS talk by Francis Everitt page 21, you will see the successive error ellipses honing down on a value about 30 mas away from the GR prediction. Perhaps it was wishful thinking on their part - we shall wait and see! Garth

wolram Blog Entries: 1 Recognitions: Gold Member	Does every one consider this test (clean) it seems to me like trying to make some sense out of white noise, i mean it is not like a some number registers on a dial, if the test was repeated with the same results it would be more convincing. Or is this the one test that has been done with so much care that no other conclusions are probable.

JonathanK	I meant look at the numbers at a shortcut (this can help!). Then only look further if the two discrepancies come out roughly the same, when allowing for orbit differences. But it might be worth waiting for all possible sources of error to be removed from the GP-B data first.