particles influencing each other

[gib65]

I'm trying to think of an example of two fundamental particles that could coexist very close to each other and have no influence on each other. That is, out of the four forces, neither of the particles exerts any of the forces on the other (and visa-versa). For example, protons and electrons have mass and therefore exert gravitational force upon each other, and they also have charge so they exert an electromagnetic force upon each other. But are there any particles that exert no force on each other? I'm not asking if there are any particles that don't exert force period, just not on each other. For example, one particle may have mass but no charge, and another may have charge but not mass. So long as this exhausts all the forces they have values for, these particles would not influence each other. Are there such particles?

[swansont]

AFAIK the relevant value in GR is not mass, it's the terms in the stress-energy tensor, so you really can't get away from a gravitational interaction, but it's usually very, very, very small on the particle scale.

 

Further, uncharged particles can interact via the electromagnetic interaction, e.g. the photon.

[gib65]

AFAIK the relevant value in GR is not mass' date=' it's the terms in the stress-energy tensor, so you really can't get away from a gravitational interaction, but it's usually very, very, very small on the particle scale.

 

Further, uncharged particles can interact via the electromagnetic interaction, e.g. the photon.[/quote']

 

So does this mean that zero-mass particles can still exert gravitational force upon each other? Would all particles exert gravitational force?

 

What kind of effect would the interaction of photons have on uncharged particles? I know that electrons emit and absorb photons as a means of moving between energy levels, and this has nothing to do with charge AFAIK. Is this the kind of interaction that can go on between uncharged particles?

[swansont]

So does this mean that zero-mass particles can still exert gravitational force upon each other? Would all particles exert gravitational force?

 

So far as I know, yes.

 

What kind of effect would the interaction of photons have on uncharged particles? I know that electrons emit and absorb photons as a means of moving between energy levels, and this has nothing to do with charge AFAIK. Is this the kind of interaction that can go on between uncharged particles?

 

Yes. You should be able to excite a neutron in a nucleus with a photon of the appropriate energy.

[gib65]

Ah, I see.

 

Just for fun though, here's a trick question: Do gravitons exert gravity on each other?

[swansont]

Ah' date=' I see.

 

Just for fun though, here's a trick question: Do gravitons exert gravity on each other?[/quote']

 

Seeing as they're hypothetical virtual force-carriers, probably not.

[timo]

I´d think they´d do:

Reason 1 (from GR): Gravitons contribute to the energy-density tensor and couple to it.

Reason 2 (from gauge theory): The Lorentz Group is non-abelian.

[Klaynos]

The below is what I can just about remember form studying this a couple of years ago mixed with some of my own speculation (mainly the second part) from what I understand your saying about gravitons interacting using gravity...

 

As far as I know no force carrier interacts using the force with which it carries (else this would lead to a continuality of infinitie interactions i.e. photon 1 interacts using photon 2 which then interacts using photon 3 creating a continued line of force carriers.

 

In the case of a graviton interating using gravity, the gravitons used to interact would also have to interact using gravity leading to an infinite force...

[timo]

As far as I know no force carrier interacts using the force with which it carries [...]

Gluons do.

http://en.wikipedia.org/wiki/Gluons

 

In contrast to QCD where you get confinement due to the gluon-gluon interaction I could imagine that you wouldn´t get such a scenario in quantum gravity. Reason: While the number of color-anticolor pairs is not limited and each gluon-coupling has the same strength, your energy is limited and therefore the coupling between the gravitons might decrease with an increaing number of them.

[Klaynos]

Gluons do.

http://en.wikipedia.org/wiki/Gluons

 

In contrast to QCD where you get confinement due to the gluon-gluon interaction I could imagine that you wouldn´t get such a scenario in quantum gravity. Reason: While the number of color-anticolor pairs is not limited and each gluon-coupling has the same strength' date=' your energy is limited and therefore the coupling between the gravitons might decrease with an increaing number of them.[/quote']

 

 

Thanks, I stand corrected (it's been a while as I said)

[Severian]

If fact, any force based on a non-abelian symmetry do. So photons are the exception - they are the only force carriers which do not self-interact (there is also no triple-Z vertex, but you could argue that the ZW+W- vertex is a sort-of self-interaction).

 

To take the original question further, even if you discount gravity, the interactions will not be entirely absent. Imagine a neutrino which has no charge (so doesn't feel electromagnetism) and is 'colorless' so doesn't feel the strong interaction. All it feels is the weak interaction. Also imagine a particle which feels electromagnetism and the strong interaction but not the weak interaction (it doesn't exist, but never mind). At first order they do not interact, but since the neutrino has a weak interaction, it can split into a virtual electron and W-boson, and then the charged electron or W-boson can inteact with the hypothetical particle before annihilating again to reform the neutrino. This would be a very very very small interaction, but not zero.

 

This is true of all particles. The only way to prevent them from interacting with each other is to prevent them from having any interactions with common particles. But then, we (who are made of quarks, etc) would never be able to observe with these particles in any way, so as far as science is concerned they do not exist.