Virtual Particles: Creation & Destruction in 10-43 Seconds?

[PhanthomJay]

If virtual particles pop into and out of existence in less than the blink of an eye,
1. How much less than a blink...less than 10^-43 seconds?
2. Where is this happening now as I speak..in front of my eyes, in my next door neighbor's house, or in places far far away..?
3. If some of these particles created matter, why is no matter being created now...or is it?
4. Is the creation and destruction of virtual particles the long sought perpetual motion machine?

Thanks.

 

[Tanelorn]

Just because an electron positron pair can spontaneously appear or similar doesn't necessarily mean that all the matter in all the galaxies in the observable universe can appear in a similar way, I think that would be stretch!

 

[Vanadium 50]

If virtual particles pop into and out fo existence in less than the blink of an eye

They don't.

Virtual particles are not real.

Simple, no?

 

[PhanthomJay]

No. I guess i'll have to read up on them again.

 

[Vanadium 50]

Unfortunately popularizations in particular seem to misunderstand that the reason we distinguish between "real" and "virtual" particles is that virtual particles are not real.

 

[PhanthomJay]

Do all virtual wave-particle pairs, which give rise to force fields, originate within the Planck length of the curled up dimensions, spawned by the Gravitional Field? Or can virtual particles originate outside of our universe and enter ours, like what the postulated Graviton might be able to do?

Maybe this question makes no sense...I'm fishing for answers...

 

[kexue]

Unfortunately popularizations in particular seem to misunderstand that the reason we distinguish between "real" and "virtual" particles is that virtual particles are not real.

PhanthomJay, I took the pain to write emails to some of the most eminent figures in the field of quantum field theory, especially after I was frustrated to get such answers on this forum as the one you got from Vanadium 50. This question is far from being simple. It goes deep down to the heart of what quantum field theory is. I received a wide range of answers, few of them I shared in https://www.physicsforums.com/showthread.php?t=75307".

For a start read Frank Wilzcek 11-page http://scipp.ucsc.edu/~dine/ph217/wilczek.pdf" on quantum field theory, especially page 3, where he states that the association of forces with particles is a general feature of quantum field theory.

 

[PhanthomJay]

kexue

Thanks for the response, i'll check out your links.

I originally posed this question on virtual particles in the Cosmology forum, after reading Hawking's latest book where he talks about the Universe originating as a quantum event, which is likey true, with the Gravitational Field being responsible for the creation of our Universe as we perceive it (I think). Anyway, it aroused my curiosity about virtual particles, gravity, sum of histories, and the Heisenberg/Quantum Uncertainty Principles. Thanks again.

 

[Born2bwire]

kexue

Thanks for the response, i'll check out your links.

I originally posed this question on virtual particles in the Cosmology forum, after reading Hawking's latest book where he talks about the Universe originating as a quantum event, which is likey true, with the Gravitational Field being responsible for the creation of our Universe as we perceive it (I think). Anyway, it aroused my curiosity about virtual particles, gravity, sum of histories, and the Heisenberg/Quantum Uncertainty Principles. Thanks again.

Well, this has already been discussed ad nauseum but it comes down to what Vanadium states. Virtual particles aren't real, they are a mathematical construct that exists in some theories of quantum mechanics. One thing to be careful of in quantum mechanics is that there are often many ways of describing a phenomenon. Just because one works does not mean that it is physically correct. The quantum vacuum's zero-point energy is one such example that comes to mind as many people take it for granted that, say, the Casimir effect is proof of existence. However, some phycisists, like Jaffe, are quick to point out that one can derive the Casimir effect without any reference to the vacuum. Virtual particles fall under this category since they are a property found in perturbation theory. But we can solve many problems without perturbation theory or the use of virtual particles.

 

[Dynamic Sauce]

PhanthomJay, I took the pain to write emails to some of the most eminent figures in the field of quantum field theory, especially after I was frustrated to get such answers on this forum as the one you got from Vanadium 50. This question is far from being simple. It goes deep down to the heart of what quantum field theory is. I received a wide range of answers, few of them I shared in https://www.physicsforums.com/showthread.php?t=75307".

