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I think a problem occured when i used long()

Andrew Dalke adalke at mindspring.com
Thu Sep 2 21:55:19 EDT 2004 
Porky Pig Jr wrote:
> Incidently, if I recall, the arguments against 'very high precision'
> was coming from scientists (e.g. those dealing with quantum mechanics
> issues) rather than from programmers. The main argument was that the
> measuring tools' precision is soo well below 53bit precision available
> as 'C double' that using anything higher than that will mistakenly
> create the impression of 'very high precision of the experiment' - but
> this is just it - *mistakenly*.

Out of curiosity, I looked for the physical constant
with the most precisely measured value.  It looks
to be the electron magn. moment to Bohr magneton ratio
   http://physics.nist.gov/cuu/Constants/Table/allascii.txt
which is
   -1.001 159 652 1859  with error of 0.000 000 000 0038

That's 1 part in 10**13.  So there are a few things
which need that sort of precision.

(53 bits is about 1 part in 10**16.  10**13 needs only
43 bits.)

I also think GIS systems need enough precision so that
single isn't good enough for large maps.  23 bits for
a 32 bit float gives about a 4m resolution while 53
bits gives about a nm resolution.

I did molecular mechanics, not QM, but I don't recall
the QM people complaining about this issue.


				Andrew
				dalke at dalkescientific.com
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