The Eternal Question of Inertia

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Discussion Overview

The discussion revolves around the nature of inertia and its relationship to gravity, particularly in the context of general relativity and alternative models. Participants explore theoretical frameworks and implications of inertia in both curved and flat spacetime, as well as the concept of weight as an inertial force.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant asserts that general relativity describes gravity as a curvature of spacetime, where objects follow geodesic paths without needing a force, and questions the origin of weight or inertial force when deviations occur.
  • Another participant briefly mentions "momentum" without further elaboration.
  • A later post reiterates the principles of general relativity and introduces an alternative model proposed by Dennis Sciama, suggesting that inertia can arise from a Newtonian perspective extended to the universe, which may conflict with general relativity.
  • The alternative model is described as aligning with Mach's principle and is noted for its elegance and plausibility, raising concerns about its compatibility with general relativity.

Areas of Agreement / Disagreement

Participants express differing views on the nature of inertia and its relationship to gravity, with no consensus reached regarding the validity of general relativity versus the alternative model presented.

Contextual Notes

The discussion includes references to complex theoretical constructs and assumptions that may not be universally accepted, particularly regarding the compatibility of different models of inertia and gravity.

e2m2a
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The general theory of relativity states gravity is not a force, but a curvature of spacetime. The geodesic equations predict what the paths of objects will be as they follow their natural geodesic paths in either flat or curved spacetime. And they do not need a force to constrain them in these paths for they are conforming to a generalized inertia path. But as soon as some agent forces the object into geodesic deviation, inertial force is manifested in the object. In the case of a gravitational field this is called weight. What causes weight or inertial force?
 
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e2m2a said:
The general theory of relativity states gravity is not a force, but a curvature of spacetime. The geodesic equations predict what the paths of objects will be as they follow their natural geodesic paths in either flat or curved spacetime. And they do not need a force to constrain them in these paths for they are conforming to a generalized inertia path. But as soon as some agent forces the object into geodesic deviation, inertial force is manifested in the object. In the case of a gravitational field this is called weight. What causes weight or inertial force?

For an actual answer (but not within General Relativity) see Dennis Sciama's beautiful 1953 paper http://www.adsabs.harvard.edu/abs/1953MNRAS.113...34S".

He shows that if you extend a simple Newtonian model of gravity to the whole universe, using concepts modeled on the electromagnetic field, then inertia arises naturally as a force opposing acceleration relative to the vector potential of the universe (and similarly angular momentum is relative to the rotation of the universe). This model also fully satisfies a form of Mach's principle.

This model is so neat and seems to be such a plausible explanation of how it all works that I find it quite disturbing that GR is totally incompatible with it.
 
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Thanks, I will look into it.
 

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