Does Gravity Gravitate? Part 2 - Comments

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Discussion Overview

The discussion revolves around the concept of gravity and its implications in the context of the Schwarzschild metric and energy-momentum tensors. Participants explore the nature of mass in gravitational fields, the validity of certain integrals related to mass calculations, and the conceptual challenges of discussing gravity in hypothetical scenarios where it is "switched off." The conversation touches on theoretical aspects of general relativity, Newtonian gravity, and the implications of gravitational energy.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Some participants clarify the trace of the energy-momentum tensor and its calculation, noting differences between various forms of representation.
  • There is a discussion about the integrals for mass, with some participants questioning whether different integrals for mass quantities should yield the same results.
  • Some participants argue that the concept of "switching off" gravity is meaningless, as gravity is an inherent aspect of physical laws.
  • Others propose that assuming a uniform density of Earth raises questions about its validity in Euclidean versus curved spacetime.
  • Participants discuss the absence of a tensor for gravitational energy and the implications of self-interaction in gravitational fields.
  • There are references to Newtonian gravity and its treatment of mass, with some participants suggesting that GR introduces corrections that are not present in Newtonian theory.
  • Some participants express enjoyment of the insights shared in the discussion and encourage further exploration of the topics raised.

Areas of Agreement / Disagreement

The discussion features multiple competing views and remains unresolved on several points, particularly regarding the implications of gravity in hypothetical scenarios and the nature of mass in different gravitational contexts.

Contextual Notes

Participants highlight limitations in understanding the implications of switching off gravity and the assumptions involved in discussing uniform density in different geometrical contexts. There are also unresolved mathematical steps related to the integrals for mass.

  • #31
This is the Hilbert action, and it's unique (up to a cosmological constant, which is not included here) in being generally covariant and leading to 2nd-order equations of motion. This is so, because ##R## is the only scalar you can build from the pseudometric tensor that is only linear in the 2nd derivative with coefficients containing no derivatives, which means that via partial integration you can show that the action is a functional of a Lagrangian that only depends on the ##g_{\mu \nu}## and its first derivatives.
 
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