# Why is gravity special ?

1. Dec 16, 2011

### Runner 1

Why is gravity "special"?

(You may think this belongs in the General Relativity section, but the question really relates more to particle physics).

As I understand it, anything with any energy whatsoever creates a disturbance in the gravitational field. So this means that the potential energy from interactions via W bosons, Z bosons, gluons, or photons all have effects on gravity (even if those effects are so small as to be undetectable).

On the other hand, the fields due to other forces (strong, weak, and electromagnetic) are -- as far as I know -- affected ONLY by the particles that correspond to those fields. In other words, no amount of charge is going to affect the strong force.

I'm not great at lucidly describing what I mean so I hope someone sees what I'm getting at. Can someone help explain this?

Thanks

2. Dec 16, 2011

### tom.stoer

Re: Why is gravity "special"?

Gravity is a universal force coupling to energy-momentum density of all fields, whereas other forces couple to special current densities of a subset of fields.

3. Dec 16, 2011

### Runner 1

Re: Why is gravity "special"?

Huh, okay. Thanks. A little over my head but I kind of think I know what you mean.

I also thought of a better way to posit my question:

Everything affects the gravitational field, but only certain things affect the electromagnetic field, the Higgs field, etc. Why?

4. Dec 16, 2011

### tom.stoer

Re: Why is gravity "special"?

Yes

Nobody knows.

One can write down a rather sophisticated mathematical model describing this behaviour in certain regimes (not yet in the quantum gravity regime); but that does not explain why this models is realized in nature, whereas others aren't.

5. Dec 16, 2011

### phinds

Re: Why is gravity "special"?

In this, as in many thing, the point of physics is to determine HOW things work. WHY is philosophy. To extend Tom's excellent point, we know all KINDS of things about the universe and the interactions that take place. A lot of the WHY is a total mystery. For example, there are 23 (I think it is) constants in the Standard Model. Things like the mass of the electron. NONE of them are what they are for any known reason, they just ARE. Maybe someday we'll know more of the why.

6. Dec 16, 2011

### Runner 1

Re: Why is gravity "special"?

That isn't really where I was going with this.

If someone asks why two electrons repel, we can describe that using a model we've developed.

If someone asks why there is a tide, we can give a reason involving the gravitational pull of the moon.

And if someone asks why everything affects the gravitational field but not everything affects the other fields, and if there is a mathematical model that describes these cases, then that would be absolutely great to know. ;)

(If you can't tell, I'm not fond of interpreting semantics in the strictest possible sense -- were this the case, communication among humans would be all but impossible. It's like the "Can I... ?" question where someone snidely responds "Of course you can...". Not to mention the fact that I even included a disclaimer in my original post to try and avoid these situations).

Last edited: Dec 16, 2011
7. Dec 16, 2011

### Staff: Mentor

Re: Why is gravity "special"?

I think the best that can be said is that some people are working along paths such as string theory and loop quantum gravity, that might eventually answer questions like this. Right now there are no generally-accepted and experimentally-tested theories that do this.

8. Dec 16, 2011

### atyy

Re: Why is gravity "special"?

In the view of gravity as a [url="http://arxiv.org/abs/astro-ph/0006423]massless spin 2[/url] particle on flat spacetime, Weinberg has an argument I'm trying to understand for why gravity is universally and identically affected by all energy. It's described in http://arxiv.org/abs/1007.0435v2 section 2.2.2:

"Lorentz invariance for the emission of a soft massless spin-two particle leads to the conservation law ... On the other side, translation invariance implies momentum conservation ... Therefore, for generic momenta, Poincare invariance requires all coupling constants to be equal ... In other words, massless particles of spin-two must couple in the same way to all particles at low energies."

9. Dec 16, 2011

### phinds

Re: Why is gravity "special"?

OK, gotcha ... I was going off in the wrong direction.