Curvature of spacetime inside hollow sphere

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Discussion Overview

The discussion revolves around the effects of mass on the curvature of spacetime, specifically focusing on a scenario involving a hollow lead sphere containing two smaller lead balls. Participants explore how the presence of the smaller balls affects the curvature of spacetime both inside and outside the hollow sphere.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation

Main Points Raised

  • One participant questions how the two smaller balls can exert their own gravity if the spacetime is already curved by the outer hollow sphere.
  • Another participant asserts that the spacetime geometry inside the hollow sphere changes with the addition of the smaller balls.
  • A different participant emphasizes that the geometry of spacetime is invariant in General Relativity, suggesting a contradiction with the previous claims.
  • One participant challenges the notion of invariance, clarifying that "invariant" refers to observations across different frames, not to the effects of varying distributions of mass and energy.
  • It is noted that changing the distribution of matter, such as adding the smaller balls, alters the geometry of spacetime.

Areas of Agreement / Disagreement

Participants express differing views on the implications of mass distribution on spacetime curvature, with no consensus reached regarding the invariance of spacetime geometry in this context.

Contextual Notes

There are unresolved assumptions regarding the interpretation of invariance in General Relativity and how it applies to different mass distributions. The discussion does not clarify the mathematical implications of these claims.

Tony Stark
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If mass curves spacetime in its vicinity, then consider the following case-
Take a heavy hollow lead sphere which has 2 smaller lead balls placed in it. The Outer Sphere will curve spacetime around itself and thus will have its own gravity, but what about the 2 balls placed in it? The spacetime already have been curved by the outer sphere, then how will the two smaller balls have their own gravity?
 
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Tony Stark said:
The spacetime already have been curved by the outer sphere, then how will the two smaller balls have their own gravity?
Space time geometry inside the big shell with the smaller balls is different than without them.
 
A.T. said:
Space time geometry inside the big shell with the smaller balls is different than without them.

Outside too, of course. (A.T. already knows this, I'm just not sure that OP does).
 
A.T. said:
Space time geometry inside the big shell with the smaller balls is different than without them.
The geometry of spacetime is taken invariant in General Relativity, so how can it be different..
 
Tony Stark said:
The geometry of spacetime is taken invariant in General Relativity
Where did you get that idea from?
 
Tony Stark said:
The geometry of spacetime is taken invariant in General Relativity

"Invariant" means "the same for all observers" (or in all frames of reference). It does not mean "the same for all possible distributions of matter and energy". If you change the distribution of matter and energy (for example, by putting two smaller lead balls inside a hollow lead sphere, instead of just having the hollow lead sphere with nothing inside it), you change the geometry of spacetime.
 
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