For a start read Frank Wilzcek 11-page http://scipp.ucsc.edu/~dine/ph217/wilczek.pdf" on quantum field theory, especially page 3, where he states that the association of forces with particles is a general feature of quantum field theory.

Kexue,are you saying they are real?

 

[weejee]

To me, claiming that virtual particles actually pop out from the vacuum and disappear after a short time sounds like saying that a resonant bond is actually oscillating in real time in the chemical bonding theory.

 

[kexue]

Kexue,are you saying they are real?

What you want call real in quantum physics or not, is, I think, a rather subtle question. If you only call things real that you can directly observe, then 'virtual' particles are not real. That is why people came up with the name 'virtual'. Kind of obvious.

According to quantum mechanics, no objects are "real" in the same sense as in classical physics; only probabilities of individual outcomes and the formulae to calculate them are "real" and predictable. No quantity characterizing a quantum physical system exists prior to the measurement. However, if you consider correct formulae for observable probabilities "real", then the virtual particles are "real" as well. Represented as internal lines (propagators) of Feynman diagrams, they are essential building blocks of the formulae for the probability amplitude.

The only difference in "reality" between virtual and asymptotic particles is that the asymptotic particles may "exist" eternally while the life of virtual particles is, by definition, transient. Because the virtual particles only live temporarily, their energy and momentum don't have to satisfy the usual E^2-p^2.c^2=m^2.c^4. In a real setup, no particle exists eternally, so every particle in the real world is, to some extent, virtual.

 

[weejee]

In a real setup, no particle exists eternally, so every particle in the real world is, to some extent, virtual.

You are again mixing up concepts here. If a particle decays, it is making a real transition. Virtual particles come in only when we talk about higher order processes in the perturbation theory.

People's negative comments on your opinion on virtual particles are not about how you define what 'being real' means in terms of ordinary language. Such matter is not even in the realm of physics, and it is why various physicists gave you seemingly different answers on your question. Still, they are all talking about the same physics, while verbalizing differently. After all, exact formulations of physics are expressed only through equations.

Trying to understand the physics in terms of ordinary language is always worthwhile, as we all are far more comfortable with everyday words than mathematics. One important caveat is that since physics in the form of ordinary language (a.k.a. physical picture) is not so precise, it is unable to tell exactly when it is valid and when it is not. Therefore, we should always be ready to assess on whether our 'physical picture' is really sound or not, and this is done only, I repeat, by EQUATIONS.

If you meet these standards, I think nothing is in principle wrong about insisting that virtual particles are real. It is just your style of using ordinary language, although some people might consider it a little bit awkward. However, when you say things based mainly on the notion of 'realness of virtual particles' and don't care enough about what the exact formulation of the theory tells us, you inevitably say blatantly wrong things at some point.

Please... study the theory in its exact form more carefully and hold back your urge to play just with pictures or words.

I'm sure that you will be able to understand why people say that virtual particles are artifacts of the perturbation theory, and whether you call them real objects or artifacts has nothing to do with the essence of the physics.

 

[Vanadium 50]

Do all virtual wave-particle pairs, which give rise to force fields, originate within the Planck length of the curled up dimensions, spawned by the Gravitional Field?

Virtual particles are not real.

Asking where they come from is like asking where the tooth fairy comes from. They are a clever and useful mathematical trick (like image charges, if you've ever used method of images), but they are not real. It is unfortunate that the authors of so many popularizations treat them as if they are real, which leaves it to PF to clean up the mess they made.

Kexue's viewpoint is not the standard one, and I fear it is muddying the waters rather than clarifying everything. As he points out, there's already a thread on his views, so let's not derail this any further.

 

[Hawkwind]

...In a real setup, no particle exists eternally, so every particle in the real world is, to some extent, virtual.

