# Most things in our universe have rotation

#### wolram

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most things in our universe have "rotation"

most things in our universe have "rotation", no matter if it is an electron, or a planet, or a galaxy.
this has me thinking, would it make a difference if the metric of spacetime is also rotating?

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#### jcsd

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Re: rotation

Originally posted by wolram
most things in our universe have "rotation", no matter if it is an electron, or a planet, or a galaxy.
this has me thinking, would it make a difference if the metric of spacetime is also rotating?
You mean if the universe itself is rotating? yes it would make a major difference as it would lead to causal violations!

The universe is not rotating however.

#### marcus

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Re: Re: rotation

Originally posted by jcsd
You mean if the universe itself is rotating? yes it would make a major difference as it would lead to causal violations!

The universe is not rotating however.
Jcsd, I for one would enjoy a bit more discussion of this. I accept what you just said but would like to hear more of your reasoning---including about the causal violations.

I guess everybody knows that both Newton and Einstein thought about this problem---can you say something is rotating in some absolute sense or only relative to some other things and if it is only relative then what is it relative to? Newton introduced the famous Bucket of Water thought experiment.
When the bucket is rotating you know because the surface curves up. Somebody (Ernst Mach?) said that this was because
the bucket was rotating with respect to the "distant stars" and Einstein purportedly took that at least partly seriously.

Einstein seems to have concluded that the changeable dynamic metric IS the gravitational field---at least the clearest mathematical representation of it. And he thought that ultimately what defined whether something was rotating or not was whether it was rotating with respect to the gravitational field.
The field is space in some sense, or the thing relative to which we rotate. These are dizzying ideas to me. They have attracted the attention of some exceptional people and there is probably some residual puzzle in them, never completely settled.

Perhaps it is only possible to define rotation relative to the gravitational field and so it wouldnt make any sense to say that the gravitational field is rotating. Hope you can eke a bit more clarity out of this jcsd.

#### jcsd

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Kurt Goedel found a GR solution for rotating universes which allowed for time loops.

In observations of the CMBR no evidence that the universe is rotating has been found.

#### marcus

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Originally posted by jcsd
Kurt Goedel found a GR solution for rotating universes which allowed for time loops.

In observations of the CMBR no evidence that the universe is rotating has been found.
this sounds like it might be an area where I might not ever
get a satisfactory understanding

I wonder how Goedel could have modeled a rotating universe

Has anybody seen Goedel's paper

maybe I can dig up a reference, or do you happen to have one, jcsd?

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#### marcus

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Originally posted by jcsd
I do somewhere, but it's in a bookcase somewhere upstairs.

Haven't looked at this paper, but from the abstract it seems to be what you want:

http://arxiv.org/ftp/gr-qc/papers/0106/0106070.pdf
Thanks! I just did a google search myself and came up with a reference to Goedel's original 1949 paper
and also a paper by Burghardt discussing the model
(it has closed timelike geodesics----time-loops that would mess up causality----as you say)

it also has a finite cut-off radius beyond which Burghardt declares it is not physically meaningful because things are whirling around too fast

remarkable mental construct

edit: I just checked. Your link is to the same paper that I found!
It is by Rainer Burghardt (Austrian physicist I think)
and he gives the original paper as

Goedel, Reviews of Modern Physics, volume 21, page 447 (1949)

and he describes a radius cutoff----the merrygoround cant be too big or else the edge is going faster than the speed of light

Burghardt seems to have published some articles in peer-reviewed physics journals in the 1970s that relate to this. apparently his interest in this goes back a ways.

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#### marcus

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Jcsd,
here is an idea you may be able to help me track down.

I've read that Einstein said that "space is the gravitational field"

that is, there is no absolute space within which things happen
the idea of space is relational

the metric describes spatial relations between things
the metric (which can undergo expansion, contraction, undulation) is the gravitational field
the gravitational field is space

anyway, supposedly that is the idea

Do you know if Einstein actually said "space is the gravitational field" or words to that effect?

"Ja natuerlich mein Kind, der Raum is eben das Gravitationsfeld!"

