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-   -   When Virtual Becomes Real (http://www.physicsforums.com/showthread.php?t=449013)

Kyoma Nov18-10 04:25 AM

When Virtual Becomes Real
 
Virtual Particles appear due to the Weisenberg Uncertainty Principle.

So if [tex]\Delta[/tex]t is prolonged or something like that, virtual particles become real. Am I right?

tiny-tim Nov18-10 05:29 AM

Hi Kyoma! :smile:
Quote:

Quote by Kyoma (Post 2991775)
Virtual Particles appear due to the Weisenberg Uncertainty Principle.

(it's Heisenberg! :wink: Ö though I suppose you're allowed some uncertainty in the name! :biggrin:)

No, virtual particles are a mathematical device in the Dyson expansion in perturbation theory in quantum field theory, they have nothing to do with the uncertainty principle. :wink:

(Even the mathematics does not give them any location or duration.)

Kyoma Nov19-10 06:19 AM

Re: When Virtual Becomes Real
 
Then how do virtual particles become real? :confused:

xepma Nov19-10 06:23 AM

Re: When Virtual Becomes Real
 
They don't.

HallsofIvy Nov19-10 08:22 AM

Re: When Virtual Becomes Real
 
Tiny Tim said: "virtual particles are a mathematical device". Do you understand what that means?

nismaratwork Nov19-10 11:54 AM

Re: When Virtual Becomes Real
 
Tiny Tim: Who has time for "Werner Heisenberg", when you can rock the "Weisenberg". From now on I subscribe to the WUT (anyway **** that nazi Heisenberg...).

Kyoma: At no time are virtual particle intended in math from the outset to be real or "BECOME" real, and if they did they wouldn't BE virtual particles! "What if a horse had black and white stripes and lived in Africa?..." It'd be a Zebra, not a horse, that's what.

In the struggle to 'get' from point A to B in QFT, these little mathematical devices called "virtual particles" are useful in the endeavor, but have no more physical reality than the FOIL method becoming a fencing implement. The only time you could conceivably move in the "striped horse in africa" direction would be Hawking Radiation, but that's not a virtual particle except for meaningless period in the math.

pallidin Nov19-10 07:46 PM

Re: When Virtual Becomes Real
 
I would not be too quick to dismiss the concept of "virtual" particles/photons.
For example, can the extension of magnetism in free-space make any sense whatsoever without the notion of virtual photons? No.
That is just one example.

Sure, with virtual particles/photons it's all mathematical, but maybe something IS there, transitory as it may be.

tiny-tim Nov20-10 01:42 AM

Quote:

Quote by pallidin (Post 2994711)
For example, can the extension of magnetism in free-space make any sense whatsoever without the notion of virtual photons? No.

what's the extension of magnetism? :confused:

nismaratwork Nov20-10 07:51 AM

Re: When Virtual Becomes Real
 
Quote:

Quote by tiny-tim (Post 2995113)
what's the extension of magnetism? :confused:

I think the field lines use this:

http://img.alibaba.com/img/imagerepo...web17_9052.jpg

This to me, is the clear solution to problems in QFT... plus what if I drop the remote, am I supposed to haul *** to pick it up?! I NEED those calories... really I do.

lugita15 Nov20-10 08:50 AM

Re: When Virtual Becomes Real
 
I think that Kyoma is confusing virtual particles with things like W and Z bosons, which have more energy than conservation of energy would allow, but exist for a very short period of time. That is OK because of Heisenberg's uncertainty principle.

jtbell Nov20-10 10:11 AM

Re: When Virtual Becomes Real
 
There are virtual W and Z bosons, which mediate low-energy weak interactions like nuclear beta decay, and real W and Z bosons, which can be produced in accelerators with enough energy. The difference is exactly like between virtual and real photons.

Kyoma Nov21-10 08:09 AM

Re: When Virtual Becomes Real
 
Quote:

Quote by lugita15 (Post 2995379)
I think that Kyoma is confusing virtual particles with things like W and Z bosons, which have more energy than conservation of energy would allow, but exist for a very short period of time. That is OK because of Heisenberg's uncertainty principle.

