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Thread: GPS, relativity and tautology

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    GPS, relativity and tautology

    In discussions of relativity theory (special and general), when discussions of proof come up people often point to the GPS as exhibit A. GPS data has also been used to test the invariance of the speed of light (see Wolf and Petit 1997). However, it has occurred to me that we can't use the GPS data to test the invariance of c b/c the GPS incorporates the invariance of c as a methodological assumption. For example, in synchronizing the satellite clocks with ground clocks we must use light signals (Einstein synchronization). As such, any data received from the GPS will include this methodological assumption of invariant c so such data analysis, like that done by Wolf & Petit, MUST return an invariant c. Here's some info on how GPS synchronization works, which appears to support my point: https://gssc.esa.int/navipedia/index...stem_.28GMS.29.

    If this is the case, we are reduced to tautology in such analyses.

    Thoughts on this complicated set of issues?

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    Quote Originally Posted by aramis720 View Post
    In discussions of relativity theory (special and general), when discussions of proof come up people often point to the GPS as exhibit A. GPS data has also been used to test the invariance of the speed of light (see Wolf and Petit 1997). However, it has occurred to me that we can't use the GPS data to test the invariance of c b/c the GPS incorporates the invariance of c as a methodological assumption. For example, in synchronizing the satellite clocks with ground clocks we must use light signals (Einstein synchronization). As such, any data received from the GPS will include this methodological assumption of invariant c so such data analysis, like that done by Wolf & Petit, MUST return an invariant c. Here's some info on how GPS synchronization works, which appears to support my point: https://gssc.esa.int/navipedia/index...stem_.28GMS.29.

    If this is the case, we are reduced to tautology in such analyses.

    Thoughts on this complicated set of issues?
    I didn't look at the links, because I'm pretty sure it's not a tautology. Are you leaving out the information we have about satellite orbit and position?

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    The idea that a false assumption in engineering a complex and sensitive system will cause that system to produce results supporting that assumption is...questionable. Overlooked factors or false assumptions tend to make complex systems fail entirely.

    If fundamental assumptions such as invariance of c were not correct, GPS simply wouldn't work properly. At best, it would give results with systemic errors. The proper functioning of the system makes for a sensitive test of the validity of the assumptions it was designed under.

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    The Petit and Wolf paper is here: https://www.researchgate.net/publica...A_56_4405-4409

    I only have access to the abstract but I can’t see from that why the measurements would be tautological. They are (indirectly) measuring the frequency of the satellites’ clocks and using that to check the equivalence of clock synchronisation.

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    I think most people, when using GPS to support relativity, just point out that the system corrects for relativistic effects to get accurate answers. If you don't do these corrections GPS systems simply don't work.

    It looks like the Wolf and Petit work is all about testing the accuracy of the synchronisation. Which is perfectly valid, you assume the speed of light is constant, synchronise, then look for the system falling out of synchronisation. The rate of desynchronisation puts a limit on how wrong your assumption of constant light speed is. Don't see a tautology there.

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    Quote Originally Posted by aramis720 View Post
    In discussions of relativity theory (special and general), when discussions of proof come up people often point to the GPS as exhibit A. GPS data has also been used to test the invariance of the speed of light (see Wolf and Petit 1997). However, it has occurred to me that we can't use the GPS data to test the invariance of c b/c the GPS incorporates the invariance of c as a methodological assumption. For example, in synchronizing the satellite clocks with ground clocks we must use light signals (Einstein synchronization). As such, any data received from the GPS will include this methodological assumption of invariant c so such data analysis, like that done by Wolf & Petit, MUST return an invariant c. Here's some info on how GPS synchronization works, which appears to support my point: https://gssc.esa.int/navipedia/index...stem_.28GMS.29.

    If this is the case, we are reduced to tautology in such analyses.

    Thoughts on this complicated set of issues?
    Not complicated at all. You are simply mistaking self-consistency for tautology. These are not the same thing.

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    Appreciate the responses so far but no one has addressed my specific question. Here's the paper: https://www.dropbox.com/s/j18gaehctm...97%29.pdf?dl=0.

    My question was this: if data extracted from GPS is used to test the invariance of c aren't we reduced to a methodological tautology b/c an invariant c is assumed in all synchronization and re-synchronization of the satellites? So the answer of an invariant c is baked in to the methodology and we can't get anything other than an invariant c out of the data.

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    Quote Originally Posted by aramis720 View Post
    Appreciate the responses so far but no one has addressed my specific question. Here's the paper: https://www.dropbox.com/s/j18gaehctm...97%29.pdf?dl=0.

