Do Gravitons Exist? | Physics Forums

[EL]

samalkhaiat and Physics Monkey!
Thank you very much for you comments, I didn't notice them until I had finished my reply. I totally agree with both of you! Damn I'm having a hard time with writing down the correct words to describe what I mean, and here you just do it in a couple of lines...well done!

 

[Haelfix]

there's a few things that are a little artificial though about a QFT without lorentz invariance. I know its done all the time, but you might object to the uniqueness of the theory itself.

For instance, the requirement that the interaction density be formed from linear combinations of creation and annihilation operators need not be true a priori. This is a disaster! You could end up with fields that break unitarity bounds and so forth.

Its *ok* to take the limit of a QFT and contract it where c --> infinity, but the very basis goes awry if you aren't very careful with what you are doing.

I've heard people talk about this sometimes, afair there was a very famous mistake that someone did in condensed matter that was masked somehow b/c they were working outside of the continuum limit, but their theory explicitly broke lorentz invariance in a bad enough way that it introduced all sorts of unexpected errors..

 

[Ratzinger]

I think the problem I and most people have when hearing about gravitons, is that we first learn that gravity is different, that it's no force but bending of spacetime. Then we get told there are little force carriers that transmit gravity. But what does that imply for Einstein's theory and his radical different approach? How do spacetime curvature and gravitons go together?

I read here on PF that gravitons are more methapors, calculation devices and they belong to linearized gravity pertubation theory.

 

[marlon]

I think the problem I and most people have when hearing about gravitons, is that we first learn that gravity is different, that it's no force but bending of spacetime. Then we get told there are little force carriers that transmit gravity. But what does that imply for Einstein's theory and his radical different approach? How do spacetime curvature and gravitons go together?

This is completely true and covers very well what i am trying to say with "fundamental difference" between QFT and string theory.

regards
marlon

 

[marlon]

marlon,
I don't think QFT is defined as the union of QM and SR. Quantum field theory is "simply" the quantum description of a field i.e. something with an infinite number of degrees of freedom, it need not have anything to do with relativity. As EL noted, and as Weinberg has emphasized, the union of SR and QM will always look like a quantum field theory at low energies. Nevertheless, the fact that QFT is independent of relativity should be apparent from the fact that it is used in condensed matter physics all the time.

Ok i get the point. Indeed i realize now that EL was right on this one. I misinterpreted some of the things he wrote. Anyhow, i should not just have written that QFT is the unification of QM and SR without adding the proper context. Thanks for clearing this up, Physics Monkey.


regards
marlon

 

[marlon]

What I object against is that you claim that QFT is about elementary particles, or as you more precisely expresse it, about fields which fluctuations represent elementary particles.

Sorry, but this is not what i wrote. You are missing the context on this one. What i said was this : the most fundamental part of the description of nature are the fields of which the fluctuations correspond to elementary particles, not the elementary particles themselves. Them fields are described by QFT (like QED, QCD,...) This does not imply that all particles described by QFT must be elementary.

Just take some QFT used in solid state physics. The field quanta are certainly not elementary particles there.

Agreed, ie effective field theories.

I still have to say no! This is not a correct definition. And this is the crucial part where I think our opinions diverge.
You seem to be stuck with that a QFT need to describe elementary particle fields, like in QED, QCD, the Standard model, etc.
Actually it is true that the concept of QFT was born in the attempt to unite SR and QT, which lead to QED, the first succesfull QFT. And I guess this is why many takes it for granted that QFT is just about quantizing Lorentz covariant theories in order to get a desription of elementary particles. But this is just one kind of QFT.

I realize now that i was wrong on this one. You even provided the clarification in this answer and indeed my vision on this was "too limited"

No no, seriously though, i was wrong on this one.
I agree with you now.

To me it feels like that the QFT you are talking about, is just a subgroup of what I talk about. But maybe we are just talking around each other. What do you think?
(And let's go on with the string theory after this has been cleared out.)
Regards /EL

Well, in short : YOU ARE CORRECT and i am wrong

I was totally omitting the discussion on Lorentz covariance which you wrote down in your last post. I am sorry, this was my mistake...Can i blame it on fatigue ?

