Gravity question: force vs spacetime curvature

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Discussion Overview

The discussion revolves around the nature of gravity, specifically contrasting Newtonian gravity as an attractive force with Einstein's general relativity, which describes gravity as the curvature of spacetime. Participants explore whether two stationary metal spheres in a vacuum would attract each other and the underlying mechanisms of this interaction.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • One participant questions if two stationary metal spheres in empty space would attract each other and seeks to understand the mechanism behind this potential attraction.
  • Another participant asserts that, according to Einstein's theory of relativity, the spheres would create curvature in spacetime, leading to an attraction as they "roll down the hill" created by each other's mass.
  • A participant challenges the metaphor of "rolling down the hill," asking what force causes the spheres to accelerate, indicating a need for clarity on the mechanism of attraction.
  • Another participant suggests that in general relativity, gravitational effects are inertial effects, implying that the spheres feel these effects due to their inertia rather than a traditional gravitational force.
  • One participant proposes that the motion causing attraction is through time, suggesting that even stationary objects are moving temporally, which influences their interaction in spacetime.

Areas of Agreement / Disagreement

Participants express differing views on the nature of gravitational attraction, with some supporting the idea of spacetime curvature while others seek clarification on the mechanisms involved. The discussion remains unresolved with multiple competing perspectives on the topic.

Contextual Notes

There are limitations in the discussion regarding the definitions of force and inertia in the context of general relativity, as well as the implications of motion through time versus space. These aspects remain open for further exploration.

xlsdx
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So Newton says that gravity is an attractive force and some people believe in gravitons to transmit that attractive force, but Einstein says the attraction is actually due to moving along the curvature of spacetime (caused by the bodies' mass). I'm not asking which is correct, but my question is this: If space was completely empty except for two metal spheres which were completely motionless (relative to some unknown absolute coordinate frame), would those two spheres be attracted to each other? If so, what is the mechanism producing the force?

This has confused me for a long time.. Thanks
 
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Why wouldn't they?
According to the modern established theory of gravitation (Einstein's theory of relativity, not Newton's, which is a weak-field slow-varying approximation of Einstein's, nor graviton or some gauge-theory, which tries to be a theory to which Einstein's is itself a weak-field approximation) the two massive spheres would create curvature and each one would "roll down the hill" created by the other.
 
ok, but what mechanism causes them to "roll down the hill"? something must be causing a force on the spheres to accelerate them... ?
 
xlsdx said:
ok, but what mechanism causes them to "roll down the hill"? something must be causing a force on the spheres to accelerate them... ?

That "roll down the hill" metaphor isn't very good, because it just prompts people to ask exactly that question. There's an IMO much better explanation in the video in this thread: https://www.physicsforums.com/showthread.php?t=669096
 
Yeah, I might not have been very accurate. See the thread Nugatory advised you. In GR there is no 'gravitational force'. Gravitational effects are nothing more than inertial effects, and the sphere feels these effects because it has inertia. See the thread, it's very good.
 
those diagrams are making some sense. so the "motion" that actually causes two static objects to attract (roll down the hill) is actually their motion through time.. so even if they're not moving spatially, they're still moving temporally which moves them around the cone. do i have this correct?
 
xlsdx said:
those diagrams are making some sense. so the "motion" that actually causes two static objects to attract (roll down the hill) is actually their motion through time.. so even if they're not moving spatially, they're still moving temporally which moves them around the cone. do i have this correct?

Pretty much, yes.
 

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