The fundamental difference between real and virtual particles is that virtual particles do not have to be on their mass shell, i.e. they do not fulfill the fundamental energy-momentum-mass relation of Special Relativity

[URL]http://upload.wikimedia.org/math/6/3/3/6331382755056101ff11eaead25ddd1c.png[/URL]

taken from
http://en.wikipedia.org/wiki/On_shell_and_off_shell

In fact, virtual particles will never appear "alone" like a usual real particles with fixed momentum and energy but will always contribute to a process through a whole spectrum of energies and momenta folded into the momentum integrals of probability amplitudes. They contribute via a term called propagator; this propagator has a pole on mass shell but off-shell contributions are essential too. Propagators have some properties which may seem crazy; so the propagator of a photon is not purely lightlike but has timelike and even spacelike contributions, too.
OTOH, virtual particles are presented as internal lines in Feynman diagrams and these lines carry the same matter quantum numbers as the real counter parts. I guess. that's where their name is from. But that's already where the similarity ends. IMHO, they are nothing but part of a calculation recipe (Feynman rules).

 

[kexue]

Weejee, I pretty much agree with everything you say. Except that I still like to point out that really any particle we detect is "virtual" because on-shell particles are an idealization that never occurs in practice. It's really a matter of degree -- particles can be more or less "off-shell", but are never actually exactly on-shell.

To Vanadium, since you seem to know with such absolute certainty what is "real" and not in quantum physics, could you please clean up the mess that Frank Wilczek made on page three of this http://scipp.ucsc.edu/~dine/ph217/wilczek.pdf" , where he says that association of forces with particles is a general feature of quantum field theory.

(Or the mess Zee does in Quantum Field Theory in a Nutshell or Feynman in chapter 3 of QED Theory of Light.)

And let me again quote from the email that Wilzcek kindly wrote to me.

It comes down to what you mean by "really there". When we use a concept with great success and precision to describe empirical observations, I'm inclined to include that concept in my inventory of reality. By that standard, virtual particles qualify. On the other hand, the very meaning of "virtual" is that they (i.e., virtual particles) don't appear *directly* in experimental apparatus. Of course, they do appear when you allow yourself a very little boldness in interpreting observations. It comes down to a matter of taste how you express the objective situation in ordinary language, since ordinary language was not designed to deal with the surprising discoveries of modern physics.

I'm bold enough to follow Wilczek and include them in my inventory of reality.

You are not. Which is also fine as Weejee points out.

 

[lightarrow]

I'm sure that you will be able to understand why people say that virtual particles are artifacts of the perturbation theory, and whether you call them real objects or artifacts has nothing to do with the essence of the physics.

I have a lot of problems in understanding what physicists mean with "artifacts". For example, it would seem that quarks are "artifacts" too: you cannot detect single quarks. And what about a photon in fly between source and detector? At the end, a lot of things in physics are not directly detectable. Are they real or not?

 

[tiny-tim]

Quarks and photons are described by the maths as existing in ordinary space-time.

Ordinary space-time is real. ​

Off-shell virtual particles are described by the maths as existing in "momentum space".

"Momentum space" is not real. ​

 

[PhanthomJay]

It seems, then, that the spontaneous production of electron-positron pairs from the vacuum fluctuations is mathematical only, not actually particle creation-annihilation through photon exchange? Please explain my misunderstanding. Maybe I'm confusing this with matter winning out over antimatter from a zero energy universe.

 

[Drakkith]

I have a lot of problems in understanding what physicists mean with "artifacts". For example, it would seem that quarks are "artifacts" too: you cannot detect single quarks. And what about a photon in fly between source and detector? At the end, a lot of things in physics are not directly detectable. Are they real or not?

A great many things are directly detectable, including quarks. The light that hasn't hit a detector yet most certainly exists. You could detect it if you were in the right spot, say by moving the detector closer to the emitting source. Or just wait a bit and then register the light on the detector.

 

[weejee]

It seems, then, that the spontaneous production of electron-positron pairs from the vacuum fluctuations is mathematical only, not actually particle creation-annihilation through photon exchange? Please explain my misunderstanding. Maybe I'm confusing this with matter winning out over antimatter from a zero energy universe.

It is different from a real pair production caused by a strong field.
Basically real or virtual just means whether it comes in as the zeroth order or a higher order term in the perturbation theory.

Let me give you a QM example.
1) In a hydrogen atom, if you apply an electromagnetic field with its frequency corresponding to the energy difference between 1s and 2p states, it can make an electron in the 1s state jump to the 2p state. This is a REAL transition.

