Source of Virtual Particles in Space?

[A. Neumaier]

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?

The formalism of quantum field theory does not provide any way to assign a lifetime to an internal line of a Feynman diagram; all lifetimes attached to them are the result of wishful thinking, not the results of defendable quantum computations.

So a corrected - and then correct - form of your formulation would be:

''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!''

 

[JK423]

Agreed, thank you very much!

 

[Naty1]

Bill_K asks, post #31:"This definition { from Rovelli, what's 'observable'} leaves out a lot of particles! Many particles in the Standard Model have lifetimes too short to leave a visible track.

"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".

W mesons, top quarks and Higgs bosons have equally short lifetimes. If you consider these particles somehow not real, you're drawing an artificial distinction between particles that are otherwise closely related..."It 'never being on the mass shell', always an an 'internal line' in Feynman diagram, suggests it does not have a quantum state, is not a 'real' particle'...but I do not know if either of those quoted statements is correct.

"It has a mass of 91 GeV and a lifetime of 3 x 10-25 sec." means in principle it does have a quantum state, and is in principle at least 'detectable'. Are these observed or calculated?

However Wikipedia suggests mesons are routinely observed:

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.

http://en.wikipedia.org/wiki/Meson

But for me, this is not necessarily conclusive one way or another...I don't know what interpretations are assumed in such statements. For example, that the Casimir effect proves the existence of virtual particles...and that they have observable effects..has always seemed to me to be a bit of a stretch.

 

[JK423]

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?

 

[Naty1]

oh, I just saw post #51:

''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!''

Nice!...now let's wait to see who tries to refute that...Sounds like a 'go to' statement!

 

[Naty1]

However, you open Peskin & Schroeder, and from the first page they say virtual states pop out from nowhere, obeying energy-time uncertainty relations etc.

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. Neumaier]

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".

All this says that the Z-meson is a real, unstable particle (hence has a complex mass shell - it is not off-shell in the same sense as this term is used for virtual particles) and _not_ a virtual particle.
Thus of course it is not an artifact of perturbation theory.

 

[A. Neumaier]

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?

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. It is a trade-off, where many opt for the nice visualization. Many people also use quite different words for journal publications and for essays addressed to laypeople. 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.

 

[Jim Kata]

Maybe a better question is what is a particle? I guess just the inputs and outputs of a scattering process. What is a virtual particle? I just think of them as the internal lines in a Feynman diagram and real particles are the external lines of a Feynman diagram, and don't put too much more thought into it than that.

 

[JK423]

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.

They are trying to put some intuition, and as a result a huge percent of graduate students (and even researchers!) have misunderstood this concept..
Ofcourse, 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.

 

[tom.stoer]

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'...

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.

We can discuss about the interpretation or reality of "-1 car" in the calculation "1 car = 2 cars - 1 car", and perhaps we don't agree. But we DO agree that this equation does NOT violate car-conservation.

So again, I am sorry to say that, statements like non-conservation of energy due to virtual particles are rubbish.

 

[strangerep]

[...]

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.

(JK423's emboldening).

A lecturer in a QED course I attended 15yrs ago tended to say similar things. Sad.

Probably the final paragraph in that section (bottom of p14 and over to 15) is all the motivation that is really needed:

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.

Thus, P&S is perhaps best regarded as a learn-to-calculate book.

 

[strangerep]

Maybe a better question is what is a particle?

... umm,... you'd better search back through several years of previous threads before opening that can of worms again.

Alternatively, search the contents page of Arnold's FAQ for the word "particle". There's more than enough reading there to occupy several rainy days.

 

[A. Neumaier]

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.

They are careful to say ''We can think of such states as'', implying that it is just a convenient visualization, not a physical fact.

 

[Naty1]

I've tried to connect to Arnold Neumaier's FAQ here...

http://arnold-neumaier.at/physfaq/topics/unstable.html


but keeping getting a message:

Forbidden

You don't have permission to access /~neum/physfaq/physics-faq.html on this server.

Any suggestions on how to access this information will be appreciated!

 

[Naty1]

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...

I'm not familiar with the creation of 'multiparticle states'...can anyone recommend
an online source where I can learn more about this?
Thanks.

 

[A. Neumaier]

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!

There appears to be a problem with our web server. Just try again at a later time. (Web support is already in the weekend; so this might get fixed only on Monday.)

 

[xlsdx]

is this not an expected outcome from the probabilistic nature of quantum mechanics? if any particle's wave-function gives it the chance to exist anywhere in the universe, multiplied by the number of particles in the universe, surely it fits that some portion of the particles in the universe would spontaneously appear/disappear from random locations?