Do Gravitons Exist? | Physics Forums

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In summary, the existence of gravitons, the hypothetical particles that carry the force of gravity, is still uncertain and not yet proven. While there are theories and indications that suggest their existence, there is not enough evidence to confirm it. The possibility of detecting gravitons at the CERN is still being researched, but it is not the main focus of the experiments. However, there are some theories that suggest that gravitons can be observed at the LHC, and this is being explored. Ultimately, we will have to wait and see what the future experiments and advancements in technology reveal about the existence of gravitons.
  • #1
Physics101
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First of all, I’m not well versed in Calculus or other tools required to fully grasp the latest understandings of QM or Relativity, so please take my question with a grain of salt.

My question is simple. Do Gravitons exist?

My intuition impels me to think of Graviton and Gravity in terms of Particle/Wave duality (as in Photon/Light). I suppose such visualization is in line with how other particle and wave concepts that are well applied in experiments, but then I hit a wall.

Consider a Black Hole. We know that not even light can escape the Black Hole from within the Even Horizon boundary (save the Hawking’s Radiation, which is really a QM effect at the fringe of Event Horizon). Why, then, is Gravity (Graviton) allowed to “escape” or “emanate” from Black Holes? While I realize that it is precisely Gravity which makes Black Hole what it is, but what’s so special about Gravity/Graviton, a seemingly close cousin of Light/Photon, that makes it the only remnant of Black Holes?

I had an earlier post asking about the nature of Space-Time fabric and whether or not the Matter, Gravity and the Speed of Light are but manifestations of the underlying composition and characteristics of such Space-Time fabric’s interaction with Force/Energy (https://www.physicsforums.com/showthread.php?t=102640). Without providing any equations or proofs (I lack the tools), isn’t it possible that Black Hole is a region of Space-Time that counters the isotropic inflation/expansion (due to great Mass) that is the very fabric of Space-Time itself?

I must be misguided somewhere, but would appreciate some pointers on this and my other post (which is lacking reponses/guidances).

Thanks.
 
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  • #2
Physics101 said:
My question is simple. Do Gravitons exist?

My answer is simple. Nobody knows yet whether gravitons exist, as far as I know anyway.
 
  • #3
Physics101 said:
My question is simple. Do Gravitons exist?

Well, actually you should have asked : do gravitational fields exist and can they be quantisized ? In QFT, the fundamental building blocks are NOT elementary particles but the associated quantum fields of which the fluctuations correspond to particles.

We do not know if such gravitational fields can be quantisized (they DO exist ofcourse) because we do not have the "observational power" to detect manifestations of this quantization. The required distance scales are far too small for our current technology. I say, you wait 10 to 15 years and we will know a whole lot more:wink:

regards
marlon
 
  • #4
They definitely do exist, it's just that we can't see them, not yet...

Daniel.
 
  • #5
By theory they MUST exist or the theory which predicted other particle properties and force carrier particles is wrong!

CERN are building (and have nearly finished building) the new particle accelerator that will hopefully show that gravitons exist in 'the flesh' not merely on paper.
 
  • #6
NewScientist said:
By theory they MUST exist or the theory which predicted other particle properties and force carrier particles is wrong!

Sorry, but this is entirely incorrect. gravitons arise when one applies the notions of QFT onto gravity fields. But if gravitons were not to exist, this most certainly does NOT prove that the other interactions are not correctly described by QFT. How can you even say this, when you consider all the successes of QED,QCD, etc etc ?

CERN are building (and have nearly finished building) the new particle accelerator that will hopefully show that gravitons exist in 'the flesh' not merely on paper.

Err, i don't think so. Do you know what energies are required to observe gravitons. Let us just be happy if we would detect the Higgs particle, ok ?:wink:

marlon
 
  • #7
I was not saying that by a 'proof' gravitons do not exist, that the whole of QFT falls to the floor going aaaaaaah. Rather I was saying that the graviton almost certainly exists as the theory that predicts its existence has so much experimental support and theoretical support.

Yet one must remember the quantum world is weird and thus something peculiar may happen with regard to the graviton.

Gravity waves are being looked for at LIGO - by wave-particle duality is not finding one the same as finding the other!
 
  • #8
NewScientist said:
Rather I was saying that the graviton almost certainly exists as the theory that predicts its existence has so much experimental support and theoretical support.
I don't know any theory which predicts the graviton that has any experimental support. (E.g. string theory is far from experimentally verified). Although, I will agree it is very, very likely the graviton exists, since quantum field theory has been so succesfull in so many other areas.

Gravity waves are being looked for at LIGO - by wave-particle duality is not finding one the same as finding the other!
No, gravity waves are predicted already by GR, which is NOT a quantum theory of gravity. The wave-particle duality steems from quantum theory, so you can't use that argument before we have a quantum theory of gravity.
 
  • #9
EL said:
I don't know any theory which predicts the graviton that has any experimental support. (E.g. string theory is far from experimentally verified). gravity.

