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Author Topic: Can something 'spin' without involving another frame of reference?  (Read 3006 times)

Offline yor_on

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Isn't a spin defined by displacements?
So, can there be constant displacements (in this case a spin) without an outside reference defining it as spinning relative it?


 

Offline yor_on

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And how about linear displacements (walking for example), if you find it impossible to define a spin without using another frame of reference?
=

Finally, if so, what does that make a displacement?
« Last Edit: 27/11/2011 21:43:23 by yor_on »
 

Offline JP

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If a tree falls in the forest and no one's around, does it make a sound?
 

Offline Geezer

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If a tree falls in the forest and no one's around, does it make a sound?

If it's a Weeping Willow, yes.
 

Offline Soul Surfer

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Most definitely yes for extended non rigid objects.  See my reply on   http://www.thenakedscientists.com/forum/index.php?topic=42239.0

This is so important that I will copy and paste it here with a few revisions.

It is quite possible for an extended body to possess angular momentum i.e. rotate or spin without any need for reference to an external point to detect the rotation.  This is because forces and directions of motion would be disturbed from those purely due to self gravity. 

To help with understanding this consider that our galaxy was the only thing in the universe and there was absolutely nothing to see beyond its immediate confines.  We can see quite a large part of the galaxy it would be quite easy to tell it was rotating from the differential velocities of objects it would also be possible to infer the existence of dark matter (in fact that was the way it was first discovered).  This is quite obvious because in the time we have been able to look at and measure our galaxy it has not rotated to any detectable angular extent from its position.  It takes about 250million years to go round once. 

Now consider reducing our horizons to a few thousand light years and then shrinking the volume steadily down to the local group of stars it would gradually get more difficult to detect the rotation and the curvature of stellar motions in their orbits. 

A great deal of work has been done to detect the sort of systematic distortions in the real velocities (ie taking out the space expansion factor) of distant galaxies that would show that the universe as a whole was rotating but so far only lower limits can be set on the rotation.  i.e. it cannot be greater than some specific figure which would be detectable.  The CMB may have some indications of rotation and the so called "axis of evil" in it is considered by some to point towards this.



« Last Edit: 28/11/2011 08:32:49 by Soul Surfer »
 

Offline MikeS

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I agree, angular momentum would be conserved even without reference to anything else. 

I'll probably regret saying this but virtually everything within the Universe is spinning, quite possibly the Universe itself is spinning.
 

Offline yor_on

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Thanks SoulSurfer. I was wondering because it to me seems to go back to our basic ideas of how we define things. I remember reading Einstein worrying about a spin too, as that seems to make time travels possible (closed timelike curves). To me it is a question of how a spin comes to be, and if it need a 'spinning position' relative a background, or not.

Although it seems to me that in the case of it not needing a background, then where and through what makes those spinning forces, as the angular momentum, come to exist? That's what I'm trying to understand here.

Btw: As I saw your answer I looked on the net, finding this recent survey (of the universe that is).

Is the Universe Spinning?

 

Offline yor_on

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Reading up on it some more. One argument for the universe not spinning might be that any spin craves a 'accelerative effect' expressed in Newtonian physics as any spinning objects tendency to want to take a 'straight line' at a tangent away from its rotation (you spinning with a rope, letting it go), and there

"The cosmic microwave background is a sea of photons left over from the big bang. It permeates all of space. Two recent experiments (WMAP and COBE) have mapped this sea and its tiny fluctuations to a nigh degree of precision. The results of these experiements allow us to constrain any possible rotation of the universe to a high degree - if the universe was rotating the effects of non-inertial motion would show up in variations in the microwave background."

But would that argument hold for something 'spinning' without this 'still' background existing?

That is, can we prove that a 'force' created by that spin is existent without something against which we can measure (define) as being still?
 

Offline Bored chemist

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I'm fairly surre that this
http://en.wikipedia.org/wiki/Foucault_pendulum
answers the question.
I can tell the earth is spinning, without looking at anything "outside" the earth as a reference.
 

Offline yor_on

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I wish there was a definite answer to it. The Foucault pendulum is a very nice illustration for it but I'm not sure if it would work in 'empty' universe?

Here's some citations illustrating it.

"How does the Foucault Pendulum "know" to ignore local motions, and line itself up with the distant stars? Some reputable physicists say that we really don't know.

Perhaps it's a case of Newton's First Law of Motion "A body will try to keep on doing whatever it's doing, unless acted upon by an external force." So a body that is still will not move, unless a force tries to push it. In another example of Newton's First Law, a body that is moving will keep on moving, unless an external force tries to stop it. This desire of a body to keep on doing whatever it's doing, is called inertia. Nobody really understands what inertia is. The traditional explanations involve some circular reasoning. The reasoning goes like this.

A body will keep on doing whatever it's doing is because of its inertia. And inertia is the tendency of a body to keep on doing whatever it's doing. But why does it keep on doing what it's doing? Because of its inertia. But what is inertia? The tendency of a body to keep on doing whatever it's doing. And so on"

From The Foucault Pendulum

as well as

"What does non-rotating mean? What is the frame of reference in which centrifugal and Coriolis forces vanish, the frame where Newton's laws work? Observationally, we find that this Newtonian or inertial frame is one in which the distant galaxies are not rotating. But if we removed everything in the universe except the earth, how would we know if the earth were turning or not? How would the pendulum know whether to precess or not? Or, to put the question formally, is it just a coincidence that the frame in which the distant galaxies do not rotate is an inertial frame? Ernst Mach thought not, and speculated that the distant stars must somehow affect inertia (Mach's Principle), but no-one has yet come up with a successful and elegant theory. The recent cosmological hypothesis of the inflationary universe offers hope of a different resolution: if the universe expanded exceedingly rapidly in its early phase, any initial rotation will have slowed down correspondingly and so the distant objects have almost no rotation."

Does The Earth Move For You Too? (Foucault Pendulum)
 

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