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Where does it come from?by Kruger
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#1
Apr2505, 11:21 AM

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Where does a photon in vacuum come from if it creates a pair? In QFT there shoudn't be a photon available in vacuum because the number operator is zero.



#2
Apr2505, 11:44 AM

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For the quantum theory of the em field,the vacuum state has 0 particles...
But QED means INTERACTING quantum fields:Dirac & Maxwell. Daniel. 


#3
Apr2505, 01:05 PM

P: 219

Which Dirac theory? I know the Hole theory. But this theory does only describe the positron and electron, not the virtual photon. Can you tell me?
thanks dextercioby 


#4
Apr2505, 01:27 PM

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Where does it come from?
Dirac field.The vacuum for a free field has 0 energy,0 momentum,0 electric charge & 0 particles (remember the necessity of defining the normal product of creation & annihilation operators).But the vacuum for an interacting field theory is different.Remember that the quantum corrections to the Green functions are nonzero.
Daniel. 


#5
Apr2605, 03:31 PM

P: 219

Can you please explain me, why the quantum corrections for this aren't as normal. Normal we have E=hw/4pi.



#6
Apr2605, 03:59 PM

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Nope.The normal product deals with "residual" energy.The vacuum of a free theory has 0 energy.
Daniel. 


#7
Apr2605, 05:59 PM

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marlon http://www.physicsforums.com/journal...page=10&page=2 you will need to scroll a bit 


#8
Apr2705, 05:16 AM

P: 219

But if I derive the ground state of the electromagnetic field I would get the zero point energy of this quantum field. And there aren't photons in there. And that's why I think there aren't photons in the vacuum (not real photons and not virtual photons < in an empty space where only ground state oscillations can exist). If we put in a strong electric field Dirac's hole theory predicts virtual pair creation. But for this it is a virtual photon needed with many energy and ground state oscillation hasn't this energy.
I know that Heisenberg's uncertainty principle play a role in this but we used it in deriving the ground state of the em field. And that's actionally the point where I get problem. OR DO WE CALL THIS GROUND STATE OSCILLATION "VIRTUAL PHOTONS"??? 


#9
Apr2705, 09:16 AM

P: 4,006

Now, the vaccuum can have an influence on this. If the vaccuum is filled with virtual particle antiparticle pairs, each with opposite charge, these pairs will act as good old dipoles and they will try to align with the electric field of the two electrons. Basically what happens is that the vaccuum plays the role of a dielectric that screens (ie : make more interactions more difficult) the electron's charges. That is why we have the concept of bare and dressed charge in field theory. This vaccuuminfluence is expressed by the polarization insertion in QFT and this is nothing else then the self energy of the photon propagator marlon 


#10
Apr2705, 10:04 AM

P: 219

thanks dextercioby and marlon
In all books I read about the vacuum there is written something like this: ///In vacuum there are all electrons on their lowest state. There are also negative energy states which are occupied. If a virtual photon collide with such a negative energy particle then this negative particle gets positive energy and it leaves a hole back (negativ states) that behaves like a positron. After time d(t) (Uncertainty) the electron will go back in the negativ energy state. /// 


#11
Apr2705, 10:18 AM

P: 219

If I look on quantum field theory then this mattresses don't contain particles. ??? 


#12
Apr2705, 10:49 AM

P: 4,006

It's the vibrations that correspond to what we call particles regards marlon 


#13
Apr2705, 10:54 AM

P: 219

But you wrote ""So you have one particle with energy given by ...""



#14
Apr2705, 10:55 AM

P: 4,006

regards marlon 


#15
Apr2705, 01:57 PM

P: 219

Sorry marlon.
Thanks 


#16
Apr2705, 03:20 PM

P: 4,006

regards marlon 


#17
Apr2705, 03:26 PM

P: 219

Ok, I think I got the case of my problem now. Thanks marlon you're very helpful.



#18
Apr2705, 03:37 PM

P: 219

These virtual particles (photons) are they a "product (intermediate state)" of an interaction or can they arise from vacuum (in case of virtual pair creation)? In the case of pair creation the e+e pair isn't there at the beginning and the virtual photon must be an off mass shell interacting particle, so fom where does this photon come from (which particle send it)? 


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