I refer to my earlier point - the multiple theories that predcit numerous other particles, have been experimentally verified but not for the graviton - I was implying that their ability to standing up to testing so far puts the graviton in good stead!
 
  • #10
I believe that the correct statement is that, while there is no direct evidence that the graviton exists, we would all be a bit surprised if it doesn't.


As for detecting gravitons at the CERN: The LHC certainly is not being built for doing that - it's looking mainly for the Higgs, and more generally for new particles at the TeV scale that can give us insight into fundamental physics.


However, there are classes of theories - all involving extra dimensions in some way - in which graviton effects are directly observable at the LHC. It's a long shot, but there is some chance and the experimenters will certainly be searching for those effects.
 
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  • #11
EL said:
Although, I will agree it is very, very likely the graviton exists, since quantum field theory has been so succesfull in so many other areas.

But QFT does NOT predict the graviton.

The graviton is predicted by string theory, ie the adapted version of QFT that incorporates the basic fondations of general relativity. This is NOT QFT, however !

marlon
 
  • #12
marlon said:
But QFT does NOT predict the graviton.
The graviton is predicted by string theory, ie the adapted version of QFT that incorporates the basic fondations of general relativity. This is NOT QFT, however !
marlon

Eh, I don't really get you?
Of course QFT does not predict the graviton. QFT predicts nothing at all until you apply it to a field. It is the different quantum field theories that predicts stuff. QFT is more of a method which has worked very well in building the standard model, but also in other cases like in solid state physics. QFT is not just QED, QCD,...
In string theory we also quantize the fields, so why shouldn't it be called a quantum field theory?
Now string theory predicts the existence of a graviton, but of course we cannot yet say it is a well working theory, since much is left to compute, and there are no experimental verifications of the theory.
 
  • #13
EL said:
QFT predicts nothing at all until you apply it to a field.

Hu ? What is 'it' ?

In string theory we also quantize the fields, so why shouldn't it be called a quantum field theory?
Because the basic fondations of string theory are totally different in nature than those of QFT. QFT is not just about quantizising fields. In QFT you cannot describe gravity because of two reasons :

1) Heisenberg Uncertainty principle
2) superposition of wavefunctions

This is the main reason why string theory is fundamentaly different in nature when you compare it to QFT (all known QF-theories will respect the above two concepts). String Theory is not just another field theory because it is an attempt to reformulate QFT with the incorporation of the fact that gravity does not "know" the above two notions. It is a conceptually totally different theory than all QFT-theories. That is my point.

regards
marlon
 
  • #14
marlon said:
Hu ? What is 'it' ?
Quantum field theory is the application of quantum machanics to a dynamical system of fields.

Because the basic fondations of string theory are totally different in nature than those of QFT. QFT is not just about quantizising fields. In QFT you cannot describe gravity because of two reasons :
1) Heisenberg Uncertainty principle
2) superposition of wavefunctions
This is the main reason why string theory is fundamentaly different in nature when you compare it to QFT (all known QF-theories will respect the above two concepts). String Theory is not just another field theory because it is an attempt to reformulate QFT with the incorporation of the fact that gravity does not "know" the above two notions. It is a conceptually totally different theory than all QFT-theories. That is my point.
regards
marlon

But in string theory we can start from a classical theory of a string, where the space-time coordinates (seen as fields) give a map of the world-sheet (parametrised by sigma and tau) into the physical space-time. Then we quantize these fields in the usual manner, and hence achieves a 2-dimensional QFT. Why is this not a QFT?
On the other hand, if you want an analoge to how we usually assign an operator to every physical space-time point, but now to every string instead, you have to, as I have understood it, do string field theory.

Could you please elaborate more about 1) and 2)?
Regards.
 
  • #15
The direct answer to your simple question is:

We don't know if they exist.

Some strong theories certainly predict them. But prediction is not the same as detection.
 
  • #16
I don't agree with this. QFT doesn't predict 'any' particle, rather we postulate the existence of certain fields satisfying various assumptions (for instance we want our EM field to satisfy maxwells equations) and then try to write down the simplest thing we can think off under the formalism. Poof out pops electromagnetism when we require a field to be invariant under U(1) gauge symmetry. The only thing we can think off that works is a massless spin 1 force carrier, so there you go, the photon.

Well we know Einsteins equations are right, and so we look for something that satisfies them, and poof we find the only thing that works under the QFT formalism is a massless spin 2 field. Lo and behold it outputs einsteins equations exactly.

So again in that sense gravitons are more or less guarenteed to exist in some sense. Why? B/c special relativity, quantum mechanics and General relativity are correct and these three necessarily imply a spin 2 particle, at least in some linearized sense (which again has to exist at some level of reality).

Now string theory on the other hand goes further. It doesn't just postulate the existence of this field to satisfy experiment (like how we got the equation for all other particles and forces) and we don't just add it in by hand, it goes one step further and says: You give me an interaction between particle a and particle b, not only do you get whatever *that* is, you also get a spin 2 particle popping out for free. It has to show up for the theory to be consistent. Thats very aesthetically pleasing, it means that gravity in some sense knows about all the other interactions a priori, and we don't have to stick it in ad hoc.
 
