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Pioneer anomaly not gravitational

  1. Dec 17, 2009 #1

    bcrowell

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  2. jcsd
  3. Dec 23, 2009 #2
    IMO, there were really never any reason to expect that the PA should involve any
    real accelerations. That's just a specific interpretation of the PA, even if it is the popular one.
    But the fact that the anomalous acceleration is very close to cH, would suggest that
    the PA has something to do with mismodeling of null curves in an expanding Universe.
    This does not work in GR, but alternative models exist. See, e.g., gr-qc/9910054.
     
  4. Dec 23, 2009 #3

    bcrowell

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    Interesting. I took a quick, casual look at that paper, and at gr-qc/0112025, which describes his alternative theory of gravity. I don't claim to have examined it carefully enough to have even absorbed the general ideas of the theory. It is interesting that he claims to have a viable non-metric theory that is compatible with all the classic solar-system tests. However, he doesn't seem to have proposed any specific experimental tests other than the Pioneer Anomaly. It would be a lot more compelling if he could say, "Hey, here's this other empirical test where I predict something different from GR -- go measure this and see what happens."
     
  5. Dec 24, 2009 #4

    Garth

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    You may be interested in the discussion we have already had in physics forums, such as here: The Anomalous Acceleration of the Pioneer Spacecrafts, especially this analysis.

    The unexplained residual is consistent with cH.

    What actually was being detected was a residual blue shift in the radio signals, which is interpreted as an extra Sun-wards acceleration.

    It could just as well be a time drift between atomic and ephemeris clocks.

    Garth
     
    Last edited: Dec 24, 2009
  6. Dec 24, 2009 #5
    what is cH please?
     
  7. Dec 24, 2009 #6

    bcrowell

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    The speed of light multiplied by the Hubble constant. IMO it's not very impressive that cH happens to be of the same order of magnitude as the anomalous acceleration observed in the Pioneer anomaly, but that seems to be one of the main empirical facts used to support Østvang's QMR theory.
     
  8. Dec 24, 2009 #7

    turbo

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    The observations might be consistent with a variable speed of propagation of light through space that is polarized by gravitation of embedded masses. Such an interpretation of observations would be a game-changer and thus would be anathema to BB cosmology. If it's true, prepare to wait for a century or so before it can become "accepted".
     
  9. Dec 25, 2009 #8
    "The observations might be consistent with a variable speed of propagation of light through space that is polarized by gravitation of embedded masses. Such an interpretation of observations would be a game-changer and thus would be anathema to BB cosmology. "

    The use of the conditional tense is appropriate indeed, as always when pure speculation without enough experimental data is involved.
     
  10. Dec 25, 2009 #9

    turbo

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    We have plenty of experimental data, and not just from the Pioneers, but from other probes that are at least partially spin-stabilized. If the apparent "acceleration" is not real, and is not caused by gravitational forces on the probes themselves, we must be prepared to consider that the radio signals could be propagating at something other than c. Einstein's view of space as a polarizable ether was not well-received by his contemporaries nor by modern-day physicists, but it was a view that he held from 1920 onward.

    http://www.bartleby.com/173/22.html
     
  11. Dec 25, 2009 #10
    turbo-1,

    How far have you analyzed your experimental data?
    Could you make them available as well as your analysis?

    I am very far from being a specialist on this topic and I have never seen the full data. I even don't know how I could process them!
    That's why I am so sceptical, considering the many effect and sources of error to be taken into account and listed by Turyshev (see for example: http://arxiv.org/abs/physics/0502123). And this list is maybe far from being complete. For example, what would be the impact of disturbances caused by (very) small charge densities dispersed in space? Has this been studied? I would be interrested to know the impact on the experimental data.

    For me, the Pioneer anomaly is a very interresting topic, but I doubt it is a "clean enough" experiment to bring us to big conclusions, specially considering the small value of the anomaly.
     
  12. Dec 25, 2009 #11

    turbo

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    The Pioneer anomaly is being studied by the Planetary Society. A few years ago, years' worth of magnetic data-tapes were recovered, and it's unclear whether they have completed their analysis, from information on their web site. However, a recent publication by Turyshev says:

    http://arxiv.org/PS_cache/arxiv/pdf/0906/0906.0399v1.pdf

    While analysis to that level is pretty impressive, there is another more compelling reason to discount systematics. The anomaly is not confined to the Pioneer craft. Other probes also show evidence of anomalous Sunward acceleration, and when probes have different masses, cross-sectional areas and thermal signatures (due to the various ways RTGs are mounted) it is difficult to come up with scenarios in which the anomaly can be produced for each of them. As in the quote above, if thermal radiation contributed to the anomaly, one would expect that the effect would diminish as the RTGs decayed.
     
