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Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: neilep on 16/07/2025 13:03:24

Title: Does anything in the universe exist in a truly non-rotating, motionless state?
Post by: neilep on 16/07/2025 13:03:24
Dearest Spinologists and those with degrees in Spinology !


As a sheepy I of course rotate, it's so that everyone gets their fair share of viewing me....


When we look out into the universe, motion seems to be everywhere. Planets spin on their axes, moons orbit planets, stars rotate, and entire galaxies swirl through space. Even tiny particles at the quantum level appear to exhibit forms of angular momentum. With motion so deeply woven into the fabric of the cosmos, it raises a fascinating question: Is there anything in the universe that isn?t spinning or rotating in some way?


whajafink ?

sheepy
xxxxxxxxx
Title: Re: Does anything in the universe exist in a truly non-rotating, motionless state?
Post by: alancalverd on 16/07/2025 16:49:33
Quote
The Higgs boson has a spin of zero, making it the only elementary particle with no spin. It is classified as a scalar boson, which means it has no intrinsic angular momentum.

What has a mass of 125 GeV, horns and a tail?
Title: Re: Does anything in the universe exist in a truly non-rotating, motionless state?
Post by: neilep on 16/07/2025 18:10:50
Quote from: alancalverd on 16/07/2025 16:49:33
Quote
The Higgs boson has a spin of zero, making it the only elementary particle with no spin. It is classified as a scalar boson, which means it has no intrinsic angular momentum.



What has a mass of 125 GeV, horns and a tail?


Thank ewe Alan...........Higgs Bison?
Title: Re: Does anything in the universe exist in a truly non-rotating, motionless state?
Post by: paul cotter on 16/07/2025 20:52:20
The devil, obviously.
Title: Re: Does anything in the universe exist in a truly non-rotating, motionless state?
Post by: Eternal Student on 17/07/2025 19:32:20
Hi.

    As always, thanks for introducing a good topic and I guess our job is to say something interesting in reply and maybe find a few side-roads and tangents that come off from it.

    So, it's a problem isn't it?  A big problem in something that seemed like we really had it and understood it.

    If something isn't spinning then there aren't any centrifugal forces in its rest frame,  or to say it another way, its rest frame could be a genuine inertial reference frame.   So you'd think that you'd know if something was spinning.

     Let's assume for a moment that you're a person and not always a sheep, @neilep, then you know when you're spinning because there is some centrifugal force on your arms and they tend to lift out and away from your body.   Out in the field on a clear night, you can feel this in your arms even with your eyes closed.  You don't have to be looking up at the stars and seeing them whirr around you to know that you are spinning.
     However, let's suppose that we do a slightly different trick:   We'll have you remain as still as you can.    What we'll do now is just start the distant stars spinning and whirring around you.   Now when you open your eyes, it obviously looks as if you are spinning.   The question is, has a centrifugal force just appeared in your rest frame - are your arms naturally tending to be pulled outward and away from your body?   Remember, we haven't touched you at all, we've just made some distant stars spin around.

    I'll leave you (or any reader) to think about that, there are some hints under this spoiler:
Spoiler: show

   You may want to look up   "Mach's principle".
   You might also like to look up "frame dragging" in General Relativity - but that's optional.  Mach's principle doesn't have to be connected to GR,  it's just one very convenient explanation for it along with some evidence to support it (e.g. the experiments and data of Gravity Probe A and B that are in orbit around earth).


    It's relevant for your original question because you probably ought to be clear what it actually means when we think or say that something is, or is not, spinning.

   We could say more about elementary particle spins that @alancalverd mentioned and @neilep mentioned in the OP  -  but it's best just to introduce a thing and leave the other person to think about that first.