  • #17
Physics101 said:
First of all, I’m not well versed in Calculus or other tools required to fully grasp the latest understandings of QM or Relativity, so please take my question with a grain of salt.
My question is simple. Do Gravitons exist?
My intuition impels me to think of Graviton and Gravity in terms of Particle/Wave duality (as in Photon/Light). I suppose such visualization is in line with how other particle and wave concepts that are well applied in experiments, but then I hit a wall.
Consider a Black Hole. We know that not even light can escape the Black Hole from within the Even Horizon boundary (save the Hawking’s Radiation, which is really a QM effect at the fringe of Event Horizon). Why, then, is Gravity (Graviton) allowed to “escape” or “emanate” from Black Holes? While I realize that it is precisely Gravity which makes Black Hole what it is, but what’s so special about Gravity/Graviton, a seemingly close cousin of Light/Photon, that makes it the only remnant of Black Holes?

You are off on a wrong track here. Both gravity, and the electrostatic columb force, can "escape" a black hole (assuming the black hole is charged) - they are on equal footing.

Note that the "force" of gravity would be due to virtual "gravitons", just as the columb force is due to virtual photons, not real photons.

I would suggest that you read the sci.physics.faq

http://math.ucr.edu/home/baez/physics/Relativity/BlackHoles/black_gravity.html" [Broken] for more info on this particular question.

You have probably taken the virtual particle model a bit too seriously. Virtual particles, as the FAQ notes, are not constrained by the light cone - they also cannot be used to communicate information.

You are also drifting off into realms of philosophy. Before you can answer whether gravitons are real, you have to be able to answer whether virtual particles in general are "real". Before you can answer that, you have to know what "real" means - are you real? How do you know that you're real? If you answer, for instance "I think, thefore I am", you are left asking if particles think, which doesn't seem to me to be particularly productive, though I gather some philophers like Whitehead think the idea is interesting.

Possibly you aren't really interested in such philophical questions - in which case if you could come up with more specific questions (like whether or not virtual particles can carry information), we can give more definite answers. Just asking if they are "real" is both very broad, very vague, and unlikely to achieve very much except a general discussion about philosophy.

If you are interested mainly in the philophical questions, on the other hand, there is a place on the forum for philosphical questions - it is, of course, the philosophy forum.

I had an earlier post asking about the nature of Space-Time fabric and whether or not the Matter, Gravity and the Speed of Light are but manifestations of the underlying composition and characteristics of such Space-Time fabric’s interaction with Force/Energy

My best guidance is that this is an "overly speculative post". I'm sorry that you need more tools to understand mainstream theory, but a lack of understanding isn't really a good reason to go out and invent a new theory whose only merit is that you understand it (or at least think you do).
 
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  • #18
I generally don't think of gravity in terms of gravitons, nor do I particularly recommend it. I gather, however, that it is possible to do so in a reasonably well defined way. See for example http://arxiv.org/abs/astro-ph/0006423" [Broken]

A pedagogical description of a simple ungeometrical approach to General Relativity is given, which follows the pattern of well understood field theories, such as electrodynamics. This leads quickly to most of the important weak field predictions, as well as to the radiation damping of binary pulsars. Moreover, certain consistency arguments imply that the theory has to be generally invariant, and therefore one is bound to end up with Einstein's field equations. Although this field theoretic approach, which has been advocated repeatedly by a number of authors, starts with a spin-2 theory on Minkowski spacetime, it turns out in the end that the flat metric is actually unobservable, and that the physical metric is curved and dynamical.
Short sections are devoted to tensor-scalar generalizations, the mystery of the vacuum energy density, and quintessence.

The "simple" approach though, requires knowledge of group theory and Lagrangian field formalisms, something that particle physicists take for granted, but would not be particularly "simple" from the POV of someone, like the original poster, who lacked calculus.

There is one other difficulty with this theory - it is not renormalizable. This used to be a major obstacle, but I gather that this is no longer seen as a major obstacle, that one can come up with an "effective field theory" even if the underlying theory isn't renormalizable. (The resulting theory won't work everywhere, though, just at "low" energies).

Google just found an interesting reference which probably explains this better than I can (I'm in the position of learning that renormalizability was no longer particularly important for self-consistency from remarks by other posters)

"Introduction to the Effective Field Theory Description of Gravity"

This is a pedagogical introduction to the treatment of general relativity as a quantum effective field theory. Gravity fits nicely into the effective field theory description and forms a good quantum theory at ordinary energies.
 
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  • #19
pervect said:
I would suggest that you read the sci.physics.faq
http://math.ucr.edu/home/baez/physics/Relativity/BlackHoles/black_gravity.html" [Broken] for more info on this particular question.

