Alternative theories being tested by Gravity probe B

[maurol2]

Although I share your reservations and asked a question along similar lines to Francis Everitt about the error reduction at the April 2007 APS meeting, I have no doubt the signal did trace the GR prediction as can be seen here.

Let's accept you (and the GP-B) team are right. Let's assume that after removing all the noise, the remaining effects agree with GR predictions. OK with that: GR accurately predicts the small effects of frame dragging and geodetic precession.

Now, wouldn't be interesting to analyze and try to understand how and from where all that unexpected noise came from? After more than five years, has nobody done that?
I for one, find the noise (and the reason it was completely unexpected) more interesting than the data, don't you think?

Do you have any info related to this? Specifically, to theories or studies done on the noise, not with the objective to remove or cancel it, but to understand it, along with its origin?

Regards,
Mauro

 

[maurol2]

Do you have any info related to this? Specifically, to theories or studies done on the noise, not with the objective to remove or cancel it, but to understand it, along with its origin?

Of course, to be able to remove or cancel something, you must be able to understand it (to a certain degree, at least). Don't get me wrong. What I'm saying is: if these effects were standard and known, why were they so unexpected? And also: What's their cause? not in the sense of how they work and affect the results, but in the sense of from where they came from? What's their origin? Why these effects were not expected?

Thanks,
Mauro

 

[Polestar101]

Mauro – That is exactly the right question: what motion produced the noise signals?

In order to isolate the geodetic effect and the very small GR effect, the GPB team needs to separate out a number of unwanted signals. The known ones are easy whereas the unknown ones are difficult to spot unless they have a short periodicity. The most obvious one, the well-known aberration of light due to the motion of the spacecraft ’s regular orbit around the Earth (slightly changing the telescopes orientation to the guide star with each orbit), amounts to +-5” per orbit, and effectively acts as a calibration tool due to its short predictable periodicity. Another, the aberration of light due to the Earth’s annual orbit around the sun (carrying the spacecraft with it), may be less obvious because the Earth only went around the sun once during the GPB data collection period, and even this period was troubled with interrupted signals due to a number of stop and start issues. But it is a well-known effect, and quite large (+- 20.148”p/y), and therefore should also be readily identifiable.

If the Earth also has another motion, moving with the sun, curving through space due to solar system motion, producing the observable of the stars moving across the sky at about 50” p/y, then a very sensitive spacecraft orbiting the Earth should pick up this motion. However, if the earth’s changing orientation to VLBI sources is simply due to a wobbling Earth (as posited by current precession nutation theory which constrains ALL solar system motion to zero) then the GPB spacecraft , high above the so called wobbling earth, should not pick up any such signal. But the IAU has found that current precession theory is “inconsistent with dynamical theory” (IAU PO3), suggesting there is something very wrong with the current static sun precession theory. If some portion of the 50”p/y observable is indeed due to the motion of the solar system (as BRI data suggests) then the gyros should pick up this signal, unexpected under current theory. However, because the periodicity of this motion is so long (taking roughly 25,700 years to complete one cycle at the present rate) in relation to the data gathering period of the gyros, it would only reveal a tiny fraction of the waveform peak to trough during the life of the GPB experiment, and therefore it would show up as background drift rather than any obvious cycle. Nonetheless this effect will be fairly large, on the order of up to 50”p/y, if BRI is correct, thus making it immediately obvious that there is a lot of “noise” in the overall signal. This is what I think GPB found, but will we ever know it?

A methodology that is so intent on proving GR, that it effectively endorses a static solar system model, and eliminates noise without specifically identifying the source of all such unwanted noise, is not very objective in my opinion. Like you I have no problem with the existence of GR. It is the integrity of the process, one that may have thrown the baby out with the bathwater (proving solar system motion), and lost the support of NASA and many objective scientists along the way, that is so troublesome.

 

[Garth]

Final Results Published - after 5 years of analysis!