Best Wishes.
Title: Re: Does anything in the universe exist in a truly non-rotating, motionless state?
Post by: paul cotter on 17/07/2025 20:09:13
Here is a follow up question: in the total absence of external reference points would it be possible to tell if one was rotating or not? For example an imaginary planet with it's own source of light and a sky that is uniformly black with no stars at all. My guess, and it is just a guess, is that one could not tell and in the absence of an external reference can rotation even be defined?
Title: Re: Does anything in the universe exist in a truly non-rotating, motionless state?
Post by: alancalverd on 17/07/2025 20:14:03
Anyone on the surface of a completely spherical and uniform planet should be able to detect an apparent increase in gravitation as they approach the poles, due to a decrease of centrifugal force.
Title: Re: Does anything in the universe exist in a truly non-rotating, motionless state?
Post by: paul cotter on 17/07/2025 22:08:07
Very good, Alan, I did not think of that.
Title: Re: Does anything in the universe exist in a truly non-rotating, motionless state?
Post by: Eternal Student on 17/07/2025 23:31:01
Hi.

Quote from: paul cotter on 17/07/2025 20:09:13
....in the total absence of external reference points would it be possible to tell if one was rotating or not?...

     Basically, no, it shouldn't be possible to know.   There shouldn't be any centrifugal forces in your rest frame frame and it should appear in all respects that you are not spinning.    Of course, it's never been possible to do the experiment, we can't delete the rest of the universe, so all we have is the theory.   The only relevant theory I know a little about is GR, so I'll be using that but as I stated before, Mach's principle doesn't HAVE to be connected to GR - there are other theories and methods of explanation proposed.


Quote from: alancalverd on 17/07/2025 20:14:03
Anyone on the surface of a completely spherical and uniform planet should be able to detect an apparent increase in gravitation as they approach the poles, due to a decrease of centrifugal force.
    Yes and no.   That's ok for a typical planet in the sort of the universe that we do actually have.    However, let's now start doing a thought experiment.   We will steadily start deleting all the other matter in the universe until only our one planet is left.   As we start doing this, the centrifugal forces that would have been necessary to describe the motion of particles in the rest frame of the planet, should start reducing and eventually disappear.

    Let's try and do this the easy way first, just with words and an appeal to a notion of what "frame dragging" is about.   

    If a massive spherical shell of material starts to spin around a point in space, then it tends to "drag" or twist the local inertial frame centred around that point with it.   To paraphrase this, the spinning massive shell of material drags the fabric of space around with it.    This local inertial frame centred at our point in space is genuinely "inertial" in the usual sense, there will be no centrifugal forces in this frame.   However, if we were able to overlay the old local inertial frame over the top of this (before the hollow shell of material was made to spin), we would see that the new inertial frame is seemingly dragged around and turns with the spinning massive hollow shell of material.
    Now, I need to make it clear that we can't just overlay the old and new local inertial frames and actually "see" that the new frame is being dragged around:   Space has fundamentally changed once the massive hollow shell started to spin around and any frame you tried to lay down there will naturally tend to twist and follow in sympathy with the rotating massive shell of material.   None-the-less it's useful as a concept, if we could make the old frame immune to the way space has now been warped,  then we would see the new inertial frame is being dragged around with the spinning hollow shell.

    So, as we start deleting more of the matter in the universe there are less things dragging space around.   The motion of our one planet becomes a more important contribution and an inertial frame centred at the planet will be frame dragged more by the planet than by anything else.   Eventually the only thing left is our planet, it's the only thing dragging space around.   When there is only that one (previously spinning) planet left, then the local inertial frame centred at that planet will be exactly that frame which is completely dragged around in sympathy or in keeping with the planet.   That is an inertial frame in the usual sense of "inertial frame", there just won't be any centrifugal forces in the rest frame of the planet.   

    Now it's your choice:   You can either say that the planet is no longer spinning   OR ELSE   we can say that spacetime has been altered / warped appropriately so that the hallmarks of rotation / spinning have been lost.   Either way you want to view it, the practicality is the same:   In the local inertial frame centred at this planet, we don't need to include any centrifugal force terms into the mechanics that describes how test particles would move about on the surface of that planet, it's not there, it will be exactly as if the planet is not spinning.

Best Wishes.

