Physics Forums

Physics Forums (http://www.physicsforums.com/index.php)
-   Quantum Physics (http://www.physicsforums.com/forumdisplay.php?f=62)
-   -   Virtual particles (http://www.physicsforums.com/showthread.php?t=460685)

PhanthomJay Jan3-11 08:14 AM

Virtual particles
 
If virtual particles pop into and out fo 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 Jan4-11 12:28 PM

Re: Virtual particles
 
Just because an electron positron pair can spontaneously appear or similar doesnt 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 Jan4-11 06:26 PM

Re: Virtual particles
 
Quote:

Quote by PhanthomJay (Post 3064085)
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 Jan4-11 07:28 PM

Re: Virtual particles
 
No. I guess i'll have to read up on them again.

Vanadium 50 Jan4-11 07:50 PM

Re: Virtual particles
 
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 Jan5-11 08:29 AM

Re: Virtual particles
 
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 Jan5-11 10:06 AM

Re: Virtual particles
 
Quote:

Quote by Vanadium 50 (Post 3066403)
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 this (unfortunately very long!) thread.

For a start read Frank Wilzcek 11-page survey article 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 Jan5-11 11:02 AM

Re: Virtual particles
 
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 Jan5-11 10:49 PM

Re: Virtual particles
 
Quote:

Quote by PhanthomJay (Post 3067116)
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 Jan6-11 02:43 AM

Re: Virtual particles
 
Quote:

Quote by kexue (Post 3067054)
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 this (unfortunately very long!) thread.

For a start read Frank Wilzcek 11-page survey article 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 Jan6-11 03:25 AM

Re: Virtual particles
 
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 Jan6-11 04:30 AM

Re: Virtual particles
 
Quote:

Quote by Dynamic Sauce (Post 3068215)
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 Jan6-11 07:12 AM

Re: Virtual particles
 
Quote:

Quote by kexue (Post 3068283)
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 Jan6-11 09:58 AM

Re: Virtual particles
 
Quote:

Quote by PhanthomJay (Post 3066958)
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. :wink:

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 Jan6-11 10:24 AM

Re: Virtual particles
 
Quote:

Quote by kexue (Post 3068283)
...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

http://upload.wikimedia.org/math/6/3...ead25ddd1c.png

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 Jan6-11 10:43 AM

Re: Virtual particles
 
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 survey, 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 Jan6-11 12:01 PM

Re: Virtual particles
 
Quote:

Quote by weejee (Post 3068403)
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 Jan6-11 12:29 PM

Quarks and photons are described by the maths as existing in ordinary space-time.
Ordinary space-time is real. :smile:
Off-shell virtual particles are described by the maths as existing in "momentum space".
"Momentum space" is not real. :frown:


All times are GMT -5. The time now is 05:30 PM.

Powered by vBulletin Copyright ©2000 - 2014, Jelsoft Enterprises Ltd.
© 2014 Physics Forums