# Equivalence principle to other forces?

1. Nov 16, 2009

### Gerenuk

Why is it that to derive general relativity you use the equivalence principle on gravity and not electromagnetism for example?

2. Nov 16, 2009

### Al68

Because the equivalence principle would be obviously false if applied to electromagnetism instead of gravity.

3. Nov 16, 2009

### Gerenuk

Electromagnetism is a force and acceleration exhibits a force. So at least from the basic ideas that I heard it doesn't make a difference.

4. Nov 16, 2009

### Nabeshin

Gravitational mass and inertial mass are the same.

What equivalent statement could you possibly make about any of the other forces?

5. Nov 16, 2009

### Gerenuk

I see. I was thinking about the force argument only. I have the feeling that's a good answer.

And I cannot apply the same concept to electric charge? (converting the units appropriately)

6. Nov 16, 2009

### Al68

I don't know what a "Becauso so." answer is, but the "force" of electromagnetism isn't equivalent to a pseudoforce in accelerated reference frames the way gravitational "force" is.

The "basic idea" of the equivalence principle isn't about forces being equivalent, they generally aren't. It's about the equivalence of reference frames. Two accelerated reference frames are equivalent if the only difference between them is the presence of a gravitational field. No local experiment can even detect the presence of the gravitational field. In both cases a ball thrown "up" will "fall", for example. The ball "falls" because of the acceleration of the reference frame, not because of a real force.

An electromagnetic field can easily be detected by local experiments, so the equivalence principle is not valid for electromagnetism.

7. Nov 16, 2009

### DrGreg

I think you've more or less worked this out now. In Newtonian terminology, the "acceleration due to gravity" of a particle does not depend on the particle's mass (or any other property of the particle). The "acceleration due to electromagnetism" depends on the particle's charge-to-mass ratio.