That really explains quite a bit in layman's terms (perfect for guys like me). Thanks.

pervect said:
If you are interested mainly in the philophical questions, on the other hand, there is a place on the forum for philosphical questions - it is, of course, the philosophy forum.
My best guidance is that this is an "overly speculative post". I'm sorry that you need more tools to understand mainstream theory, but a lack of understanding isn't really a good reason to go out and invent a new theory whose only merit is that you understand it (or at least think you do).

With all due respect, I think the modern-day science, particularly physics, has taken a turn which makes it nearly impossible for layperson to understand. We all know that reality can be stranger than fiction (QM, Relativity, etc.) and unless you have the necessary tools to work out the equations, these "facts" do resemble philosophy. In this respect, I think the scientists can do a better job of relaying the information to other non-scientists in terms that they can relate to. I think Richard Feynman did an excellent job as best he could, but even that I'm sure threw off many people. As I've already stated in my original post, I do lack the necessary tools to better grasp these concepts, but I certainly have the interest and try to keep up at "layman's" levels. As for your comment on lack of understanding not being a good reason to invent new theory/ideas, I disagree. Humans are prone to organize and categorize their understanding (however lacking) into some sort of model in their mind. All great discoveries, I'm sure, involved lack of understanding which was overcome with some imagination and an attempt to theorize what was going on. My intention is not to make new discoveries. Rather, these "theorizations" help me to better understand what's going on by my attempts to tie them together (as I'm sure the experts do). Perhaps I should be more prudent on what I post (or how I post them), but these forums are certainly great source of information.

Thanks for all you replies.
 
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  • #20
EL said:
Quantum field theory is the application of quantum machanics to a dynamical system of fields.
This is not entirely accurate. Just check out the thread on "the difference between QM and QFT", where we have debated this issue recently.

Applying QM does not equal quantizising. QM cannot be applied onto a system with infinite or non-fixed degrees of freedom because one of the basic requirements of QM is a fixed finite number of particles. This explains the difference in interpretation of "an annihilation operator" in QM and QFT.

But in string theory we can start from a classical theory of a string,
What exactly is that ?

Then we quantize these fields in the usual manner, and hence achieves a 2-dimensional QFT. Why is this not a QFT?

Ok, "1) and 2)" mean the two notions i summed up in my previous post (eg HUP and superposition).

String Theory is a little more than that. What i meant with 1) and 2) is the fact that the gravitational interaction doees not know the concepts of superposition and the HUP. But these two notions are the basic ingredients of QFT, otherwise we could have omitted the letter Q in QFT. When you "quantizise" a field, this means that 1) and 2) become valid for these fields. But than again, how would you describe an interaction that does not recognize 1) and 2) in terms of fields that do. It is this manifest contradictio in terminis that determines the very foundation of string theory.

Besides, the most basic property of QFT are the fields of which the fluctuations correspond to elementary particles. In string theory, this basic property are indeed also fields of which the fluctuations produce strings. One of the clues of string theory is how to link quantum fields and strings. The reason that strings are used comes from the theory that governs gravity : General Relativity...Also keep in mind that in QFT, elementary particles are described in a fixed space time, while in string theory the fluctuations of the fields actually express the fluctuations of space time. I would say there is a fundamental difference here

regards
marlon
 
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  • #21
marlon said:
This is not entirely accurate. Just check out the thread on "the difference between QM and QFT", where we have debated this issue recently.
Well, they aren't my own words. They belong to Peskin and Schroeder.
In fact QM can be seen as a quantum field theory (see Zee).

Applying QM does not equal quantizising. QM cannot be applied onto a system with infinite or non-fixed degrees of freedom because one of the basic requirements of QM is a fixed finite number of particles.
Applying QM to fields means using the same quantization procedure as in non-relativistic quantum mechanics but now generalized to an infinite number of degrees of freedom. E.g. like the cannonical quantization is generalized to fields in Mandl and Shaw, or like the path integral formulation in Zee.

What exactly is that ?
"The classical theory of a string" is what you usually start with in a string theory course. A classical string can be described by the N-G action, or equivalently by the Polyakov action (see e.g. Green, Schwarz, Witten). What did you start your string theory courses with? (This was not sarcastic, I'm just honnestly curious.)

String Theory is a little more than that. What i meant with 1) and 2) is the fact that the gravitational interaction doees not know the concepts of superposition and the HUP.
How do you mean? How could any classical field know about superposition and HUP before quantization?

But these two notions are the basic ingredients of QFT, otherwise we could have omitted the letter Q in QFT. When you "quantizise" a field, this means that 1) and 2) become valid for these fields.
Exactly!

But than again, how would you describe an interaction that does not recognize 1) and 2) in terms of fields that do. It is this manifest contradictio in terminis that determines the very foundation of string theory.
You lost me here.

Besides, the most basic property of QFT are the fields of which the fluctuations correspond to elementary particles.
I don't agree here. For example I would not call field excitations like "phonons" elementary. Remember that QFT can be used in many different areas, including solid state physics.

