# Does a massive object interact with its own gravitational field?

1. Feb 5, 2005

### touqra

Does an electrically charged particle's own electric field affect its own path in space? i.e., does the particle's eletrical nature interact with its own field?

Does a massive object interact with its own gravitational field?

2. Feb 5, 2005

### marlon

yes, a charged particle interacts with its own field and this is called the self energy. Indeed, this self energy has an influence on possible trajectories of this particle. Keep in mind that in field theory, the trajectory of a particle between two points, really is the "superposition" over all possible trajectories between those two points. You know, the Path-integral formalism...

A massive object curves space time, that is all there is to it according to general relativity.

regards
marlon

3. Feb 5, 2005

### touqra

Why is it that you have self energy for electromagnetism but not for gravitational force?

Both electromagnetism and gravity use the field concept. And both is pretty much anologous to the other, from the view of a single particle.

4. Feb 5, 2005

### marlon

Self energy does not come from EM, it comes from QED !!! This is the quantummechanical approach to EM-phenomena. Self energy arises when you are working with perturbation theory and this is an approximative way to solve a Schrodinger equation. This way of working, however does not apply for general relativity. The fields in QED are quantum-fields, those in GTR are NOT. General Relativity and GTR are totally different in nature because of the Heisenberg uncertainty and the superposition principle for example. You don't have those in GTR

marlon

5. Feb 6, 2005

### touqra

So is there the existence of self energy in the theory for strong force, ie, in QCD?

If perturbation theory is only an approximative way to solve the Schrodinger
equation, would the concept of self energy be actually an illusion, if we have the exact way of solving the Schrodinger equation?

Wouldn't a good theory of gravity should also have this concept of self energy? since it is also a force, and should be equivalent to the EM force.

6. Feb 6, 2005

### dextercioby

Yes,in the SM at least,every QFT has self-energy terms in its (non necessarily convergent) perturbation series.Of course,Quantum Chromodynamics cannot make an exception.

How would that happen?Please,give an example of an INTERACTING quantum field theory (i put the word "interacting" explicitely just to make sure you wouldn't cheat and come up with a free theory,which is very "solvable" and whose connected Green functions are only the propagators ).

Yes,all known (to have failed due to nonrenormalization (e.g. HE) or incapability to produce Newton's equations in the classical limit (e.g.Weyl gravity)) quantum gravity theories have self-energy diagrams for the graviton.As you have been told already,it's a pure QM effect.

Daniel.

7. Feb 7, 2005

### touqra

What I meant was, if perturbation theory is only an approximation, and the necessary consequence of perturbation is self energy, then, would this thing called self energy is only a mere theoretical flaw due to this approximation, and does not exist in nature?

What do you mean by "solvable" and that the connected Green functions are only the propagators?

8. Feb 7, 2005

### dextercioby

We have no experimental evidence so far which would claim the the perturbation series for any of the QFT-s involved in the SM (renormalizable theories) would lead to catastrophic results...There's a problem with the QCD (the strong interaction has a large coupling constant,which is non suitable with our classic idea of perturbative expansion (c QM textbooks)),but nontheless,we have no doubt regarding the correctness of self-energy of quarks or gluons for example...

I mean that in the operator approach to QFT,one has to solve the classical field equations b4 passing onto quantization.That's what i meant by "solvable".It doesn't apply to Yang-Mills fields (which are not really free,but self-interacting,just like the gravitational field).
Yes,for a FREE theory,the Connected Green Functions are only the propagators.

Daniel.

9. Feb 14, 2005

### Haelfix

With gravity the self-energy problem is subtle and not quite understood, outside of weak field approximations. General gravitons (by that I mean those that are output by the *real* theory outside of the approximation) should not only have self energy graphs, but they presumably back-react with the actual metric that they themselves generate.

Its very tough to conceptualize, harder still to write down sensible equations for.

In the weak field case, there are various selfconsistency measures that are often imposed. In string theory these mechanisms can actually induce topology change.