2) When the frequency doesn't match (e.g. static E-field), the 2p state slightly mixes in with the 1s state [ |1s> -> |1s> + (small number)|2p>] , according to the perturbation theory. In this case, we call the 2p state VIRTUAL.

The notion that the vacuum produces virtual electron-positron pairs amounts to the case 2).

 

[Vanadium 50]

The fundamental difference between real and virtual particles is that virtual particles do not have to be on their mass shell, i.e. they do not fulfill the fundamental energy-momentum-mass relation of Special Relativity

I don't like thinking of that as fundamental, because it gives people - especially those who get their information from popularizations - is that you have electrons with many different masses. If the mass is 511 keV, they are "real", otherwise they are virtual. You end up with misconception on top of misconception.

The key idea that is missing is that there is no calculation performed with virtual particles that cannot be performed some other way. They are convenient, but not necessary. As I said before, a good analogy is image charges. They make certain calculations much simpler, but you wouldn't go around trying to collect them with a battery.

 

[weejee]

Weejee, I pretty much agree with everything you say. Except that I still like to point out that really any particle we detect is "virtual" because on-shell particles are an idealization that never occurs in practice. It's really a matter of degree -- particles can be more or less "off-shell", but are never actually exactly on-shell.

To Vanadium, since you seem to know with such absolute certainty what is "real" and not in quantum physics, could you please clean up the mess that Frank Wilczek made on page three of this http://scipp.ucsc.edu/~dine/ph217/wilczek.pdf" , where he says that association of forces with particles is a general feature of quantum field theory.

(Or the mess Zee does in Quantum Field Theory in a Nutshell or Feynman in chapter 3 of QED Theory of Light.)

And let me again quote from the email that Wilzcek kindly wrote to me.

It comes down to what you mean by "really there". When we use a concept with great success and precision to describe empirical observations, I'm inclined to include that concept in my inventory of reality. By that standard, virtual particles qualify. On the other hand, the very meaning of "virtual" is that they (i.e., virtual particles) don't appear *directly* in experimental apparatus. Of course, they do appear when you allow yourself a very little boldness in interpreting observations. It comes down to a matter of taste how you express the objective situation in ordinary language, since ordinary language was not designed to deal with the surprising discoveries of modern physics.

I'm bold enough to follow Wilczek and include them in my inventory of reality.

You are not. Which is also fine as Weejee points out.

I'm worried a little bit here.

You seem to think that I said, "BELIEVING IN THE REALNESS OF VIRTUAL PARTICLES IS ACCEPTABLE only if you understand the exact formulation behind it", but what I meant is more like "Believing in the realness of virtual particles is acceptable ONLY IF YOU UNDERSTAND THE EXACT FORMULATION BEHIND IT". Look carefully at where it is written in upper cases.

Moreover, what other people are saying to you is not "YOU SHOULD NOT CLAIM THAT VIRTUAL PARTICLES ARE REAL because you don't seem to understand how that statement is connected to the exact formulation and often naively draw blatantly wrong conclusions from it ", but more like "You should not claim that virtual particles are real BECAUSE YOU DON"T SEEM TO UNDERSTAND HOW THAT STATEMENT IS CONNECTED TO THE EXACT FORMULATION AND OFTEN NAIVELY DRAW BLATANTLY WRONG CONCLUSIONS FROM IT".

Please don't think that I'm being too harsh here. I, too, often NAIVELY DRAW BLATANTLY WRONG CONCLUSIONS out of my not-so-exact "physical pictures". It is only that we need to be extremely conservative on believing that such conclusions are actually true and should always try to see whether the exact theory gives you the same result.Back to the claim that all real particles are slightly off-shell (per Susskind):

If you think this claim means that a real particle has definite energy and momentum $$E$$ and $$\textbf{p}$$, and they slightly violate the relation $$E^2 - \textbf{p}^2 c^2 - m^2 c^4 = 0$$ , I would say it is a misconception. It also has nothing to with particles not existing eternally.

It should really mean

(actual 'out'-state) = (asymptotic out-state, which is on-shell) + (perturbative corrections, which are off-shell).