In string theory, this basic property are indeed also fields of which the fluctuations produce strings.
Now I have to ask you again what string theory books you've used in your courses? This is not the impression I have got, or maybe I just don't understand what you mean.

Also keep in mind that in QFT, elementary particles are described in a fixed space time
And so is the excitations of the fields (i.e. the physical spacetime coordinates) in the quantization of a string (here the "fixed spacetime" is parametrized by sigma and tau). Right?

while in string theory the fluctuations of the fields actually express the fluctuations of space time.
So you are saying a QFT needs to have fields which are functions of the physical spacetime?

Regards /EL
 
  • #22
EL said:
In fact QM can be seen as a quantum field theory (see Zee).

I have Zee's book and indeed what you state here is correct. However, this does not change anything because you need to interprete this correctly. What the above sentence means is this : QM can be seen as a "limited or smaller" version of QFT. Limited means (and here i go back to the actual descripancy that is also described in the Wikipedia website and Zee's book) that the number of particles are constant, more generally : the number of degrees of freedom is constant !

Applying QM to fields means using the same quantization procedure as in non-relativistic quantum mechanics but now generalized to an infinite number of degrees of freedom. E.g. like the cannonical quantization is generalized to fields in Mandl and Shaw, or like the path integral formulation in Zee.

I agree, but the interpretation of concepts like creation and annihilation operators changes. I mean the concept of "creating a particle" is not covered in QM. This is one reason why the beta decay cannot be described in terms of QM. One needs QFT to do the job properly.

"The classical theory of a string" is what you usually start with in a string theory course. A classical string can be described by the N-G action, or equivalently by the Polyakov action (see e.g. Green, Schwarz, Witten). What did you start your string theory courses with? (This was not sarcastic, I'm just honnestly curious.)
:smile:

Don't worry, it is your very right to ask this question. Actually, i used a version of Gerardus t' Hooft's course on String Theory because the professor at my home university that was in charge with this subject, has a close professional relationship with t' Hooft. The course can be found on his website (or at least an updated version of it, since i took it about 2 years back:blushing: )

How do you mean? How could any classical field know about superposition and HUP before quantization?
This is true, but my point was this : how do you describe an interaction that does not know both the HUP and superposition in terms of a theory that does. You see ?

For example I would not call field excitations like "phonons" elementary.

Photons are force carriers ! Not matter particles. Besides, how do you define "elementary". I would say a particle is elementary when there is no "deeper" structure that you can use to describe an interaction. I mean something like :"when it is no longer possible to split it up".

Photons are elementary force particles because of this reason. This is an uniform convention that you will find in any textbook that covers the difference between elementary field theories and EFFECTIVE field theories.

Elementary particles are not the very base of how nature works (according to the Standard Model). It are the fields of which the fluctuations behave like particles (but also like waves:wink: ) that are the very base of it all.

Remember that QFT can be used in many different areas, including solid state physics.

Ofcourse, you are right. What do you mean by this ?
And so is the excitations of the fields (i.e. the physical spacetime coordinates) in the quantization of a string (here the "fixed spacetime" is parametrized by sigma and tau). Right?

Sorry, i do not understand your question. Could you please clarify ?

So you are saying a QFT needs to have fields which are functions of the physical spacetime?
Regards /EL
No no, just the opposite. QFT works independently from space time. Besides, there is NO coupling between space and time, that is a general relativity thing

regards
marlon
 
  • #23
NewScientist said:
falls to the floor going aaaaaaah.

...an appropriate expression in a discussion about the nature of gravity. :p
 
  • #24
marlon said:
I have Zee's book and indeed what you state here is correct. However, this does not change anything because you need to interprete this correctly. What the above sentence means is this : QM can be seen as a "limited or smaller" version of QFT.
More precise, it is a 0+1 dimensional QFT. A QFT need not be 3+1 dimensional.

Actually, i used a version of Gerardus t' Hooft's course on String Theory
And it didn't say anything about classical strings? I find that very strange.

This is true, but my point was this : how do you describe an interaction that does not know both the HUP and superposition in terms of a theory that does. You see ?
Not really. Do Maxwells equations know about HUP and sp? And yet we can formulate QED...
No, I don't think I get your point...

Photons are force carriers ! Not matter particles. Besides, how do you define "elementary". I would say a particle is elementary when there is no "deeper" structure that you can use to describe an interaction. I mean something like :"when it is no longer possible to split it up".
Photons are elementary force particles because of this reason. This is an uniform convention that you will find in any textbook that covers the difference between elementary field theories and EFFECTIVE field theories.
Elementary particles are not the very base of how nature works (according to the Standard Model). It are the fields of which the fluctuations behave like particles (but also like waves:wink: ) that are the very base of it all.
I totally agree with all this! Nice comments, but it had nothing to do with what I wrote.:wink:
Note that I wrote phonon, not photon.:tongue2: That's why I mensioned solid state physics.
(Goddamn, I know what a photon is...:wink: )

Sorry, i do not understand your question. Could you please clarify?
Well actually it was more of a statement I just wanted you to agree on.
I wanted to say that when quantizing a classical string, one also uses a "fixed spacetime", namely the worldsheet of the string parameterized by two parameters sigma (spacelike) and tau (timelike). In this case the "fixed spacetime" does not coinside with the physical spacetime (the physical spacetime coordinates are now considered as fields). Hence the quantization of a string gives a 1+1 dimensional QFT.

