# Rotation - is it absolute?

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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?

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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.

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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)

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If it's rotating it isn't an inertial frame

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11 minutes ago, swansont said:

If it's rotating it isn't an inertial frame

Could you elaborate on this with an example?

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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.

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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?

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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.

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 by studiot

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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.

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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.

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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 by J.C.MacSwell

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If it’s rotating, it’s not inertial. If it’s inertial, it’s not rotating.

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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."

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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.

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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.

This is true locally but is it true in a more global geometry of GR spacetime? (is it true absolutely?)

Edited by J.C.MacSwell

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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.

Edited by pavelcherepan

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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 by StringJunky

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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?

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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 by geordief

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8 hours ago, pavelcherepan said:

P.S. There's actually a wiki article on Absolute rotation.

Thank you.  I should have looked there before I posted.  Greater minds than mine have thought about this!

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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.

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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."

This would be a pretty trivial statement if the frame referred to is not inertial.

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39 minutes ago, J.C.MacSwell said:

What isn't an inertial frame?

Within a rotating frame, what coordinates are changing ?

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1 hour ago, J.C.MacSwell said:

What isn't an inertial frame?

Gravity, causing frame-dragging.

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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 by J.C.MacSwell