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Gravity Probe B / precession question

  1. Sep 8, 2011 #1
    I've been reading up on gravity probe B and the results, but I have a question about precession in general - I've heard it said that since the gyroscopes are in free fall there is no classical precession to be expected.

    So then, what explanation was there for earth's precession before general relativity? We knew about earth's precession long before einstein was around. Surely there was a classical model for earth's precession, right? That seems like a big hole in newton's theory if we knew about the earth's precession but couldn't explain it.
  2. jcsd
  3. Sep 8, 2011 #2


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    In Newtonian theory, the Earth precesses because it's not perfectly spherical - i.e because it has an equatorial bulge. Goldstein, "Classical Mechanics", has quite a bit about it. Tidal forces won't cause a perfect sphere to precess, but they will cause an imperfect one to precess.

    I'm pretty sure that this is the dominant cause of precession even in GR (though I'm not positive on this point).

    Great care was taken to make the gravity probe gyroscopes almost perfectly spherical, so that this effect wouldn't interfere with the experiment.
  4. Sep 8, 2011 #3


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    I may be wrong, but I think precession of earth is due to the earth being not spherical but oblate, so there are forces which try to move the equator back to the eclipsis and which are responsible for the precession. In GP-B this mechanism should be absent, as the rotators are almost perfectly spherical.
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