Summary of Final GP-B Experimental Results

GP-B STATUS UPDATE — May 4, 2011
After 31 years of research and development, 10 years of flight preparation, a 1.5 year flight mission and 5 years of data analysis, our GP-B team has arrived at the final experimental results for this landmark test of Einstein’s 1916 general theory of relativity. Here is the abstract from our PRL paper (see next section) summarizing the experimental results.

Gravity Probe B, launched 20 April 2004, is a space experiment testing two fundamental predictions of Einstein's theory of General Relativity (GR), the geodetic and frame-dragging effects, by means of cryogenic gyroscopes in Earth orbit. Data collection started 28 August 2004 and ended 14 August 2005. Analysis of the data from all four gyroscopes results in a geodetic drift rate of -6,601.8±18.3 mas/yr and a frame-dragging drift rate of -37:2±7.2 mas/yr, to be compared with the GR predictions of -6,606.1 mas/yr and -39.2 mas/yr, respectively ('mas' is milliarc-second; 1 mas= 4.848 X10-9 radians or
2.778 X10-7 degrees).

The table and diagram below show the individual gyroscope results, the weighted average results for all four gyroscopes combined, and the theoretical predictions for both effects from Einstein's general theory of relativity, as calculated by Stanford physicist, Leonard Schiff.

Note: The individual and combined statistical uncertainties are corrected for the "over" and "under" dispersion using the Χ2 of the individual estimates in the N-S and W-E directions. Please see our PRL paper (next section below) for more detailed information about these results and their derivation.

Gravity Probe B — Final Experimental Results

rN-S (Geodetic Measurement) rW-E (Frame-Dragging Measurement)

Gyroscope #1 -6,588.6±31.7 mas/yr -41.3±24.6 mas/yr
Gyroscope #2 -6,707.0±64.1 mas/ yr -16.1±29.7 mas/yr
Gyroscope #3 -6,610.5±43.2 mas/yr -25.0±12.1 mas/yr
Gyroscope #4 -6,588.7±33.2 mas/yr -49.3±11.4 mas/yr

Weighted-Average Results for All Four Gyroscopes
All Gyroscopes -6,601.8±18.3 mas/yr -37.2±7.2 mas/yr

Schiff-Einstein Predicted Theoretical Values
Theoretical Gyroscope -6,606.1 mas/yr -39.2 mas/yr

The results are consistent with GR and also those theories that are not tested against GR by this experiment because they make the same Geodetic and frame-dragging predictions.

The list that I had compiled now looks like this (no change from the last summary):

1. Einstein's General Relativity(GR)
2. Brans-Dicke theory (BD)
3. Moffat's Nonsymmetric Gravitational theory (NGT)
4. Stanley Robertson's Newtonian Gravity theory (NG),
5. F. Henry-Couannier's Dark Gravity theory (DG).
6. Alexander and Yunes' prediction for the Chern-Simons gravity theory (CS).
7. Kris Krogh's Wave Gravity theory (WG)
8. Hongya Liu & J. M. Overduin prediction of the http://www.journals.uchicago.edu/ApJ/journal/issues/ApJ/v538n1/50681/50681.text.html?erFrom=5252751197746712308Guest#sc8 gravity theory (KK).
9. Kerr's Planck Scale Gravity: Predictions of Experimental Results from a Gravity Theory (PSG)
10. My Self Creation Cosmology (SCC),

The predictions are now:

GP-B Geodetic gross precession (North-South).

1. GR = -6606 mas/yr.
2. BD = -(3\omega + 4)/(3\omega + 6) 6.606 arcsec/yr. where now \omega >60.
3. NGT = -(6606 - a small \sigma correction) mas/yr.
4. SCC = -6606 mas/yr.
5. NG = -6606 mas/yr.
6. DG = -6606 mas/yr.
7. CS = -6606 mas/yr.
8. WG = -6606 mas/yr.
9. KK = -(1 + b/6 - 3b² + ...) 6606 mas/yr. where 0 < b < 0.07.
10. PSG = -6606 mas/yr.

GPB gravitomagnetic frame dragging gross precession (East-West).

