Register to reply

Renormalization Group for dummies

by waterfall
Tags: dummies, renormalization
Share this thread:
waterfall
#37
Feb22-12, 06:20 PM
P: 381
Quote Quote by bhobba View Post
The physical reason the coupling constant gets larger is the the shielding effect of the virtual particles around an electron. Landau showed that because of that for any finite charge at any distance the charge would measure zero. This created the famous zero charge problem. The only way around it is for the charge to actually be infinite and to grow to an infinite value the closer and closer you get to it - this is called the Landau pole. That is the reason for the infinities - the closer you get the higher the energy you must use. If I remember correctly this is not just theory at 90gv it measured 1/127 (OK to avoid egg on my face I checked it)
You talked as if the virtual particles were real and really there around electrons. These are just figurative right? The perturbation series terms are given fancy names like 2nd order or 3rd order virtual particles when these don't have factual existence. So the reason the coupling constant gets bigger is simply due to the perturbation series alone and not to any nonexistent virtual particles (unless you mean they really exist?).

As one other person posted the fact it blows up to infinity means the theory is sick and incorrect - but we already know that because it gets replaced by the electroweak theory which will probably also get replaced by something else someday and that something else will hopefully be free of these problems.

Thanks
Bill
The message before this is due to your mentioning the "electroweak theory", what is it connection with the landau pole, is there such thing as electroweak pole? what are you talking about, pls refer to the message before this where I asked it in details. Thanks.
bhobba
#38
Feb22-12, 06:24 PM
Sci Advisor
PF Gold
P: 2,816
Quote Quote by waterfall View Post
You talked as if the virtual particles were real and really there around electrons. These are just figurative right?
They however have real effects - the shielding of the charge of an electron is one of them as well as things like the Casmir Effect.

Regarding the Landau pole although it is an issue it is calculated perturbatively and the region in blows up is one where perturbation theory will not work because it is so large. However re-normalisation group theory does and it shows it blows up in a different way - but blow up it does.

Thanks
Bill
waterfall
#39
Feb22-12, 06:39 PM
P: 381
Quote Quote by bhobba View Post
They however have real effects - the shielding of the charge of an electron is one of them as well as things like the Casmir Effect.
Prof. Neumaier mentioned how they are caused directly by the fields and not by any virtual particles which he refers to as Multivariate Integrals. Maybe he can give details when he comes here later.

Regarding the Landau pole although it is an issue it is calculated perturbatively and the region in blows up is one where perturbation theory will not work because it is so large. However re-normalisation group theory does and it shows it blows up in a different way - but blow up it does.

Thanks
Bill
Laudau pole is in the center of the planck length. It may not have factual existence because the planck scale may not even contain any spacetime so points lose its meaning.

Do you agree that we only do renormalization group calculations because the theory we have is only an effective field theory. Meaning when we come to the final true theory, we don't have to use any renormalization group, can anyone refute this?
bhobba
#40
Feb22-12, 07:14 PM
Sci Advisor
PF Gold
P: 2,816
Quote Quote by waterfall View Post
Do you agree that we only do renormalization group calculations because the theory we have is only an effective field theory. Meaning when we come to the final true theory, we don't have to use any renormalization group, can anyone refute this?
The re-normalisation group tells us how parameters such as the coupling constant varies with the regulator (conceptuality the cut-off - usually in energy) and also tells us where we would expect new physics to appear. If would be very unwise to push a theory beyond that IMHO - so yes I believe in the EFT approach as the only reasonable thing to do.

Thanks
Bill
waterfall
#41
Feb22-12, 07:21 PM
P: 381
Quote Quote by bhobba View Post
The re-normalisation group tells us how parameters such as the coupling constant varies with the regulator (conceptuality the cut-off - usually in energy) and also tells us where we would expect new physics to appear. If would be very unwise to push a theory beyond that IMHO - so yes I believe in the EFT approach as the only reasonable thing to do.

Thanks
Bill
Right. You mentioned below that the electroweak thing which I can't understand the context:

"As one other person posted the fact it blows up to infinity means the theory is sick and incorrect - but we already know that because it gets replaced by the electroweak theory which will probably also get replaced by something else someday and that something else will hopefully be free of these problems."

But QED is because of the broken symmetry where electromagnetism and the weak force is not united because of the low energy. Are you saying that near the planck length, the electroweak force is active and this QED probe can still touch it? I know we need 100 Gev probe to touch it.. but in ordinary em field, can it probe the electroweak scale?
waterfall
#42
Feb23-12, 10:58 PM
P: 381
There's this very interesting book called: "The Infinity Puzzle: Quantum Field Theory and the Hunt for an Orderly Universe"

http://www.amazon.com/Infinity-Puzzl...0058226&sr=8-1

It's a laymen book and Renormalization Group was not mentioned however it seems to be related to the older Mass and Charge Renormalization used by Feynman, Schwinger, Tomonaga mentioned up to Chapter 3 which I just finished. Going to wiki.

