Locally Maximally Symmetric Spacetimes

  • Context: Graduate 
  • Thread starter Thread starter HomerSimpson
  • Start date Start date
  • Tags Tags
    Spacetime Symmetric
Click For Summary

Discussion Overview

The discussion revolves around the concept of locally maximally symmetric spacetimes in the context of general curved spacetimes. Participants explore the implications of curvature and symmetry, particularly in high curvature scenarios, and the relationship between local flatness and local symmetry.

Discussion Character

  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • One participant questions whether every general curved spacetime can be considered locally maximally symmetric, noting that while spacetimes can be locally flat, this does not imply they are maximally symmetric with nonzero curvature.
  • Another participant argues that most general curved spacetimes lack symmetries, specifically pointing out the absence of Killing vector fields.
  • It is mentioned that while every general curved spacetime is locally Lorentz, this does not imply that the Riemann curvature tensor is zero.
  • A participant suggests that Riemann normal coordinates might relate to the discussion of local symmetry.
  • There is a proposal to consider whether the Riemann curvature could be assumed to be almost constant in a manner similar to maximally symmetric spacetimes.
  • One participant concludes that a general high curvature spacetime cannot be described as "locally" maximally symmetric, which is affirmed by another participant.

Areas of Agreement / Disagreement

Participants generally agree that a general curved spacetime is not locally maximally symmetric, but there is disagreement on the implications of local flatness and the nature of curvature in relation to symmetry.

Contextual Notes

The discussion highlights the complexity of defining local symmetry in the presence of curvature and the limitations of applying concepts from maximally symmetric spacetimes to general curved spacetimes.

HomerSimpson
Messages
6
Reaction score
0
Can one say that every general curved spacetime, locally is maximally symmetric?
I know that one can say that every general curved spacetime is locally flat (and therefore maximally symmetric with R=0), but I'm talking about a very high curvature spacetime, where still we can consider nonzero curvatures.
 
Physics news on Phys.org
HomerSimpson said:
Can one say that every general curved spacetime, locally is maximally symmetric?

What do you mean by "maximally symmetric"? Most general curved spacetimes have no symmetries at all (i.e., no Killing vector fields).

HomerSimpson said:
I know that one can say that every general curved spacetime is locally flat (and therefore maximally symmetric with R=0)

No, you can't say that. What you can say is that every general curved spacetime is locally Lorentz, meaning that at a given event, you can choose a coordinate chart that makes ##g_{ab} = \eta_{ab}## at that event, and makes all of the connection coefficients (first derivatives of ##g_{ab}##) vanish at that event. But that doesn't make ##R = 0##.
 
  • Like
Likes   Reactions: 1 person
Thanks for you replay Peter, but I'm still a little bit confused.

PeterDonis said:
What do you mean by "maximally symmetric"? Most general curved spacetimes have no symmetries at all (i.e., no Killing vector fields).

I know it, but I meant "locally" not globally.


PeterDonis said:
No, you can't say that. What you can say is that every general curved spacetime is locally Lorentz, meaning that at a given event, you can choose a coordinate chart that makes ##g_{ab} = \eta_{ab}## at that event, and makes all of the connection coefficients (first derivatives of ##g_{ab}##) vanish at that event. But that doesn't make ##R = 0##.

What you mention here, looks like the Riemann normal coordinate, is it?

PeterDonis said:
But that doesn't make ##R = 0##.

Can we assume that R is almost constant as in maximally symmetric spacetimes and therefore [itex]R_{ab}\propto Rg_{ab}[/itex]?


What I get from your answer, is we can not say that a general high curved spacetime, is "locally" maximally symmetric. Am I right?
 
HomerSimpson said:
What I get from your answer, is we can not say that a general high curved spacetime, is "locally" maximally symmetric. Am I right?
That's right. And you can strike the word "high". A general curved spacetime is not locally maximally symmetric.

Since the Riemann tensor has dimension L-2, curvature only sets a length scale, and you can only say it's large or small in comparison to some other length.
 
  • Like
Likes   Reactions: 1 person

Similar threads

  • · Replies 31 ·
2
Replies
31
Views
2K
  • · Replies 5 ·
Replies
5
Views
922
  • · Replies 4 ·
Replies
4
Views
1K
  • · Replies 12 ·
Replies
12
Views
2K
  • · Replies 38 ·
2
Replies
38
Views
3K
  • · Replies 12 ·
Replies
12
Views
3K
  • · Replies 22 ·
Replies
22
Views
2K
  • · Replies 7 ·
Replies
7
Views
3K
  • · Replies 3 ·
Replies
3
Views
2K
  • · Replies 3 ·
Replies
3
Views
1K