1. GR = -39 mas/yr.
2. BD = -(2\omega + 3)/(2\omega + 4) 39 mas/yr.
3. SCC = -39 mas/yr.
4. NGT = -39 mas/yr.
5. NG = -39 mas/yr.
6. DG = 0 mas/yr.
7. CS = -39 mas/yr. + CS correction
8. WG = 0 mas/yr.
9. KK = -39 mas/yr.
10. PSG = -39 mas/yr.

Those theories being tested against GR by this experiment are:
BD – Brans Dicke theory; with \omega > 660.
KK - Kaluza-Klein gravity theory; with b < 1.5 x 10^-4.
NGT – Nonsymmetric Gravitational theory; it depends on how ‘small’ the \sigma correction is!And the ones not tested against GR by this experiment, and are still consistent with GP-B are:
SCC - Self Creation Cosmology theory.
NG - Newtonian Gravity theory.
CS - Chern-Simons gravity theory.
PSG - Planck Scale Gravity theory.

Despite the disparaging remarks made the reason the NASA funding was withheld in the final stage was because the funds had run dry, with the cut backs etc. not because they thought the result was untrustworthy. The extra error reduced the final accuracy to that originally hoped for but it had been modeled by two independent methods and the methodology was not intent on proving GR but making an unbiased and independent set of measurements.

The problems with the experiment have been openly discussed, the only problem being IMHO is that should the results have been different from GR then the rest of the GR world would have had found it difficult to accept them.

Garth

 

[TrickyDicky]

Final Results Published - after 5 years of analysis!

of Final GP-B Experimental Results/url]The results are consistent with GR and also those theories that are not tested against GR by this experiment because they make the same Geodetic and frame-dragging predictions.

The list that I had compiled now looks like this (no change from the last summary):

1. Einstein's General Relativity(GR)
2. Brans-Dicke theory (BD)
3. Moffat's Nonsymmetric Gravitational theory (NGT)
4. Stanley Robertson's Newtonian Gravity theory (NG),
5. F. Henry-Couannier's Dark Gravity theory (DG).
6. Alexander and Yunes' prediction for the Chern-Simons gravity theory (CS).
7. Kris Krogh's Wave Gravity theory (WG)
8. Hongya Liu & J. M. Overduin prediction of the http://www.journals.uchicago.edu/ApJ/journal/issues/ApJ/v538n1/50681/50681.text.html?erFrom=5252751197746712308Guest#sc8 gravity theory (KK).
9. Kerr's Planck Scale Gravity: Predictions of Experimental Results from a Gravity Theory (PSG)
10. My Self Creation Cosmology (SCC),

The predictions are now:

GP-B Geodetic gross precession (North-South).

1. GR = -6606 mas/yr.
2. BD = -(3\omega + 4)/(3\omega + 6) 6.606 arcsec/yr. where now \omega >60.
3. NGT = -(6606 - a small \sigma correction) mas/yr.
4. SCC = -6606 mas/yr.
5. NG = -6606 mas/yr.
6. DG = -6606 mas/yr.
7. CS = -6606 mas/yr.
8. WG = -6606 mas/yr.
9. KK = -(1 + b/6 - 3b² + ...) 6606 mas/yr. where 0 < b < 0.07.
10. PSG = -6606 mas/yr.

GPB gravitomagnetic frame dragging gross precession (East-West).

1. GR = -39 mas/yr.
2. BD = -(2\omega + 3)/(2\omega + 4) 39 mas/yr.
3. SCC = -39 mas/yr.
4. NGT = -39 mas/yr.
5. NG = -39 mas/yr.
6. DG = 0 mas/yr.
7. CS = -39 mas/yr. + CS correction
8. WG = 0 mas/yr.
9. KK = -39 mas/yr.
10. PSG = -39 mas/yr.

