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Re: cosmology and quantum gravity



Since I don't know how you define "virtual photons", it is not
possible for me to answer your question. I don't use the term when I
am doing perturbation theory in QED.
The photon propagators that occur in the Feynman
formalism are gauge dependent quantities, but they are usually defined
to be purely transverse objects, so they do not contain the Coulomb
field. They are, however, depicted (in Feynman diagrams) as representing
the transfer of "virtual photons".
Don't forget that the notion of "virtual interactions" was
introduced to provide a picturesque representation of perturbation theory.
If one knew how to go directly from a Lagrangian to and S-Matrix (as one
tries to do in lattice-gauge calculations) the notion would not occur.

Regards,
Jack

Adam was by constitution and proclivity a scientist; I was the same, and
we loved to call ourselves by that great name...Our first memorable
scientific discovery was the law that water and like fluids run downhill,
not up.
Mark Twain, <Extract from Eve's Autobiography>

On Sat, 29 Apr 2000, William J. Larson wrote:

At one point I thought that I understood the distinction to be made
was between virtual & real photons (and I assume virtual & real
gravitons). And I thought real photons are detectable
by, e.g. producing an image on photographic film, whereas
virtual photons are detectable by, e.g. static electric forces.

Please explain how I am wrong. [Please limit your response
to 2000 words or less. :-) ]

==>The gravitational force, like electrical force, can be divided
==>into two parts: one part is time dependent, has a corresponding flux
==>vector (transporting energy), and can be quantized. The other part
==>is time independent and has nothing to do with energy transport - this
==>part, accordingly, can not be quantized. In gravity the time independent
==>force corresponds to the 1/r^2 law of Newton (and, incidentally, some
==>predecessors). In electricity the time independent part corresponds to
==>the coulomb field.

Dr. William J. Larson
Bill_Larson@csi.com
Institut Monte Rosa
Montreux, Switzerland

----- Original Message -----
From: Jack Uretsky <jlu@HEP.ANL.GOV>
To: <PHYS-L@lists.nau.edu>
Sent: 2000 April 29 5:01 AM
Subject: Re: cosmology and quantum gravity


I will answer you, presuming that you are seeking an answer
suitable for high school students.
Forget the curvature of space part, because that has not yet been
fitted into quantum theory. So your question is intelligent because it
probes an area of physics that presently is only dimly understood. It
nevertheless is directed to an aspect of the force laws that are really
understood very well.
The gravitational force, like electrical force, can be divided
into two parts: one part is time dependent, has a corresponding flux
vector (transporting energy), and can be quantized. The other part
is time independent and has nothing to do with energy transport - this
part, accordingly, can not be quantized. In gravity the time independent
force corresponds to the 1/r^2 law of Newton (and, incidentally, some
predecessors). In electricity the time independent part corresponds to
the coulomb field.
"Time independence" is a relativistically ambiguous expression.
What I mean is, time independent when viewed in the frame of the source
of the field (mass, for gravity, charge, for electricity). It is as
though the time independent field, and its accompanying source, have
(and will) exist together through all eternity.
Note that both of the examples, in their quantum aspects, involve
the transport of massless bosons. The arguments that I have given you
would not apply, for marvelous technical reasons, to fields involving the
exchange of massive bosons.
Pedants may add many more details, but I think that this addresses
the nub of your question.
Regards,
Jack
Adam was by constitution and proclivity a scientist; I was the same, and
we loved to call ourselves by that great name...Our first memorable
scientific discovery was the law that water and like fluids run downhill,
not up.
Mark Twain, <Extract from Eve's Autobiography>

On Fri, 28 Apr 2000, Zach Wolff wrote:

I may be completely missing the point here, but I feel
like this answer has strayed from the original
question. The answer here talks about a static
gravitational field as a curvature of space. In my
limited understanding this is a relativistic
description of the force, as opposed to a quantum
description in which forces are mediated by particles.
If this is not true, what am I missing? If this is
true, is there any way a force mediated by a particle
(or particles) could be created by an interaction
between something inside the event horizon and
something outside the event horizon?

Zach

--- Jack Uretsky <jlu@HEP.ANL.GOV> wrote in part:
What's going on is that you have to look at
the entire universe,
not just the black hole (something like donuts).
The Schwarzchild
geometry is the geometry of a universe with a large
mass at the center.
The static gravitational effects that we can observe
are embedded in the
metric (more strictly, the curvature) outside of the
horizon. In the
usual metric, the gravitational effects get larger
and larger as we
approach the black hole until they blow up at the
horizon, where there is
a coordinate singularity. So the answer is that the
mass at the center
of the black hole has distorted all of space; part
of that distortion is
the event horizon that bounds the black hole.
Regards,
Jack


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