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Einsteins GR and the rotating earth

  1. Nov 26, 2012 #1
    Einstein, for his GR theory he made a last important effort to make it valid also for rotating objects. So consider the following thought experiment.
    There is an observer A standing at the North Pole and another observer B sitting on a platform, turning clockwise once every 24 hours, both with a camera.
    The earth is hollow, with little gravity and a thin crust, turning like the normal earth.
    At the equator there is a man sending short laser pulses through an hole in the earth crust to the opposite side, about 12.000 km away. It takes 40 msec. for the pulses the reach the other side and inside the hollow earth there is a bit of fog as to show the course of the laser beam.
    During the 40 msec. objects at the equator move on for about 18 meters.
    At the North Pole there is also a hole in the crust. Both A and B make a pictures during the laser beam crossing the inner earth.
    Now, considering the rotating earth as a reference frame for GR, observer A sees the laser beam going straight.
    But B in rotation gets a picture from a laser beam slightly bent.
    But then, consider B as part of the non-rotating universe, following the laser beam with his camera, he should find a straight line. Because why should light bent when there is no gravity involved. How to solve this dilemma.
  2. jcsd
  3. Nov 26, 2012 #2


    Staff: Mentor

    Is the earth rotating? And is the turntable for B counteracting the rotation or adding to it?

    Assuming that the earth is rotating then A will not see a straight path.
  4. Nov 26, 2012 #3


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    Staff Emeritus
    Science Advisor

    If the Earth is hollow and the crust is light, this is essentially a SR experiment, because you are saying gravity is negligible. And this experiment basically tells us that rotation is absolute, not relative.

    We can measure the rate of absolute rotation with a ring laser gyro, for instance, as well as your scheme.

    To do a GR experiment we would need to consider a case where gravity would make a diference. For example we might consider, as Einstein did, consider a still hollow but massive rotating Earth. Or in any case a "shell".

    Then you can ask questions such as "will the frame that you determine to be nonrotating be fixed relative to the distant stars". Einstein's theory answers this one - no it will not.

    The general class of effects due to rotating massive bodies is called "frame dragging". Wiki as a short summary on it http://en.wikipedia.org/w/index.php?title=Frame-dragging&oldid=518801991
  5. Nov 27, 2012 #4
    Yes, I was already wondering why there was no return after I send the laser beam to the centre of the earth on to the mirror positioned at the other side.
    After reading Einstein’s biography in respect to GR there was talking about rotating objects, gyroscopes etc., which was confusing. And of course there would have been many more conflicts in the universe. The reference article you pointed at was very helpful. Thanks.
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