Not really...

In wikipedia, it states that if a single virtual particle is detected, then the consequences of its existence are prolonged to such a degree that it cannot be virtual... I don't understand this. And then there's the Hawking Radiation... which apparently allows virtual particles to become real...

tiny-tim Nov21-10 02:26 PM

Quote:

Quote by Kyoma (Post 2996665)
In wikipedia, it states that if a single virtual particle is detected, then the consequences of its existence are prolonged to such a degree that it cannot be virtual... I don't understand this.

That's the second sentence of the second paragraph of the introduction to http://en.wikipedia.org/wiki/Virtual_particle, and I think most PF members would completely disagree with that introduction Ö
In physics, a virtual particle is a particle that exists for a limited time and space, introducing uncertainty in their energy and momentum due to the uncertainty principle. Because energy and momentum in quantum mechanics are time and space derivative operators, then due to Fourier transforms their spans are inversely proportional to time duration and position spans, respectively.Ö
virtual particles exist only in the maths (and even in the maths they have no creation event, annihilation event, or duration, whether limited or otherwise).

A "particle that exists for a limited time and space" is a real particle Ö that's what "real" means!! :rolleyes:

If it exists because it has "borrowed enough energy", then the amount of energy it has borrowed is exactly the amount for a real particle (eg, if it's a "borrowed-energy" electron, it has the correct wikipedia rest-mass of an electron), and therefore cannot be a virtual particle (which can have any rest-mass, even according to wikipedia, see below :rolleyes:) Ö it can only be a real particle!
(Actually, that's not completely correct Ö there are two types of virtual particle in the maths, those in the position representation, and those in the momentum representation Ö the first type do have the correct rest-mass, but they only obey conservation of 3-momentum, not of 4-momentum.

Since wikipedia defines "A virtual particle is one that does not precisely obey the m2c4 = E2 − p2c2 relationship for a short time.", ie it defines it as not having the correct rest-mass, that means that wikipedia does not define that first type as a virtual particle! :smile:)

JoaoPais Nov21-10 04:49 PM

Re: When Virtual Becomes Real
 
Quote:

Quote by tiny-tim (Post 2991821)
No, virtual particles are a mathematical device in the Dyson expansion in perturbation theory in quantum field theory,
(Even the mathematics does not give them any location or duration.)

Quote:

Quote by HallsofIvy (Post 2993747)
Tiny Tim said: "virtual particles are a mathematical device". Do you understand what that means?

Iím sorry, correct me if Iím wrong, but virtual particles arenít only a mathematical device, once, as far as I know, are used to explain the Casimir effect. The virtual particles ďcome to existenceĒ only to be annihilated again in an infinitesimal variation of time and space, with is associated with the mass of the particle. Either way, they need some space for the appearing and annihilation to occur, and bitten two very close plates (10 nm) there are less virtual particles than outside the plates and this explains the force that appears to attract the plates (Casimir effect).
Also Iím not familiar with Dyson expansion, so I hope this doesnít become some stupid question =P

nismaratwork Nov22-10 05:36 AM

Re: When Virtual Becomes Real
 
Quote:

Quote by JoaoPais (Post 2997348)
Iím sorry, correct me if Iím wrong, but virtual particles arenít only a mathematical device, once, as far as I know, are used to explain the Casimir effect. The virtual particles ďcome to existenceĒ only to be annihilated again in an infinitesimal variation of time and space, with is associated with the mass of the particle. Either way, they need some space for the appearing and annihilation to occur, and bitten two very close plates (10 nm) there are less virtual particles than outside the plates and this explains the force that appears to attract the plates (Casimir effect).
Also Iím not familiar with Dyson expansion, so I hope this doesnít become some stupid question =P

That's an extension of a mathematical model, but it doesn't mean whatever is happening between those plates involved (it kills me to say this) "real" virtual photons... that's just how we describe the interaction. Remember, the action of virtual particles are used to explain REAL physical effects, but they're the mathematical intermediary between two states that ARE real.

