Dear Ian,
The iteration seems to be converging :-)) .
Regarding your last paragraph, however, I do not think that we have to
forbid ourselves to call a spade a spade because of possible confusions that
might be caused by misleading abbreviations. If the unit is "an electron's
worth of scattering", its abbreviation to "electron" has to be accompanied
by a suitable annotation so that it does not get confused with some physical
attribute of an electron such as its charge. I don't think that anticipated
inadequacies of notation should stand in the way of correct terminology. If
necessary, we should draft a one-liner that could be added as a footnote to
every Table 1 (like the formula for the R-factor!) and would explain that
the word "electron", as used in the context of that Table, actually means
"an electron's worth of scattering"; and to show how important we are, we
could even have the exact wording reviewed and approved by the Computing
Commission of the IUCr! But this would start sounding like Russian history
in the 1920's ... .
With best wishes,
Gerard.
--
On Mon, Mar 01, 2010 at 04:14:57PM +0000, Ian Tickle wrote:
> Dear Gerard
>
> I would certainly agree that in general, provided one takes sufficient
> care over dimensions and units, paradoxes can never appear. However,
> in this particular case I was pointing out the dimensionality error of
> writing equations such as "f = 10e", and equivalent ones for the
> structure factor and electron density, given that 'f' is defined as a
> dimensionless ratio (as I believe it usually is). Even if you
> replaced the 'e' with whatever unit represents an electron's ability
> to scatter X-rays (which would be the amplitude of the scattered
> wave), you still have the same problem. I only focused on electric
> charge because 'e', the elementary unit of charge, was being posited
> as the unit of 'f'.
>
> The alternative solution that you suggested of using the word
> 'electron' as an abbreviation for "an electron's worth of scattering",
> is likely to cause just as much confusion and probably would be
> further abbreviated to 'e' anyway, thus leading people to believe it
> represented the electronic charge! The correct solution, as you, Marc
> and myself have pointed out, is to treat f as a pure number, with
> corresponding treatment of any other quantity that depends on f.
>
> Cheers
>
> -- Ian
>
>
> On Mon, Mar 1, 2010 at 1:13 PM, Gerard Bricogne <[log in to unmask]> wrote:
> > Dear Ian,
> >
> > Perhaps I should have made a more explicit connection to your message
> > in what I wrote yesterday. I do not think there is any paradox, or apples
> > vs. oranges problem, in this situation.
> >
> > The structure factor is a count of "electrons as X-ray scatterers", so
> > that the Fourier synthesis computed from them is a number density for these
> > unit scatterers. The density can get clothed with a charge a-posteriori,
> > because we know what the charge of an electron is, but it is not that charge
> > as such that is sensed by the diffraction experiment: it is the complicated
> > combination of charge and mass and various physical constants that ends up
> > determining an electron's ability to scatter X-rays.
> >
> > I think that if one bears this in mind at all times, paradoxes never
> > appear.
--
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