#### jcsd

Gold Member
This is the only paper I could find with that quote in:

http://arxiv.org/PS_cache/gr-qc/pdf/9608/9608043.pdf [Broken]

The metric describes the curvature of space, which is caused by gravity so it's not implausible to say that space is the graviational field as it's curvature represents properties such as intensity of the graviational field in that location.

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#### wolram

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i wish you whales would rember us tiddlers

#### marcus

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Originally posted by wolram
i wish you whales would rember us tiddlers
LOL wolram quite frankly I do not know what space is or whether it could rotate

i suspect it cannot rotate (in spite of what Kurt Goedel may have said) because relative to what could it rotate? But I dont know.
And when it comes to really big questions maybe even einstein was a tiddler

#### jcsd

Gold Member
What I take it to mean is that the universe as a whole has non-zero angular momentum in a Goedel universe.

#### Imagine

Originally posted by marcus
... and he describes a radius cutoff----the merrygoround cant be too big or else the edge is going faster than the speed of light...
Relative to Hubble relation, V=R*H:
- Do we have the same problem?
- How is it resolved?-
- Is this resolution could apply to this case?
- Is centripetal acceleration, w2r, could be related to Hubble relation?
Originally posted by marcus
... the gravitational field is space ...
IMHO, I would said that "the gravitational field is 1/time", and the particular empty space-time property, second per meter, equal to "1/c".

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#### marcus

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Originally posted by Imagine
Relative to Hubble relation, V=R*H:
- Do we have the same problem?
- How is it resolved?-
- Is this resolution could apply to this case?
- Is centripetal acceleration, w2r, could be related to Hubble relation?

IMHO, I would said that "the gravitational field is 1/time", and the particular empty space-time property, second per meter, equal to "1/c".
Imagine, a reply from only one person, myself, may be insufficient. Interestingly enough we do NOT have a problem with superluminal recession speeds-------they simply say that the physical distance between some locations is increasing at a rate > c. Nothing is traveling at such speeds, no transmission of information, no motion as we normally understand it. There is no A that is passing thru the neighborhood of B and observed by B to being going faster than light.

A simple calculation shows that volume-wise the greater part of the observable universe is receding > c. Light now reaching us from objects in some 90% of the observable volume was emitted when those objects were receding at superluminal speed and was initially swept backwards. It is an amazing fact but part of the standard picture cosmologists have agreed on---in this sense it is not controversial, just part of the consensus model.

NON-cosmologists, however, are apt to find it controversial and to want to argue about it.

a short paper by Tamara Davis and Charles Lineweaver discusses this

http://arxiv.org/astro-ph/0011070 [Broken]

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Now about what Goedel said in 1949, and what Burghardt said in 1996, I have not understood yet. It is possible that Jcsd has understood what they are talking about.

I dont see WHY Burghardt should have declared there was a cutoff. He is not a person of any great stature or reputation as far as I know and maybe he simply made a mistake which no one bothered to correct. I know of no reason to give particular weight to what he says. The case of Goedel is different, but I have not read Goedel-----Goedel may very well have thought that he did not have to apply a cutoff.

I dont yet understand how anyone can model the rotation of the gravitational field, that is to say the metric. Expansion is easy: you just have the metric get larger.

I think Jcsd may understand this and he may be willing to put it in terms you can understand. But I cant.

However I can respond to what you said about the Hubble relation. No cutoff needed there! Just look at the sky and reflect that most of what can in principle be seen there (with the proper instruments) is at present receding at over the speed of light and
was already doing so when it emitted the light now reaching us!

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#### jeff

The rotation in a godelian spacetime M is intrinsic in the sense that M is rotationally symmetric about every point. This is highly non-intuitive and difficult to visualize for more than one point.

With respect to some point p through which runs the r = 0 axis, we think about the causal flow in terms of the behaviour of light cones: Those on the axis contain the vertical t-direction but not the horizontal r- and &Phi;-directions, and as one moves away from the axis, the light cones open out and tilt in the &Phi;-direction until at a certain radius this direction becomes null with the circle of this radius about the origin being a closed timelike curve.

#### marcus

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that is intriguing
may have to visit the library for that one

#### wolram

Gold Member
godelian space is non intuitive, but very interesting,
what happens to the "time dimention" or is it seperate
from godlian space?

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