Giorgio T. Posted February 22, 2018 Share Posted February 22, 2018 It seems to be possible to detect rotation in an object without reference to anything external. This can be done from within the object with a gyroscope, a ring laser or by detection of centrifugal forces. Is there a state of absolute non rotation? 1 Link to comment Share on other sites More sharing options...
swansont Posted February 22, 2018 Share Posted February 22, 2018 5 minutes ago, Giorgio T. said: It seems to be possible to detect rotation in an object without reference to anything external. This can be done from within the object with a gyroscope, a ring laser or by detection of centrifugal forces. Is there a state of absolute non rotation? Yes, since rotation is detectable, in principle. Link to comment Share on other sites More sharing options...
J.C.MacSwell Posted February 22, 2018 Share Posted February 22, 2018 A closely related question would be "can any inertial frame rotate with respect to any other?" I believe it may be possible, but am not certain. If it can then perhaps the answer to the PO might be no. (though I am not absolutely certain of that either) Link to comment Share on other sites More sharing options...
swansont Posted February 23, 2018 Share Posted February 23, 2018 If it's rotating it isn't an inertial frame Link to comment Share on other sites More sharing options...
koti Posted February 23, 2018 Share Posted February 23, 2018 11 minutes ago, swansont said: If it's rotating it isn't an inertial frame Could you elaborate on this with an example? Link to comment Share on other sites More sharing options...
pavelcherepan Posted February 23, 2018 Share Posted February 23, 2018 4 minutes ago, koti said: Could you elaborate on this with an example? Inertial frame is a frame that moves at a constant velocity without acceleration. On the other hand, rotation (or for that matter any motion on a closed trajectory) has to involve acceleration. Therefore, any FoR that exhibits such motion is not inertial by definition. When you are in a rotating FoR you can, in principle, measure the acceleration due to rotation. One such example is the Coriolis force or Coriolis effect. https://en.wikipedia.org/wiki/Coriolis_force 1 Link to comment Share on other sites More sharing options...
Giorgio T. Posted February 23, 2018 Author Share Posted February 23, 2018 9 hours ago, swansont said: If it's rotating it isn't an inertial frame Rotating in relation to what? Are we relying on internal signals, the external Universe or the inertial frame? Or is it all of those? Link to comment Share on other sites More sharing options...
studiot Posted February 23, 2018 Share Posted February 23, 2018 (edited) 9 hours ago, koti said: Could you elaborate on this with an example? A roundabout and good old classical mechanics. 16 minutes ago, Giorgio T. said: Rotating in relation to what? Are we relying on internal signals, the external Universe or the inertial frame? Or is it all of those? This is Einstein's principle of equivalence and the roundabout 9 hours ago, pavelcherepan said: Inertial frame is a frame that moves at a constant velocity without acceleration. On the other hand, rotation (or for that matter any motion on a closed trajectory) has to involve acceleration. Therefore, any FoR that exhibits such motion is not inertial by definition. When you are in a rotating FoR you can, in principle, measure the acceleration due to rotation. One such example is the Coriolis force or Coriolis effect. https://en.wikipedia.org/wiki/Coriolis_force Agreed, +1 Note that it is simpler to understand that observed 'gravity' is greater at the poles than the equator due to this effect Edited February 23, 2018 by studiot Link to comment Share on other sites More sharing options...
J.C.MacSwell Posted February 23, 2018 Share Posted February 23, 2018 18 hours ago, swansont said: If it's rotating it isn't an inertial frame Here is my thinking: If something is rotating it is rotating with respect to an inertial frame. If something somewhere else is rotating it is also rotating with respect to an inertial frame. These frames may be moving with respect to each other, but if they are also rotating with respect to each other then at least one of those rotations can not be considered absolute...but which one? ...so if ("if" is the key to the question, if this is known to be impossible the point is moot) any inertial frames can rotate with respect to each other then (perhaps?, obviously based on some assumptions, but similar in some respects to those for no absolute motion) there can be no absolute rotation. Link to comment Share on other sites More sharing options...
