# I Inertial mass and charge

1. Jun 30, 2017

### nojustay

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.

2. Jun 30, 2017

### Staff: Mentor

"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.

3. Jun 30, 2017

### nojustay

4. Jun 30, 2017

### pixel

Maybe you could clarify your question, since charge and mass are two different things.

5. Jun 30, 2017

### Staff: Mentor

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.

6. Jul 1, 2017

### nojustay

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: Jul 1, 2017
7. Jul 1, 2017

### Orodruin

Staff Emeritus
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.

8. Jul 1, 2017

### nojustay

Yeah, that what I meant.

9. Jul 1, 2017

### Orodruin

Staff Emeritus
@mfb gave an appropriate answer to this in post #2.

10. Jul 1, 2017

### SlowThinker

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.