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

B A is slower than B, B is slower than A. Where did it go?

  1. Jan 2, 2018 #1
    As we know, according to Special Relativity, if A and B move relatively to each other, A’s clock will be slower from the point of view of B and vice versa!

    Well, there is Champeney and Moon experiment. Two observers (detectors) rotate on opposite sides of a rim of a centrifuge:


    The experiment vividly demonstrates, that there was no relative time dilation. That probably means, that the detectors slow down at the same magnitude bacause motion is absolute, or what?

    This source


    (just below the diagram) also claims:

    “Qualitatively this applies equally to both the classical and the relativistic treatments. (Since both emitter and receiver have the speed v relative to this system of reference, there is no differential time dilation.)”

    It is according to Lorentz, not Einstein, isn't it?

    But what about the Great Science - A is slower than B and B is slower than A?
  2. jcsd
  3. Jan 2, 2018 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member
    2017 Award

    This holds as stated when you discuss inertial frames with a well defined standard simultaneity convention. This is not the case for the observers in the experiment you quote. The results are perfectly compatible with SR.
  4. Jan 2, 2018 #3
    The article at Mathpages speaks about inertial observers. I added it intentionally. Our Earth is not inertial and Sun system not inertial also. Do you think if there is slightest curvature and purely inertial motion physics is different?
  5. Jan 2, 2018 #4

    Vanadium 50

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor
    2017 Award

    Bartolomeo, your anti-relativity crusade is becoming tedious. There used to be a sticky here saying this was not what this forum is for; I think removing it did not improve anything. Your first link involved observers in non-inertial frames, as was pointed out, and your second link is about users in inertial frames. These are not interchangable.
  6. Jan 2, 2018 #5


    Staff: Mentor

    This is only true if A and B are inertial observers using their respective inertial rest frames. It is not true for all observers using all reference frames.

    When saying “according to Special Relativity” it is important to accurately represent what SR claims. Otherwise your argument becomes a fallacious “straw man” argument, as is the case here.

    While you are away, a worthwhile exercise would be to calculate what result SR actually predicts for this experiment.

    Another worthwhile (but more difficult) calculation would be to determine how much error the earth or the sun introduce into this experiment.

    Approximations are justified if and only if they do not introduce large errors. Run the calculations and see which approximations are justified in this experiment.
  7. Jan 4, 2018 #6


    User Avatar
    Staff Emeritus
    Science Advisor

    It looks like the OP is on vacation, but I thought I'd mention <<this old PF thread>> , something I wrote a while back, on how symmetrical time dilation (A is slower than B, B is slower than A) implies the relativity of simultaneity, henceforth ROS.

    When one understands ROS, the seeming paradox disappears.
  8. Jan 4, 2018 #7


    User Avatar
    Science Advisor

    Actually, I think Bartolomeo's problem is this. For any object, including those in circular motion, you can always find (or define) an inertial object that is instantaneously in the same position and state of motion. So their observations at that time must be the same. Which is true.

    However he then concludes that relativity must be broken because the inertial and circling objects come to different conclusions about the clock rate of any other object. But this is just what we were arguing about on Alfredo Tifi's thread about Roemer's light speed measurement. Time dilation or not is purely a coordinate effect. It depends entirely on choices you are free to make in any way you like. It has no physical consequence, and the obvious choice(s) for inertial and non-inertial observers are different.

    So the answer to Bartolomeo's question
    ...is that it depends what you mean by physics. Two observers' interpretations of what's going on at a remote location can be radically different without affecting anything measurable.

    Is interpretation, modelling what's happening based on indirect evidence, part of physics? I flip-flop on that. Others - notably vanhees71 and Paul Colby on the Roemer thread - seem to say an emphatic NO. I don't particularly care either way - it's just words. But the fact that multiple interpretations are possible seems to me to be what Bartolomeo can't, or won't, swallow.

    At least, that's the only interpretation (:wink:) I can see for his questions in this thread.
    Last edited: Jan 4, 2018
  9. Jan 4, 2018 #8


    Staff: Mentor

    Since the OP is gone, I think we can close this thread.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook