The Graviton

[jordan]

I picked up Brain Greene's book "The Elegent Universe" a few days ago (I'm on page 236 for those who have the book) and something just hit me. The graviton seems to be coming a pretty acceptable concept. But what I wondered is how the graviton (actually, as I'm writing this I realized any of the four force particles could be put in here) accounts entirely for gravity. As it's a particle and, according to string theory, made of a string, then they wouldn't be in infinite supply. If they aren't, how can everything be assured of being affected by gravity? Could it ever be that two objects pass without gravitational influence because the gravitons didn't hit each other? And also, do more massive objects have more gravity because they have more particles to release more gravitons?

 

I'm not saying that string theory is all right and the TOE, but I'm wondering how string theory is planning on accounting for this should it be the TOE. Thanks.

[swansont]

Why wouldn't they be in infinite supply? They're virtual particles.

[jordan]

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

 

Virtual particles are often popularly described as coming in pairs, a particle and antiparticle, which can be of any kind. These pairs exist for an extremely short time, and mutually annihilate in short order.

 

Hmmmm...so I guess I'm not entirely sure how these virtual particles fit into the theory. It wasn't explained very well (or at all) in the book. And the above description doesn't mesh very well with my traditional thoughts about the graviton. What's the anti-graviton and how does gravity work when the particles cancel out quickly?

[Severian]

It is exactly the same ideas as we have already proven for the photon. All gravitons which mediate gravity, must be virtual by definition (just as the virtual photons mediate electromagetism). Also the graviton is its own antiparticle, just like the photon is its own antiparticle.

[Daecon]

That's one of the reasons why I don't think the graviton is real and you don't *need* a force to explain gravity, as it's not a particle-based phenomenon.

[Royston]

That's one of the reasons why I don't think the graviton is real and you don't *need* a force to explain gravity, as it's not a particle-based phenomenon.

 

I agree, I remember reading about gravitons (or the theory of gravitons) in a 'Brief of history of time' and remember finding it a little tenuous. I'm still under the assumption gravity is a result of the curvature of space-time.

 

It's not a force in it's own right, it's an observable effect due to the universe's structure. (could be worded better...I'm trying to do four things at once at the moment.)

[Severian]

I agree' date=' I remember reading about gravitons (or the theory of gravitons) in a 'Brief of history of time' and remember finding it a little tenuous. I'm still under the assumption gravity is a result of the curvature of space-time.

 

It's not a force in it's own right, it's an observable effect due to the universe's structure. (could be worded better...I'm trying to do four things at once at the moment.)[/quote']

 

That would be possible if gravitational forces were fixed, and never changed, but the curavture changes in response to the stress-energy tensor, so you need a mechanism of translating that into curvature. This is what the graviton is for.

[Daecon]

It could just be "pressure" acting on that 4th dimension...

[Royston]

Thanks for the explanation Severian, so they affect each other... the string theory explanation for gravity seems more logical to me in that case, albeit I only understand the theory's basic principles.

[sergey500]

That great book. Anyway, gravitons aren't even confirmed by most phyisists

[ydoaPs]

That great book. Anyway, gravitons aren't even confirmed by most phyisists

iirc, gravitons aren't confirmed by ANY physicists.

 

 

 

edit: image tags....[ img ]url of pic[ /img ] without the spaces

[Royston]

iirc, gravitons aren't confirmed by ANY physicists.

 

Exactly, they havn't been observed...only the effects of gravity have been observed.

 

I can't help but think that gravity isn't a massless force carrying particle...especially as it acts on long distances, and as Severian pointed out has a direct relation with the curvature of space.

 

It seems to me there is a duality with particles and gravity, that the force of gravity is more underlying than inherent to atomic structure, like a force inbetween atoms maybe.

[tholan]

The notion of particles is given on flat spacetime as well as on other simple geometries. As soon as one wants to set up a theory of matter on a complex or arbitrary geometry, this notion becomes meaningless and is replaced just by the one of excitations of a quantum field. From this point of view I think that the idea of the graviton as the carrier of the gravitational field must be fundamentally wrong. The graviton might be, however, a “low energy” phenomenon of gravitation.

[Severian]

As soon as one wants to set up a theory of matter on a complex or arbitrary geometry, this notion becomes meaningless and is replaced just by the one of excitations of a quantum field.

 

That is exactly what particles are. No-one is suggesting anything different for a graviton.

[Royston]

This is just a stab in the dark, and forgive my lack of knowledge on the subject, but could it is possible to use the string model of gravity alongside quantum mechanics.

 

This way we uphold observational data on particles, and retain a model of gravity that logically (IMO) that compensates for curvature and long distances. Or are the equations that support either theory totally incompatible.

[Severian]

String theory is supposed (hoped) to provide a theory which merges quantum mechanics and gravity, so you don't need anything along side it! String Theory is a quantum theory, so it has to be consistant with the ideas of QM. The problem with String Theory is that it is extremely difficult to work with mathematically, so it is difficult to make testable predictions.

[Royston]

String theory is supposed (hoped) to provide a theory which merges quantum mechanics and gravity, so you don't need anything along side it! String Theory is[/b'] a quantum theory, so it has to be consistant with the ideas of QM. The problem with String Theory is that it is extremely difficult to work with mathematically, so it is difficult to make testable predictions.

 

Thanks for putting me straight Severian, I suppose I got confused because some people favor string theory over other quantum theories of gravity...time to do some reading.

[tholan]

That is exactly what particles are. No-one is suggesting anything different for a graviton.

Particles are excitations of quantum fields on flat spacetime, or on some other simple spacetimes. But the point is that excitations are not always particles. This is valid for curved spacetimes in general. It makes no sense to speak about gravitational interaction via gravitons in such cases in which the notion of particle is not well defined.

[Severian]

Particles are excitations of quantum fields on flat spacetime' date=' or on some other simple spacetimes.

[/quote']

 

Locally, all (reasonable) space-times are flat.

 

But the point is that excitations are not always particles. This is valid for curved spacetimes in general. It makes no sense to speak about gravitational interaction via gravitons in such cases in which the notion of particle is not well defined.

 

What do you mean by "excitations are not always particles"? How are you defining a 'particle'? I would regard any localised field excitation as a particle. (Actually many people go further, and call non-local excitations particles too!)

 

Can you give me an example of a localised excitation which is not a particle?

[tholan]

Yes, every spacetime is locally flat but you cannot always find a unique set of positive frequency modes that allow you to unequivocally define a representation of the commutation relations. Worst, these representations may not be equivalent. Particles in one reference frame may not be in the other, but, nevertheless, interactions should be invariantly identified by both observers as being caused by the field(s). On the other hand things get also weird in dynamic spacetimes, for example in QFT on a de-Sitter background, in which superhorizon fluctuations take place. How to explain superhorizon fluctuations in terms of particles?