    My question was this: if data extracted from GPS is used to test the invariance of c aren't we reduced to a methodological tautology b/c an invariant c is assumed in all synchronization and re-synchronization of the satellites? So the answer of an invariant c is baked in to the methodology and we can't get anything other than an invariant c out of the data.

    aramis702, The ATM section is not for other members to answer your questions.
    It is YOUR claim that there is a tautology and therefore YOU will have to defend that statement.
    You have gotten answers from several persons. If you don't think their comments are correct, then you have to explain so why, and not just link to a paper.
    You will have to present your case here and not tell the people to go and read a paper by some person in you dropbox.
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    Quote Originally Posted by aramis720 View Post
    Appreciate the responses so far but no one has addressed my specific question. Here's the paper: https://www.dropbox.com/s/j18gaehctm...97%29.pdf?dl=0.

    My question was this: if data extracted from GPS is used to test the invariance of c aren't we reduced to a methodological tautology b/c an invariant c is assumed in all synchronization and re-synchronization of the satellites? So the answer of an invariant c is baked in to the methodology and we can't get anything other than an invariant c out of the data.
    Are you sure no one has? You haven't answered my question:
    Quote Originally Posted by grapes View Post
    Are you leaving out the information we have about satellite orbit and position?
    If you include the satellite orbit and position information (including timing), then we can test whether c is invariant, no?

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    Quote Originally Posted by aramis720 View Post
    Appreciate the responses so far but no one has addressed my specific question. Here's the paper: https://www.dropbox.com/s/j18gaehctm...97%29.pdf?dl=0.

    My question was this: if data extracted from GPS is used to test the invariance of c aren't we reduced to a methodological tautology b/c an invariant c is assumed in all synchronization and re-synchronization of the satellites? So the answer of an invariant c is baked in to the methodology and we can't get anything other than an invariant c out of the data.
    What data do you think is being extracted from GPS that would make this tautological?

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    Quote Originally Posted by aramis720 View Post
    My question was this: if data extracted from GPS is used to test the invariance of c aren't we reduced to a methodological tautology b/c an invariant c is assumed in all synchronization and re-synchronization of the satellites? So the answer of an invariant c is baked in to the methodology and we can't get anything other than an invariant c out of the data.
    I have answered this question. The tests in question can show if the assumption that c is invariant is wrong because any change in the speed of light induces a desynchronisation that can be measured. The systems are not continuously recalibrated during the process.

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    Quote Originally Posted by grapes View Post
    Are you sure no one has? You haven't answered my question:

    If you include the satellite orbit and position information (including timing), then we can test whether c is invariant, no?
    I don't think so b/c any measure of position is also based on Einstein synchronization, so position info bakes in an invariant c in the same way that timing information bakes in an invariant c. In other words, the way that satellite positions are calculated relies on Einstein synchronization in the same way that time stamps rely on Einstein synchronization. This link describes how satellite clocks are re-synchronized on an ongoing basis: https://gssc.esa.int/navipedia/index...stem_.28GMS.29.

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    Shaula, how often are GPS satellite clocks re-synchronized? And how exactly is this done?

    More generally, how would the Wolf & Petit methodology discover a desynchronization and a resulting deviation from invariant c? Here's the highly-cited paper at Sci-Hub: https://sci-hub.tw/10.1103/physreva.56.4405.

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    Here's the highly-cited paper at Sci-Hub: https://sci-hub.tw/10.1103/physreva.56.4405.

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    Quote Originally Posted by aramis720 View Post
    Shaula, how often are GPS satellite clocks re-synchronized? And how exactly is this done?

    More generally, how would the Wolf & Petit methodology discover a desynchronization and a resulting deviation from invariant c? Here's the highly-cited paper at Sci-Hub: https://sci-hub.tw/10.1103/physreva.56.4405.
    The clocks are resynchronised when they drift beyond the system specification of a microsecond. The clocks are left alone for as long as is possible.

    I don't know what you are asking with regards to the paper, their methodology is set out clearly in it. They use a difference approach along an orbital segment. Do you have specific issues with their method? Can you indicate which bits of their working you disagree with?

    If you want to ask basic questions about how GPS works you'll get better participation in the Q&A or Science and Tech fora.

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    Quote Originally Posted by Shaula View Post
    The clocks are resynchronised when they drift beyond the system specification of a microsecond. The clocks are left alone for as long as is possible.

    I don't know what you are asking with regards to the paper, their methodology is set out clearly in it. They use a difference approach along an orbital segment. Do you have specific issues with their method? Can you indicate which bits of their working you disagree with?

    If you want to ask basic questions about how GPS works you'll get better participation in the Q&A or Science and Tech fora.
    My specific issues with the Wolf and Petit paper are as follows, but my more general question is as stated in the OP. The authors warn of the risk of presupposing the validity of SR in testing the assumptions and predictions of relativity, and they make a number of methodological adjustments to avoid doing so:

    Additionally one has to ensure that corrections applied to the raw timing data used for orbit determination and the measurement of T do not presuppose the validity of special relativity. In fact, two corrections are routinely applied to GPS timing data, which are of relativistic origin and therefore do imply del c = 0: the correction for the gravitational redshift and the second-order Doppler shift of the rate of the satellite clock with respect to coordinate time, and the correction for the so-called Sagnac effect, which is due to the rotation of the Earth during signal transmission.

