Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Is Einstein's theory of GR shaky

  1. May 13, 2012 #1
    This is from Robert March's book Physics for Poets:

    How accurate is the above statement?
     
  2. jcsd
  3. May 13, 2012 #2

    jtbell

    User Avatar

    Staff: Mentor

  4. May 13, 2012 #3

    HallsofIvy

    User Avatar
    Staff Emeritus
    Science Advisor

    It depends upon what you mean by "shaky". No physical theory can be perfect. There is always going to be some, as yet unknown, data that will not be accounted for by the theory. But relativity fits the known facts so well that whatever does supercede relativity will have to include it as a "special case" or "approximation".
     
  5. May 13, 2012 #4
    The author said that the measurements were rather crude and not very precise. Is that right?
     
  6. May 13, 2012 #5

    Mentz114

    User Avatar
    Gold Member

    Those who maintain this are wrong. The precession of the perihelion of Mercury is a very tiny relativistic effect and it is explained to a high accuracy by GR.

    If Robert March cares as much for GR as I do for poetry, then you can disregard what he says about GR.
     
  7. May 13, 2012 #6

    phinds

    User Avatar
    Gold Member
    2016 Award

    No. The initial eclipsed-sun measurements WERE "shaky", even to the extent that the experimenters disagreed on the interpretation of the results, but that was nearly 100 years ago.

    Perhaps March's knowledge of physics is 100 years old. It certainly isn't current.
     
  8. May 13, 2012 #7
    Who in the world is Robert March?

    EDIT:

    Oops, he's a high-energy (emeritus) professor at University Wisconsin-Madison.
     
  9. May 13, 2012 #8

    Matterwave

    User Avatar
    Science Advisor
    Gold Member

    Is this book published before the Gravity Probe B results were? I don't think you can call those "crude" by any means.
     
  10. May 13, 2012 #9

    Mentz114

    User Avatar
    Gold Member

    Ah, that explains it.:wink:
     
  11. May 13, 2012 #10
    I don't know when 'crude' got identified with 'shaky'.

    The author (supposedly) objects that some of the earlier measurements were not precise enough. Of course, if you judge by the future standards, every past experiment is too 'crude'. For example, is Coulomb's experiment with the torsion balance the end all proof of Coulomb's Law in electrostatics?

    One needs to remember the distinction between 'precise' and 'accurate'. The Gran Sasso neutrino time-of-flight measurements were very precise. However, they weren't accurate, because they had a large systematic error. I would say this is more 'shaky'.
     
  12. May 13, 2012 #11

    Dale

    Staff: Mentor

    I really like "there are those who maintain that". It is a phrase which can be used to justify absolutely anything. "There are those who maintain that werewolves are real." Or "There are those who maintain that the earth is flat."
     
  13. May 13, 2012 #12

    atyy

    User Avatar
    Science Advisor

    The first measurements of the bending of starlight may have been shaky - I don't believe there is consensus on that. The current solar system tests are pretty tight, and the bending of starlight outside of the solar system has even been used to study dark matter. A very famous test of GR is the binary pulsar observations of Taylor and Hulse. Of course, if dark matter doesn't exist, that would mean GR is wrong.

    Example of an evaluation of early experiments as "ambiguous": http://aapt.org/doorway/TGRUTalks/Weiss/WeissTalk1of9.htm (p3)
    Pretty up-to-date Review of GR tests: http://relativity.livingreviews.org/Articles/lrr-2006-3/fulltext.html [Broken]
    Bending of light or "gravitational lensing" to study dark matter: http://home.slac.stanford.edu/pressreleases/2006/20060821.htm
     
    Last edited by a moderator: May 6, 2017
  14. May 13, 2012 #13
    Surely the significance of the starlight bending observations was not a precise agreement with GR, but rather that any bending occurred at all. Sure, I have heard modern arguments for the bending of light in Newtonian gravity (although still using the relativistic relationship between mass and energy), but at the time many people expected that there should be zero bending of light by gravity, if I remember correctly. Therefore the observation of any bending at all would be a big deal.
     
  15. May 13, 2012 #14
    ok, it looks like the board is unanimous that March is wrong. Someone mentioned the gravity probe B project. I've heard of that. I looked up the measurements on wiki and it said

    I think that book was written in something like 1979, so definitely before gravity probe b
     
  16. May 13, 2012 #15

    atyy

    User Avatar
    Science Advisor

    Gravity Probe B had some problems http://arxiv.org/abs/1106.1198

    Among the tests of GR available in 1979 were:
    http://en.wikipedia.org/wiki/Shapiro_delay
    http://en.wikipedia.org/wiki/PSR_B1913+16

    But there's interesting stuff about the correctness of GR from that period, eg. the solar oblateness story: http://www.astrosociety.org/pubs/mercury/9404/dicke.html [Broken]

    Again, a good review is http://relativity.livingreviews.org/Articles/lrr-2006-3/ [Broken]
     
    Last edited by a moderator: May 6, 2017
  17. May 13, 2012 #16
  18. May 14, 2012 #17
    "There have been a few somewhat shaky astronomical confirmations"
    ^^^^^^^^^^^

    To be fair, precisely what he wrote is true (given that interpretations of the term "shaky" may vary). The initial solar eclipse measurements and comparison from experiment to theory were extremely crude. Much of the reason for that is that the refractive index around the sun is so highly variable that the deviation of the results for that reason swamped any other observables.

    There was a study done by a french scientist, in the 60's I think, that applied the same methodology for determining Mercury's precession of the perihelion to other planets and satellites. None of the calculations were close to observed values. I make no judgement on the merits of that as I haven't read the book in which the results were published (and don't at the moment remember the author's name), however they were mentioned by the very eminent physicist Leon Brillouin.
     
  19. May 14, 2012 #18
    To be fair, there has never been (or at least their hadn't when i last looked for such a things a few years back) a strong field test of GR. Only first-order, weak-field corrections.
     
  20. May 14, 2012 #19
    Please predict black holes in the weak-field limit of GR.
     
  21. May 14, 2012 #20
    Please give evidence for GR's black holes (with singularities?) that cannot be called "shaky". :tongue2:
    [STRIKE]Does that book say anything about black holes?[/STRIKE]

    PS I looked it up at Amazon, and I see that he does write about them (but I can't access those pages)

    Anyway, I see, a bit to my surprise, that it is a textbook complete with exercises and answers.
    Near the end of the introduction he writes "What science should teach us is to doubt - to consider that many of the beliefs [..] may simply be wrong." - very true! :smile:
    And he ends that section with: "if this book can contribute to teaching that lesson, it will have done its job".
    That explains the sentence with which this thread started.
     
    Last edited: May 14, 2012
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Is Einstein's theory of GR shaky
  1. GR by Einstein himself (Replies: 31)

Loading...