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A question about how to understand the relativity

  1. Mar 10, 2012 #1
    Suppose that there exists a special universe,in this universe, only have two congruent stars A and B, stars A side always facing the star B (similar to the moon is always only one side toward the Earth), star B in a certain angular velocity of rotation.

    In a super observer's vision, the two stars around the same center of site O in circular motion, then the centrifugal force and gravitational forces just cancel each other out.

    I want to kow:

    1.In the star A, a observer A, what will he understand the movement of the two stars?

    2.In the star B, a observer B, what will he understand the movement of the two stars?
  2. jcsd
  3. Mar 10, 2012 #2


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    This is a classic example used by Einstein, related to Mach's principle. See http://www.lightandmatter.com/html_books/genrel/ch03/ch03.html#Section3.5 [Broken] , subsection 3.5.2. You get one answer in general relativity, another answer in Brans-Dicke gravity. The one you get in GR is not the one Einstein thought/hoped you would get in GR. Brans-Dicke gravity is currently not very viable, http://www.lightandmatter.com/html_books/genrel/ch08/ch08.html#Section8.3 [Broken] , so Mach's principle doesn't seem to apply in our universe.
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  4. Mar 10, 2012 #3
    Thank you for your reply.
    In fact, what I want to say is:
    1, Observer O in space, he saw the scene in full compliance with the laws of physics;
    2, in the view of the observer A, the law of universal gravitation is wrong, because he will feel that the two stars are stationary, but not close to each other;
    3, in the view of the observer B, Newtonian mechanics is wrong, because he could not draw the same mechanics formula to the observer O's view .
    But I do not believe that ,So:
    1, can we consider that the existence of absolute space and time system?Observer O's system.
    2, or all the laws of physics are not universally applicable?
    3, or there are other answers ? I want to hear your views.
    4 My view is the laws of physics are generally applicable, but there is not real physical universe, the three observers in three different universe, each observer is an absolute time and space . Classical universe is the universe of the observer O; in universe of observer A , the two stars will be close to each other the final collision; in universe of observer B , star A circular motion around the star B, but the two stars look like in accordance with the physical constraints of the law of the quality as well as other physical quantities were adjusted accordingly.
  5. Mar 10, 2012 #4


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    Ok, let's start over. And I will answer in the framework of general relativity since it is backed by reliable evidence. Clearly what you are thinking is that the observers A and B will not realize they are rotating, because there are no distant stars in this universe to compare their motion to. This is false.

    Rotation in general relativity is absolute. It does not require any outside observation to determine a local nonrotating frame. You can do it very simply in a sealed room, using for example what they call a photon gyroscope. Aim a photon at a mirror and let it come back at you. If you think the received direction is different from the direction it was transmitted, you are rotating. If you think they are the same direction, you are not.

    Observer A, who constantly faces the other star, will measure his own rotation and use it to determine that both stars are in orbit about each other, including the plane of the orbit. If he is sufficiently accurate he will also be able to notice the frame dragging effect caused by the rotation of the surface of his own star. (These guys live on stars??) Likewise for B.
  6. Mar 10, 2012 #5


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    I agree with your #4. I would also point out that Brans-Dicke gravity is backed by all the same reliable evidence as GR, since BD gravity is equivalent to GR for large values of the unitless parameter [itex]\omega[/itex]. BD gravity gives the Machian result in this thought experiment. However, the value of [itex]\omega[/itex] is constrained to be so high by solar system measurements that it is no longer considered a competitive theory, since there is no good motivation for any value of [itex]\omega[/itex] that is not of order unity.
  7. Mar 10, 2012 #6
    Thank you, but If the three observers individually do the Michelson - Morley experiment in the respective location, what happens?

    My view is three experiments are the same results.
    What about your view?
    Last edited: Mar 10, 2012
  8. Mar 10, 2012 #7


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    Timeanalyser, what you're trying to define with the angles in the MM experiment is probably equivalent to the Sagnac effect. The Sagnac effect has been thoroughly studied both experimentally and theoretically, so you might want to try using it instead of a hypothetical and poorly defined variation on the MM experiment.
    Last edited: Mar 10, 2012
  9. Mar 10, 2012 #8
    I am glad that someone can agree the world is unreal not quite in line with common sense, but I do not know that there are no more mature point of view to establish a set of theoretical system from this perspective.
    If you have , can you tell me?
    In fact, from this point of view(the world is unreal ), with only a small number of reasonable assumptions and simple geometry will be able to deduce all of the special theory of relativity.I thinke the universe should be simple.
    We can take some time to discuss as follow:
    1, Assume that each different point is different time and space system, just like a whole universe.(A universe = a Planck length area)I think this is a reasonable assumption, your opinion?
    2, On the basis of that the universe is illusion, I think that each observer named A0 are independent of absolute space and time system, other observers (time and space ) relative to the observer A0 for relative time and space system, which is a reasonable assumption.

    First discuss these two issues, and there are more.
  10. Mar 10, 2012 #9
    What I mean is only to the observer O(if the guy has enough good vision), observers A and B see the instrument will draw the desired result(Line with the known laws of physics) , but to the observer A or B, they observed the instruments will be different results with observer O's.

    Is that possible?
  11. Mar 11, 2012 #10
    A could equally claim to be at rest and in this view B is spinning at a constant distance away, or B could be orbiting them and spinning at a faster rate then they themselves are. B could come to the exact same conclusions and there's no contradiction.
  12. Mar 11, 2012 #11
    But this is tantamount to admitting the existence of absolute space and time.
    Moreover,your laws of physics can be arbitrarily prescribed.
  13. Mar 12, 2012 #12
    How is it tantamount to admitting the existence of absolute space and time? Just the opposite. You view them as orbiting each other in which case both are spinning, just at different rates. Or you could view B as spinning and A as not spinning and remove orbit.

    The laws of physics cannot be arbitrarily prescribed. The two views are equivalent.
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