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Experimental verification

  1. Jun 13, 2007 #1
    A simple question of a curious person: How well is the general relativity verified experimentaly? In particular, are most of the tests conserned merely with the linear approximation of the gravitation? Or is there more or less direct evidence for the validity of strong gravitation fields, or weak with high accuracy, behaving according to Einstein's theory also?
     
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  3. Jun 13, 2007 #2
    Most of the experimental tests of GR are in the weak field limit. These use the PPN formalism.
    These include (http://relativity.livingreviews.org/Articles/lrr-2001-4/)

    Weak field:

    1. The deflection of light which passes near the sun. (see Eddington in circa 1919)
    2. The perihelion shift of the planets, especially, Mercury.
    3. The time delay of light as it is sent to a planet or moon and returned to earth.
    4. Gravitational redshift experiements. For example, The Pound and Rebka Experiment
    http://world.std.com/~sweetser/quaternions/gravity/redshift/redshift.html
    5. There is also Gravity Probe B which has measured the geodetic precession caused by the earth to high precision-better than 1%. Results for the dragging of inertial frames(Lense-Thirring effect) of the earth will be available sometime later this year, I think.
    http://www.space.com/businesstechnology/technology/gravity_probe_b_031231.html
    https://www.physicsforums.com/showthread.php?t=104694
    6. GPS would not be very accurate without GR corrections.
    see here:
    http://www.eftaylor.com/pub/projecta.pdf

    Strong Field:
    1. GR predicts that any relativistic system should radiate energy in the form of gravity waves-leading to the inspiral. Gravitational radiation is causing the binary pulsar PSR 1913+16 orbits to shrink at a rate of about 3.5 meters per year. The equation for energy loss due to gravitational waves gives an answer that matches observations of Hulse and Taylor, for which they received the Nobel Prize.
    2. Gravitational waves are a prediction but not yet detected.

    I guess it could be said that the solutions to Einstein equations also predict or atleast allow an expanding universe which is what is observed. Other theories of gravity might come close to describing SOME of these effects but not all of them and not to the same precision.
     
    Last edited: Jun 13, 2007
  4. Jul 25, 2007 #3
    I could guess that the weak field approximation is quite well tested, but I'm still a bit skeptical about the full theory. Is this Hulse and Taylor observation so far the only evidence supporting the Einstein's theory of gravity with strong fields? I'm not able to estimate how reliable it is myself... but well, is it?
     
  5. Jul 25, 2007 #4

    George Jones

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    Roger Penrose says that for the Taylor/Hulse pulsar, theory and "experiment" differ by at most one part in 10^14, which makes this one of the most accurate experiments in the history od science.
     
  6. Jul 25, 2007 #5
    :surprised All right
     
  7. Jul 25, 2007 #6

    Jonathan Scott

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    That figure is presumably related to the relative accuracy to which we can measure the time dilation effects, because of the accuracy of the pulsar and the length of time over which it can be observed. It is truly astonishing. However, I don't think it directly relates to the original question.

    As far as I know, the only aspect of this case which relies on calculation using higher order terms than the weak field approximation is the rate of loss of gravitational energy, which appears to be consistent with GR to within observational error which I believe is now down to somewhat less than 1 per cent (I'm sure that a search on the web will turn up more details).

    However, even this "strong field" case is very weak compared with the interestingly non-linear cases, such as the borderline between neutron stars and what GR would predict to be black holes. In such cases, it usually seems possible to explain the observations using GR, but I get the impression that the explanations push it rather hard in some cases, and I personally often suspect that GR might not be the whole story.
     
  8. Jul 25, 2007 #7

    George Jones

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    Most physicists think that GR is not the end of the story, but most physicists also are amazed by the gradually, but continually, growing body of indirect "experimental" evidence that is in accord with GR's predictions for stuff like neutron stars and black holes.

    From the Conclusions of Clfford Will's Living Review (the update line at blumfeld0's site doesn't seem to work):

    "We find that general relativity has held up under extensive experimental scrutiny. The question then arises, why bother to continue to test it? One reason is that gravity is a fundamental interaction of nature, and as such requires the most solid empirical underpinning we can provide. Another is that all attempts to quantize gravity and to unify it with the other forces suggest that the standard general relativity of Einstein is not likely to be the last word. Furthermore, the predictions of general relativity are fixed; the theory contains no adjustable constants so nothing can be changed. Thus every test of the theory is either a potentially deadly test or a possible probe for new physics. Although it is remarkable that this theory, born 90 years ago out of almost pure thought, has managed to survive every test, the possibility of finding a discrepancy will continue to drive experiments for years to come."
     
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