LATE EDITING:    The gist is OK but we might actually need the planet to be just a hollow shell.  A solid ball can mess it up a little because there will be bits of space with momentum and stress that differ in numerical value, for example the bit of space at the centre of ball is very different to the edge of the ball.
Title: Re: Does anything in the universe exist in a truly non-rotating, motionless state?
Post by: alancalverd on 18/07/2025 09:58:18
My scenario derives from theoretical physics, not planetary engineering: my planet was a uniform solid sphere spinning in an infinite vacuum. My test particle has mass m << M of the sphere.

There is one unique point in a stationary sphere - the center. If it spins, there are two other unique points at the poles.

Gravitational force = GMm/rp2 everywhere on the surface, but the test particle wants to fly off tangentially (Newton) so experiences a centrifugal force mrrω2 opposing gravity.

Annoyingly, this invokes Hamdani's concept of "rotational radius"  that I spent so much time decrying in the discussion of torque! It is clear that the radius of rotation at the equator is the radius of the planet rr = rp, but rr = 0 at the poles.

There are, of course, plenty of engineering examples.  Surface tension acts very similarly to gravitation, and oldfashioned window glass was made by spinning a spherical blob of viscous material into a thin disc. All the forces involved are much greater than any due to the environment, so this approximates to the ideal planet with a test mass on the surface.

It gets more complicated if the spin axis is also precessing, which provides two more unique points in space where the axis of precession exits the sphere, but these are not unique points on the surface of the sphere. Calculation of the resultant net force on m at an arbitrary point on the surface is left as an exercise to the reader but if I recall correctly (it's in "takeoff characteristics of taildraggers" but that exam was a long time ago!) it involves something sinusoidal,
Title: Re: Does anything in the universe exist in a truly non-rotating, motionless state?
Post by: paul cotter on 18/07/2025 10:10:04
A two axis rotation would eliminate the poles.
Title: Re: Does anything in the universe exist in a truly non-rotating, motionless state?
Post by: alancalverd on 18/07/2025 10:14:00
You beat me to the draw by 2 minutes - see the last paragraph edit of reply #9!
Title: Re: Does anything in the universe exist in a truly non-rotating, motionless state?
Post by: paul cotter on 18/07/2025 10:41:51
Guess i'm guilty of carelessness. I should have read your post completely before commenting.
Title: Re: Does anything in the universe exist in a truly non-rotating, motionless state?
Post by: Eternal Student on 18/07/2025 12:00:28
Hi,

Quote from: alancalverd on 18/07/2025 09:58:18
My scenario derives from theoretical physics, not planetary engineering:

    No it doesn't,  it's almost exactly the opposite of that.  It derives from an engineers bias that the universe should always be modelled with the equations and formulae they learnt at school and college.

   Half the problem is that when there is only that bit of mass in the universe that is (was) the planet,  then space is very different.   As you know, mass bends space.   So, it's not straightforward to know what sort of geometry will apply or exist in this space.  In particular, it's not clear that the planet would still look spherical or have properties like a sphere in Euclidean space.  Moreover, it's not clear that conventional rotational dynamics would apply (e.g. using formula like mω2r  for the centrifugal force at the surface of the planet).

     I've tried to overlook this issue myself and continued talking about "the planet" as if it will still be something like our planet, just to keep the discussion as simple as possible.  However, at least I am mentioning that space changes as you start eliminating matter from it.   Meanwhile, you ( @alancalverd ) are simply not doing this but instead just ploughing on as if conventional rotational dynamics derived in flat Minkowski space will somehow always continue to apply.    For example, the whole spacetime manifold has probably degenerated into something vaguely like the Schwarzschild solution (or the Kerr Solution if we maintain that the planet-like object was rotating) and so a small change in the r co-ordinate,  dr,  doesn't even have to describe a change in an amount of space.   Where r < rs (the Schwarzschild radius), then the r co-ordinate has become time-like and not space-like.   Pause for a moment and recognise that the underlying geometry of this space is thoroughly messed up and conventional formulae for rotational dynamics like mω2r (that were derived for circular motion in flat space) have long since "gone out of the window".

