Re: Bill Bryson and the big bang

From: Franz Heymann (notfranz.heymann_at_btopenworld.com)
Date: 06/25/04 

Date: Fri, 25 Jun 2004 19:17:56 +0000 (UTC)

"greywolf42" <mingstb@marssim-ss.com> wrote in message
news:10dlpq56oh4oub6@corp.supernews.com...
> Bjoern Feuerbacher <feuerbac@thphys.uni-heidelberg.de> wrote in
message
> news:cbe2c6$aq3$7@news.urz.uni-heidelberg.de...
> > greywolf42 wrote:
> > > Franz Heymann <notfranz.heymann@btopenworld.com> wrote in
message
> > > news:cba025$6jv$1@sparta.btinternet.com...
> > >
> > >>"Franz Heymann" <notfranz.heymann@btopenworld.com> wrote in
message
> > >>news:cb92s2$3q2$1@sparta.btinternet.com...
> > >>
> > >
> > > {snip}
> > >
> > >
> > >>>The SLAC experiment of which I know used an electron beam
incident
> > >>
> > >>on
> > >>
> > >>>a lead target to produce bremsstrahlung. The photons in the
> > >>>bremsstrahlung were tagged with the electron incident energy
and the
> > >>>scattered electron's energy, so that the photon energies were
known
> > >>>These were allowed to interact head-on with a laser beam .
> > >>
> > >>I need to do a big search. It is possible that I misremembered
how
> > >>the high energy photon beam was made.
> > >>I realise that in the latest experiments, the HE photon beam was
> > >>essentially made up of the virtual photons of the electric
field of a
> > >>fast electron. For very peripheral interactions, these photons
are
> > >>very nearly real. (Shades of the Weiszacker-Williams
approximation)
> > >
> > >
> > > Is 'very nearly real' like being 'almost observable' or 'a
little bit
> > > pregnant?'
> >
> > No. It means plain and simple that for them, sqrt(E^2/c^4 -
p^2/c^2) is
> > very close to zero.
>
> A trivial an meaningless observation.

You are memrly showing your ignorance by that assertion.

The amount by which a particle deviates from its mass shell is given
quite precisely by the numerical value of
sqrt(E^2/c^4 - p^2/c^2)
If that quantity is zero, the particle is on its mass shell.
Such a particle is known as a free particle.
If it is close to zero, the particle is close to its mass shell.
The closer a particlt is to its mass shell, the more closely its
behaviour approaches to that of a free particle.

I hope you have learnt something today, but I am dubious.

> > Did you really not know this, or did you just wanted to make fun
of
> > Frantz anyway?
>
> I wasn't making fun of Franz, per se. I was making fun of the
absolutely
> silly notions of "virtual" photons 'almost' becoming real.

The notion is not absurdly silly. The nearer the virtual particle is
to its mass shell, the more closely its behaviour approaches that of a
free particle.

Have you ever heard of the common physical procedure of "extrapolating
to zero"?

Franz

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