Rotation - is it absolute?

[studiot]

5 minutes ago, J.C.MacSwell said:

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)

 

The coordinates of a frame refer to points in space, as indicated against the frame axes.

If this is the only frame you have (eg a roundabout) are say the coordinates of the handrails of the roundabout changing in the roundabout frame?

Now consider the centre of that roundabout as fixed at a point in the playground frame.

Are the coordinates of the roundabout handrails changing in the playgound frame?

What are the transformations between the two frames?

Are such transformations inertial?

 

[J.C.MacSwell]

16 minutes ago, studiot said:

The coordinates of a frame refer to points in space, as indicated against the frame axes.

If this is the only frame you have (eg a roundabout) are say the coordinates of the handrails of the roundabout changing in the roundabout frame?

Now consider the centre of that roundabout as fixed at a point in the playground frame.

Are the coordinates of the roundabout handrails changing in the playgound frame?

What are the transformations between the two frames?

Are such transformations inertial?

 

Is there a spinning black hole in the vicinity? If not, or something similar, how is this pertinent? You have a non inertial frame with the roundabout and want to compare it to the playground (essentially inertial). The question I put forward yesterday was whether two inertial frames could rotate wrt one another and the answer  according to the link I found seems to indicate that yes, it is possible (albeit extreme circumstances) This does not prove absolute rotation cannot exist but it does (for me at least) raise the possibility and I have heard no arguments against it, Swansonts proclamations notwithstanding. 

Edited February 24, 2018 by J.C.MacSwell

[studiot]

21 minutes ago, J.C.MacSwell said:

Is there a spinning black hole in the vicinity? If not, or something similar, how is this pertinent? You have a non inertial frame with the roundabout and want to compare it to the playground (essentially inertial). The question I put forward yesterday was whether two inertial frames could rotate wrt one another and the answer  according to the link I found seems to indicate that yes, it is possible (albeit extreme circumstances) This does not prove absolute rotation cannot exist but it does (for me at least) raise the possibility and I have heard no arguments against it, Swansonts proclamations notwithstanding. 

 

Forgive me but isn't the meat of the answer to your restated question in your answer above.

Sorry but I regard black holes as red herrings.

 

The first question is:

Is the roundabout rotating or or the playground?

The rotation is relative.

 

And the second observation is that the requirement for inertial system is that Newton's first law holds, you know the one about straight lines.

Straight line Motion in one inertial system implies straight line motion in any other, by definition.

So can this happen with contra rotating inertial systems?

[J.C.MacSwell]

3 minutes ago, studiot said:

 

Forgive me but isn't the meat of the answer to your restated question in your answer above.

Sorry but I regard black holes as red herrings.

 

The first question is:

Is the roundabout rotating or or the playground?

The rotation is relative.

 

And the second observation is that the requirement for inertial system is that Newton's first law holds, you know the one about straight lines.

Straight line Motion in one inertial system implies straight line motion in any other, by definition.

So can this happen with contra rotating inertial systems?

Again this is clearly true in Newtonian physics and SR/Minkowski space but as I asked earlier is it necessarily true in GR? 

The Wiki link seems to indicate no, it is not necessarily true in every case.

It is very brief, did you read it?

[pavelcherepan]

31 minutes ago, J.C.MacSwell said:

Again this is clearly true in Newtonian physics and SR/Minkowski space but as I asked earlier is it necessarily true in GR? 

There's no concept of inertial and non-inertial reference frames in GR. You have flat and curved space-time instead.

[J.C.MacSwell]

4 minutes ago, pavelcherepan said:

There's no concept of inertial and non-inertial reference frames in GR. You have flat and curved space-time instead.

I think the concept is there. The fact that they have limited realm should support, not detract, from my line of reasoning, should it not?

[pavelcherepan]

9 minutes ago, J.C.MacSwell said:

I think the concept is there.

Not sure about that. How would you describe a non-inertial FoR in GR?

[J.C.MacSwell]

29 minutes ago, pavelcherepan said:

Not sure about that. How would you describe a non-inertial FoR in GR?

One where Newtonian physics, to a close approximation, does not apply.

If you mean more precisely than that, LOL, I would not!

You mentioned this earlier, can you elaborate on the point you are trying to make and how it relates to absolute rotation?

 

Edited February 24, 2018 by J.C.MacSwell

[geordief]

5 hours ago, swansont said:

 

Combined, yes. Summmed, no.

What if the rotations have different centres? (it is what I was thinking of)

Very difficult and pointless?

[swansont]

21 minutes ago, J.C.MacSwell said:

One where Newtonian physics, to a close approximation, does not apply.

Why would Newtonian physics have to not apply? 

1 minute ago, geordief said:

What if the rotations have different centres? (it is what I was thinking of)

Very difficult and pointless?

Probably difficult. pointless depends on context.

[geordief]

10 minutes ago, swansont said:

 

Probably difficult. pointless depends on context.

The context (which made me wonder) was whether we could assign an overall rotation to the universe by "adding up" ** all the individual "sub rotations".