    Let's just approach the situation the other way around, start from the very specific for which we have an exact solution to the EFE and then generalise to our situation.   What is it that happens in the Kerr solution that describes the spacetime around a rotating black hole?   Can we generalise this to a universe where the only thing in it is a rotating planet (that hasn't necessarily collapsed to a proper BH)?

The Kerr metric is a generalization to a rotating body of the Schwarzschild metric ......
According to the Kerr metric, a rotating body should exhibit frame-dragging (also known as Lense?Thirring precession), a distinctive prediction of general relativity. ....... Roughly speaking, this effect predicts that objects coming close to a rotating mass will be entrained to participate in its rotation, not because of any applied force or torque that can be felt, but rather because of the swirling curvature of spacetime itself associated with rotating bodies.
[Extracts taken from  Wikipedia.  https://en.wikipedia.org/wiki/Kerr_metric ]

    There you go.   That's how a theoretical scientist does it.   There may be some practical difficulties when trying to delete all other matter from the universe but that's a separate issue.   If we assume that we can delete all the other matter in the universe and leave only the (previously spinning) planet, then local inertial frames centred at some point on the surface of the planet should do as previously discussed.  In particular, test masses on or just above the surface of the planet are "entrained" to follow the rotation of the planet without needing to have any centripetal or centrifugal forces included in the equations that will govern their motion.

Best Wishes.
Title: Re: Does anything in the universe exist in a truly non-rotating, motionless state?
Post by: paul cotter on 18/07/2025 12:30:34
I am not in a position to argue with many of your points as my level of GR understanding is abysmal(tensor math with all it's sub/super scripts just confuses me). However, space as it currently exists is locally close to flat Minkowski space and deletion of all other mass in the universe apart from our test planet would surely make it even flatter?  NB: engineering argument as opposed to a physicist's argument.
Title: Re: Does anything in the universe exist in a truly non-rotating, motionless state?
Post by: alancalverd on 18/07/2025 18:43:05
Quote
In particular, it's not clear that the planet would still look spherical or have properties like a sphere in Euclidean space.

It will because, like a good mathematician, I said it does, in reply #6
Quote
a completely spherical and uniform planet
Quote
In particular, test masses on or just above
I said on, not just above. And the entrainment, whether by frame dragging or a piece of string, means that it will be accelerating towards the center of the planet, so the gravitational force will be offset by the fact that it is in permanent free fall.   
Title: Re: Does anything in the universe exist in a truly non-rotating, motionless state?
Post by: Bored chemist on 18/07/2025 19:13:39
I'm curious...
Unfortunately, I'm a curious chemist, rather than a physicist.

So...
Imagine I'm on a perfectly spherical planet which is alone in the whole infinite universe.
I can check it's a sphere. I just need a big one of these.
https://en.wikipedia.org/wiki/Spherometer

And I can choose a planet which is made from a single practically rigid material.
I can get a pendulum clock and a quartz clock.
I can put them next to each other and compare how fast they run.
If I plot the rate of change of difference between the two clocks as a function of position, I think I should get a pattern where the pendulum clock runs faster at the poles and slower at the equator.
Not some strange relativistic thing, just the effect of centrifugal force meaning the pendulum isn't pulled down so hard at the equator.

If this
https://www.engineeringtoolbox.com/acceleration-gravity-latitude-d_1554.html
is to be believed, the effect is such that g changes from 9.78 to 9.83 m/s/s here on earth, and the error in timing is comparable (About 0.25% if I have got the arithmetic correct),

There will be some tiny effect of local gravity on the quartz clock, but I can pretty much ignore that if all I want to do is find the equator.

If the two clocks are consistent over the whole of the sphere's surface I can say it's not rotating.

