Can General Relativity Accommodate Spaces Without Killing Vectors?

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Discussion Overview

The discussion revolves around the implications of the Killing equation in general relativity and whether spaces can exist without Killing vectors. Participants explore the relationship between symmetries in the metric and the existence of conserved quantities, questioning the foundational assumptions regarding the inner product and the nature of points and vectors in a manifold.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • Some participants propose that the requirement for the inner product to be invariant leads to the Killing equation, questioning if general relativity forbids spaces without Killing vectors.
  • Others argue that a Killing vector represents a symmetry in the metric, suggesting that if the metric lacks symmetries, then Killing vectors cannot exist.
  • One participant asserts that general relativity does not forbid spaces where the Killing equation cannot be satisfied.
  • There is a challenge regarding the interpretation of the inner product, with some participants questioning the initial usage of terms and the nature of points versus vectors in a manifold.
  • A later reply clarifies that points in a manifold are not vectors, emphasizing the distinction between points and vectors in the context of curvature.
  • Another participant suggests that the entire discussion is based on a mistaken premise regarding the nature of points and vectors.

Areas of Agreement / Disagreement

Participants express disagreement regarding the interpretation of the inner product and the foundational premises of the discussion. There is no consensus on whether general relativity can accommodate spaces without Killing vectors, and the discussion remains unresolved.

Contextual Notes

Limitations include potential misunderstandings of mathematical definitions and the implications of curvature on the relationship between points and vectors in a manifold.

Tio Barnabe
By requiring the inner product in two points ##x## and ##x'## having metrics ##g## and ##g'## to be invariant, i.e. ##g'(x') = g(x)##, one is lead to the Killing equation. Does general relativity forbiddes spaces where the Killing equation cannot be satisfied?

It seems obvious that we want conserved quantities in our theories. But, is there a way around in which we can consider a space-time having no Killing Vectors at all?
 
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Tio Barnabe said:
By requiring the inner product in two points ##x## and ##x'## having metrics ##g## and ##g'## to be invariant, i.e. ##g'(x') = g(x)##, one is lead to the Killing equation. Does general relativity forbiddes spaces where the Killing equation cannot be satisfied?

It seems obvious that we want conserved quantities in our theories. But, is there a way around in which we can consider a space-time having no Killing Vectors at all?
A Killing vector represents a symmetry in the metric. If the metric has no symmetries then there are no Killing vectors.

Although the gravity affecting the Earth's solar orbit is nearly symmetric, if you include all the factors, then at a certain level of accuracy it won't be symmetric at all.
 
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Tio Barnabe said:
By requiring the inner product in two points ##x## and ##x'## having metrics ##g## and ##g'## to be invariant, i.e.##g'(x') = g(x)##, one is lead to the Killing equation.

How so?

Tio Barnabe said:
Does general relativity forbiddes spaces where the Killing equation cannot be satisfied?

Certainly not.
 
Inner product, so far as I know, is an operation on two vectors producing a scalar. Your usage does not appear to jive with this. Please clarify
 
PeterDonis said:
How so?
PAllen said:
Please clarify
I just considered that the two points ##x## and ##x'## were vectors themselves, in which my notation ##g(x)## means the inner product of ##x## with itself (similarly for ##g'(x')##). Couldn't I do that?
 
Tio Barnabe said:
I just considered that the two points ##x## and ##x'## were vectors themselves, in which my notation ##g(x)## means the inner product of ##x## with itself (similarly for ##g'(x')##). Couldn't I do that?
Points in a manifold are not vectors. Vectors live in the tangent space to a manifold at a given point. In flat space, position vectors do happen to form a vector space, but not if there is any curvature. Also, for flat space, you can choose to treat the space as it’s own tangent space, but again you cannot if there is any curvature.
 
Tio Barnabe said:
I just considered that the two points ##x## and ##x'## were vectors themselves

Which, as @PAllen has pointed out, is incorrect. So this entire thread appears to be based on a mistaken premise in your OP.

Thread closed.
 

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