You do not need to discuss the intricacies of the Lamb shift and virtual
particles in order to illustrate the basic property of INDISTINGUISHABILITY
shared by quantum-mechanical particles. From Quantum Mechanics by Liboff, p.
577:
"There is a very fundamental distinction between the quantum and classical
descriptions of such systems < my note: he is referring to identical particles,
like electrons>. At the quantum-mechanical level of description, identical
particles are also indistinguishable. In the classical description of a system
of identical particles, one may conceptually label such particles and follow
their respective motion. THIS IS IMPOSSIBLE <emphasis mine> at the quantum
level. There is not experimental result that distinguishes between two states
obtained by exchange (interchange) of identical particles."
Now Glenn, I believe that your original intent was to show the impossibility of
predicting where a given electron would be in the future, because your
conclusion was:
>Since this Lamb-Rutherford shift has been experimentally
>observed, I would contend that not only can you not predict the location of
>the electron, you can't even be sure it is the same electron next year.
>Quantum is very, very strange stuff.
>What you can predict (and I may be wrong here) is that the electron or its
>offspring will be somewhere within 1 light year of me next year.
However, in light of the fundamentally INDISTNGUISHABLE nature of identical
particles at the quantum level, your point seems hollow. We already know,
apart from the Lamb shift and apart from any virtual particles, that there is
no experimental way to distinguish the situations you described, in which two
electrons have effectively exchanged places. So why lay such emphasis on the
inability of theory to predict something that we can never hope to observe
anyway?
I agree that quantum mechanics is strange stuff. But what you did not point
out in your note is that the Lamb shift, discovered by Willis Lamb and his
student Retherford (not Rutherford) in 1947, is a small but barely measurable
effect of the quantum nature of the electromagnetic field on the energy levels
of the hydrogen atom. In fact, the announcement of this discovery at the
Shelter Island Conference in 1947 led to a flurry of theoretical activity by
the likes of Tomonaga, Feynman, and Schwinger. They worked out the rudiments
of quantum electrodynamics (QED) in order to explain Lamb's results, which were
in conflict with the predictions of the Dirac equation of quantum mechanics
(the discrepancy is due to the classical treatment of the electromagnetic
field). My reason for pointing this out is to give the context for the
following quote from "From X-rays to Quarks", p. 273, by E. Segre:
"The present agreement (1976) between theory and experiment is admirable,
perhaps the most precise in all physics. For instance, the magnetic moment of
the electron <my note: whose value is affected by the quantum nature of the
electromagnetic field> can be measured to 2 parts in 10^9, and the result of
the measurement agrees with the calculation to 1 part in 10^9. Also, the
calculation has a residual uncertainty of about 3 parts in 10^9 because of
uncertainties in higher approximations."
So the history of the Lamb-Retherford shift is actually a good example of the
difference between the interaction of theory/experiment between physics and
evolutionary biology. In the one case, a focus (even a preoccupation) on
specificity of predicition (here, retrodiction), with a willingness to modify
what seems to be a successful theory (classical electromagnetism) in order to
reconcile the theoretical model with the experimental data. In the other case,
an extreme amount of hand waving, "just-so" story-telling, and a paucity of
quantitative description or prediction.
Reading what I have written again, I suppose that I am vulnerable to the charge
that my recapitulation of scientific history is hopelessly naive and
positivist. I am well aware of the deficiencies of positivism in science, so
please don't respond to my post in that vein. My main point is that to those
of us trained in the quantitative sciences (you too, Glenn), it is amazing how
low the burden of proof is in the area of evolutionary biology. It's hard to
be impressed by Dawkins and company when you have grown up reading Einstein
and Feynman.
With my professional bias showing,
Stan Zygmunt
Dept. of Physics and Astronomy
Valparaiso University