Or doesn't that work?
Title: Re: Does anything in the universe exist in a truly non-rotating, motionless state?
Post by: alancalverd on 18/07/2025 23:04:23
It's a pretty basic form of gravimeter, but an even simpler one just* uses a helical spring and a weight.

http://www.cleonis.nl/physics/phys256/eotvos.php  discusses some interesting experiments where a gravimeter in an airship flying east to west measured a different value from one flying west-east, the difference being due to the difference in centrifugal force.


*physicist's shorthand for hundreds of hours in an instrumentmaker's workshop.
Title: Re: Does anything in the universe exist in a truly non-rotating, motionless state?
Post by: Bored chemist on 18/07/2025 23:42:42
I just want the excuse to have a good pendulum clock.
(Even if it's just  in a thought experiment)

https://en.wikipedia.org/wiki/Shortt%E2%80%93Synchronome_clock

The great advantage to using clocks is that, if they are reasonably well built, I can improve the accuracy of my measurement by simply waiting longer.


Title: Re: Does anything in the universe exist in a truly non-rotating, motionless state?
Post by: geordief on 19/07/2025 17:17:01
So if we have two rotating objects with mass (obviously in a vacuum)  does the rotation of  the one affect the rotation of the other?
Title: Re: Does anything in the universe exist in a truly non-rotating, motionless state?
Post by: Eternal Student on 20/07/2025 11:50:47
Hi,

Quote from: geordief on 19/07/2025 17:17:01
So if we have two rotating objects with mass (obviously in a vacuum)  does the rotation of  the one affect the rotation of the other?

   A brief answer:    Not usually.

   Our universe is big and contains many things:   uncounted galaxies,  dark energy spread over the place   etc.         This is what will determine the background, the curvature of spacetime.   The contribution from the two objects that you're focusing will be utterly insignificant by comparison.   We can approximate the situation as if space is just a static background:  Your two objects are simply in the space but do not really change the nature of space in any significant way.

     If your two objects were the only material in the universe, then that's different,  each one will now strongly influence the background geometry of spacetime.   Similarly, if your two objects were unusually dense, say two rotating neutron stars in close proximity, then they can be the dominant sources / causes of spacetime curvature in a small region of space around them.

     We have this phrase which is usually credited to  John Wheeler:
Spacetime tells matter how to move; matter tells spacetime how to curve.
   So, if one of your objects is the significant cause of spacetime curvature in a region, then it will influence how everything in the region can possibly move.

Best Wishes.
Title: Re: Does anything in the universe exist in a truly non-rotating, motionless state?
Post by: alancalverd on 20/07/2025 17:26:09
Given that angular momentum is conserved, I wonder whether the net angular momentum of the observable universe is zero, some entirely arbitrary number (i.e. the observable universe has an axis and two unique points) , or nonzero and precisely balanced by that of  dark matter?
Title: Re: Does anything in the universe exist in a truly non-rotating, motionless state?
Post by: geordief on 20/07/2025 22:20:46
Quote from: alancalverd on 20/07/2025 17:26:09
Given that angular momentum is conserved, I wonder whether the net angular momentum of the observable universe is zero, some entirely arbitrary number (i.e. the observable universe has an axis and two unique points) , or nonzero and precisely balanced by that of  dark matter?
Well,since the earliest known  period in the universe is said to  have  been t+ 10^-43 seconds is there any indication whether any rotation  might have  occured then
?
If so ,perhaps that  rotation (angular momentum) might have been conserved  into later periods.

Would there be a connection between any primordial rotation and the initial degree of assymmetry  that I have heard talk of?
Title: Re: Does anything in the universe exist in a truly non-rotating, motionless state?
Post by: Eternal Student on 20/07/2025 23:55:14
Hi,

   You  ( @alancalverd ) are doing this deliberately, aren't you?

Quote from: alancalverd on 20/07/2025 17:26:09
Given that angular momentum is conserved,.....

       Why would we assume angular momentum is conserved?   I mean to say, it's not as if we're sure that every spacetime and every solution of the EFE will have the sort of rotational symmetry that Noether's theoreom requires.

       We've had discussions about whether "Energy" is conserved in general relativity   (.... final conclusion:  it doesn't have to be).  "Angular Momentum" is much the same.   At best, it's a grey area.