 

If at the time around BB  there was  such a rotation of the material at that time ,would that rotation have continued as the universe expended?

 

A possibly similar question is asked as to whether there were initial assymmetries of  different kinds (eg matter v antimatter) that had effects later on in the evolution of the Universe.

 

**"combining?

Edited February 25, 2018 by geordief

[J.C.MacSwell]

30 minutes ago, swansont said:

Why would Newtonian physics have to not apply? 

Probably difficult. pointless depends on context.

In there simplest and most straight forward form? Because it is not an inertial frame.

[MigL]

This all reminds me of Newton's pail and Mach's principle...

If you take a pail half full of water and spin it, you have various effects that arise because of that rotation, such as the water 'rising' along the edges.
Now take that same pail and spin the universe around it. Does the water still rise along the edge ?

Ernest Mach proposed that it did, and that this 'attraction' from the far-flung masses of the universe were responsible for inertia.

[pavelcherepan]

1 hour ago, J.C.MacSwell said:

You mentioned this earlier, can you elaborate on the point you are trying to make and how it relates to absolute rotation?

 

My point is that in GR there's an "absolute reference frame" - the spacetime itself and geodesics that show object's passage through it. As far as I understand, geodesics are invariable, whatever reference frame you calculate them in, the answer should be the same.

[J.C.MacSwell]

32 minutes ago, pavelcherepan said:

My point is that in GR there's an "absolute reference frame" - the spacetime itself and geodesics that show object's passage through it. As far as I understand, geodesics are invariable, whatever reference frame you calculate them in, the answer should be the same.

I would certainly agree with that part. Did not know there was an absolute reference frame in GR. Do you have a link for this?

[pavelcherepan]

17 minutes ago, J.C.MacSwell said:

Did not know there was an absolute reference frame in GR

There's no absolute time, but there is essentially an absolute space in relation to distant stars. So when I said "absolute reference frame" I meant it spatially only. Apologies for confusion.

https://en.wikipedia.org/wiki/Absolute_space_and_time#General_relativity

[Bender]

7 hours ago, pavelcherepan said:

My point is that in GR there's an "absolute reference frame" - the spacetime itself and geodesics that show object's passage through it. As far as I understand, geodesics are invariable, whatever reference frame you calculate them in, the answer should be the same.

Can some timelike geodesics "twist" with respect to others?

[pavelcherepan]

5 minutes ago, Bender said:

Can some timelike geodesics "twist" with respect to others?

No idea. Hopefully someone who understands Riemannian geometry can answer.

[studiot]

5 hours ago, Bender said:

Can some timelike geodesics "twist" with respect to others?

Do you mean as in torsion or do you mean curvature along the third degree of freedom afforded in 4D spacetime?

 

No one has yet mentioned rotational inertia, which is different from linear inertia (mass) in that it can be transferred from one body to another.

 

Here is an interesting demonstration (sorry I can't video it)

Take a large fruit bowl or similar.

Place two billiard balls in the bottom jut touching.

Give each ball a twist to spin them in contrarotating directions, still touching.

The balls will come to rest at the same instant, even if one is initially set rotating much faster than the other.

[J.C.MacSwell]

14 hours ago, pavelcherepan said:

There's no absolute time, but there is essentially an absolute space in relation to distant stars. So when I said "absolute reference frame" I meant it spatially only. Apologies for confusion.

https://en.wikipedia.org/wiki/Absolute_space_and_time#General_relativity

Agree essentially but still perhaps not absolutely. We do not know what is beyond our fixed stars (observable universe), so cannot say definitively that it is not rotating wrt something greater or other, if given that inertial frames can be produced that rotate wrt it as the Wiki article suggests.

 

This was what I meant when I posted previously:

On 2/24/2018 at 1:43 AM, J.C.MacSwell said:

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

 

Edited February 25, 2018 by J.C.MacSwell

[Bender]

3 hours ago, studiot said:

Do you mean as in torsion or do you mean curvature along the third degree of freedom afforded in 4D spacetime?

Torsion. Eg two geodesics (locally) describing  (part of) a double helix. If that is possible, a bowl of water in between those geodesics could have a flat surface, even though its orientation with respect to the distant stars is changing.  

[Bender]

Kerr metric:

Quote

The Kerr metric or Kerr geometry describes the geometry of empty spacetime around a rotating uncharged axially-symmetric black hole with a spherical event horizon.

I can't say I understand much of the article, but it mentions that spacetime itself can rotate.

[A. GEORGES]

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?

I do not know if anyone has already answered that, so I will allow myself to do that. Sorry my limited language skills, I am French.

Already, this sentence is interesting: "It seems to be possible to detect rotation in an object without reference to anything external."

Indeed, it is possible in General Relativity. The rotation of an object deforming the geometry of the space-time alters the curvature of said object. This is called the Lense-Thirring Effect. Measuring this variation of curvature makes it possible to determine whether the object is rotating and to determine the speed of this rotation.

In addition, I do not know what you mean concretely by "absolute non-rotation", but if I understood your question, you are asking if an object may not be rotating. If that's the case, yes, it's possible in theory.