"In theoretical physics, the renormalization group (RG) refers to a mathematical apparatus that allows systematic investigation of the changes of a physical system as viewed at different distance scales."

My questions are how come the Renormalization Group was attributed to Kenneth Wilson when in fact even Feynman, Schwinger, Tomonaga used it called Mass and Charge Renormalization. And what would happen if you use the Kenneth Wilson's "Renormalization Group" on QED. Noting Wiki:

"They effectively devised the theory of mass and charge renormalization, in which the infinity in the momentum scale is cut-off by an ultra-large regulator, Λ"

Anyone has other perspective of this or can in a very sentences give some quick insight?
What I'm saying is that the infinity-infinity thing is related to mass and charge renormalization as mentioned by "The Story of Light" But wiki seems to be saying that this also used the regulator thing used in Wilson full Renormalization Group.

Thanks.
waterfall
#43
Feb23-12, 11:47 PM
P: 381
Quote Quote by waterfall View Post
There's this very interesting book called: "The Infinity Puzzle: Quantum Field Theory and the Hunt for an Orderly Universe"

http://www.amazon.com/Infinity-Puzzl...0058226&sr=8-1

It's a laymen book and Renormalization Group was not mentioned however it seems to be related to the older Mass and Charge Renormalization used by Feynman, Schwinger, Tomonaga mentioned up to Chapter 3 which I just finished. Going to wiki.

"In theoretical physics, the renormalization group (RG) refers to a mathematical apparatus that allows systematic investigation of the changes of a physical system as viewed at different distance scales."

My questions are how come the Renormalization Group was attributed to Kenneth Wilson when in fact even Feynman, Schwinger, Tomonaga used it called Mass and Charge Renormalization. And what would happen if you use the Kenneth Wilson's "Renormalization Group" on QED. Noting Wiki:

"They effectively devised the theory of mass and charge renormalization, in which the infinity in the momentum scale is cut-off by an ultra-large regulator, Λ"

Anyone has other perspective of this or can in a very sentences give some quick insight?
What I'm saying is that the infinity-infinity thing is related to mass and charge renormalization as mentioned by "The Story of Light" But wiki seems to be saying that this also used the regulator thing used in Wilson full Renormalization Group.

Thanks.
Reading the archives at PF to see the difference between Feynman Renormalization and modern day Wilson Renormalization, I came across the following by Eugene (I heard this guy has his own theories so I can't differentiate if what he is saying is mainstream or his own.. or rather.. what topics where he created his own ideas versus that of mainstream so I can avoid topics where he made it up.. this is because the archives has a lot of his contributions):

http://www.physicsforums.com/showthread.php?t=183903 msg #2 by Eugene:

"In the end of 1920's Dirac, Pauli, Weiskopff, and Jordan formulated a quantum theory of interactions between electrons and photons in a loose analogy with Maxwell's classical electrodynamics. This early quantum electrodynamics (QED) was very successful in calculations of various scattering processes in lowest orders of the perturbation theory. Unfortunately, all contributions to the S-matrix in higher orders came out infinite.

In late 1940's Tomonaga, Schwinger and Feynman found the way to fix this problem of infinities by renormalization. The renormalization basically adds certain infinite counterterms to the Hamiltonian of the early QED. The form of these counterterms was selected such that the resulting theory satisfied two physical principles. First, the calculated electron's mass should be equal to the measured electron's mass. Second, the calculated interaction energy between two electrons at large distances should be equal to the classical expression e2/r. These two requirements lead to two types of renormalization counterterms in the Hamiltonian - the mass and charge renormalization counterterms."

And http://www.physicsforums.com/showthread.php?p=3628984 in msg #20 by atyy:

"Probably the chief conceptual advance since Feynman's original work (not the book, which is late), is why renormalization works. This is provided by "renormalization group" and "effective field theory ideas". The basic idea is that we don't need theories that are consistent at all energies. They just need to work at low energies. Renormalization flow is the process of seeing what a theory given only its symmetries "looks like" at lower and lower energies. A "renormalizable theory" like QED is one in which the flow converges to a fixed point, about which perturbation theory can be done."

My questions. What happens if you apply the modern Renormalization Group" idea to the Feynman era QED problem? How would it differ then to the mass and charge renormalization techniques?
bhobba
#44
Feb24-12, 01:34 AM
Sci Advisor
PF Gold
P: 2,816
Quote Quote by waterfall View Post
My questions. What happens if you apply the modern Renormalization Group" idea to the Feynman era QED problem? How would it differ then to the mass and charge renormalization techniques?
The difference would be in understanding whats going on - that's all. The early users of re-normalization did not understand what was going on - this led to jokes about looking for infinities under physicists rugs. Now we know the answer - some parameters like the coupling constant depend on the cut-off. In order for the perturbation method to work you have to use a parameter to perturb about that is small. The re-normalization group allows you to see exactly how it depends on the cutoff and and at what energies you can reasonably be confident the theory is valid - that is the EFT approach. It not hard really.

