- **Physics Forum**
(*http://www.molecularstation.com/forum/physics-forum/*)

- - **A version of QED without ultraviolet divergences**
(*http://www.molecularstation.com/forum/physics-forum/34276-version-qed-without-ultraviolet-divergences.html*)

A version of QED without ultraviolet divergencesPhysicists tried to get rid of ultraviolet divergences in quantum field theories, in particular in QED, for a long time. Without much success. The divergences are still there. I think I know how to eliminate them from the theory. Wanted to know your opinion. All this started in the late 1920's when Heisenberg and Pauli invented QFT. The Hamiltonian H of QED was written and it was soon discovered that scattering amplitudes are described well in the 2nd order of the perturbation theory and turn out to be infinite in the 4th and higher orders. Then came Tomonaga, Schwinger, and Feynman who said (that's not what they actually said, but that's a rather accurate translation of their words to a different language) that Hamiltonian H is not correct and one must add certain renormalization corrections R to H in each order of the perturbation theory. The corrections were chosen so as to make the calculated mass of the electron equal to the experimental mass and to fit the low-energy electron-electron scattering to experimental data (charge renormalization). The corrections (or counterterms) R appeared to be infinite, but the S-matrix calculated with the new Hamiltonian H+R turned out to be finite and perfectly agreed with experiment. In calculations of the S-matrix the different infinities happily cancelled each other. This is what we have in the renormalized QFT today: Infinite Hamiltonian and finite and accurate S-matrix. Theoretically this is a disaster, but from the practical point of view there is not much trouble, because in high energy physics we care only about the energies of bound states (poles of the S-matrix) and scattering cross-sections (squares of the matrix elements of the S-matrix). We will get in trouble in the future when the resolution of experiments will allow us to measure the time evolution of interacting particles during their collisions. The S-matrix is not enough to describe such time evolution. We need the Hamiltonian, but the Hamiltonian is bogus. My idea is that the Hamiltonian with counterterms H+R is still wrong. We need to find a better Hamiltonian of QED, let me call it H'. Apparently, this new Hamiltonian must satisfy at least two conditions: (1) it must be finite (2) the S-matrix calculated with H' must be exactly the same as the accurate S-matrix calculated with H+R. The condition (2) is satisfied by unitary transforms of H+R. I.e., if we choose H' = U(H+R)U^{-1} where U is a unitary operator satisfying some mild restrictions, we can guarantee that H' yields exactly the same S-matrix as H+R. Now, what about the finiteness? It appears that the class of transformations U preserving the S-matrix is so large that we can find transformations U which make H' finite. These operators U are infinite (more exactly, their phase is infinite), but we couldn't care less, because U do not have any physical meaning. If all this is done with some mathematical rigor, we obtain a FINITE Hamiltonian H' describing interactions of charged particles and yielding a FINITE S-matrix. Now we can forget about preliminary bad Hamiltonians H and H+R, and about the infinite transformation operator U. We have a complete theory with the Hamiltonian H' which allows us to calculate time evolution, scattering, bound states, etc. according to usual rules of quantum mechanics without regularization, renormalization, and other tricks. The details of the derivation of the true QED Hamiltonian H' are given in my paper E. V. Stefanovich, `` Quantum Field Theory without Infinities'' Ann. Phys. (NY) 292 (2001), 139-156. As a sanity check, in the 2nd perturbation order in the v^2/c^2 approximation, I obtained the well-known Breit Hamiltonian for the electron-electron interaction. 4-th and higher order radiative corrections can be also readily obtained. Eugene. |

