Why do we use gravitons without a corresponding theory?

[Ivan Seeking]

I hear many models for particle interactions described that use gravitons, virtual particles, and that reference many concepts that I thought were highly controversial; but these models don't seem to be treated as such.

Mainly though the question of gravitons is what attracted me. If we have no quantum theory of gravity, then how can we effectively use these models? How can we know the nature of a graviton? Clearly any theories of Quantum Gravity fail thus far. Aren't we virtually assured that our models are incorrect since we can't produce a theory?

 

[Tail]

I thought there was a pretty good theory about gravitons, but we just cannot observe them because of their low energy or something.

The classical attitude towards gravitons is that they SHOULD exist, no?

 

[Ivan Seeking]

Originally posted by Tail
I thought there was a pretty good theory about gravitons, but we just cannot observe them because of their low energy or something.

The classical attitude towards gravitons is that they SHOULD exist, no?

I thought that until we have a unified theory of GR and QM, it is all speculative at best.

This is a comment posted by Jeff. I recognize Jeff as highly accomplished and I treat what he says with the highest regard. But I don't understand the apparent certitude in examples such as this one and hundreds of others.

It's of course perfectly alright to view directional changes in the paths of photons moving in a curved spacetime as a result of successive interactions with gravitons. But in terms of single scattering events, it's the energy of interaction that's important: Large deflections result from the emission or absorption of a single high energy graviton.

 

[Mr. Robin Parsons]

Just out of curiosity, but aren't gravitons considered
THEORETICAL Particles?

They are "The Theory".

 

[jeff]

Originally posted by Ivan Seeking
If we have no quantum theory of gravity, then how can we effectively use these models?

A theory of a self-interacting massless helicity-2 field is obtained in the weak field limit of GR in which the metric is approximated as a sum of a flat background part and the aforementioned field living on it. In a QGT the corresponding quantized field would represent the graviton, just like the potential in maxwells equations represents the photon in QED.

At present string theory is the only quantum theory we know of whose spectrum includes a massless helicity-2 particle whose behaviour is governed by GR in the classical limit. In this sense string theory - whether or not it's correct - is our only known QGT.

 

[selfAdjoint]

And string theory, at least so far as I have seen, is not prepared to calculate the phenomonolgy to support all these conjectures about gravitons. GR, although only an effective theory, remains our only source of usable calculations of gravitic effects.

 

[jeff]

Originally posted by selfAdjoint
And string theory, at least so far as I have seen, is not prepared to calculate the phenomonolgy to support all these conjectures about gravitons.

That string theory contains GR is not a conjecture, it's a fact. GR emerges from string theory as a consistency requirement that the theory on the world-sheet be weyl-invariant. This requirement takes the form of the vanishing of the beta-function, which to 1st order yields GR.

 

[LURCH]

Originally posted by jeff
In a QGT the corresponding quantized field would represent the graviton, just like the potential in maxwells equations represents the photon in QED.

IOW; without knowing whether or not gravitons actually exist, we have found that the mathematical models used to represent them make predictions that match well with observation, right? So until we can prove or disprove their existence, we will use the model because it works.

 

[Ivan Seeking]

Originally posted by jeff
That string theory contains GR is not a conjecture, it's a fact. GR emerges from string theory as a consistency requirement that the theory on the world-sheet be weyl-invariant. This requirement takes the form of the vanishing of the beta-function, which to 1st order yields GR.

Is ST considered to be mostly correct but just incomplete? Is this considered to be only a matter of loose ends and exact solutions, or does ST itself qualify as speculative? What kind of consensus exists here? I know that Kaku thinks we have THE final theory in ST but that is just unfinished; ie it evades exact solutions for the moment. However, Kaku seems to come off as slightly towards to fringe so I don't tend to trust his claims entirely.

 

[Ivan Seeking]

Originally posted by LURCH
IOW; without knowing whether or not gravitons actually exist, we have found that the mathematical models used to represent them make predictions that match well with observation, right? So until we can prove or disprove their existence, we will use the model because it works.

I realize that this must be the case. I was alluding to more fundamental questions of certitude and context. Note also that many theories can be made to fit with a little nudging; which of course says nothing of correctness. The epi-cycles of Mars fit the observations for a time also, but they were of course completely without any basis in reality.

 

[jcsd]

Gravitons are more of an educated guess at what a quantum field theory of gravitation would include, rather than part of a comphrehensive theory that explains gravitation.

 

[Ivan Seeking]

Originally posted by jcsd
Gravitons are more of an educated guess at what a quantum field theory of gravitation would include, rather than part of a comphrehensive theory that explains gravitation.

As a related issue, and this has come up several times, I was taught that photons have a mass of hνC^-2; that no distinction can be made in GR between energy and mass energy. Am I remembering this incorrectly? Is this a notion out of favor [if so by what argument?], or is this now known to be wrong and why? Obviously I don't mean to ask for a complete explanation, but a well defined pointer would be nice.

 

[jcsd]

Originally posted by Ivan Seeking
As a related issue, and this has come up several times, I was taught that photons have a mass of hμC^-2; that no distinction could be made in GR about energy and mass energy. Am I remembering this incorrectly? Is this a notion out of favor [if so by what argument?], or is this now known to be wrong and why? Obviously I don't mean to ask for a complete explanation, but a well defined pointer would be nice.

The quantity hvc^-2 for a phton is what is known as it's relativistic mass, many moons ago this was the accepted definition of mass but nowdays when a physicist talks about the mass of a particle he will almost certainly mean the rest mass unless otherwise qualified. The reason for this is simply it is a more convient definition. Obviously the two are not indistiungishable as by simply (at least in relativity)measuring the momentum and velocity of an object you can work out it's rest mass and kinetic energy and keeop the two terms separate.

