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JK423
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Well i am waiting for Bill_K to respond to these counter-arguments against his views, but he seems to ignore me.Naty1 said:jk423...good point you beat me to posting...
Well i am waiting for Bill_K to respond to these counter-arguments against his views, but he seems to ignore me.Naty1 said:jk423...good point you beat me to posting...
Didn't you see the tears in his initial post in this thread? There's a reason for that...JK423 said:Well i am waiting for Bill_K to respond to these counter-arguments against his views, but he seems to ignore me.
Great summary!strangerep said:Arnold Neumaier went to great lengths a couple of years ago to explain it all, but such threads inevitably recur. For a good (imho) explanation of the distinction between the concepts of "stable, unstable, resonance, virtual, etc" in this context, you'll find it hard to do better than this:
http://arnold-neumaier.at/physfaq/topics/unstable.html
It is more concise and far superior to most of the "virtual particle" garbage spewing endlessly onto this forum.
Mentioning DIS gives me the chance to explain one Common misconception. DIS is often quoted as application of perturbation theory. This is partially wrong, b/c perturbation theory is limited to the QED part and to the Q2-behavior of the structure functions F(x,Q2), whereas the x-dependency is entirely non-perturbative for finite energies and cannot be calculated using perturbation theory. The x-dependency is not calculated, but perturbative treatment of other effects is used to extract it from the data. So within DIS the understanding for the reason of a specific x-behavior is zero.Haelfix said:If you insist that they do, and give them one anyway, for instance as you ramp up the energies of the collider during deep inelastic scattering experiments then I assure you the distinction between real and virtual really does become a matter of convention .
This is an excellent, well-reasoned exposition of the matter. I agree with about 96 percent of what he says.Arnold Neumaier went to great lengths a couple of years ago to explain it all, but such threads inevitably recur. For a good (imho) explanation of the distinction between the concepts of "stable, unstable, resonance, virtual, etc" in this context, you'll find it hard to do better than this:
http://arnold-neumaier.at/ph.../unstable.html
Unfortunately although many a discussion starts with a well-reasoned exposition, the followup often deteriorates into emotional, unprofessional, unreasoning.Didn't you see the tears in his initial post in this thread? There's a reason for that...
This is because the creation and annihilation operators were taken to be the Fourier transforms of a free field. This choice is made for its simplicity. One could define things otherwise.A stable particle can be created and annihilated, as there are associated creation and annihilation operators that add or remove particles to the state. According to the QFT formalism, these particles must be on-shell...]multiparticle states are always composed of on-shell particles only...States involving virtual particles cannot be created for lack of corresponding creation operators in the theory.
This may say more about lattice gauge theory than the particles!In diagram-free approaches to QFT such as lattice gauge theory, it is impossible to make sense of the notion of a virtual particle.
Renormalization is an accepted part of field theory, unpleasant to deal with, but not an argument against virtual particles. Bringing it up in this context is a red herring.Indeed, a single Feynman diagram usually gives an infinite (and hence physically meaningless) contribution to the scattering cross section.
This is IMO the largest error in Neumaier's reasoning, and one I've heard expressed many times. But Feynman diagrams represent amplitudes, and QM tells us that amplitudes must be summed over. Always we sum/integrate over all possible histories. However this does not cause a particle, in going from point A to point B, to somehow lose its reality, and does not justify regarding its intermediate path as an "artificial construct." Yes, even real things must sometimes be summed over.The finite, renormalized values of the cross section are obtained only by summing infinitely many such diagrams. This shows that a Feynman diagram represents just some term in a perturbation calculation, and not a process happening in space-time. Therefore one cannot assign physical meaning to a single diagram but at best to a collection of infinitely many diagrams.
Certainly true, diagram methods are applied in other contexts. And it's also true that this has no relevance to the subject of virtual particles!Feynman-type diagrams arise in any perturbative treatment of statistical multiparticle properties, even classically, as any textbook of statistical mechanics witnesses. But in the literature, one can find no trace of a suggestion that classical multiparticle physics is sensibly interpreted in terms of virtual particles.
Haelfix said:I think the problem I have with that entry is that it leads to statements like this written by JK423.
"Real particles, no matter how SMALL a lifetime they have, you can in principle interact with them because they have a quantum state"
This is deeply wrong for a number of reasons. The first is that mathematically this is fantasy. Most Interacting particles in 4d do not have well defined quantum states, especially ones that are not well separated, that haven't undergone clustering and that have arbitrarily small lifetimes. So pathological example.. Low energy quarks do NOT have well defined particle number operators. This is completely independent of perturbation theory and is indeed a nonperturbative statement. If you insist that they do, and give them one anyway, for instance as you ramp up the energies of the collider during deep inelastic scattering experiments then I assure you the distinction between real and virtual really does become a matter of convention (in this case the convention of energetics to contrast to the usual convention of time explained in the other thread).
The second is you have to define what you mean by 'interaction'. You can rewrite all of the contributions of virtual particles in certain specific theories (like QED) as 'dressed' particle interactions. This 'dressing' absolutely, quantitatively makes a separate and very real contribution to physical processes like scattering cross sections, decay times and so forth. So again, you simply can't be consistent and argue that they have nothing to do with interactions at all.
Haelfix said:The second is you have to define what you mean by 'interaction'.
Observable particles. ... At energies larger than the real part of the mass, the imaginary part determines its decay rate and lifetime; at smaller energies, the unstable particle cannot form for lack of energy, but the existence of the pole is revealed by a ….resonance in certain cross sections. From its position and width, one can estimate the mass and the lifetime of such a particle before it has ever been observed. Indeed, many particles …are only resonances.
