Why Isn't Gravity a Repulsive Force Based on Energy Arguments?

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Discussion Overview

The discussion centers around the nature of gravity, specifically questioning why it is not considered a repulsive force based on energy arguments. Participants explore the implications of energy density in gravitational fields and how it relates to observed forces, with a focus on theoretical and conceptual aspects.

Discussion Character

  • Exploratory
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant argues that gravity should be a repulsive force based on energy arguments, drawing a parallel with electrostatic forces between electrons.
  • Another participant questions the energy density of the gravitational field, suggesting that understanding this concept is crucial to the discussion.
  • A different participant asserts that using a field argument to claim a force should be opposite to observed behavior seems flawed, emphasizing the role of fields as mathematical constructs for describing forces.
  • One participant expresses comfort with the idea of negative energy density in the gravitational field as a potential explanation for gravity's attractive nature.
  • Another participant suggests that the existence of a force, such as that required for building muscle, does not necessarily imply a conservative force associated with energy density.

Areas of Agreement / Disagreement

Participants express differing views on the nature of gravitational forces and energy density, indicating that multiple competing perspectives remain without a clear consensus.

Contextual Notes

There are unresolved questions regarding the energy density of gravitational fields and its implications for the nature of gravitational forces. The discussion also touches on the relationship between force and energy in different contexts, such as lifting weights.

Who May Find This Useful

This discussion may be of interest to those exploring theoretical physics, gravitational theories, and the conceptual foundations of forces and energy in physics.

Antiphon
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Frankly, I think gravity should be repulsive force on the basis of a simple energy argument which I will now make. I request anyone with insight to find the flaw in my argument. (there has to be one since I think we'd all agree that gravity is an attractive force.)

One way to compute forces is to take a differential of a field energy along some spatial displacement. The force will point in the direction that decreases the energy of the field. Simple example: two electrons near one another. If you integrate the energy of the electrostatic fields through space 1/2*epsilon*E^2 in MKS units, you'll find that there is less energy in the field if the electrons are further apart. Therefore the force they experience is repulsive.

Why is this not the case for gravity? By this reasoning, masses should repel one another.

What am I missing?
 
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What is the energy density of the gravitational field?
 
DaleSpam said:
What is the energy density of the gravitational field?

Good question. But there has to be one otherwise lifting weights at the gym wouldn't build muscles, right?
 
Using a field argument to claim that a force should be opposite to what is observed seems backwards. Isn't a field essentially a mathematical construct for describing the force that a body would feel at every point in a space?
 
johng23 said:
Using a field argument to claim that a force should be opposite to what is observed seems backwards. Isn't a field essentially a mathematical construct for describing the force that a body would feel at every point in a space?

Yes.

I found a thread with a good review of the question at https://www.physicsforums.com/showthread.php?t=316784&page=3

I'm comfortable with a negative energy density in the gravitational field as the origin of it.
 
Antiphon said:
Good question. But there has to be one otherwise lifting weights at the gym wouldn't build muscles, right?
Building muscles requires a force, not necessarily a conservative force that would be associated with an energy density and a conserved energy.
 

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