Subject: Re: orbital and reality

From: Stefan Bromley <s.bromley/at/qf.ub.es>
Date: Wed, 02 Feb 2005 10:20:51 +0100

Dear all,

I have been following the evolution of the discussion on the reality of 
orbitals with interest. I think it is important, however, to be careful 
to distinguish claims about the possible physical reality of the orbital 
model and the possible physical reality of actual wavefunctions.

Malcolm Davis for example writes:
"Folks have tried to show that there is some "normal" reality underlying 
QM that we just cannot measure.  Such "Hidden Variable" theories have 
been ruled out by "Bell's Inequality" and the "Aspect" experiments.  So, 
wave functions are as real as it gets".

This is a very common misconception and is one I hope that does not 
pervade throughout the whole of the CCL community. Bell's inequality and 
the Aspect experiments rule out only *local* hidden variable theories. 
Based on the work of de Broglie, David Bohm in the 1950s developed an 
example of a theory not ruled out by the Bell inequality (A suggested 
interpretation of the quantum theory in terms of "hidden" variables. I & 
II. D. Bohm, Phys. Rev. 85, 166-193, 1952). In fact Bell's inequality 
came about largely through contemplation of Bohm's work - in particular 
its non-local aspect. Importantly, Bohm's theory, by the necessity of 
its construction, it is exacty equivalent in its predictions to standard 
quantum mechanics. The extra given by such a theory is many-fold in 
terms of physical insight but arguably little in terms of  present day 
measurement. You can find a large resource of literature on the theory 
(see e.g. The Quantum Theory of Motion: An account of the de 
Broglie-Bohm Causal Interpretation of Quantum Mechanics by Peter 
Holland, Cambridge University Press 1995). In this theory (or 
interpretation) the wavefunction is a real field which gives rise to a 
quantum potential acting on a real particle (e.g. an electron) which 
follows a *non-classical* trajectory in real space.

I do not wish to go into the merits or physical validity of such an 
interpretation but just to note that it is perfectly possible to have a 
quantum theory where wavefunctions and electrons do have "an underlying" 
physical reality.

It is, perhaps, of further note that Bohm's causal interpretation of 
quantum mechanics based on the wavefunction as a guiding field to 
particles has lead to some recent advances in the 
modelling/interpretation of scattering and tunnelling processes and 
Quantum Field theory (see e.g. O. V. Prezhdo et al. Phys.Rev. Lett. 86, 3521, 2001, C. L. Lopreore 
et al. Phys. Rev. Lett. 82, 5190, 1999,  A. S. Sanz et al. Phys. Rev. B 
69, 11513, 2004, Phys. Rev. Lett. 93, 090402, 2004)

Best regards,

Stefan Bromley