Those theories being tested against GR by this experiment are:
BD – Brans Dicke theory; with \omega > 660.
KK - Kaluza-Klein gravity theory; with b < 1.5 x 10^-4.
NGT – Nonsymmetric Gravitational theory; it depends on how ‘small’ the \sigma correction is!And the ones not tested against GR by this experiment, and are still consistent with GP-B are:
SCC - Self Creation Cosmology theory.
NG - Newtonian Gravity theory.
CS - Chern-Simons gravity theory.
PSG - Planck Scale Gravity theory.

Despite the disparaging remarks made the reason the NASA funding was withheld in the final stage was because the funds had run dry, with the cut backs etc. not because they thought the result was untrustworthy. The extra error reduced the final accuracy to that originally hoped for but it had been modeled by two independent methods and the methodology was not intent on proving GR but making an unbiased and independent set of measurements.

The problems with the experiment have been openly discussed, the only problem being IMHO is that should the results have been different from GR then the rest of the GR world would have had found it difficult to accept them.

Garth

But that last remark should also work then the other way, at least for frame-dragging effect, given the large errors of the 4 gyroscopes, from a non-biased POV results compatible with the frame-dragging prediction should also then be difficult to accept.

Of the 4 gyroscopes (centering on the frame-dragging effect) 3 of them (#1,#2, and #3) show errors that admit values compatible with predictions closer to 0 mas/yr than to the -39 mas/yr prediction. One of them (#2) is compatible with a null result. Gyroscope #4 is compatible with -60.6 mas/yr . And these are the numbers achieved after more than 5 years of fitting the raw results to something tolerable.
IMHO, GPB experiment, in the case of the geodetic effect confirms it with little room to doubt of its existence (this had been pretty much been settled by other experiments, but in the case of the Lense-Thirring effect this results are of course compatible with GR but also compatible with very different predictions, including a null effect, and given the error margins wrt the smallness of the effect and the disparity among the 4 gyroscopes, the experiment can't be used either to confirm or reject the effect.
The experiment of course wasn't designed to prove GR, no experiment can do that. It could only falsify or confirm predictions of GR, it confirmed the geodetic effect, and it neither confirmed nor falsify the Lense-Thirring effect due to the lack of accuracy obtained by the gyroscopes.
Apparently all theories but DG and WG are compatible with GBP results, even those two are compatible with the results of one of the 4 gyroscopes.

 

[Polestar101]

Is there a detailed analysis available that explains how they canceled out each of the 50 plus unwanted signals? I am particularly interested to learn how they canceled out the "unpredictable" polhode noise and other "unidentified" signals. Presumably there will be some follow-up report that provides this data?

 

[JonathanK]

Thanks Garth, and perhaps I should say thanks from all of us for what you've done on this over the last few years.

NB. The predictions you posted for my theory PSG are correct, but not the link to them, which is here http://journalgp.awardspace.com/journal/0202/020203.pdf
'A derivation of the geodetic effect without space curvature'.

I agree with the point made above that the geodetic effect result from GP-B is on much more solid ground than that for frame dragging. Best wishes to all.

 

[Garth]

Thank you Jonathan, the Final results have today been published in Physical Review Letters, which you may access through the GP-B website: Gravity Probe B: Final Results of a Space Experiment to Test General Relativity and a Viewpoint article by Professor Will: Finally, results from Gravity Probe B.

In reply to Polestar101 I quote from Professor Will's article for a summary:

However, three important, but unexpected, phenomena were discovered during the experiment that affected the accuracy of the results.

First, because each rotor is not exactly spherical, its principal axis rotates around its spin axis with a period of several hours, with a fixed angle between the two axes. This is the familiar “polhode” period of a spinning top and, in fact, the team used it as part of their analysis to calibrate the SQUID output. But the polhode period and angle of each rotor actually decreased monotonically with time, implying the presence of some damping mechanism, and this significantly complicated the calibration analysis. In addition, over the course of a day, each rotor was found to make occasional, seemingly random “jumps” in its orientation—some as large as 100 milliarcseconds. Some rotors displayed more frequent jumps than others. Without being able to continuously monitor the rotors’ orientation, Everitt and his team couldn’t fully exploit the calibrating effect of the stellar aberration in their analysis. Finally, during a planned 40-day, endof-mission calibration phase, the team discovered that when the spacecraft was deliberately pointed away from the guide star by a large angle, the misalignment induced much larger torques on the rotors than expected. From this, they inferred that even the very small misalignments that occurred during the science phase of the mission induced torques that were probably several hundred times larger than the designers had estimated.