Best Wishes.
Title: Re: Does anything in the universe exist in a truly non-rotating, motionless state?
Post by: Eternal Student on 21/07/2025 01:15:00
Hi again,

   I've just re-read your post and noticed something else:      Did you mean to say "observable universe" ?

New particles enter the observable universe as time progresses, so they bring angular momentum in with them.   The system doesn't seem suitably closed.

Best Wishes.
Title: Re: Does anything in the universe exist in a truly non-rotating, motionless state?
Post by: alancalverd on 21/07/2025 09:39:13
This surely allows us to explore the "unobservable" parts of the universe - a powerful insight - or predict the future of the observable universe.

If angular momentum is conserved then the net change in am of the observable universe with time tells us something about the source of said new particles - at least how its am is changing.

If am is not conserved, then the observable universe will either explode as the increase in am drives orbiting bodies further apart, or collapse as gravity overpowers a reducing am.
Title: Re: Does anything in the universe exist in a truly non-rotating, motionless state?
Post by: Eternal Student on 21/07/2025 12:38:10
Hi,

   What are you talking about @alancalverd ?   

Quote from: alancalverd on 21/07/2025 09:39:13
....If am (angular momentum) is not conserved, then the observable universe will either explode as the increase in am drives orbiting bodies further apart, or collapse as gravity overpowers a reducing am...

    The notion of the observable universe coming apart doesn't actually seem all that unlikely.  We do think the universe is expanding, distances between galaxies are generally increasing and this rate of expansion seems to be accelerating.

Best Wishes.
Title: Re: Does anything in the universe exist in a truly non-rotating, motionless state?
Post by: alancalverd on 21/07/2025 13:56:54
Newtonian mechanics. If you increase the angular momentum ∑mr2ω of an assembly, it either gets heavier, spins faster, or gets wider.
Title: Re: Does anything in the universe exist in a truly non-rotating, motionless state?
Post by: Bored chemist on 22/07/2025 19:24:21
Quote from: alancalverd on 18/07/2025 23:04:23
It's a pretty basic form of gravimeter, but an even simpler one just* uses a helical spring and a weight.

http://www.cleonis.nl/physics/phys256/eotvos.php  discusses some interesting experiments where a gravimeter in an airship flying east to west measured a different value from one flying west-east, the difference being due to the difference in centrifugal force.


*physicist's shorthand for hundreds of hours in an instrumentmaker's workshop.
This seems to have somewhat distracted people from answering my question.
Am I right in thinking that I can tell if the planet I'm on, alone in the universe is rotating?
Title: Re: Does anything in the universe exist in a truly non-rotating, motionless state?
Post by: paul cotter on 22/07/2025 20:28:49
The simple answer seems to be that if it is a single axis rotation then it should be possible. Generally rotation is specified in relation to some other object- does rotation have a valid meaning if there is nothing else in our imagined infinite empty universe to reference this rotation against?
Title: Re: Does anything in the universe exist in a truly non-rotating, motionless state?
Post by: geordief on 22/07/2025 22:01:40
Quote from: paul cotter on 22/07/2025 20:28:49
The simple answer seems to be that if it is a single axis rotation then it should be possible. Generally rotation is specified in relation to some other object- does rotation have a valid meaning if there is nothing else in our imagined infinite empty universe to reference this rotation against?
If  it was the only object then wouldn't  it comprise  all there was ?
In the same way that  we cannot view our "present universe" from an outsider's viewpoint then perhaps the observer of this object would have to also be an integral part of it

If the object had a structure the observer would be somewhere in that structure and (I think)  be able to judge  whether the object was rotating by making observations of different regions of the object.
Title: Re: Does anything in the universe exist in a truly non-rotating, motionless state?
Post by: alancalverd on 23/07/2025 13:19:07
Quote
Am I right in thinking that I can tell if the planet I'm on, alone in the universe is rotating?
Certainly, if you know it is a homogeneous sphere, by using gravimetry.
Title: Re: Does anything in the universe exist in a truly non-rotating, motionless state?
Post by: Bored chemist on 23/07/2025 19:53:01
Did I misunderstand this?
Quote from: Eternal Student on 17/07/2025 23:31:01
Hi.