No no, just the opposite. QFT works independently from space time.
So then you must agree on my last line?

Besides, there is NO coupling between space and time, that is a general relativity thing
Sorry? What do you mean?

Regards
 
  • #25
EL said:
Do Maxwells equations know about HUP and sp? And yet we can formulate QED...

This is true, but again you make the same mistake: The interactions described by the Maxwell equations govern the dynamics of the force carriers of the EM-interaction. The clue is that photons, electrons,... all can be described in terms of wavefunctions.

I mean that you are creating a field theory for an interaction of which the particles can be described in terms of QM (hence they know 1) and 2)). In the case of gravity, you do not have that !

Note that I wrote phonon, not photon.:tongue2:

Opps, my mistake. A phonon exists because one can quantisize the wavelike motion of lattice atoms in a crystal. One does not need field theory to describe this. Ofcourse QFT knows these "particles" because all results coming out of QM must be respected by QFT. Even, one can describe phonons in terms of field-fluctuations, but why would one do that ?

The many body physics is described in terms of Hartree Fock or Density Funtional Theory. One does not need QFT to describe many body interactions. Ofcourse one can do this in QFT but than i do not see what your point is ?

I wanted to say that when quantizing a classical string, one also uses a "fixed spacetime", namely the worldsheet of the string parameterized by two parameters sigma (spacelike) and tau (timelike). In this case the "fixed spacetime" does not coinside with the physical spacetime (the physical spacetime coordinates are now considered as fields). Hence the quantization of a string gives a 1+1 dimensional QFT.
So then you must agree on my last line?

This is a very strange approach that i am not familiar with or at least that i do not understand. Check out the course of 't Hooft on his website to see how i was introduced to string theory. But this should not be an issue to be honest. When i say that space and time are uncoupled in both QM and QFT, than i mean that there is no space-time coordinate. You only have this in General Relativity.

Besides, i want to ask you how do you define a 1+1 QFT ?

marlon
 
  • #26
Ok Marlon, we seem to have a pretty different view of what a QFT is. Let's try to clear this out, but first here is the link to t´Hooft's Introduction to String theory notes: http://www.phys.uu.nl/~thooft/lectures/stringnotes.pdf
They clearly take up the Classical string, and follow about the same route I'm used to, so they must have been changed since you took the course. (I.e. it follows Green-Schwarz-Witten pretty close, although not exactly.)
(In GSW you can also read about this 1+1 dimensional field I have talked about.)

Anyway, let's turn to the definition of a QFT. I will make some statements, so please tell me if you agree or not:

A QFT need not be the unification between Quantum Theory and Special Relativity. However, the unification of QT and SR needs to be a QFT.

Start from any field theory described by an action. Quantize it (e.g. through cannonical quantization by identifying the fields conjugate and imposing commutation relations). Now we have a QFT. (Wheter it makes sense or not, e.g. normalizability, is another question.)

The fields which we are quantizing need not be functions of physical spacetime coordinates. Any configuration space is ok. (E.g. the sigma-tau space parametrizing the worldsheet of a classical string.)
The number of parameters defines the dimensionality of the QFT. So QED is 3+1 for example. (It is convenient to separate spacelike and timelike coordinates.)

A QFT need not be relativisticly invariant.

A QFT need not have to do with elementary particles at all.

marlon said:
The interactions described by the Maxwell equations govern the dynamics of the force carriers of the EM-interaction. The clue is that photons, electrons,... all can be described in terms of wavefunctions. I mean that you are creating a field theory for an interaction of which the particles can be described in terms of QM (hence they know 1) and 2)). In the case of gravity, you do not have that !
I'm sorry that I don't really get this.
What I do when I quantize the EM-field (and the Dirac-field) is just to take the classical actions and pull them through the quantization proceedure. I could do the same with the Einstein-action, but the problem is that the theory I arrive at then is not normalizable, i.e. doesn't make sense. Isn't this correct? Are you saying the deeper reason for this is that gravity somehow cannot be expressed in wavefunctions, or? I'm just not getting it, sorry.

A phonon exists because one can quantisize the wavelike motion of lattice atoms in a crystal. One does not need field theory to describe this.
But phonons arise (at least I think they are called phonons) as field exitations in QFT's in solid state physics. However these are not "elementary particles", which you claimed was the most basic property of QFT. What I wanted to point out was that QFT need not have anything to do with elementary particles. It depends on what QFT you are talking about.

Even, one can describe phonons in terms of field-fluctuations, but why would one do that ?
I guess the solid state people have some good reasons for using QFT in their work, but I'm not into it, so I cannot tell why.