 

[lethe]

Originally posted by Ivan Seeking
As a related issue, and this has come up several times, I was taught that photons have a mass of hνC^-2; that no distinction can be made in GR between energy and mass energy. Am I remembering this incorrectly? Is this a notion out of favor [if so by what argument?], or is this now known to be wrong and why? Obviously I don't mean to ask for a complete explanation, but a well defined pointer would be nice.

Photons are massless:

relativistic mass

Of the two, the definition of invariant mass is much preferred over the definition of relativistic mass. These days, when physicists talk about mass in their research, they always mean invariant mass. The symbol m for invariant mass is used without the subscript 0. Although the idea of relativistic mass is not wrong, it often leads to confusion, and is less useful in advanced applications such as quantum field theory and general relativity. Using the word "mass" unqualified to mean relativistic mass is wrong because the word on its own will usually be taken to mean invariant mass. For example, when physicists quote a value for "the mass of the electron" they mean its invariant mass.

"Ouch! The concept of `relativistic mass' is subject to misunderstanding. That's why we don't use it. First, it applies the name mass--belonging to the magnitude of a four-vector--to a very different concept, the time component of a four-vector. Second, it makes increase of energy of an object with velocity or momentum appear to be connected with some change in internal structure of the object. In reality, the increase of energy with velocity originates not in the object but in the geometric properties of space-time itself."

-wheeler

"It is not good to introduce the concept of the mass M = m/(1-v2/c2)1/2 of a body for which no clear definition can be given. It is better to introduce no other mass than `the rest mass' m. Instead of introducing M, it is better to mention the expression for the momentum and energy of a body in motion."

-einstein

you only find the concept of relativistic mass used in popular science books these days, and on internet physics boards or newsgroups. in real science textbooks, it is simply not found at all.

photons have energy hν. and mass 0.

 

[Ivan Seeking]

First, thanks everyone for your answers thus far.

So the notion of mass is defined as meaning rest mass. What about gravity? Am I not safe to argue that by conservation of momentum, if a planet pulls on a photon, the photon pulls on the planet? If we have gravity then we must have mass - mass that agrees with the total momentum as mV, and mass that agrees with the forces observed between the planet and the photon? By this, can I argue that whatever we mean by the word mass, the thing represented by this concept is present within the photon?

Edit: I almost did it again! The key question is one of gravity. By this reasoning, is it still safe to conclude that photons have a gravity field? Or has something considered "safe" now ruled this out as a proper interpretation?

 

[LURCH]

Originally posted by Ivan Seeking
First, thanks everyone for your answers thus far.

So the notion of mass is defined as meaning rest mass. What about gravity? Am I not safe to argue that by conservation of momentum, if a planet pulls on a photon, the photon pulls on the planet?

I remember this idea being brought up a few times before. The example (IMHO) only proves that the photon has momentum, and the planet has gravity. It does not make a very good case for the photon having gravity.

I actually started a Topic that addressed the question of relativistic mass having gravitational influence. The example was a neutron star with a mass of two Solar Masses, and a radius smaller than Earth's Moon. This object moves through space at such a valocity (relative to us) that it's relativistic mass is 20-30 SM. An object with a mass of 30 SM should be a Black Hole with an event horizon radius greater than the Moon's radius. So does the object become a black hole? The answer is of course, "no, it doesn't". After all, if it's not a black hole for objects that are stationary relative to it, then things inside the event horizon (as we perceive it) could climb out!

So, if relativistic mass will not make a black hole out of a moving object, it cannot have a gravitational consequence.

 

[Ivan Seeking]

Originally posted by LURCH
I remember this idea being brought up a few times before. The example (IMHO) only proves that the photon has momentum, and the planet has gravity. It does not make a very good case for the photon having gravity.

So, if relativistic mass will not make a black hole out of a moving object, it cannot have a gravitational consequence.

But we do feel the gravity from the relativistic mass. So in this way the gravity would seem absolutely tied to relative mass.

Also, how do we avoid the need for conservation of momentum with your statement about photons? If we observe a force between the photon and the planet, and if we use gravity [spacetime curvature] to explain the force on the photon, then what other mechanism is left for mediating the required momentum change to the planet? By all accounts the photon does experience a momentum change, so the planet must also...no?

 

[LURCH]

Originally posted by Ivan Seeking
But we do feel the gravity from the relativistic mass. So in this way the gravity would seem absolutely tied to relative mass.

Not following you here. How do we feel or measure the gravitational influence of relativistic mass?

Or do you mean that there is attraction between the photon and the planet? What I was attempting to say with my earlier post was that the attraction between the photon and the planet can take place without the photon exerting gravitational influence on the planet, so long as the planet exerts gravitational influence on the photon.

Also, how do we avoid the need for conservation of momentum with your statement about photons? If we observe a force between the photon and the planet, and if we use gravity [spacetime curvature] to explain the force on the photon, then what other mechanism is left for mediating the required momentum change to the planet? By all accounts the photon does experience a momentum change, so the planet must also...no?

Oh no, I am not proposing that we "avoid the need for conservation of momentum", on the contrary; I completely agree that the planet must experience a change in momentum as a result of this interaction. However, you say, "If we observe a force between the photon and the planet, and if we use gravity (space-time curvature) to explain the force on the photon, then what other mechanism is left for mediating the required momentum change to the planet? ". My point is, this model of curved space around the planet altering the course of the photon works equally well whether or not the photon curves space. The curvature of space caused by the planet will alter the course of the photon, and will also compose the medium by which conservation of momentum is transferred, with or without any curvature of space being caused by the photon itself.