When does Metric contain curved spacetime?

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Discussion Overview

The discussion revolves around understanding when a metric indicates the presence of curved spacetime. Participants explore the relationship between metric components, curvature, and gravitational effects, with references to various mathematical tensors and their implications in general relativity.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant suggests that if metric components mix time and space in a complex manner, it indicates the presence of gravity.
  • Another participant proposes calculating the Riemann tensor to determine curvature, noting that a zero Riemann tensor indicates flat spacetime.
  • A different viewpoint mentions the possibility of using Ricci and Weyl tensors for curvature analysis but questions the complexity of these methods.
  • One participant challenges the idea that the metric contains information about the coordinate system, asserting that it can be expressed independently of coordinates.
  • A participant expresses a desire for clarification on the distinction between their understanding and that presented in a textbook regarding the metric's information content.
  • Another participant invites reference to a specific post for further insight into the discussion.

Areas of Agreement / Disagreement

Participants express differing views on the relationship between metric components and curvature, with some asserting that a more complex mixing of coordinates indicates gravity, while others emphasize the necessity of curvature tensors for definitive conclusions. The discussion remains unresolved with multiple competing perspectives.

Contextual Notes

Some participants acknowledge the limitations of their understanding and the challenges of learning general relativity without direct guidance, which may affect the clarity and depth of their arguments.

Mad Dog
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I’m aware that the metric contains information regarding both the coordinate system and the curvature of spacetime, and have been trying to understand how a glance at it could tell one if the spacetime it expressed is curved.
At this point, I suspect that:
If any of the metric components involve mixing time and space in any way more complicated than ax+bt you’ve got “gravity.”
Can anybody give me some help with this line of reasoning?
 
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Thanks atyy. One could also get there calculating the Ricci and Weyl tensors, but... but... but... ain't there no easier way? The simple rule I gave worked in the 7 examples I've checked. Still wondering.
 
Mad Dog said:
I’m aware that the metric contains information regarding both the coordinate system

It doesn't. The metric as such contains no information about coordinates, as can be evinced by the fact that one can provide an entirely coordinate-independent expression for the action of the metric on vectors.

Mad Dog said:
and the curvature of spacetime, and have been trying to understand how a glance at it could tell one if the spacetime it expressed is curved.

Without a great deal of experience, you can't.

Mad Dog said:
At this point, I suspect that:
If any of the metric components involve mixing time and space in any way more complicated than ax+bt you’ve got “gravity.”
Can anybody give me some help with this line of reasoning?

Besides pointing out that this line of reasoning is nonsense, no. If you want to determine whether or not a given manifold-metric pair is curved, you've got to determine the curvature tensors. By definition, there's no way around this.
 
Thank you Shoehorn- Please understand that I'm trying to learn GR (down here in the bowels of Mexico) from books and without the aid of either a teacher or fellow students. This forum is my only contact with those who know more than I, so I appreciate every response from anyone who offers their help by responding to my questions.
Now: Considering the information contained in the Metric - Foster and Nightingale's book "A Short Course in General Relativity", 3rd Edition, Section 3.5, page 112 says:
"The metric tensor contains two separate pieces of information:
(i) the relatively unimportant information concerning the specific coordinate system used (e.g., spherical coordinates, Cartesian coordinates, etc.);
(ii) the important information regarding the existence of any gravitational potentials."
Can you explain the difference between your point of view and theirs?

With regard to "this line of reasoning is nonsense": It may be, and you could convince me easily if you could point me to an example where a mixing of only spatial coordinates can result in a spacetime where movement through time "spills over" into movement in space.

Thanks again, Mexican Mad Dog
 

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