Quote from: paul cotter on 17/07/2025 20:09:13
....in the total absence of external reference points would it be possible to tell if one was rotating or not?...

     Basically, no, it shouldn't be possible to know.   
Title: Re: Does anything in the universe exist in a truly non-rotating, motionless state?
Post by: Eternal Student on 24/07/2025 11:37:37
Hi  @Bored chemist ,

    Your earlier post  (post #16)  implied that you weren't considering any effects from relativity.
Quote from: Bored chemist on 18/07/2025 19:13:39
Not some strange relativistic thing, ......

    I think everyone then assumed that you wished only Newtonian mechanics and a corresponding notion of spacetime as Newton's spacetime to apply.
    In that situation,  the usual  mechanics holds and the equation describing the time period of pendulum,  T = 2π √(L/g) , will apply.    So an appropriate answer to your question would be "yes",  you could determine a planet was spinning by taking your pendulum clock to different places on the planet.   As @alancalverd suggested,  an even simpler "gravity meter" could be used.   

      Post #8 which you recently quoted describes the situation that is, in my opinion, actually more likely.   Specifically, the universe is under no obligation to comply with Newtonian mechanics and Newton's notion of spacetime.    Newton is a good approximation quite often but when your planet is the only thing in the universe,  then it ceases to be a good model of the situation.    Relativity, especially the theory of General Relativity, suggests that spacetime would be very different.   In particular, "frame dragging" effects should occur so that it would no longer be possible to determine if the planet was rotating.    To use your specific method of identifying planetary rotation:   A pendulum clock could be put anywhere on the surface of the planet and it would still have the same time period  (e.g.  T = 1 second).  It won't have a longer period somewhere (like at a place you would describe as the pole of the planet) and a shorter time period at the equator.   It would be as if the planet wasn't rotating.    This would happen even if we started from a standard universe in which we knew the planet was rotating and tried not to touch or adjust the motion of that planet at all but instead just deleted everything else around it.  Under GR, spacetime is NOT a static background.   Spacetime changes and reacts to any material in it and it isn't just the property of "mass" and its location that is important.   The motion (or momentum) of things is also important,  spacetime reacts to momentum within it and more generally to what could be called "stress-energy-momentum" to imply all the assorted things that are important and spacetime will react to.   Also the motion of matter must change and react to the nature of the spacetime.   Spacetime is poetically described as being an actor on the stage in its own right rather than just merely being the static background of the stage with lumps of matter being the actors that move around on it.

     Under Newtonian notions, spacetime is just a static background and the planet would still have been rotating in that standard static background even if we did delete everything else around it.  In particular a pendulum clock would still have had different time periods when you took it to different places on the surface of that planet.

    I hope that makes some sense.

Best Wishes.
Title: Re: Does anything in the universe exist in a truly non-rotating, motionless state?
Post by: alancalverd on 24/07/2025 14:33:41
This argument implies that the existence of centrifugal force depends on the proximity of other objects, since it disappears in an otherwise vacant universe. It would be interesting to calculate the magnitude of the effect and compare it with a simple experiment in which we send a Watt governor https://extrudesign.com/wp-content/uploads/2019/09/Governor-in-Engine-extrudesign.com-002.jpg or similar mechanism into deep space and measure the centrifugal force as a function of rotational speed as departs from the solar system.

Frame dragging surely applies to any moving object, so a further implication is that if we start an object spinning and gradually remove all other objects from its vicinity, it will gradually lose its unique poles. It seems counterintuitive that uniqueness (a mathematical concept) is dependent on the proximity (physics) of anything, and is in fact a continuous function rather than a discrete property.

But surely the whole business of relativity depends on there being at least two objects in the universe, else what is anything relative to?