Perturbative corrections arise since real particles are not infinitely separated from one another after scattering, and therefore, they have some residual interaction.

Furthermore, the notion of 'being on(off)-shell' only occurs in Feynman's formulation of the perturbation theory. In the old-fashioned perturbation theory (= time-independent perturbation theory we learn in QM), there is no such thing. Instead, we talk about whether the energy is conserved or not ('real' if conserved and 'virtual' if not'). That means we can as well say that real particles slightly violate the energy conservation law. 'Being off-shell' is just a way of loosely expressing the exact physics in terms of ordinary language.

It is zillion times more important to know where you have a good understanding of the exact formulation and where you don't, than defending yourself by saying "It is just a matter of wording!" and support it using famous physicists' 'wordings' that happen to look similar to yours, when others are actually pointing out your misconceptions on the exact physics. It is just a matter of wording ONLY IF YOU UNDERSTAND THE EXACT FORMULATION BEHIND IT CORRECTLY.

p.s. Do you now understand why virtual particles don't appear in non-perturbative contexts?

Kexue's viewpoint is not the standard one, and I fear it is muddying the waters rather than clarifying everything. As he points out, there's already a thread on his views, so let's not derail this any further.

I'm sorry Vanadium 50, I think I've derailed this quite a bit. I'll stop now.

 

[PhysDrew]

Do virtual particles exert an influence on the 'real' particles? If they do doesn't that imply that they may be 'real?'. Didn't the Lamb shift allude to this earlier? Or is that unrelated?

 

[weejee]

I have a lot of problems in understanding what physicists mean with "artifacts". For example, it would seem that quarks are "artifacts" too: you cannot detect single quarks. And what about a photon in fly between source and detector? At the end, a lot of things in physics are not directly detectable. Are they real or not?

In this case, people call virtual particles "artifacts" since they appear only in the context of the perturbation theory. I think many people here agree that something "real" needs to have an independent existence from whether it is described perturbatively or not.

 

[lightarrow]

Quarks and photons are described by the maths as existing in ordinary space-time.

Ordinary space-time is real. ​

Off-shell virtual particles are described by the maths as existing in "momentum space".

"Momentum space" is not real. ​

Ok. "Real" or "not real" is equivalent to "on shell" or "out shell"?

Edit: not "out shell" but "off shell", sorry.

 

[lightarrow]

A great many things are directly detectable, including quarks.

Quarks are directly detectable? I mean, single quarks?

The light that hasn't hit a detector yet most certainly exists.

But I haven't written "the light", I have written "a photon". You can trace the trajectory of a, say, high energy proton, not of a photon which coherence length is greater than the distance source-detector. In that case the photon as spatially localized corpuscle between source and detector doesn't exist at all. If instead with "real" you mean it satisfies E² = (cp)² + (mc²)², it's another story.

You could detect it if you were in the right spot, say by moving the detector closer to the emitting source.

In this way you did nothing else than move your detector in another position, so where is the photon between the source and this new detector's position?

Or just wait a bit and then register the light on the detector.

This is not clear to me.

 

[tiny-tim]

Ok. "Real" or "not real" is equivalent to "on shell" or "out shell"?

No.

 

[lightarrow]

No.

Then it's equivalent to "existing in ordinary space-time" and "non existing in ordinary space-time"?

 

[tiny-tim]

Then it's equivalent to "existing in ordinary space-time" and "non existing in ordinary space-time"?

On-shell virtual particles, like epicycles, are described by the maths as existing in ordinary space-time, but, like epicycles, they're still not real.

Off-shell virtual particles are described by the maths as existing in "momentum space", so from that fact alone they're not real.

 

[kexue]

The key idea that is missing is that there is no calculation performed with virtual particles that cannot be performed some other way. They are convenient, but not necessary. As I said before, a good analogy is image charges. They make certain calculations much simpler, but you wouldn't go around trying to collect them with a battery.

When we collide an electron and a positron in an accelerator, you can calculate the scattering result without the corrections coming from virtual particles?

Virtual particles are nothing but quantum field excitations that do not obey energy-momentum relation. Though they never appear in the initial or final condition of the experiment, they are necessary, since they have important impact on the result of the experiment.