Anyhow, shall we go back to strings ?

regards
marlon

 

[EL]

Anyhow, shall we go back to strings ?

Ok, now I think we more or less agree! (There may be some minor things left over, but let's not dig too deep. )Back to the strings:

I will make a statement so please tell me if you agree or not:

Start with a classical string in flat physical spacetime. As it moves through the physical space it sweeps out a (2-dimensional) worldsheet. Let two parameters parametrize this worldsheet: sigma (spacelike) and tau (timelike). The physical spacetime coordinates are now functions of the sigma and tau parameters.
The equations of motion for the classical string can be derived from an action, either the Nambu-Gutu action, or equivalently the Polyakov action. In those actions the physical spacetime coordinates works as fields, while the parameters sigma and tau takes the same role as the physical spacetime coordinates do in for example QED. I.e. we considering a 2-dimensional spacetime (parametrized by the coordinates sigma and tau), with a number of fields (equal to the number of physical spacetime dimensions) defined on it.
Now we quantize the theory, which means we identify the fields (i.e. the physical spacetime coordinates) and the fields conjugate, and impose commutation relations on them.
Vola! We have a 1+1 dimensional QFT.
Agree?

 

[marlon]

In light of the latest revellations i can only say : AGREED.

But my problem remains : how unify gravity with superposition and HUP ?

marlon

 

[Blam]

Ratzinger

How do spacetime curvature and gravitons go together?

I would guess that the Gravitons are smaller then Photons so the gravitons would curve toward the Earth because larger particles attract smaller ones.

Also could we detect gravitons in minute gravity fluctuations of a body?

 

[Haelfix]

"But my problem remains : how unify gravity with superposition and HUP ?"

Well the superposition problem sort of makes canonical quantization impossible to deal with, since Einsteins field equations are nonlinear, so you have to work iwth the path integral from the getgo. there's no way aruond it.

But really there is no big difficulty in writing it down and then calculating loop effects and so forth (just write the most general lagrangian consistent with the symmetries of a spin 2 particle). The quantization procedure goes through as usual and the HUP is build in (*). The whole thing makes perfect sense so long as the approximation of splitting the metric into background + perturbation remains valid (and the appropriate parameter goes something like E^2/M^2, it makes good sense at least where the curvature doesn't get too large or the energy too high) .

This field theory of the graviton then reproduces Einsteins equations (up to a constant), and everyone is happy with this effective field theory description. But since its nonrenormalizable, it obviously can't be the last word. Enter String theory and very high energy QG to try to make it UV complete.

The bigger problem is not so much the HUP, but rather which observables do you pick, and which variables make good sense globally. There are issues in curved manifolds and their asymptotics that make this choice very difficult and ambiguous and considerable confusion remains. This is a *tough* subject, and why I chose a career outside QG.

 

[EL]

how unify gravity with superposition and HUP ?

Well, let's proceed with the string and see if it solves your problem.
At the moment we can at least agree on that the quantized string respects HUP, since HUP actually is imposed on the theory when quantizing it. Agree?

From varying the action, we can see that the theory allows for open (with either Neuman or Dirichlet boundary conditions) as well as closed strings. Without going into any details (and please correct me on this if I'm saying something wrong, I'm not at all a string theorist) one mode of the closed string may be identified as the graviton. The reason for this is because if you consider propagation in a background consisting of many of those closed strings, it will effectivley be the same as propagation in i curved spacetime. I.e. it is possible to derive Einsteins Equations from the interaction caused by a background of those closed strings.

I don't know if this was of any help, since I'm still not really sure if I interpret your question correctly?

regards /EL

 

[GOD ENTITY]

How could gravity exist if it wasn't made out of gravitons. Obviously, according to physics and just simple logic if something can exert a force then it must be made of matter. F=ma. something can't be massless if it has a force. Gravity is force thus it is made out of something, this something being gravitons. THEREFORE IT MUST EXIST.

 

[ZapperZ]

How could gravity exist if it wasn't made out of gravitons. Obviously, according to physics and just simple logic if something can exert a force then it must be made of matter. F=ma. something can't be massless if it has a force. Gravity is force thus it is made out of something, this something being gravitons. THEREFORE IT MUST EXIST.

