Re: How real are the "Virtual" partticles?

From: Franz Heymann (notfranz.heymann_at_btopenworld.com)
Date: 03/15/05 

Date: Tue, 15 Mar 2005 18:17:35 +0000 (UTC)

"Arnold Neumaier" <Arnold.Neumaier@univie.ac.at> wrote in message
news:422EB74C.5020806@univie.ac.at...
> Franz Heymann wrote:
> > "Arnold Neumaier" <Arnold.Neumaier@univie.ac.at> wrote in message
> > news:422AC49F.3030502@univie.ac.at...
> >
> >>Franz Heymann wrote:
> >>
> >>>>Unstable particles are modelled exactly like stable particles,
> >>>>namely as external lines in a Feynman diagram.
> >>>>Virtual particles in Feynman diagrams are exactly those
> >>>>which are not given by external lines.
> >>>>
> >>>>Hence what is real and what is virtual is not affected by a
> >>>>diagram rotation - this only affects what is input oand what is
> >>>
> >>>output.
> >>>
> >>>I am not happy with that.
> >>>Let me illustrate my unhappiness with a specific example.
> >>>Take the resonant elastic scattering case:
> >>>
> >>>pi + p --> Z --> pi + p
> >>>
> >>>I hope we both agree that the Z is produced as a real but
unstable
> >>>particle in the s-channel.
> >>
> >>No. Z is a virtual particle since it is an intermediate line.
> >
> > You are doing nothing to alleviate my unhappiness.
> > If the Z were a virtual prticle, it would not be on its mass shell
and
> > a mass measurement would not be possible by studying the reaction.
>
> It is not on its (complex) mass shell when it is in an internal line
as
> in your diagram.
>
> When measured, one has instead a diagram where the Z is detected,
> and hence on an external line of Feynman diagrams associated to
> the coresponding cross section!
>
> In ordinary scattering experiemnts the existence of the Z shows in
> resonances of the cross section. This is evidence for the presence
> of the Z in the action, and can be used to calculate properties of
Z,
> but it is Not a measurement of Z as a particle.
>
>
> > My undertanding is that the line corresponding to the Z is the
> > trajectory of a real Z travelling from its birth to its decay.
>
> This interpretation of Feynman diagrams is popular but completely
> unfounded, and gives misleading conclusions if carried beyond the
> most superficial view.
>
>
> > But it is observed, and its (complex) mass is actually measured.
>
> It is _not_ observed in an experiment where the Feynman diagram
> you drew is part of the calculation.
>
> It _can_ be observed, but only in a process analyzed with
> Feynman diagrams where the Z is an external line.

I maintain that it is an external line which has its far end
terminated in the decay of the resonance.

In atomic physics, a photon may be absorbed by an atom in its ground
state. The excited state very quickly decays back to the ground state
by the emission of a photon.

Are you now saying that the excited state is not a real, but a virtual
state?

> >>One can also see it by computing
> >>the momentum balance and check that the momentum does not lie
> >>on the Z's (complex) mass shell, as would be needed for a real Z.
> >
> > Oh? Sorry, but the Z is no less real than the pi0.
> > If the pi0 can decay into two photons, then it can also be created
> > bytwo photons.
>
> For the pi0 the situation is the same. If measured, the cross
> section for pi0 production or decay comes from an analysis with
> external pi0 lines, while in other processes it is a virtual
> particle without objective meaning.

I fear me I will never understand you, and you will never understand
me. I suggest we discontinue this conversation after your next reply
to me, if indeed you wish to make one. 

-- 
Franz
"A first-rate laboratory is one in which mediocre scientists can
produce outstanding work"
P.M.S. Blackett

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