# A question about the difference between mass and charge?

1. Aug 21, 2010

### jeebs

Hi,
I was wondering about the difference between mass and charge. As far as my understanding goes, these are just some names we've given to these 2 quantities of "stuff" that determines the strength of the field each one causes in the space around it. However, I was thinking about Newton's equation that comes in handy for all sorts of problems, F=ma. If we increase the mass of an object, then all other things being equal, it will be more difficult to change its state of motion. On the other hand, there is no F=Qa. What I mean by that is, as far as I am aware, you could put as much charge onto that object as you wanted and, all other things being equal, it would be no harder to change its state of motion.

So, my question is, why is this the case? why does mass seem to "drag" through "empty" space (i know empty space isn't strictly empty) but charge does not? I can't help feeling this is a bit of an ignorant question, but I've gone through 3 years of my 4 year physics course and this hasn't been covered in any way and I can't think of anything obvious I've overlooked.

Thanks.

2. Aug 21, 2010

### diazona

Well, mass really serves two functions in physics: it determines inertia (how hard an object is to move) and it determines how much an object responds to gravity (or how much gravity it produces). It's conceivable that in some alternate universe with different physics, these two might be different. An object's gravitational mass determines the strength of the gravitational field it produces, just like its charge determines the strength of the electric field it produces, whereas the inertial mass is the one that appears in ΣF=ma. If inertial mass and gravitational mass were not related, there would be no confusion.

However, in our universe they are (as far as anyone can tell) the same. You're not the first to wonder why gravitational mass is linked to inertial mass but there is no "inertial charge." According to general relativity, this is because gravity and electrical forces are actually completely different phenomena. The electric force works by exerting a force to actually change a particle's trajectory through space and time, according to Newton's law, ΣF=ma, and Coulomb's law, F=kq1q2/r2[/sub]. But gravity instead changes the "shape" of the underlying spacetime. There's no force involved. The particle continues to move in a straight line (or at least what it thinks is a straight line) but because spacetime is distorted, that line no longer seems straight to observers like us. It seems like there's a gravitational force acting on the particle, but in the GR view, that force is just an illusion.

3. Aug 22, 2010

### Gerenuk

@jeebs: That's a good question and the reason it wasn't answer in your course is, that only very few people are able to think outside the box.
I don't know the heavy theoretical physics to give sophisticated answers, but here are some simple differences between mass and charge:
- there are opposite charges which attract. if you have three bodies, then by gravity they will all attract, but by EM they surely won't all attract
- charge is quantized and all particles have a small multiple of the unit charge
- the mass inertia changes with velocity; whereas the charge field converts from being electrical to magnetical when in motion; it's complicated

In a way both a still comparible, since both effects are just transfering momentum. In fact, maybe it's GR which connects charge and mass, because it explains how light is bent?!

4. Aug 22, 2010

### diazona

um... I dunno about that. Light isn't charge, it's light. They're different. And charge and mass don't have any special connection in GR.

5. Aug 22, 2010

### Gerenuk

Light is photons and charge is the source of photons. So it's the same.

And GR and mass is also the same.

6. Aug 22, 2010

### jumpjack

This remembers me of Coriolis and Centrifugal force: they do act, but they do not exist: we THINK they exist if we are in a rotating system.

So, which could be the X in these equations?!?

Coriolis : Rotation = Gravity : X

or

Centrifugal : Rotation = Gravity : X

How can a person in a rotating system determine that Coriolis and Centrifugal forces actually do not exist? Can he?!?

7. Aug 22, 2010

### Gerenuk

It's easy. If you put an object into empty space, where no other forces should exist, and the object still starts moving, then you are not in an inertial frame and some kind of virtual force exists.

8. Aug 22, 2010

### jumpjack

So, we SEE this force (gravity) but it CAN'T exist (for General Relativity). So we "know" we are in a "non-GR" system (~=non inertial system). So I wonder how things work in the "real" world out there... out of the "rotating disk" we live on?!? A world where gravity does not exist. Maybe even mass does not exist, there.

9. Aug 22, 2010

### diazona

Absolutely not...

10. Aug 22, 2010

### Gerenuk

Wow, you really baffle me with your knowledge and arguments you present.
If you want to make a point, then write some content.

11. Aug 22, 2010

### diazona

X would be "spacetime curvature". I'm not sure it's a perfect analogy but it's reasonably close.
"Non-GR system" doesn't make sense, you were right with "non-inertial system".

The equivalent of the non-rotating disk would be going into orbit or freefall, where you feel weightless.

12. Aug 22, 2010

### diazona

Well, I'd hoped it would be obvious. But if you want more, think of it like this: charge is to light as a boat is to its wake. Light is the "wake" left behind by a moving charge.

The wake is not the boat; similarly, light is not charge.

13. Aug 22, 2010

### jumpjack

I know, I just created it to give a name to something I don't even understand.

14. Aug 22, 2010

### stevenb

This is an important point and it is a well known fundamental difference between electromagnetics and GR. Photons are not charge (or better said, have no charge), so light is not considered a source of fields in electromagnetics. In GR, gravity waves (or you could say gravitons, to compare to photons) are a source of gravitational fields (or spacetime curvature). For that matter, photons are also a source of gravitational field. So the idea that photons are charge is doubly wrong. Photons are more akin to mass, even though they have zero rest mass. They are a gravity source, as part of the the stress energy tensor.

15. Aug 22, 2010

### diazona

Fair enough. Sorry if I was a little hard on you there.