One does not need QFT to describe many body interactions. Ofcourse one can do this in QFT but than i do not see what your point is ?
As said, my point is that QFT need not have to do with elementary particles.

When i say that space and time are uncoupled in both QM and QFT, than i mean that there is no space-time coordinate. You only have this in General Relativity.
Sorry, now I'm feeling a little stupid. I don't get you. What do you mean by "no space-time coordinate"?

Regards.
 
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  • #27
EL said:
A QFT need not be the unification between Quantum Theory and Special Relativity. However, the unification of QT and SR needs to be a QFT.

I disagree, QFT is DEFINED as the unification of QM and special relativity. The unification of QM and SR does not need to be a QFT because of the fixed number of particles that can be required.

I all i would say that all results of both QM and SR will be reproduced by QFT when you look at QFT at the appropriate energy scale.

Start from any field theory described by an action. Quantize it (e.g. through cannonical quantization by identifying the fields conjugate and imposing commutation relations). Now we have a QFT. (Wheter it makes sense or not, e.g. normalizability, is another question.)

Agreed

The fields which we are quantizing need not be functions of physical spacetime coordinates. Any configuration space is ok. (E.g. the sigma-tau space parametrizing the worldsheet of a classical string.)
What do you mean by any configuration space ?
All i ever said was that in QFT, you do not have a space time continuum like in general relativity. Spatial coordinates and time cannot be treated in the same (Lorentz-covariant) way. Besides, the concept of world line and world sheet is introduced in order to be sure that both space and time coordinates ARE in fact Lorentz covariant.

The number of parameters defines the dimensionality of the QFT. So QED is 3+1 for example. (It is convenient to separate spacelike and timelike coordinates.)
Agreed

A QFT need not be relativisticly invariant.

Disagree, what is the physical relevance of such a theory ?

A QFT need not have to do with elementary particles at all.

Correct, that is the difference between QFT and effective QFT

What I do when I quantize the EM-field (and the Dirac-field) is just to take the classical actions and pull them through the quantization proceedure. I could do the same with the Einstein-action, but the problem is that the theory I arrive at then is not normalizable, i.e. doesn't make sense.

Well this is true but the main point is this : when you do this procedure in for the three interactions besides gravity, you define your QFT with respect to three spatial coordinates and one time coordinate. In the case of gravity, you are quantisizing space-time-continuum but to what base do you define your QFT ? Don't you see the fundamental difference in that ?

But phonons arise (at least I think they are called phonons) as field exitations in QFT's in solid state physics.
No, this is not how phonons are defined. I explained that to you in the previous post.

However these are not "elementary particles", which you claimed was the most basic property of QFT.
No i did not say that. I said this : the most fundamental property of how nature works are not elementary particles but the fields of which the fluctuations behave like particles (and waves ofcourse). QFT governs the dynamics of such fields.

I never said that QFT needs to describe elementary particles only. I gave the reference to effective field theories (like QHD) several times here.

Again, keep in mind that the attempt to quantisize gravity (ie string theory) wants to describe this interaction at a very small level because the energy at which all interactions have the same coupling constant is very big. The problem is that how do you know that gravity will behave quantummechanically at this energyscale ? It most probably will do so but this is one of the fundamental difficulties with string theory. This is what i am repeatedly trying to say here...That is all...

What I wanted to point out was that QFT need not have anything to do with elementary particles.

But why do you always say this ? What does have to do with anything concerning string theory ? I mentionned effective field theory several times...so ? this does not change anything.

As said, my point is that QFT need not have to do with elementary particles.
True, but i never said the opposite. I think you have misinterpreted what i tried to say, or maybe i was not clear enough. Anyhow, i tried to explain it again in this post.

Sorry, now I'm feeling a little stupid. I don't get you. What do you mean by "no space-time coordinate"?
Regards.

Space and time are NO continuum

regards
marlon
 
  • #28
EL, you are absolutely right.And, you must also know that superposition principle is neither necessary nor sufficient condition for quantization.
In general,(classical & quantum) superposition is satisfied if,and only if the theory is LINEAR.
Now, all non-abelian (classical) field theories, such as Einstein's GR and Yang-Mill gauge theories, are nonlinear.i.e. NO SUPERPOSITION. But, Don't we all know that, at least, Yang-Mill's theory can be ,correctly, quantized? What is QCD then?
Given a classical system (point particle or field), the corresponding (canonical) quantum system (QM or QFT) is achieved by:
1) letting the relevant dynamical variables--->operators,
2) setting up the canonical (anti)commutation relations between the relevant dynamical variables and their conjugate momenta(the quantum conditions). Or:
Poisson Brackets--->commutators.
Quantizing classical linear theory leads to Quantum LINEAR theory with superposition principle.
Quantizing a nonlinear theory gives Quantum NONLINEAR theory with NO SUPERPOSITION PRINCIPLE.
SO, TO SAY THAT QFT HAS TO SATISFY A SUPERPOSITION PRINCIPLE IS NOTHING BUT GARBAGE.
One last thing, Classical Mechanics can be regarded as (0+1)-dimensional Classical field theory with spacetime manifold; R^1={t is real}.