It is not simple logic. You are assuming that the QFT description is valid even for gravity. This has nothing to do with logic since it requires the existence of (i) theoretical description and (ii) experimental verification. There is a lack of BOTH for gravitons. There's nothing a priori to indicate that a "quanta" of gravitational interaction should exist. We only speculate that it does based on inference of what is going on with the 3 other remaining forces. But nothing in nature or logic indicates that our view should work! That's why people are still doing reserach on it. If this is a done deal simply based on "logic", then why bother?

Zz.

 

[Physics101]

How could gravity exist if it wasn't made out of gravitons. Obviously, according to physics and just simple logic if something can exert a force then it must be made of matter. F=ma. something can't be massless if it has a force. Gravity is force thus it is made out of something, this something being gravitons. THEREFORE IT MUST EXIST.

I wish it were that simple. Really, because the concept of gravity has been pulled, stretched, meshed and torn to pieces by various disciplines of physics. Each new theory that comes along seems to deliver different message, so a layperson like myself is bound to get confused.

Having said that, what Einstein described makes the most "sense" to me so far. Gravity is a product of Matter while Matter is a product of Gravity. They are mutually dependent on each other through Einstein's Space-Time concept. We'll have to wait and see if other theories can better explain what Gravity/Graviton exactly is. Perhaps Space-Time can be quantized to reveal something resembling Graviton, but even then, it would appear that Space-Time will be tightly coupled (IMHO).

 

[DaveC426913]

How could gravity exist if it wasn't made out of gravitons. Obviously, according to physics and just simple logic if something can exert a force then it must be made of matter. F=ma. something can't be massless if it has a force. Gravity is force thus it is made out of something, this something being gravitons. THEREFORE IT MUST EXIST.

Photons aren't made out of matter and yet they can exert a force.
You make too many assumptions.
You also draw inocorrect conclusions, such as that these particles have mass - which they do not.

 

[Blam]

Gravitons don't exist because they don't fit into current General Relativity or QFT. When they fulfill the QFT prerequisites then we can scientifically quantify it but until then it is just a generalization or feeling. Yes I feel something hitting my arm and bringing it down but I just call it gravity and don't know what it is really made up of although we don't really know what sub atoms are made of but I'm trying to fit it into current scientific thought for the sake of logic or 'the argument' as is said. Also we can assume gravity is matter according to Spacetime. I'll have to study QFT and String and see how I can help prove it for scientific purposes of course. I am thinking maybe a faster computer then PetaFLOP.

Also I think right now Photons are 0 mass but wouldn't that change if a smaller particle was discovered?

Having said that, what Einstein described makes the most "sense" to me so far. Gravity is a product of Matter while Matter is a product of Gravity. They are mutually dependent on each other through Einstein's Space-Time concept.

 

[Norman]

Gravitons don't exist because they don't fit into current General Relativity or QFT. When they fulfill the QFT prerequisites then we can scientifically quantify it but until then it is just a generalization or feeling. Yes I feel something hitting my arm and bringing it down but I just call it gravity and don't know what it is really made up of although we don't really know what sub atoms are made of but I'm trying to fit it into current scientific thought for the sake of logic or 'the argument' as is said. Also we can assume gravity is matter according to Spacetime. I'll have to study QFT and String and see how I can help prove it for scientific purposes of course. I am thinking maybe a faster computer then PetaFLOP.

Also I think right now Photons are 0 mass but wouldn't that change if a smaller particle was discovered?

Just because something doesn't fit into our current paradigms of theory doesn't mean it cannot exist. And you should really not comment on whether something fits or doesn't into a theory then admit you haven't studied the theory.

What do you mean we don't know what "subatoms" are. We know nuclei are composed of protons and neutrons. These baryons are composed of quarks. Quarks are point particles and have no substructure (that has been seen as of yet).

Photons ARE zero mass (of course we are talking about rest mass here), this will not change. The only way a particle can travel at c is if it has zero rest mass. So once we see light in vacuum travel slower than c, then the photon has rest mass. What do you mean by a smaller particle? A less massive particle? Compared to what?