Janus Posted February 23, 2018 Share Posted February 23, 2018 50 minutes ago, J.C.MacSwell said: Here is my thinking: If something is rotating it is rotating with respect to an inertial frame. If something somewhere else is rotating it is also rotating with respect to an inertial frame. These frames may be moving with respect to each other, but if they are also rotating with respect to each other then at least one of those rotations can not be considered absolute...but which one? ...so if ("if" is the key to the question, if this is known to be impossible the point is moot) any inertial frames can rotate with respect to each other then (perhaps?, obviously based on some assumptions, but similar in some respects to those for no absolute motion) there can be no absolute rotation. I'm not sure what you are getting at. But let's say that you have one frame that is rotating a 1 rad/sec relative to an inertial frame and you have another frame that is rotating at 2 rad/sec relative to an inertial frame that has a velocity of 10m/s to the first inertial frame. Then the second rotating frame is rotating at 2 rad per sec relative to the first inertial frame and the first rotating frame is rotating at 1 rad /sec relative to the second inertial frame and both rotating frames are absolute. Link to comment Share on other sites More sharing options...
J.C.MacSwell Posted February 23, 2018 Share Posted February 23, 2018 (edited) 22 minutes ago, Janus said: I'm not sure what you are getting at. But let's say that you have one frame that is rotating a 1 rad/sec relative to an inertial frame and you have another frame that is rotating at 2 rad/sec relative to an inertial frame that has a velocity of 10m/s to the first inertial frame. Then the second rotating frame is rotating at 2 rad per sec relative to the first inertial frame and the first rotating frame is rotating at 1 rad /sec relative to the second inertial frame and both rotating frames are absolute. I am getting at " can any inertial frame rotate with respect to any other inertial frame?". If the answer is no, then I would say (under the right set of assumptions, those I think implied in the OP) that rotation is absolute. But if any inertial frames can rotate wrt any other inertial frames then it is less clear, unless there is a set of inertial frames that is preferred (with respect to rotation, not absolute motion) Edited February 23, 2018 by J.C.MacSwell added a ? Link to comment Share on other sites More sharing options...
swansont Posted February 23, 2018 Share Posted February 23, 2018 If it’s rotating, it’s not inertial. If it’s inertial, it’s not rotating. Link to comment Share on other sites More sharing options...
J.C.MacSwell Posted February 24, 2018 Share Posted February 24, 2018 9 hours ago, swansont said: If it’s rotating, it’s not inertial. If it’s inertial, it’s not rotating. Clearly true in Newtonian physics or SR/Minkowski Space but is that absolutely true in GR? Perhaps I am taking this too far, but how do we know for sure our little portion of the Universe (the observable universe), everything we measure rotation against, is "Absolutely" non rotating? "This frame is rotating with respect to the fixed stars and counter-rotating with respect to the black hole." https://en.wikipedia.org/wiki/Frame-dragging Link to comment Share on other sites More sharing options...
Bender Posted February 24, 2018 Share Posted February 24, 2018 10 hours ago, swansont said: If it’s rotating, it’s not inertial. If it’s inertial, it’s not rotating. To rephrase the question: can one portion of spacetime rotate with respect to another? Frame dragging to me seems to suggest the answer is yes, but I'm not familiar enough to be sure. 1 Link to comment Share on other sites More sharing options...
J.C.MacSwell Posted February 24, 2018 Share Posted February 24, 2018 (edited) 13 minutes ago, Bender said: To rephrase the question: can one portion of spacetime rotate with respect to another? Frame dragging to me seems to suggest the answer is yes, but I'm not familiar enough to be sure. Thanks. Much more succinctly stated than what I was posting. That carried to extreme suggests the possibility of no absolute rotation, at least to me. On 2/22/2018 at 8:26 PM, pavelcherepan said: Inertial frame is a frame that moves at a constant velocity without acceleration. On the other hand, rotation (or for that matter any motion on a closed trajectory) has to involve acceleration. Therefore, any FoR that exhibits such motion is not inertial by definition. When you are in a rotating FoR you can, in principle, measure the acceleration due to rotation. One such example is the Coriolis force or Coriolis effect. https://en.wikipedia.org/wiki/Coriolis_force This is true locally but is it true in a more global geometry of GR spacetime? (is it true absolutely?) Edited February 24, 2018 by J.C.MacSwell Link to comment Share on other sites More sharing options...
pavelcherepan Posted February 24, 2018 Share Posted February 24, 2018 (edited) 1 hour ago, J.C.MacSwell said: This is true locally but is it true in a more global geometry of GR spacetime? (is it true absolutely?) In GR there is no concept of inertial FoR, but it has curved space-time and you would either be rotating relative to your local geodesic or moving on a closed geodesic. In ether case it should be absolute. P.S. There's actually a wiki article on Absolute rotation. https://en.wikipedia.org/wiki/Absolute_rotation Edited February 24, 2018 by pavelcherepan Link to comment Share on other sites More sharing options...
