Subject: Re: CCL:orbitals and reality

From: Eric Scerri <scerri () chem.ucla.edu>
Date: Sat, 19 Feb 2005 15:13:59 -0800

In response to my recent posting shown below with double lines,

>>Many useful things are unobservable!
>>
>>The question was indeed whether one can observe an orbital.
>>I don't think anybody really doubts the usefulness of the concept.
>>
>>Electrons and nuclei are at least observable in principle.
>>The usual understanding is that orbitals are not.

Konrad Honsen responded,

>Electrons and nuclei are not observable, its some of their 
>properties  that are. The position, velocity, energy, charge, etc. 
>of an electron  are observables. Other properties, such as its wave 
>function, are not  observables. When we say "electrons are 
>observable", this is really  just a shorthand for saying "electrons 
>have observable properties".
>
>Note also that the status of a quantity as an "observable" is part 
>of  the model, not of "reality". The model of an electron with all 
>the  properties listed in physics textbooks is the outcome of a long 
>process  of experimentation and model making. There is no operation 
>that allows  to establish the list of all observable properties of 
>an electron. They  might become more numerous in the future, but 
>future research might  also lead us to abandon the concept of the 
>electron, or at least  downgrade its status in our understanding of 
>nature.
-------------------------------------------------------

First an apology to those who find these continuing discussions on 
orbitals somewhat tedious and who wish to return to the important 
business of talking about the details of computational methods.

To my mind this is a rather disappointing attitude.  The emergence of 
philosophical discussions among scientists is surely a sign of change 
occurring and should be welcomed as a respite from the day-to-day 
work and as an opportunity for practitioners from different parts of 
the field to engage in fruitful discussion, more widely than in the 
normal course of events.
When QM faced a crisis in the 1920s the leading physicists turned to 
philosophical discussion.  Not that I believe that this issue will 
lead to a similar big breakthrough.

It seems to be that there are some rather formidable grounds for 
believing in the real physical existence of electrons as opposed to 
orbitals even though neither are directly observable. Yes we only 
observe properties of electrons but we then infer the existence of a 
particle that gives rise to these properties.

Starting with J.J. Thomson in 1897, if not before, many properties of 
electrons have been 'observed' such as its mass, charge and spin. 
Would it not be a miracle if all these experiments pointed towards 
the existence of the electron and yet there was no such particle (The 
no miracle argument for realism).

There have been a number of very influential theories of the electron 
starting with Thomson, Rutherford, Bohr, Schrodinger, Dirac etc. 
Even though these theories may have described the electron in 
different ways there is little doubt about the continuity of 
reference.  In other words there is little doubt that Thomson's 
electron is the same as Bohr's and Dirac's electron.

In addition there has never been any feature within these theories 
that have specifically ruled out the physical existence of the 
electron.

Now orbitals.
By no stretch of the imagination is there the same mass of evidence 
for the physical existence of orbitals!  Zuo et al. made a strident 
claim in 1999 which made the front page of Nature magazine but was 
duly criticized in a number of other publications.

Why has it had to wait until 2005 for the first serious claim for 
observational evidence for orbitals (apart from Brion and other such 
claims of a different kind) ?  Why is it that unlike in the case of 
electrons the theory does dictate that wavefunctions do not have a 
physical existence and that they are just mathematical fictions?

Is it really appropriate to claim that the status of electrons vis a 
vis their physical existence is the same as that of atomic and 
molecular orbitals just because none of them are directly observable?

Most philosophers of science take a realistic attitude towards 
electrons and other fundamental particles.  Only a die-hard 
anti-realist like Bas van Fraassen would want to deny the existence 
of electrons because they cannot be observed directly.

I have not taken a poll of philosophers of science on this question 
but I suspect that most of them would not be realists about orbitals, 
regardless of the recent images from Villneuve et al.

Scientific realism does not rest on actually being able to observe 
the entity in question but is precisely a view about unobservable 
scientific terms and entities.  It is only naive realism that 
requires actually observing the entity directly.

regards,
-- 

Dr. Eric Scerri ,
UCLA,
Department of Chemistry & Biochemistry,
607 Charles E. Young Drive East,
Los Angeles,  CA 90095-1569
USA

E-mail :   scerri ~~ chem.ucla.edu
tel:  310 206 7443
fax:  310 206 2061
Web Page:    http://www.chem.ucla.edu/dept/Faculty/scerri/index.html

Editor  of  Foundations of Chemistry
http://www.kluweronline.com/issn/1386-4238

Also see International Society for the Philosophy of Chemistry
http://ispc.sas.upenn.edu/