What ensued during the data analysis phase was worthy of a detective novel. The critical clue came from the calibration tests. Here, they took advantage of residual trapped magnetic flux on the gyroscope. (The designers used superconducting lead shielding to suppress stray fields before they cooled the niobium coated gyroscopes, but no shielding is ever perfect.) This flux adds a periodic modulation to the SQUID output, which the team used to figure out the phase and polhode angle of each rotor throughout the mission. This helped them to figure out that interactions between random patches of electrostatic potential fixed to the surface of each rotor, and similar patches on the inner surface of its spherical housing, were causing the extraneous torques. In principle, the rolling spacecraft should have suppressed these effects, but they were larger than expected. The patch interactions also accounted for the “jumps”: they occurred whenever a gyro’s slowly decreasing polhode period crossed an integer multiple of the spacecraft roll period. What looked like a jump of the spin direction was actually a spiraling path—known to navigators as a loxodrome. The team was able to account for all these effects in a parameterized model.

The original goal of GP-B was to measure the frame-dragging precession with an accuracy of 1%, but the problems discovered over the course of the mission dashed the initial optimism that this was possible. Although Everitt and his team were able to model the effects of the patches, they had to pay the price of the increase in error that comes from using a model with so many parameters. The experiment uncertainty quoted in the final result—roughly 20% for frame dragging—is almost totally dominated by those errors. Nevertheless, after the model was applied to each rotor, all four gyros showed consistent relativistic precessions (Fig. 1, bottom). Gyro 2 was particularly “unlucky”—it had the largest uncertainties because it suffered the most resonant jumps.

A full description of the analysis may be found here Post Flight Analysis — Final Report

Garth

 

[Polestar101]

Thanks for the excerpt from Professor Wills (head of review board) who has looked at many of the efforts the team has gone through in trying to identify and separate the various unexpected sources of noise. His own work depends on Einstein’s GR, so it is not surprising that he would support the conclusions, nonetheless he was very clear that a lot of assumptions were made in arriving at the final conclusions.

It is impossible to know if all the assumptions in the parameterized model are ideal for properly categorizing and canceling each noise without knowing for certain the source of each unwanted signal, but the GPB team seems to have done the best they could based on the circumstances.

The inability to continuously monitor the rotors’ orientation meant the stellar aberrations could not be utilized as calibrating tools (as planned) hints at the amount of noise in the overall experiment. 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 issue I would still like to understand is if any of the “noise” might be evidence of solar system motion. If for example we discovered the solar system is just slightly accelerating, then it might explain the anomalous acceleration of Pioneer 10 and 11 and possibly shed light on the http://www.space.com/5037-nasa-baffled-unexplained-force-acting-space-probes.html" acting on our spacecraft flybys (Galileo, NEAR, Rosetta, Cassini, etc.), that could theoretically be explained by an accelerating solar system.

Maybe what Everitt calls “strange anomalous torques” are not all “criminals”, as he put it? The point is it probably makes sense to very carefully examine all the assumptions behind the unwanted cancelled-out noise to make sure we didn’t eliminate something even more fundamental than GR, that is, solar system motion.

 

[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.

 

[Garth]

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

 

[Garth]

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

 

[jumpjack]

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

Thanks, very interesting link.
And what about the moon moving around the earth? Should it also cause the same effect?

I also found this cool picture:

 

[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.

 

[jumpjack]

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.

 

[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.

 

[jumpjack]

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?

 

[JonathanK]

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.

 

[marcus]

A paper about GP-B came out today. I'll get the link

 

[laowang]

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

 

[laowang]

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.

 

[afalck]

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