Conservation of angular momentum

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Discussion Overview

The discussion revolves around the conservation of angular momentum in the context of the universe, particularly focusing on whether the universe can possess net angular momentum and how this relates to the angular momentum of galaxies during cosmic collapse or contraction scenarios.

Discussion Character

  • Exploratory
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • One participant references a paper discussing momentum conservation in collapsing nebulae and questions if similar principles apply to a collapsing universe without reaching a singularity.
  • Another participant suggests that the universe may have no net angular momentum, although this is not definitively established.
  • A participant expresses intrigue about the possibility of the universe having net angular momentum, noting the lack of a clear reason against it and inviting further input from others.
  • It is proposed that if the universe had overall rotation, it would induce frame-dragging effects, which could make such rotation unobservable, referencing Mach's principle and general relativity (GR).
  • Another participant clarifies their focus on the angular momentum of galaxies and questions its conservation during a hypothetical contraction of space related to quantum gravity theories.
  • A further contribution discusses the implications of GR in relation to apparent rotation and frame-dragging, suggesting that observers might not perceive the universe as rotating due to these effects, while acknowledging the limitations of GR in fully satisfying Mach's principle.

Areas of Agreement / Disagreement

Participants express differing views on whether the universe can have net angular momentum, with some proposing it could while others suggest it may not. The discussion remains unresolved, with multiple competing perspectives presented.

Contextual Notes

Participants highlight the complexity of the concepts involved, including the assumptions underlying quantum gravity theories and the implications of general relativity in relation to angular momentum and frame-dragging effects.

wolram
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I think i have found the ansewer, in that the universe has no (net) angular momentum?
 
wolram said:
I think i have found the answer, in that the universe has no (net) angular momentum?

Wolram, I really like this question. I haven't responded because I'm not sure enough. I'm hoping someone else will reply. My hunch is that at least according to the presently available theory the universe could have net angular momentum. I find the idea a curious one, almost bizarre. I would like to be told a reason that it could not have net angular momentum, or that the idea of a global angular momentum is meaningless. But I don't know of such a reason. So I remain intrigued, hoping some others will intervene.
 
marcus said:
Wolram, I really like this question. I haven't responded because I'm not sure enough. I'm hoping someone else will reply. My hunch is that at least according to the presently available theory the universe could have net angular momentum. I find the idea a curious one, almost bizarre. I would like to be told a reason that it could not have net angular momentum, or that the idea of a global angular momentum is meaningless. But I don't know of such a reason. So I remain intrigued, hoping some others will intervene.

If the universe did have an overall rotation, that would induce frame-dragging in the same direction. According to Mach's principle, that would be expected to have the effect that the rotation would be unobservable. GR only approximately satisfies Mach's principle, but it seems that within GR the frame-dragging effect would be of at least the right order of magnitude to make the rotation unobservable.
 
Jonathan Scott said:
If the universe did have an overall rotation, that would induce frame-dragging in the same direction. According to Mach's principle, that would be expected to have the effect that the rotation would be unobservable. GR only approximately satisfies Mach's principle, but it seems that within GR the frame-dragging effect would be of at least the right order of magnitude to make the rotation unobservable.

Johnathan, i am not sure if we are talking about the same thing, i was wondering about the angular momentum of galaxies ,and if this is conserved in a shrinking space down to the limits of the hypothetical bounce in qg theory.
 
wolram said:
Johnathan, i am not sure if we are talking about the same thing, i was wondering about the angular momentum of galaxies ,and if this is conserved in a shrinking space down to the limits of the hypothetical bounce in qg theory.

I haven't a clue what that bounce stuff assumes, but the general idea should hold; if the universe is described by GR, and appears to be rotating on average relative to some point in some frame of reference, then GR predicts that an observer at the origin of that apparent rotation will have their frame dragged around with it at a rate which increases with the fraction of the universe that is involved and is of the order of 1 to 1 when the whole universe is involved. That means that according to that observer, the universe will not appear to be rotating, as predicted by Mach's principle (although for GR the Mach's principle effect cannot possibly be exact - which seems somewhat irritating, when it could be so neat).
 

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