Many SAs and Mentors are sick to death of the endless "virtual particle" threads and the endless misunderstandings therein.
Naty1 said:Can someone help me with this from the Neumaier paper:
[namely: ''but the existence of the pole is revealed by a ….resonance in certain cross sections.'']
Is the boldface statement an experimentally observed effect or a calculated, theoretical one?
edit: Looks like it IS an observation...
Bill_K said:This is because the creation and annihilation operators were taken to be the Fourier transforms of a free field. This choice is made for its simplicity. One could define things otherwise.
JK423 said:A. Neumaier, would you agree with my understanding of the distinction between real/virtual particles summarized in this small post
https://www.physicsforums.com/showpost.php?p=4286488&postcount=32 ?
Or have i understood something wrong?
However, such particles are regularly created in experiments, in order to understand the nature of the heavier types of quark which compose the heavier mesons...While no meson is stable, those of lower mass are nonetheless more stable than the most massive mesons, and are easier to observe and study in particle accelerators or in cosmic ray experiments.
''Internal lines in Feynman diagrams of perturbation theory: you cannot in principle interact with them because they have no quantum state to interact with! In this sense, they do not exist.
Real particles, no matter how SMALL a lifetime they have: you can in principle interact with them because they have a quantum state!''
However, you open Peskin & Schroeder, and from the first page they say virtual states pop out from nowhere, obeying energy-time uncertainty relations etc.
Naty1 said:Consider the Z meson. It has a mass of 91 GeV and a lifetime of 3 x 10-25 sec. Implying, at velocity c it can travel at most a tenth of a fermi before it decays, less than the diameter of a proton. And thanks to its short lifetime the Z meson has a width of 2.5 GeV. GEV! It is never on the mass shell. It always appears as an "internal line" in some Feynman diagram.
So what do you say - is the Z meson a real particle? Or is it merely an "artifact of perturbation theory".
JK423 said:A. Neumaier, i'd like to ask you one more thing, taking advantage of the fact that you visited the forum :)
You are a researcher and professor in Vienna, and you must have talked to many other experts in the field. If the argument
virtual particles don't have a state ==> they don't exist
is correct, why is there such a confusion among the experts? This argument is so simple(!), that i cannot believe that experts do not understand it! Every QFT textbook, should emphasize this, so that generations of students don't get confused. However, you open Peskin & Schroeder, and from the first page they say virtual states pop out from nowhere, obeying energy-time uncertainty relations etc.
How can you explain this phenomenon?
They are trying to put some intuition, and as a result a huge percent of graduate students (and even researchers!) have misunderstood this concept..A. Neumaier said:I tried to explain this towards the end of
http://arnold-neumaier.at/physfaq/topics/unstable.html
Most people try to give some intuition to the abstract matter, which virtual particles do very well, and accept as its cost the resulting confusion. Those who care about clear concepts are more careful with their language. For example, Weinberg's QFT book avoids the whole concept of virtual particles and still covers everything of importance in QFT.
Also, if I remember correctly, Peskin and Schroeder in their book never claim that virtual states pop out from nowhere, obeying energy-time uncertainty relations etc.. If you want to uphold your claim above, you'd cite page and line numbers.
Naty1, I think we do not agree on all aspects regarding virtual particles, and especially not on all aspects regarding their interpretation. But I think we all DO agree that statements like "virtual particles violate energy conservation" are WRONG. This is due to the fact that it's not a wrong interpretation, but that is in contradiction to exact math.Naty1 said:yes, and that such activity violates conservation of energy! Even the quote I posted from Lisa Randall [Harvard] says that...and I have repeatedly read such things...and repeatedly not understood whether such can be 'correct'...
A lecturer in a QED course I attended 15yrs ago tended to say similar things. Sad.JK423 said:[...](JK423's emboldening).Peskin & Schroeder said:Even when there is not enough energy for pair creation, multiparticle states appear, for example, as intermediate states in second-order perturbation theory. We can think of such states as existing only for a very short time, according to the uncertainty principle ΔΕΔt=h. As we go to higher orders in perturbation theory, arbitrarily many such "virtual" particles can be created.
Peskin & Schroeder said:QFT provides a natural way to handle not only multiparticle states, but also transitions between states of different particle number. It solves the causality problem by introducing antiparticles, then goes on to explain the relation between spin and statistics. But most important, it provides the tools necessary to calculate innumerable scattering cross sections, particle lifetimes, and other observable quantities. The experimental confirmation of these predictions, often to an unprecedented level of accuracy, is our real reason for studying QFT.
... umm,... you'd better search back through several years of previous threads before opening that can of worms again.Jim Kata said:Maybe a better question is what is a particle?
JK423 said:Of course, have a look at Peskin & Schroeder, page 13, 3rd paragraph,
Even when there is not enough energy for pair creation, multiparticle states appear, for example, as intermediate states in second-order perturbation theory. We can think of such states as existing only for a very short time, according to the uncertainty principle ΔΕΔt=h. As we go to higher orders in perturbation theory, arbitrarily many such "virtual" particles can be created.
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Even when there is not enough energy for pair creation, multiparticle states appear, for example, as intermediate states in second-order perturbation theory. We can think of such states as existing only for a very short time, according to the uncertainty principle ΔΕΔt=h. As we go to higher orders in perturbation theory, arbitrarily many such "virtual" particles can be created...
Naty1 said:I've tried to connect to Arnold Neumaier's FAQ here...
http://arnold-neumaier.at/physfaq/topics/unstable.htmlbut keeping getting a message:
Any suggestions on how to access this information will be appreciated!