A version of QED without ultraviolet divergences"Eugene" < [Only registered and activated users can see links. Click Here To Register...]> wrote in messagenews: [Only registered and activated users can see links. Click Here To Register...]...| Physicists tried to get rid of ultraviolet divergences in quantum | field theories, in particular in QED, for a long time. | Without much success. The divergences are still there. | I think I know how to eliminate them from the theory. | Wanted to know your opinion. | | All this started in the late 1920's when Heisenberg and Pauli | invented QFT. The Hamiltonian H of QED was written and it was | soon discovered that scattering amplitudes are described well in | the 2nd order of the perturbation theory and turn out to be | infinite in the 4th and higher orders. | | Then came Tomonaga, Schwinger, and Feynman who said | (that's not what they actually said, but that's a rather | accurate translation of their words to a different language) | that Hamiltonian H is not correct and one must add certain | renormalization corrections R to H in each order of the | perturbation theory. The corrections were chosen so as to | make the calculated mass of the electron equal to the | experimental mass and to fit the low-energy electron-electron | scattering to experimental data (charge renormalization). | The corrections (or counterterms) R appeared to be infinite, | but the S-matrix calculated with the new Hamiltonian H+R | turned out to be finite and perfectly agreed with experiment. | In calculations of the S-matrix the different infinities | happily cancelled each other. | | This is what we have in the renormalized QFT today: | Infinite Hamiltonian and finite and accurate S-matrix. | Theoretically this is a disaster, but from the practical | point of view there is not much trouble, because in | high energy physics we care only about the energies of | bound states (poles of the S-matrix) and scattering cross-sections | (squares of the matrix elements of the S-matrix). We will get | in trouble in the future when the resolution of experiments will | allow us to measure the time evolution of interacting particles | during their collisions. The S-matrix is not enough to describe | such time evolution. We need the Hamiltonian, but the Hamiltonian | is bogus. | | My idea is that the Hamiltonian with counterterms H+R is | still wrong. We need to find a better Hamiltonian of QED, | let me call it H'. Apparently, this new Hamiltonian must satisfy | at least two conditions: | | (1) it must be finite | (2) the S-matrix calculated with H' must be exactly the same as | the accurate S-matrix calculated with H+R. | | The condition (2) is satisfied by unitary transforms of H+R. I.e., | if we choose | | H' = U(H+R)U^{-1} | | where U is a unitary operator satisfying some mild restrictions, we | can guarantee that H' yields exactly the same S-matrix as H+R. | Now, what about the finiteness? It appears that the class of | transformations U preserving the S-matrix is so large that we can | find transformations U which make H' finite. | These operators U are infinite (more | exactly, their phase is infinite), but we couldn't care less, because | U do not have any physical meaning. If all this is done with some | mathematical rigor, we obtain a FINITE Hamiltonian H' describing | interactions of charged particles and yielding a FINITE S-matrix. | Now we can forget about preliminary bad Hamiltonians H and H+R, and | about the infinite transformation operator U. We have a complete | theory with the Hamiltonian H' which allows us to calculate time | evolution, scattering, | bound states, etc. according to usual rules of quantum mechanics | without regularization, renormalization, and other tricks. | | The details of the derivation of the true QED Hamiltonian H' are given | in my paper | | E. V. Stefanovich, `` Quantum Field Theory without Infinities'' | Ann. Phys. (NY) 292 (2001), 139-156. Is this the same as the one at [Only registered and activated users can see links. Click Here To Register...] ?| As a sanity check, in the 2nd perturbation order in the v^2/c^2 | approximation, I obtained the well-known Breit Hamiltonian for | the electron-electron interaction. 4-th and higher order radiative | corrections can be also readily obtained. How does your idea compare with this? [Only registered and activated users can see links. Click Here To Register...]FrediFizzx |

A version of QED without ultraviolet divergencesFrediFizzx wrote: The published paper is on [Only registered and activated users can see links. Click Here To Register...]The link [Only registered and activated users can see links. Click Here To Register...]is to yet unpublished paper in which I apply this formalism to a simple model theory where calculations can be performed without much pain. Thank you for the link. My idea has nothing to do with this approach. Does this web-site has a continuation? At the end of the page I found "And how does renormalization relate to all of this? That's what's to follow..." Kind of disappointing. Eugene. |

A version of QED without ultraviolet divergencesEugene: Why is a cutoff not a _physical_ requirement? QED is not a theory unto itself. It's a low energy effective theory of E&M. [...] It's only theoretically a disaster if you haven't considered the validity of the assumptions that go into proving the ``disaster''. The ``no interaction'' theorem's (e.g., haag's theorem) depend upon several assumptions that aren't neccessarily valid assumptions. One is the unitarity of the S-matrix. Another is that the hamiltonian can be split into terms containing free fields and interacting fields. Another is whether or not the infinities are physically meaningful. In QED, for example, the divergences cancel order by order and the result of calculations do not depend upon the cut off procedure, only that a cut off is imposed. If the cut off has a physical basis, then insisting the theory not depend upon one would be ignoring the physics. How do you know that? By your hypothesis, we have not made those measurements. Don't you think that sort of puts the formalism ahead of the data? What if unitarity is the _wrong_ assumption? |

A version of QED without ultraviolet divergencesBilge wrote: Where is this _general_ theory whose low energy approximation is QED? Strings? quantum gravity? I do not consider QED as "effective theory" of something non-existent yet. I present you a finite, mathematically sound theory of interacting particles today. We can now calculate all physical properties (time evolution, scattering, bound states, etc.) using normal rules of quantum mechanics (no renormalization tricks) without ever encountering divergent integrals. I didn't write the Hamiltonian H' in all perturbation orders (I stopped at the 2nd order in QED and at the 4th order in a simple model theory), but there is a proof that such a construction is possible, and there are rules how to do it. The disaster is that the Hamiltonian is infinite and we cannot study time evolution. I presume there is real time-dependent dynamics behing scattering events. I hope that some day we will be able to measure that (For example, there is a rapid progress in time-dependent dynamics of wave packets made of Rydberg atomic states. Some day, these wave packets will include low-energy states affected by the Lamb shifts. Existing theory cannot predict their time evolution.) Unitarity is needed to preserve the good S-matrix which we got from the usual renormalized QED. Eugene. |

