Subject: RE: orbitals and reality

From: "Phil Hultin" <hultin..at..cc.umanitoba.ca>
Date: Thu, 20 Jan 2005 08:58:39 -0600

This is a copy of a note I sent to Sengen Sun.  I am posting it now because
there does seem to have been some interest in the issue he raised on CCL -
at the time there had been no response.  Dr. Sun has promised to respond to
my comments in the future.
====================================
Dr. Sun:

I noted your request for further response to the "orbital debate" that has
arisen from the Nature paper by Villeneuve et al.

The orbital model is undeniably a key part of modern electronic structure
theory, and it is therefore understandable that experimentalists would seek
physical confirmation (if not proof) of its validity.  On the other hand, it
is equally true that theoreticians have always maintained that orbitals are
"merely" mathematical constructs, that any arbitrary set of orbitals does
not represent a unique solution to the Schroedinger equation, and that
orbitals are therefore non-physical and non-observable.

So, to get to the argument over Villeneuve et al: Having read the paper I
must confess that I do not fully understand the experiment that they did.
They assert that the overtone spectra that they acquire encode symmetry
information about the electronic state (which they call an orbital) from
which the electron is ejected and to which it returns.  Since they cite
literature support for this assertion I am prepared to accept it for the
sake of argument until someone provides a persuasive counter argument.

So, the question is, what did they actually observe and how should it be
interpreted?  I suggest that denying their experiment on theoretical grounds
is scientifically inappropriate.  Observation must have priority over
theory, or we are talking about religion instead of science.  This doesn't
mean that they actually observed an orbital - but rather, if you are going
to criticise their interpretation of the data you should present an
alternative explanation of the origin of their spectra that is consistent
with your view of electronic theory as well as with their observations.

While the fact that orbitals are "only a model" means we must not assume
that they are "real", I suggest that it is inappropriate to take it as
axiomatic that the orbital model cannot embody at least some aspect of
physical "reality".  It is entirely possible that electronic structure
behaves under certain specific probes as if orbitals were real, that is,
that our artifical model fortuitously captures an observable aspect of
electrons.

I don't want to argue that Villeneuve et al are correct.  But please, if you
really want to conduct a scientific debate, address their experimental
observations directly and don't simply say that "because they are
inconsistent with theory they must be false". 

Dr. Philip G. Hultin
Associate Professor of Chemistry,
University of Manitoba
Winnipeg, MB
R3T 2N2
hultin..at..cc.umanitoba.ca
http://umanitoba.ca/chemistry/people/hultin