 

[Blam]

The smaller the particle the faster it goes? Is a graviton perceived to be a FTL particle like a Tachyon?

Also, Gravitons fit into QM?

 

[Norman]

The smaller the particle the faster it goes? Is a graviton perceived to be a FTL particle like a Tachyon?

Also, Gravitons fit into QM?

Smaller in what sense? The less massive a particle is the easier it is to accelerate. But special relavtivity tells us that it would take an infinite amount of energy to accelerate a particle with mass (no matter how little or how close to zero the mass is) to the speed of light, c. The graviton is thought to be a spin 2, massless particle. It has zero rest mass and therefore travels at the same speed as a photon, c.

 

[dextercioby]

One of the nicest recent results of theoretical physics [1] is that there is only type of gravitons, namely PAULI-FIERZ ones. Hence SUGRA with N>8 cannot exist, as such a theory would advocate the existence of cross-interactions of multiple types of gravitons.

Daniel.

[1]N. Boulanger, T. Damour, L. Gualtieri, M. Henneaux "Inconsistence of interacting multi-graviton theories" Nucl. Phys. B97, 127, 2001.

 

[Physics101]

Smaller in what sense? The less massive a particle is the easier it is to accelerate. But special relavtivity tells us that it would take an infinite amount of energy to accelerate a particle with mass (no matter how little or how close to zero the mass is) to the speed of light, c. The graviton is thought to be a spin 2, massless particle. It has zero rest mass and therefore travels at the same speed as a photon, c.

Since Mass and Energy are different manifestation of the same thing, yet Mass and Gravity are inter-related, I guess the better question would be "What is Mass?" Given that Mass, however small, is bound by speed of light and does contribute to Gravity while its altered-ego (Energy) does not (travels at c and causes no Gravity, although it does follow Geodesic path), what is it about Mass that shows these properties?

Also, just for fun, is this possible? If one were to be able to convert Energy to Mass and back at will, wouldn't a Gravity-Drive be possible by repeatedly creating Mass (and converting back to Energy) in front of a spaceship? Of course, we are talking about huge amount of Energy and efficiency is always a concern, but I suppose it is possible.

 

[Blam]

You could create the ship in graviton data then recreate it anywhere you wanted. I don't think you would need to do it redundantly.

About mass, so we know of the larger structure of it but how do its smaller faster properties exist. It could be maybe infinite that as we look closer and closer at it (dimensions 6 through 11?) somehow attract more matter to itself or our perspective somehow would bend throughout the universe. So at dimension 11 it would loop back or we would be at someplace else but never an open circuit. Or it could be nothing at all.

Also that's how there is allot of perfection in the universe through dimensions like the number 10, 7 and 6. 7 representing perfection, 6 imperfection, 10 a whole, 11 a change. Like fundamental numbers.

Maybe the underlying structures exist other places so like a timing thing where we couldn't perceive it all at once which seems to blend into the relativity theories. This creates diversity.

 

[Crazy8s]

This has been such a fascinating discussion! I wish to thank all of the contributors. It is rewarding to know that even amongst such very highly literate, and seemingly intelligent people, there can reside such vastly conflicting viewpoints of subject that's origins have not been verified. Some of you have said that gravitons MUST exist, and others have said that they do not have to exist, both opinions being based on exactly the same theory.

I especially liked the avoidance of associating philosophy with physics. It would be my opinion that philosophy and physics are nearly identical concepts, with the exception is that one prefers to measure concepts once they have been established. All physics came from philosophy, and that statement in itself interminably intertwines the two. Doubt me? In order to measure something, you have to have formed a concept of why you are measuring it. If physics was only the measurement of concepts, then Einstein, Newton, Galileo, Copernicus and so many others would have only been philosophers.

Again I thank you all for such an enlightening read!

 

[Ernies]

fundamentals of qt

Despite watching for the last 50-odd years i have never come across a convincing explanation of why one half of the solution of Schrödinger's equation (and of course the matrix formulation) should be ignored. NOW I come to think of it, it also applies to Kelvin's e.m. classical equation. I mean the advanced and retarded waves. Oh, i know it works but that is not what I'd call a good reason.
ernie