    Nevertheless, they fall into the trap of tautologically presupposing the validity of SR by their use of slow clock synchronization and Einstein clock synchronization as an ongoing re-synchronization technique to maintain synchronization during the operation of the GPS system (indirectly in both cases, since they simply used available data from the GPS system rather than conducting their own experiment). This seems to be a fatal flaw. Results that are tautologically determined are by definition unscientific and invalid.

    The paper states: “del c is the deviation from c of the observed velocity of a light signal traveling one way along a particular spatial direction with the measuring clocks synchronized using slow clock transport.” Slow clock transport is by definition equivalent to Einstein synchronization in the same inertial frame. And under Einstein synchronization the constant speed of light, regardless of the motion of the observer, is assumed. This is an operational assumption made in order to provide a simple and reliable way to synchronize distant clocks. It is important to note also that ongoing re-synchronization cannot, of course, be done using slow clock transport; Einstein synchronization (using light signals) must be used.

    This technique does of course provide a concrete method for defining simultaneity and thus for synchronizing distant clocks, but we must be careful to not use this technique and then forget that we have from the outset assumed an isotropic c in order to achieve synchronization. Unfortunately, Wolf & Petit overlooked this issue in their methodology.

    The 1997 paper is often cited (over 100 citations) as strong support for relativistic effects. While finding the methodological tautology in this paper is not readily apparent to the casual reader, it seems to be there nonetheless.

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    Quote Originally Posted by aramis720 View Post
    If this is the case, we are reduced to tautology in such analyses.

    Thoughts on this complicated set of issues?
    This is not a tautology because anyone who uses GPS to test the invariance of c will know about the SR and GR corrections. They will either not do the test or remove the corrections or use GPS signals that do not have the corrections.
    The corrections for SR are done by the receiver. The corrections for GR are the satellite clocks ticking as a different rate (easily corrected for).

    Wolf, P., & Petit, G. (1997). Satellite test of special relativity using the global positioning system. Physical Review A, 56(6), 4405–4409. doi:10.1103/physreva.56.4405 correct for "for the variable part of the
    gravitational redshift and second-order Doppler shift, the Sagnac effect, the ionospheric delay (using an ionosphere-free combination of the two frequencies), and the tropospheric delay".
    Last edited by Reality Check; 2018-Oct-09 at 02:42 AM.

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    Quote Originally Posted by aramis720 View Post
    The 1997 paper is often cited (over 100 citations) as strong support for relativistic effects. While finding the methodological tautology in this paper is not readily apparent to the casual reader, it seems to be there nonetheless.
    Ok, addressing your concerns. You are conflating two things here, the first is a bulk calibration of the clocks, a full resynchrinisation. This is not an ongoing process. It happens outside the time frame of the experiment. The second is GR corrections applied to timing data. In stating that this becomes a tautology you are missing the point of the experiment.

    What it is doing is applying a predicted transform from GR to the data based on orbital and kinematic parameters. This transform should be one that keeps the GPS clock and the ground clock synchronised if you assumed the speed of light is constant. If the transform were not accurate you'd see a consistent difference correlated with the apparent velocity - ie the clocks would desynchronise. This is what the experiment is testing and by showing that the desynchronisation is not seen it shows that the underlying assumptions are good.

    Edit to add: the reason I break these two effects out is that the bulk resynchronisation is done by measuring the observed difference and correcting the clock. The GR corrections are the predictions from the theory under test. So if they were continually applying bulk corrections based on observations you might have a case for declaring the experiment useless. But they are not, they are simply predicting, observing and testing based on the theory. Standard scientific method.
    Last edited by Shaula; 2018-Oct-09 at 05:38 AM. Reason: Added clarification

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    Quote Originally Posted by aramis720 View Post
    My question was this: if data extracted from GPS is used to test the invariance of c aren't we reduced to a methodological tautology b/c an invariant c is assumed in all synchronization and re-synchronization of the satellites? So the answer of an invariant c is baked in to the methodology and we can't get anything other than an invariant c out of the data.
    Right, and it still gives the correct data, which means the "baked in assumptions" you are having issue with are shown to be correct, thus this being evidence that the speed of light is invariant. If the speed of light did vary, and you did this exact same thing, you would get wrong data all the time and GPS would not work. So yes, they are assuming a constant C for this experiment. And yes, their data agrees with reality. Thus, it shows that C is constant.

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    Over 3 weeks and nothing further from aramis720; we're done.

    aramis720, if you decide to continue this discussion before your 30 days are up, Report this post and we can reopen the thread. In either case, this is your last opportunity to discuss your idea on CQ.
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