Subject: Re: CCL:Dirac's dictum

From: "Jens Spanget-Larsen" <[email-masked]>
Date: Thu, 5 Sep 1996 13:07:33 +0100

Georg:

> I haven't looked up the original paper myself, but I found the quote
> cited as P.A.M. Dirac, Proc.R.Soc.London Ser.A 123, 714 (1929). The
> qote is now known as "Dirac's dictum". Appareantly, it is from a speech that
> Dirac gave in Cambridge in 1929.
>    The paragraph before the "dictum" is equally interesting, because
> Dirac thought that the newly developed theory of relativistic quantum
> mechanics would be of no importance for chemical applications.

Thanks for your reply. I have requested the volume at our university 
library and is looking forward to read the whole story.  What is your 
opinion of the merits of the paragraph before the "dictum", as 
referred to in your reply? 

I have another and more recent quotation that is less provoking, perhaps, 
but I find it very elegant and wise:

"Quantum chemistry is faced with two fundamental problems: how to
reduce chemistry, in a practical way, to numerics; and how to avoid
doing so and still benefit from theory."

K. Wittel, S. P. McGlynn: Chem. Rev. 77, 745 (1977)

Yours, Jens >--< 

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JENS SPANGET-LARSEN
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From: <[email-masked]>
Date: Thu, 5 Sep 1996 10:03:30 -0600 (MDT)

Dear Jens,

o.k., I was, so far, too lazy to type the mentioned other paragraph. But
since you ask for it, here it is:

"The general theory of quantum mechanics is now almost complete, the
imperfections that still remain being in connection with the exact fitting of 
the theory with relativity ideas. These give rise to difficulties only when
high-speed particles are involved, and are therefore of no importance in
the consideration of atomic and molecular structure in which it is, indeed, 
usually sufficiently accurate if one neglects relativity variations of mass
with velocity and assumes only Coulomb forces between the various electrons
and atomic nuclei."

[same reference]

Pyykko has a nice little discussion as to why Dirac is wrong here
(P.Pyykko, Chem.Rev.1998,88,563). This paper is also the place where I read
the "dictum" for the first time.
   Another question is, of course, whether "the general theory of quantum
mechanics" is (almost) complete by now or not. I have heard all kinds of opinions
on this question.

Yours, Georg
--
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From: Fred Dulles <[email-masked]>
Date: Thu, 5 Sep 1996 22:03:10 -0500 (CDT)

> mechanics" is (almost) complete by now or not. I have heard all kinds of opinions
	By way of adding a bit of historical context to this question,
it's worth noting that at this point in time relatively little was known
about the nucleus (or smaller paticles and related forces), at least as
compared to what was found later.  I believe the neutron for example was
discovered a bit later in 1930.  So Dirac might have altered his view
as things developed.  I don't know if he did in fact. 

	Alternatively, if this is a discussion of the Copenhagen
interpretation (or QED), I will stay out of it, thanks.

					-Fred Dulles  

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