Is Charge Equivalent to Inertial Mass?

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Discussion Overview

The discussion centers around the question of whether charge is equivalent to inertial mass, exploring the nature of gravitational mass and its perceived uniqueness compared to other forces, particularly electromagnetic interactions. Participants examine the implications of General Relativity and the behavior of objects under different forces.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Exploratory

Main Points Raised

  • Some participants question why gravity is considered special compared to other forces, particularly in the context of mass equivalence.
  • It is noted that in General Relativity, the equivalence of inertial and gravitational mass is linked to the behavior of objects in curved spacetime, but the reason for this equivalence compared to charge remains unclear.
  • One participant emphasizes that while a proton and a positron accelerate the same in a gravitational field, they do not in an electric field, suggesting that charge is not equivalent to inertial mass.
  • Another participant raises the question of whether there is a fundamental reason for the observed behavior of mass under gravity, or if it is simply an observational fact.
  • Clarifications are made regarding the distinction between charge and mass, with some participants suggesting that the question of equivalence needs to be framed correctly.
  • General Relativity is mentioned as a theory that explains the equivalence of gravitational and inertial mass, but it does not provide an answer for why charge behaves differently.

Areas of Agreement / Disagreement

Participants express differing views on the nature of mass and charge, with no consensus reached regarding the reasons behind the observed behaviors of these properties under different forces.

Contextual Notes

The discussion highlights the lack of a clear theoretical framework explaining the differences between gravitational and electromagnetic interactions, particularly in relation to mass and charge equivalence.

nojustay
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I've read about the equivalence between inertial mass and gravitational mass. But i can't undestand why is gravity more special then other kinds of force. I mean, why isn't charge equivalent to inertial mass? After all charge plays the same kind of role of gravitational mass in another context.
 
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"Why is gravity special?" is one of the open questions in physics.
In General Relativity, it is not surprising that the two masses are identical - it is equivalent to the statement "things fly in straight lines in curved spacetime". But you can ask why gravity can be expressed that way, and the other interactions cannot, and then we don't have an answer.
 
Thanks for the answer.
 
nojustay said:
I mean, why isn't charge equivalent to inertial mass?

Maybe you could clarify your question, since charge and mass are two different things.
 
nojustay said:
I mean, why isn't charge equivalent to inertial mass? After all charge plays the same kind of role of gravitational mass in another context.
A proton and a positron in a uniform gravitational field will accelerate the same. A proton and a positron in a uniform electric field will not accelerate the same. Charge is not equivalent to inertial mass.
 
A proton and a positron in a uniform electric field will not accelerate the same. Charge is not equivalent to inertial mass.

That's the point. Is there a reason for this behaviour? Why do two different objects accelerate the same with gravity? Is there a fundamental reason (coming from some kind of theory) or is that to be taken as granted from observation?
 
Last edited:
pixel said:
Maybe you could clarify your question, since charge and mass are two different things.
In Newtonian mechanics, there is no a priori reason for inertial and gravitational mass to be the same either. I believe the question is why they are the same but charge is not.
 
Yeah, that what I meant.
 
nojustay said:
Yeah, that what I meant.
@mfb gave an appropriate answer to this in post #2.
 
  • #10
nojustay said:
That's the point. Is there a reason for this behaviour? Why do two different objects accelerate the same with gravity? Is there a fundamental reason (coming from some kind of theory) or is that to be taken as granted from observation?
To make a more precise answer, we noticed that the two masses seem to be the same, and General Relativity was developed to explain it. As a side product, it explained and correctly predicted many other things.
The reason GR gives is that a straight path is the same thing, whether it's followed by a car, a pedestrian, or an apple.
 

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