You seem to stuck on the re-normalization group - its not required to do re-normalization - simply to better understand whats going on. It does not change the method one whit which is a way of adding counter-terms to ensure what you get is finite. But the math of that is quite complicated - good luck in finding someone to explain it - you really need a textbook.

Thanks
Bill
waterfall
#45
Feb24-12, 02:15 AM
P: 381
Quote Quote by bhobba View Post
The difference would be in understanding whats going on - that's all. The early users of re-normalization did not understand what was going on - this led to jokes about looking for infinities under physicists rugs. Now we know the answer - some parameters like the coupling constant depend on the cut-off. In order for the perturbation method to work you have to use a parameter to perturb about that is small. The re-normalization group allows you to see exactly how it depends on the cutoff and and at what energies you can reasonably be confident the theory is valid - that is the EFT approach. It not hard really.

You seem to stuck on the re-normalization group - its not required to do re-normalization - simply to better understand whats going on. It does not change the method one whit which is a way of adding counter-terms to ensure what you get is finite. But the math of that is quite complicated - good luck in finding someone to explain it - you really need a textbook.

Thanks
Bill
Yes Renormalization Group is easy to understand as it is related to Effective Field Theory, taking the words of nrqed when he replied to Eugene in msg # 20 of the thread http://www.physicsforums.com/showthr...183903&page=2:

"I may be wrong but it sounds as if you imply that an effective field theory approach implies the assumption of granularity of spacetime (I may have misinterpreted your words, if so I apologize). Saying that a theory is an eft does not imply that. It just implies that at some scale "new physics" arises. The nature of this new physics is quite arbitrary, it could be granularity of spacetime but it could be a new force, inner structure to the particles (including string-like structure) etc etc etc. So in that sense it is quite general."

There is another possibility, related to the foundations of QM. If one take the view of Bohmian Mechanics, there may be no virtual particles taking all paths and the infinity problem resulting from all this amplitude path-integral approach like stuff because in the world of Bohmian Mechanics, particles are always particles. Hope a Bohmian for example Demystifier can confirm whether this is true.

Btw.. Feynman being a genius that he was. How come he didn't arrive at the Renormalization Group idea himself (about coupling constant depending on the cutoff) while mulling about it. Could there be a development in physics later on that popularize the idea which Feynman didn't dare think about? Anyone got a clue?
atyy
#46
Feb24-12, 02:17 AM
Sci Advisor
P: 8,658
Quote Quote by waterfall View Post
My questions. What happens if you apply the modern Renormalization Group" idea to the Feynman era QED problem? How would it differ then to the mass and charge renormalization techniques?
http://web.physics.ucsb.edu/~mark/ms-qft-DRAFT.pdf p 193
The final results, at an energy scale E well below the initial cutoff λ0, are the same as we would predict via renormalized perturbation theory, up to small corrections by powers of E/λ0.
"
The advantage of the Wilson scheme is that it gives a nonperturbative definition of the theory which is applicable even if the theory is not weakly coupled. ..."

"The Wilson scheme also allows us to give physical meaning to nonrenormalizable theories. Given an action for a nonrenormalizable theory, we can regard it as an effective action. ..."
bhobba
#47
Feb24-12, 02:48 AM
Sci Advisor
PF Gold
P: 2,816
Quote Quote by waterfall View Post
Btw.. Feynman being a genius that he was. How come he didn't arrive at the Renormalization Group idea himself (about coupling constant depending on the cutoff) while mulling about it. Could there be a development in physics later on that popularize the idea which Feynman didn't dare think about? Anyone got a clue?
Maybe because he was busy doing other stuff such as superconductivity and the theory of Partons.

Thanks
Bill
strangerep
#48
Feb24-12, 07:55 PM
Sci Advisor
P: 1,927
Quote Quote by waterfall View Post
Prof. Neumaier mentioned how [supposed effects of virtual particles] are caused directly by the fields and not by any virtual particles which he refers to as Multivariate Integrals. Maybe he can give details when he comes here later.
He already talked a lot about this sort of thing. See these threads:

http://www.physicsforums.com/showthread.php?t=75307

http://www.physicsforums.com/showthread.php?t=460685


Register to reply

Related Discussions
Renormalization group and universality Quantum Physics 14
Does Renormalization group tell you if a theory is Renormalizable or not ? Beyond the Standard Model 3
The idea behind renormalization group. Quantum Physics 3
Renormalization group and cut-off Quantum Physics 1
Group Theory For Dummies Linear & Abstract Algebra 210