  13. Dec 25, 2009 #12

    JesseM

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    Do all the different probes show the same magnitude of anomalous acceleration as a function of distance from the Sun, or is it different for different probes?
     
  14. Dec 25, 2009 #13

    Garth

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    The Pioneer Anomaly acceleration is constant from the orbit of Saturn outwards and equal on both the Pioneer probes.

    Inside the orbit of Saturn other effects would have masked the effect, as indeed they have on other deep space probes, although signs of the effect have been detected on the Voyager probes as well. The Voyager spacecraft's three-axis stabilization maskes the effect.

    The Flyby anomalies are different, though they may be connected.
     
  15. Dec 25, 2009 #14

    JesseM

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    If there's no clear evidence that the magnitude of the anomalies for different probes are too similar to be explained by quirks of engineering, would you then disagree with turbo-1's statement that "when probes have different masses, cross-sectional areas and thermal signatures (due to the various ways RTGs are mounted) it is difficult to come up with scenarios in which the anomaly can be produced for each of them"?
     
  16. Dec 25, 2009 #15

    turbo

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    Part of my skepticism of design-dependent systematics arises from the fact that even though the anomaly can be masked in some probes that are not fully spin-stabilized, the anomaly still results in some blue-shifting. If off-gassing, leaking thruster fuel, or thermal radiative pressures can somehow be responsible for the anomalies, why is the thrust always directed away from the Sun resulting in braking? Why no acceleration out of the system resulting in red-shifting?

    We don't have enough probes out there to make the lack of red-shifting a "smoking gun". Dedicated spin-stabilized probes with centrally-located RTGs might help clear this up.
     
  17. Dec 27, 2009 #16
    Sounds like a worthwhile experiment send out 10 probes in different directions. All the probe needs to do is ping once per ten seconds and point towards Earth (that is point the antenna part towards Earth). Say 20Kg per probe 200Kg total. But at so respectable velocity (we don't have all century). Anyone have a feel for the cost?
     
    Last edited: Dec 27, 2009
  18. Dec 27, 2009 #17
    Will Pluto Express gives us usable data on this question? On their web page it says it is three axis AND spin stabilized. I guess it depends how often they fire the reaction wheel dumping thrusters? Anyone know how they will use the probe?
     
  19. Dec 27, 2009 #18

    bcrowell

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    If you look at http://en.wikipedia.org/wiki/New_horizons#Spacecraft_subsystems you can get a feel for sizes of the different stuff that has to go aboard this type of probe. New Horizons is 465 kg, and a lot of the equipment looks like it would also be needed for a dedicated pioneer-anomaly probe. E.g., it's got a really huge radio dish.

    There's some more info here about possible dedicated missions or other ways of exploring the Pioneer anomaly: http://en.wikipedia.org/wiki/Pioneer_anomaly#Further_research_avenues

    If I were peer-reviewing a proposal for a new dedicated probe, the question in my mind would be this: if it's not a gravitational effect, then what are the chances that it's even interesting physics at all?
     
  20. Dec 27, 2009 #19
    We have a total of five spacecraft outside Saturn's orbit, two of them Pioneers, and that's really not enough data to go on.

    One common theme is that each one of the five probes (two Pioneers, two Voyagers and New Horizons) necessarily consists of a big-*** parabolic dish facing the Earth (i.e., with good precision, the Sun), and some hardware bolted on the back side of the dish.
     
    Last edited: Dec 27, 2009
  21. Dec 28, 2009 #20
    Well, that would depend on your definition of "interesting physics". Some people apparently
    think that new physics that is not "mainstream new physics", i.e., not coming from some simple extension of mainstream theory, is uninteresting. Since most gravitational
    explanantions of the PA may be classified as mainstream new physics, evidence that the PA
    is not gravitational may diminish the general interest in the PA and thus the chances for
    a dedicated mission to test it.

    More radical explanations of the PA may still be viable, e.g., based on the notion that the
    cosmic expansion has observable consequences in the solar system. This confronts the
    "well-known" mainstream position that the cosmic expansion does not apply to
    gravitationally bound systems. But it is very difficult to unlearn something that is taken for
    granted. Thus some radical explanations may seem even more farfetced than what is
    really the case, also reducing the motivation for a dedicated test mission.
     
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