In other words, I might talk about the tires of my car "biting" the surface of the road as a means of describing what happens vis a vis friction and the road, but the tires don't bite. You can talk about virtual photons between casimir plates or virtual pair creation at the event horizon of a BH, but it's just a MATHEMATICAL artifact used to describe an "in-between" interaction current physics has no physical explanation for.

JoaoPais Nov22-10 08:41 AM

Re: When Virtual Becomes Real
 
Iím sorry but, for me, the virtual particles seem as real as Higgs Boson, gluons or strings. A physical materialization of mathematic postulates that embody the best hypothesis to explain a certain observed physical phenomenon.
I understand what you mean, but it seems to me the real difference is what we account as real, being your real only the observed, described and experimentally documented.
Maybe Iím extending incorrectly my notion of ďrealĒ, that is true =)

nismaratwork Nov22-10 09:48 AM

Re: When Virtual Becomes Real
 
Quote:

Quote by JoaoPais (Post 2998276)
Iím sorry but, for me, the virtual particles seem as real as Higgs Boson, gluons or strings. A physical materialization of mathematic postulates that embody the best hypothesis to explain a certain observed physical phenomenon.
I understand what you mean, but it seems to me the real difference is what we account as real, being your real only the observed, described and experimentally documented.
Maybe Iím extending incorrectly my notion of ďrealĒ, that is true =)

I understand what you're saying, but the issue isn't that these are unobservable by direct means as is the case with a gluon... the gluon being to the the strong force as the photon is to EM. When the word "virtual" is said, in regards to a boson of any type we're talking about something that has no physical reality as you do when you omit the word "virtual".

In fact, here's the feynman diagram:
[img]http://wpcontent.answcdn.com/wikipedia/commons/thumb/1/1f/Feynmann_Diagram_Gluon_Radiation.svg/287px-Feynmann_Diagram_Gluon_Radiation.svg.png[/img0

SOMETHING happens between the annihilation of the real particles and the emission of the quarks and gluon, but what is only described by a mathematical formalism that has NOTHING to do with physical reality. Something with a physical reality (a force, the exchange of energy, etc) is described down to the point where we have to use "tricks" to fudge a bit to the next step.

I recommend reading this: http://en.wikipedia.org/wiki/Static_...ticle_exchange

Especially portions dealing with the Coulomb Force, and have this up while you do: http://en.wikipedia.org/wiki/Virtual_particles

This isn't an issue of observed vs. unobserved, this is an issue of a bit of "filler" to do the job of something taking place according to the HUP which can't be described any other way, yet. SOMETHING is going on, yes, but there's no virtual photon there at least.

TrickyDicky Nov30-10 04:39 PM

Re: When Virtual Becomes Real
 
Quote:

Quote by jtbell (Post 2995450)
There are virtual W and Z bosons, which mediate low-energy weak interactions like nuclear beta decay, and real W and Z bosons, which can be produced in accelerators with enough energy. The difference is exactly like between virtual and real photons.

This could be confusing for newcomers, the fact that the gauge bosons come in real and virtual versions, for instance the W boson is defined in the wikipedia as the particle that as well as the Z boson mediaes the weak force, so the doubt arises as to wether is referring to the virtual W or the real boson W and when it describes its basic properties it states that "in practice it can be considered to be a virtual particle". So I would like to ask the experts which are the force mediators the virtual gauge bosons or the "real" ones because often it's not very clearly defined. In case the mediators are virtual bosons, what is the role of the real particles? In the case of the photons it's obvious they are the EM radiation , but what about the W and Z particles? ( I believe the gluon hasn't been found yet)
Also I read the mean-life of the W and Z bosons it's around 10E-25 seconds so I guess they can't be directly detected , and were discovered by indirect means, can someone explain in lay terms how? Thanks


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