gjphw@ihuxm.UUCP (04/11/84)
This is both a criticism of QED and a request for some precision.
Quantum field theory (QFT) and quantum electrodynamics (QED) are not
interchangeable. The theory with the impressive computational successes for
the hyperfine transition in hydrogen is QED (18 significant figures at my last
count). QED was started in 1933, shortly after the establishment of quantum
mechanics itself. The best aspect of QED is that it handles the particle-wave
duality of the photon in a consistent fashion. Unfortunately, the infinities
that arose in QED, plus the absence of experimental results for comparison
with theory, stopped most work in QED until after World War II. Then came
microwave measurement techniques (e.g., the hyperfine transition) and
renormalization to handle the infinities.
QFT is an extension of QED. Where QED involves only electromagnetic
interactions, QFT attempts to include the two nuclear interactions (strong
nuclear and weak nuclear). The Weinberg-Salam theory, showing that the weak
nuclear and the electromagnetic interactions are merely manifestations of a
single interaction (now being called the electroweak interaction), received
the Nobel Prize because of the increasing experimental support for this union.
Due to the apparent infinite order required for strong nuclear interactions,
QFT has been virtually useless for many body interactions (e.g., nuclear
physics) and has been relegated to few-significant-figure precision
calculations with elementary particles. According to an article that I
recently read, written by Weinberg, QFT may be wholely inappropriate as the
starting point for quantization of general relativity.
While I took a year of QFT, I did not do well in the course. It appears to
require a few "leaps of faith" to proceed, and I wasn't up to the task. This
statement is being made so that you might know my biases and prejudices.
After consulting my Encyclopedia of Physics (1980) for the article on QED
(written by Bjorken of Bjorken and Drell fame), I have a criticism to offer.
QED cannot be the final theory covering electromagnetic interactions, despite
its substantial computational successes. QFT is even further removed from
nature. According to Bjorken, the presence of "bare" masses and charges has
never been considered a satisfactory treatment of what classically seemed
easy. This conflict between bare properties and experimentally observable
quantities is resolved through renormalization, which is also an unsatisfying
technique. Some of the more subtle issues around QED are ungoing experimental
investigation in an effort to resolve these unsatisfying features.
It is difficult to accept the concept that nature performs a renormalization
during all interactions. Though, to be fair, it is equally unlikely that a
particle explores alternate paths in the search for the path of least action
before making a move (perform a variational calculation to discover the
appropriate lagrangian as required in classical mechanics). At some point, no
matter how successful a theory has been up til then, some consideration must
be given to the "metaphysical" or conceptional aspects of a theory. Newton's
theories and definitions for dynamics worked well (except for Mercury) but
also required an absolute time (at least according to Newton). The success of
Einstein's special relativity (a theory that really treats the transformations
of absolutes such as mass and charge) shows the value of its conceptual
foundations, though it does have clear limitations (no accelerations). We
should recognize that the usual impetus for revising a theory has always been
the degree of inadequacy for explaining experimental results and how
uncomfortable the peer group feels about it.
QED has been greatly successful for calculating electromagnetic
interactions. QFT has been less successful, but is generally considered
adequate for most elementary particle interactions. Both require
renormalizations to work, and carry masses and charges that are different than
the observable quantities. I don't like the need for renormalizations, and
this makes these theories unattractive. Let us render unto the theories their
due (calculational successes) and look askance at their conceptional
implications (renormalization).
--
Patrick Wyant
AT&T Bell Laboratories (Naperville, IL)
*!ihuxm!gjphwclt@pur-phy.UUCP (Carrick Talmadge) (04/12/84)
I think the previous author has his notation slightly confused:
QFT is NOT an extension of QED any more than calculus is an extension
of classical mechanics! (The analogy is inexact, but so what?) QED is
based upon the techniques used in Quantum Field Theory, just as modern
Solid State Physics uses QFT for rather typical lattice calculations
(as a side note, one is just as likely to come across infinities in
Solid State Physics as in a theory of elementary particles such as QED).
I believe the author was referring either to the so-called "Standard Model"
(which incorporates the Glashow-Weinberg-Salam theory of electro-weak in-
teractions plus the theory of strong interactions), or possibly one of the
"Grand Unified Theories", which attempt unify electro-weak interactions with
strong interactions.
Carrick Talmadge
UUCP: {decvax,ucbvax,harpo,allegra,inuxc,seismo,teklabs}!pur-ee!Physics:clt
INTERNET: clt @ pur-phy.UUCPgwyn@brl-vgr.ARPA (Doug Gwyn ) (04/12/84)
I essentially agree with Mr. Wyant's dissatisfaction with QED & QFT, for similar reasons. However, it does not bother me that the basis for a theory may be a variational principle. Indeed, if there is to be any distinction between what might happen but doesn't and what does actually happen in the world, one may have to consider the alternatives that could happen but don't in order to determine which alternative is followed; this is effectively what a variational principle does. One doesn't have to imbue fundamental entities (particles or whatever) with either decision-making intelligence or understanding of OUR mathematical formulations in order for our mathematical description to nonetheless correctly describe what the entities do. I find the distinction between human-produced theoretical constructs and the behavior of absolute existents to be crucial when one is considering fundamental physical theories. The best example I know of so far is the development of the final field equations of Einstein/Schr"odinger unified field theory from a minimal set of physical assumptions. Some of the readers of this list already have a copy of my Master's thesis wherein I spell out the steps in this development. It is notable that neither the General Theory of Relativity nor the Unified Field Theory were the result of a general dissatisfaction with the predictions of existing theory; instead, they grew from Einstein's intuition about what constitutes a satisfactory explanation of fundamental phenomena.
crummer%AEROSPACE@sri-unix.UUCP (05/19/84)
From: Charlie Crummer <crummer@AEROSPACE> I think you haven't understood renormalization. (Read Bogoliubov and Shirkov for maybe the only truly lucid treatment. It requires work though.) Nature doesn't have to do renormalization since it, (or He or She), has the correct Lagrangian. It is the basic fun of QFT to work on getting the right one. When we get it, everything will fall out. This is the game of QGFT (Quantum Gauge Field Theory). --Charlie