cheers

sam
 
  • #29
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. Let me illustrate with the phonon example, for small displacements one can expand the ionic motion into normal modes. These normal modes are quantized and they give you the usual phonon operators, etc. However, when considering the long wavelength excitations of a crystal, it is reasonable and useful to consider the deformation of the ions as depending on a continuous variable i.e. a deformation at every point in space. In this case, the description of the ionic deformation in terms of a finite number of degrees of freedom is replaced with a description in terms of a continuum of degrees of freedom. This field theory is nice because it comes with a length scale, the lattice spacing, where you know your effective description will break down. Nevertheless, the field description works and is useful (basically because integrals are easier to do than sums) for describing all the low energy physics.
 
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  • #30
So this discussion is getting very wide now. I really want to discuss all details, but I think we should restrict to a few things at the moment. I'll leave some things about string theory out for the moment, but hope to get back to them later after we have cleared out this QFT thing, ok? (Otherwise I think the string theory discussion will not be very fruitful.)

First of all, what you wrote earlier was, as you indicated:
Besides, the most basic property of QFT are the fields of which the fluctuations correspond to elementary particles.
So, maybe my response to this wasn't that clear. I'll make a new try:
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.
My claim is that in a QFT, the field fluctuations need not correspond to elementary particles. Just take some QFT used in solid state physics. The field quanta are certainly not elementary particles there.

This subject is closely related to your respons to my claim that a QFT need not be a unification between QT and SR:
I disagree, QFT is DEFINED as the unification of QM and special relativity.
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.
So how to make other QFT's then? Well just take any field theory described by an action and quantize it (e.g. according to cannonical quantization). The action need not be Lorentz covariant, and hence a QFT need not be "a unification between SR and QT". The symmetries of the resulting QFT are mainly determined by the symmetries of the Lagrangian.

About non relativistic QFT's you say:
Disagree, what is the physical relevance of such a theory ?
I'm not into any such theory, but maybe some solid state physicist can tell? In fact, the physical relevance does not really matter. What matters is that even when we start from a Lagrangain which is not Lorentz covariant, what we get after quantization is still a QFT.


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
 
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  • #31
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!:biggrin:
 
  • #32
Theres 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..
 
  • #33
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.
 
  • #34
Ratzinger said:
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
 
  • #35
Physics Monkey said:
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
 
<h2>1. What are gravitons?</h2><p>Gravitons are hypothetical particles that are believed to be the carriers of the force of gravity. They are predicted by the theory of quantum mechanics and are thought to be responsible for the attraction between massive objects.</p><h2>2. How do gravitons work?</h2><p>Gravitons are thought to work by interacting with other particles, such as protons and neutrons, to create a gravitational force. This force is what causes objects to be attracted to each other and determines the trajectory of objects in space.</p><h2>3. Can gravitons be detected?</h2><p>Currently, there is no experimental evidence for the existence of gravitons, and they have not been directly detected. However, scientists are working on experiments, such as the Laser Interferometer Gravitational-Wave Observatory (LIGO), that may be able to indirectly detect gravitons in the future.</p><h2>4. What is the significance of discovering gravitons?</h2><p>If gravitons are discovered, it would provide strong support for the theory of quantum mechanics and help bridge the gap between it and the theory of general relativity. It would also open up new avenues for understanding the fundamental forces of the universe.</p><h2>5. Are there any alternative theories to gravitons?</h2><p>Yes, there are alternative theories to gravitons, such as modified gravity theories, that attempt to explain the force of gravity without the need for gravitons. However, these theories have not been as successful in explaining certain phenomena, such as the behavior of black holes, as the theory of gravitons.</p>

1. What are gravitons?

Gravitons are hypothetical particles that are believed to be the carriers of the force of gravity. They are predicted by the theory of quantum mechanics and are thought to be responsible for the attraction between massive objects.

2. How do gravitons work?

Gravitons are thought to work by interacting with other particles, such as protons and neutrons, to create a gravitational force. This force is what causes objects to be attracted to each other and determines the trajectory of objects in space.

3. Can gravitons be detected?

Currently, there is no experimental evidence for the existence of gravitons, and they have not been directly detected. However, scientists are working on experiments, such as the Laser Interferometer Gravitational-Wave Observatory (LIGO), that may be able to indirectly detect gravitons in the future.

4. What is the significance of discovering gravitons?

If gravitons are discovered, it would provide strong support for the theory of quantum mechanics and help bridge the gap between it and the theory of general relativity. It would also open up new avenues for understanding the fundamental forces of the universe.

5. Are there any alternative theories to gravitons?

Yes, there are alternative theories to gravitons, such as modified gravity theories, that attempt to explain the force of gravity without the need for gravitons. However, these theories have not been as successful in explaining certain phenomena, such as the behavior of black holes, as the theory of gravitons.

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