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Whatever happened to testing relativity?

  1. Sep 22, 2004 #1
    Correct me if i am wrong but..... I remember quite some time ago reading an article about scientists sending gyroscopes up into the earths outer atmosphere to check for frame dragging... does nyone know what happened about that experiment?
     
  2. jcsd
  3. Sep 22, 2004 #2
    I read that the test was a success, and that frame-dragging was observed and was consistent with predictions made from the theory.
     
  4. Sep 22, 2004 #3

    pervect

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    The experiment, called gravity probe B, is still ongoing. There's a status report at it's website:

    http://einstein.stanford.edu/

    Note that the results will not be released until the experiment is complete. This is going to take a year. Current status is that the mission has been collecting science data for some time on three gyros for some time, recently the fourth and last gyro has been brought online so it can also collect data.
     
  5. Sep 22, 2004 #4

    chroot

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    don.engel@hines.com,

    I don't know where you read that, but it's false.

    The experiment was launched on April 20, 2004 -- about six months ago. The experiment was stabilized and began taking science data on or about September 17, 2004 -- five days ago. It will not produce final results for another 16 months. The effect it is observing is quite small, and it takes a rather long time for the effect to accumulate clearly observable results. We'll know whether or not it was successful in early to mid 2006.

    http://www.nasa.gov/missions/highlights/launch_update_gpb.html
    http://einstein.stanford.edu/

    - Warren
     
  6. Sep 22, 2004 #5

    Garth

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    [Edit - Crossed with chroot and pervect]

    Unless I have missed another experiment you read wrong! (with respect)
    The experiment does exist and it is happening right at this moment. It is called Gravity Probe B and you can link to its weekly highlights here:- http://einstein.stanford.edu/ .
    It is a very sophisticated experiment in a polar orbit 400 miles up with four of the world's most accurate spheres spinning at about 3-4,000 r.p.m. aligned on a guide star IM Pegasi.
    There are two precessions the experiment is measuring. The first is a north-south geodetic precession caused by (excuse my 'hand waving explanation') the gyroscope pointing down the slope of the gravitational well around the Earth. The second is the much smaller frame dragging or Lense-Thirring precession in an east-west direction, caused by the spinning Earth dragging space-time with it.
    Although the data is coming in now and they will have a good idea about the gyros' behaviour in about another month or so, they rely on an independent data set that tracks the guide star in its peculiar motion across the sky. This will not be released until the data is complete and then compared with the precession data. In 2006 sometime! So we will have to wait.

    We have had a thread in these forums about GPB, whether it is worth doing or not, as it has been very expensive and one school of thought thinks GR must be correct and so the answer is a forgone conclusion.

    I believe it is very worthwhile as it is the first laboratory test of GR, apart from tests of the EEP, which are in a different class. All other tests of GR have been astronomical observations, the perihelia of Mercury, bending of light near the Sun etc. that test the question, "Do particles and photons travel on the geodesics of GR in vacuo?" The problem is that some other theories predict the same result for these previous tests.
    Garth
     
    Last edited: Sep 23, 2004
  7. Sep 22, 2004 #6

    chroot

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    :rofl: Great work, guys. :wink:

    - Warren
     
  8. Sep 22, 2004 #7

    LURCH

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    Perhaps what you read was a refference to this discovery, which claims to render GP-B obsolete.
     
  9. Sep 22, 2004 #8

    Garth

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    A claim that is contested - by Francis Everitt the project director amongst others including myself!

    As I have pointed out several times most cosmological and astronomical observations depend on taking data and then interpreting it with some theory or other. The deduced observation is theory dependent; change the theory and the observation changes too.

    In this case the theory in question is GR. The binary pulsar consists of two neutron stars in orbit around each other. A neutron star is degenerate, its material is highly relativistic and can only be analysed using the theory of GR, nothing else will do. So the observation we derive that appears to validate GR is dependent on GR for its validation. It is a circular argument. This does not necessarily mean the deduction is wrong, after all GR might well be perfectly adequate for describing such a situation, but it does not prove that it is correct.

    In the GPB experiment much better control has been obtained over the parameters that are being fed into the experiment, this leads to a greater confidence in the result.

    - Garth
     
  10. Sep 22, 2004 #9

    Nereid

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    Untangling all the effects which go to produce data from astronomical telescopes (and the like) can be quite a challenge (a recent example). At first glance it can sometimes appear the whole exercise is circular, as Garth suggests.

    However, by doing a lot of work, being very careful in your analyses, and always checking for consistency, quite solid results can be obtained. In particular, as long as the data are publicly available, anyone can do their own analyses - to check the published conclusions, to test one's own alternative ideas, to combine with other data to arrive at new, tighter constraints, and so on.

    In the case of Stairs et al, I don't think there's any doubt that their conclusions are a bit of a stretch from the data.

    Readers may be interested in some of the presentations from the recent Cosmic Vision 2015-2025 workshop, particularly those announcing plans for tests of GR (and much more); if the missions are launched, we can expect quite mind-bogglingly tight constraints on a wide range of fundamental physics, from GR to the constancy of constants, to the Casimir force, to quantum gravity, and more. (unfortunately, the presentations are PDF files, hardly any <1MB in size)
     
    Last edited: Sep 23, 2004
  11. Sep 23, 2004 #10

    Garth

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    In case I may be misunderstood, I fully accept that most astronomical and cosmological observations are interpreted with extreme care and are robust. My remark about a circular argument was specifically referring to a verification of GR by observation of a relativistic system in which the parameters, angular momentum, mass and the curvature that mass generates, which are to be fed into the relationship under test, are themselves derived using GR. Such an observation is not 'clean' and therefore the verification of GR is not robust. A 'clean' experimental test of GR requires experimental verification using parameters independently established such as that of GPB.

    Garth
     
    Last edited: Sep 23, 2004
  12. Sep 23, 2004 #11

    Garth

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    Nereid, I (and many other readers) would be interested in your second link, however it seems to be the same as the first. - Garth
     
  13. Sep 23, 2004 #12

    Nereid

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    :redface: Oops, sorry. Fixed now. :smile:
     
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