A version of QED without ultraviolet divergences"Eugene" < [Only registered and activated users can see links. Click Here To Register...]> wrote in messagenews: [Only registered and activated users can see links. Click Here To Register...]...Electroweak theory. And your position on the breaking of the elctroweak symmetry about 246 GV is? What trick? We know that it breaks down at energies around 246 GV when the electoweak theory dominates. insisting This disaster is that we now know that QED is but an approximation to a more fundamental theory. Bill |

A version of QED without ultraviolet divergencesEugene: The standard model. The electroweak sector is SU(2)_L x U(1)_Y, where the SU(2)_L is given by a weak isotriplet of vector currents, W_1, W_2, W_3, and U(1)_Y is a hypercharge singlet, B. QED is the U(1)_em symmetry obtained from the generator Q = T_3 + Y/2, which remains invariant in the electroweak ground state. The photon and Z are linear combinations of W_3 and B. QCD is an asymptotically free theory, so when you combine the electroweak and the qcd lagrangian, you have a field theory which is believed (but not proven) to be asymptotically free. The standard model has been around for about 20-25 years. Actually, you haven't presented anything. You've just made some rather general statements to which I've attempted respond in general terms to the best I have been able to figure out what you are saying. I think I missed something, since you appear to be claiming here, that your model, like qed, is also perturbative. Perturbatively, there is no problem with qed. I have a reference I was going to give that discusses this, but I just discovered some and perhaps all of both volumes are available on line from the institute for advanced study. The lecture notes from Lecture 3 by david gross, esp. ppg 2-3,5 give the details: [Only registered and activated users can see links. Click Here To Register...]In short, the ``non-existence'' you are concerned about becomes an issue for energies in the regime of 10^250 MeV, which is not an issue for the universe qed inhabits. You keep saying ``time evolution'', but I don't see the problem. What do you expect to measure other than scattering amplitudes? Are you expecting to observe something bizarre like single particle states off mass shell (which would seem to be unphysical)? The observables in the experiment being what? It almost sounds like you are trying to cheat quantum mechanics. That doesn't mean it's a good _physical_ assumption. It probably is, at least for this purpose, but it might not be and insisting that it must be just to preserve a feature in the formalism, seems to invert the priority a bit. The only justification for anything is that it is needed to explain real data. |

A version of QED without ultraviolet divergencesBill Hobba wrote: Electroweak theory has the same divergences as QED. It just adds other particles (W, neutrinos, etc.) and interactions into the mix. QED is not a low-energy approximation of the electroweak theory. Sorry, I have no position on this one. My humble goal is to correctly describe the time evolution of two interacting electrons at energies below, say, 10 MeV. Eugene. |

A version of QED without ultraviolet divergences"Eugene" < [Only registered and activated users can see links. Click Here To Register...]> wrote in messagenews: [Only registered and activated users can see links. Click Here To Register...]...Bullcrap. The electoweak theory is the theory that is supposed to operate at energies above which the symmetry between the weak and EM interactions is no longer broken and are part of a single theory that is not the same as QED ie it has SU(2) x U(1) symmetry not the U(1) symmetry of QED. Obviously QED is a low energy approximation to this theory. Also I never claimed it did not also require renormalization - what I claimed is that your proposal is for a problem that no longer exists. Of course that does not prove your proposal wrong (I think they are a crock of crap but I will leave that for experts to look into before passing a final opinion). Now if your proposing a way to resolve the divergences of the electroweak theory without renormalization that would be another matter - I still think your reasoning is erroneous - but at least it would be addressing a problem that actually exists. GV In that case since you are imposing a cutoff there is no problems with divergences. Bill |

A version of QED without ultraviolet divergencesBilge wrote: Yes, QED is a part of electroweak theory. Neither QCD nor electroweak solved the divergence problem. They have the same (or similar) divergence problems as QED. Actually, my approach to QFT is rather general and applicable to QCD and electroweak as well. I thought, when people say that QED is an "effective field theory" they presume that there is some "grand theory" in which the divergence problems are fixed, and which reveals new features somewhere at the "Planck scale". QCD and electroweak are not these "grand theories". Though, here, I admit, I am leaving my area of expertise. Please, correct me if you know better. It is all written in my paper in Ann. Phys. I don't think it is practical to write here bunch of formulas. The idea of my approach is very simple: One just need to make a unitary transformation of the Hamiltonian and infinities are gone! What do you mean there is no problem? You still need to perform renormalization. Renormalization means that you introduce infinite counterterms in the Hamiltonian. This is the problem. I expect to measure time dependence of particle observables (position, momenta, energies, spin projections). Currently, in scattering experiments these observables change abruptly during the collision. This is because the collisions are too short for modern experimental techniques. I believe that with much improved time resolution we will see what exactly happens in the short time interval of the collision. Current QFT with its emphasis on the S-matrix cannot predict this time dynamics. My modified version of QFT can. Eugene. |

All times are GMT. The time now is 09:40 AM. |

Powered by vBulletin® Version 3.8.4

Copyright ©2000 - 2015, Jelsoft Enterprises Ltd.

Copyright 2005 - 2012 Molecular Station | All Rights Reserved