StringJunky Posted February 24, 2018 Share Posted February 24, 2018 (edited) On 23/02/2018 at 12:15 AM, koti said: Could you elaborate on this with an example? Acceleration is a vector i.e it has two components consisting of magnitude and direction. It should be clear , because the direction is always changing in that situation, that the frame is accelerating and not inertial, which requires that there be no acceleration. Edited February 24, 2018 by StringJunky Link to comment Share on other sites More sharing options...
studiot Posted February 24, 2018 Share Posted February 24, 2018 On 22/02/2018 at 2:12 PM, Giorgio T. said: It seems to be possible to detect rotation in an object without reference to anything external. This can be done from within the object with a gyroscope, a ring laser or by detection of centrifugal forces. Is there a state of absolute non rotation? Consider the rotating cruciform arms in the Michelson Morley experiment. How does the fact that th arms were rotating affect the inertial status in respect of s. relativity and your question? Link to comment Share on other sites More sharing options...
geordief Posted February 24, 2018 Share Posted February 24, 2018 (edited) Can (distinct) rotations be summed? What if rotations overlap? Can the interaction be calculated purely in a "rotational way" or does the interaction simply destroy the rotation? Edited February 24, 2018 by geordief Link to comment Share on other sites More sharing options...
Giorgio T. Posted February 24, 2018 Author Share Posted February 24, 2018 8 hours ago, pavelcherepan said: P.S. There's actually a wiki article on Absolute rotation. https://en.wikipedia.org/wiki/Absolute_rotation Thank you. I should have looked there before I posted. Greater minds than mine have thought about this! Link to comment Share on other sites More sharing options...
swansont Posted February 24, 2018 Share Posted February 24, 2018 11 hours ago, Bender said: To rephrase the question: can one portion of spacetime rotate with respect to another? Frame dragging to me seems to suggest the answer is yes, but I'm not familiar enough to be sure. That's not an inertial frame. 6 hours ago, geordief said: Can (distinct) rotations be summed? What if rotations overlap? Can the interaction be calculated purely in a "rotational way" or does the interaction simply destroy the rotation? Combined, yes. Summmed, no. Link to comment Share on other sites More sharing options...
J.C.MacSwell Posted February 24, 2018 Share Posted February 24, 2018 34 minutes ago, swansont said: That's not an inertial frame. What isn't an inertial frame? 13 hours ago, J.C.MacSwell said: "This frame is rotating with respect to the fixed stars and counter-rotating with respect to the black hole." https://en.wikipedia.org/wiki/Frame-dragging This would be a pretty trivial statement if the frame referred to is not inertial. Link to comment Share on other sites More sharing options...
studiot Posted February 24, 2018 Share Posted February 24, 2018 39 minutes ago, J.C.MacSwell said: What isn't an inertial frame? Within a rotating frame, what coordinates are changing ? Link to comment Share on other sites More sharing options...
swansont Posted February 24, 2018 Share Posted February 24, 2018 1 hour ago, J.C.MacSwell said: What isn't an inertial frame? Gravity, causing frame-dragging. Link to comment Share on other sites More sharing options...
J.C.MacSwell Posted February 24, 2018 Share Posted February 24, 2018 (edited) 2 hours ago, studiot said: Within a rotating frame, what coordinates are changing ? Not sure of the context of what you are asking. Can you elaborate? The coordinates of a local inertial frame are rotating wrt another (the fixed stars) 1 hour ago, swansont said: Gravity, causing frame-dragging. Causing frame dragging rotation of an inertial frame... From the same: https://en.wikipedia.org/wiki/Frame-dragging (bolded by me) "There exists a particular rotation rate that, should she be initially rotating at that rate when she extends her arms, inertial effects and frame-dragging effects will balance and her rate of rotation will not change. Due to the Principle of Equivalence gravitational effects are locally indistinguishable from inertial effects, so this rotation rate, at which when she extends her arms nothing happens, is her local reference for non-rotation. This frame is rotating with respect to the fixed stars and counter-rotating with respect to the black hole. " Edited February 24, 2018 by J.C.MacSwell Link to comment Share on other sites More sharing options...
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