As for the perturbative vs. non-perturbative argument, note that there are problems in interpreting non-perturbative processes in terms of particles but this is due to the breakdown of the particle concept itself. For example in a proton, there are perturbatively three quarks. If one shoots high-energy electrons onto the proton on will find this idea quite well confirmed. But that is because the interaction is very small at those energies. If one take lower energies it will become less clear, how the proton is made up of quarks, or whether it is at all because the interaction between the quarks gets to strong. Being somewhat pedantic one could even say that the particle concept does not even work if there is any interaction at all, but to see this is a complicated matter.

Since the words of Frank Wilczek has been disregarded by some here, let me quote for your convenience again Gerad t'Hooft.

Virtual particles have little to do with perturbation expansion. They "are really out there" in the sense that their contribution certainly affects the amplitudes of particle transitions. But all of quantum mechanics is based on "states" that are not usually there in the classical sense. It's just like the two slit experiment. The particle goes through one slit or through the other, while*nevertheless*the behavior afterwards is determined by the fact that there were two slits. Similarly, virtual particles may have been present or absent.
Some scattering events may be entirely due to the exchange of a virtual particle; in that case, it is hard to deny that the particle was there. Sometimes, you don't know whether it was a particle going from A to B, or an antiparticle going from B to A, this happens for instance when charged particles attract or repel one another by the exchange of a photon.

Virtual particles are allowed by the uncertainty principle, though they are not directly observable, they have real and observable effects.

 

[lightarrow]

On-shell virtual particles, like epicycles, are described by the maths as existing in ordinary space-time, but, like epicycles, they're still not real.

Off-shell virtual particles are described by the maths as existing in "momentum space", so from that fact alone they're not real.

Sorry for the naive question but is there a definition of "real" particle?

 

[tiny-tim]

Not that I'm aware of.

 

[pallidin]

Sorry for the naive question but is there a definition of "real" particle?

This is not an answer, rather an opinion:

"Real" is what "virtual" is not.

 

[weejee]

Virtual particles are nothing but quantum field excitations that do not obey energy-momentum relation. Though they never appear in the initial or final condition of the experiment, they are necessary, since they have important impact on the result of the experiment.

No. They are not the excitations themselves. They are really perturbative corrections to 'excitations'.

Of course they have important impact on the result of the experiment.
Even for something as simple as electron-electron scattering, the lowest order contribution to the scattering amplitude is second order in the perturbation theory. We'd better include virtual particles(=perturbative corrections) in the calculation, since otherwise we get zero.

Virtual particles are allowed by the uncertainty principle, though they are not directly observable, they have real and observable effects.

Please tell me how in a free field theory, where both the uncertainty principle and the particle concept are valid and everything is described non-perturbatively, virtual particles can arise. (Actually you can't, since there is no virtual particle in free field theories.)

Furthermore, it is a very loose statement that virtual particles are allowed by the uncertainty principle. Can you give us any quantitative result out of that statement? What are the EQUATIONS corresponding to this statement? For example, can you calculate something like the lifetime of a virtual particle, provided that such concept ever exists?

Virtual particles have little to do with perturbation expansion. They "are really out there" in the sense that their contribution certainly affects the amplitudes of particle transitions. But all of quantum mechanics is based on "states" that are not usually there in the classical sense. It's just like the two slit experiment. The particle goes through one slit or through the other, while*nevertheless*the behavior afterwards is determined by the fact that there were two slits. Similarly, virtual particles may have been present or absent.
Some scattering events may be entirely due to the exchange of a virtual particle; in that case, it is hard to deny that the particle was there. Sometimes, you don't know whether it was a particle going from A to B, or an antiparticle going from B to A, this happens for instance when charged particles attract or repel one another by the exchange of a photon.

How would you reconcile this with Weinberg's statement that virtual particles are mere artifacts? Would you say that Weinberg is wrong?

It is pointless to defend yourself with a bunch of words, which simply aren't able to respond in anyways to what people ask. (e.g. How can virtual particles arise in non-perturbative ...)
I'm sure if t'Hooft were here, he would give reasonable answers to those questions, rather than playing around with some 'wordings'.