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

GR's Tests

  1. Nov 27, 2003 #1
    GR's Tests???

    [?]:- It that Real???

    Last edited by a moderator: Nov 27, 2003
  2. jcsd
  3. Nov 28, 2003 #2


    User Avatar
    Science Advisor

    Yes. GR makes several predictions that would distinguish its validity over other theories. The curving of light in a gravitational field and orbital precession are two notable predictions that have been verrified by observation. Time dilation is another.

    Some other major predictions of GR include "gravity waves" and "frame dragging", niether of which have been observed. We currently have LIGO looking for the gravity waves, and in the future LISA is planned to look for them even more accurately. And of course, GP-B will test frame-dragging.
    Last edited: Nov 28, 2003
  4. Nov 28, 2003 #3
    ... from Gravity Probe B 'Official' Web Site:-


    Any Comment(s) [?]
    Last edited by a moderator: Nov 28, 2003
  5. Nov 28, 2003 #4
    here it says frame dragging has been detected in two black holes in 1997:

    btw does it mean that black hole pulls everything including space time, how can it be done i thought spacetime isnt matter and that just matter can be pulled inside a blackhole?
  6. Nov 28, 2003 #5
    Wow, the technology of GP-B is absolutely amazing. I could never have dreamt of the purpose of such a project, nor knowing the purpose have imagined the experiment, nor knowing the experiment have envisioned the aparatus, nor knowing the aparatus have calculated the precision, nor knowing the precision have believed the feasibility, nor believing the feasibilty have understood the myriad of cutting-edge sciences, nor understanding the science have engineered the device, let alone the a process for constructing its componenets. Imagine the cumulative brainpower required in this test of one man's theory.

    I can see the headlines now: GP-B collides with man-made debris, drifts spacewards
  7. Nov 28, 2003 #6


    User Avatar
    Science Advisor

    RXTE found precession in the accretion disk sarounding a black hole, and this precession matched very nicely with that predicted by frame dragging, but GP-B will be more along the lines of a direct measurement.

    And (althoug you probably already realise this) it's not unique to black holes, all rotating objects with a gravitational field should pull spacetime around them. GP-B is trying to measure this effect as caused by our own humble little planet.

    If that happens, my head will probably explode !
  8. Nov 30, 2003 #7
  9. Nov 30, 2003 #8


    User Avatar
    Gold Member

    dont hold your breath, frame draging would be a vindication
    of one or two theories for gravity but it is tenuose and will be
    shown to be a misconseption, i also doubt that ligo will give
    any positive results, hope i am wrong .
  10. Dec 1, 2003 #9
    Last edited by a moderator: Dec 1, 2003
  11. Dec 2, 2003 #10
    What is your reason for saying frame dragging is 'tenuos' and why are you so confident (before experiments ) that is will be shown to be a misconception?
    I.E., do so have some basis to refute Lense-Thirring ?

  12. Dec 2, 2003 #11
    For the most part... very good article, 21toes :wink:

    The basic idea, as Lurch said, is to test competing relativistic theories. In order to do so Gravity Prbe B is actually testing two different predicted GR effects:

    One is called the geodetic effect: At the proposed orbit the gyroscope axis is predicted by GR to precess (about 6.6 arcseconds/yr.) strictly due to its passage thru the static gravitational field of the earth; i.e., strictly as a result of passing thru the earth's 'space-time curvature'. This is the same effect that produces the advance of the perihelion of Mercury, except in that case it is the solar 'geodetic' effect.

    The 2nd effect is sometimes called the 'motional' GR effect, that is, due to the motion of mass (in this case, the rotation of the earth) and is named after its discoverers Lense-Thirring. This effect is sometimes referred to as the 'gravitomagnetic field' since it is analogous to the magnetic field (in electromagnetics), which is due to the motion of electric charge.
    The precession of the gyroscope due to the motional effect is quite a bit smaller than the geodetic, being about 42 milli arcseconds/year, and requires much greater precision and technology. However, in a polar orbit the two effects are predicted to be at right angles to each other, making discrimination somewhat easier.

    Last edited: Dec 2, 2003
  13. Dec 3, 2003 #12


    User Avatar
    Science Advisor

    You are probably right; the effect will be very small, and those who do not agree with GR's prediction of frame dragging will remain unconvinced. LIGO and LISA will only gather first measurements in experiments that will have to be independently verified by other means and repeatedly reinforced before the findings can be considered reliable.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?

Similar Discussions: GR's Tests
  1. New test for GR ? (Replies: 20)