From: carlip@landau.ucdavis.edu (Steve Carlip)
Newsgroups: talk.origins
Subject: Re: Help needed to debunk creationism
Date: 27 Aug 92 00:28:30 GMT
Organization: Physics, UC Davis

In article <25147@castle.ed.ac.uk> eftv03@castle.ed.ac.uk (L R Watt) writes:
>[...]
>2. What is the evidence that radioactive isotopes have
>   constant decay rates over geological time?
[...]

If I may add a physicist's comment to some of the previous replies ---
A physicist would phrase this question as, "What evidence do we have
that the fundamental constants that determine decay rates are constant
over long time periods?"  This is an important issue, and has been
a hot topic of research, since a number of popular theories suggest
the possibility of (very slow) changes in certain constants such as
the strength of the electromagnetic interaction.  (In a futile effort
to avoid misquotation, let me emphasize that the rate of change 
relevant to string theory or Kaluza-Klein cosmology is _much_ too
slow to do the creationists any good.)  So there has been a lot of
creative work on how to look for evidence of such changes.

A nice (technical) summary is given by Sisterna and Vucetich in
Physical Review D44 (1991), p. 3096.  Among the phenomena they look
at are:
*searches for changes in the radius of Mercury, the Moon, and Mars
 (these would change because of changes in the strength of interactions
 within the materials that they are formed from);
*searches for long term ("secular") changes in the orbits of the moon 
 and the earth --- measured by looking at such diverse phenomena as 
 ancient solar eclipses and coral growth patterns;
*ranging data for the distance from earth to Mars, using the Viking
 spacecraft;
*data on the orbital motion of a binary pulsar PSR 1913+16;
*observations of long-lived isotopes that decay by beta decay (Re 187,
 K 40, Rb 87) and comparisons to isotopes that decay by different
 mechanisms;
*the Oklo natural nuclear reactor (mentioned in another posting);
*experimental searches for differences in gravitational attraction
 between different elements (Eotvos-type experiments);
*absorption lines of quasars (fine structure and hyperfine splittings);
*laboratory searches for changes in the mass difference between the
 K0 meson and its antiparticle.

While it is not obvious, each of these observations is sensitive to
changes in the physical constants that control radioactive decay.
For example, a change in the strength of weak interactions (which
govern beta decay) would have different effects on the binding energy, 
and therefore the gravitational attraction, of different elements.
Similarly, such changes in binding energy would affect orbital motion,
while (more directly) changes in interaction strengths would affect
the spectra we observe in distant stars.

The observations are a mixture of very sensitive laboratory tests,
which do not go very far back in time but are able to detect extremely
small changes, and astronomical observations, which are somewhat less
precise but which look back in time.  (Remember that processes we
observe in a star a million light years away are telling us about
physics a million years ago.)  While any single observation is subject
to debate about methodology, the combined results of such a large
number of independent tests are hard to argue with.

The overall result is that no one has found any evidence of changes
in fundamental constants, to an accuracy of about one part in 10**11
per year.  

Steve Carlip
carlip@dirac.ucdavis.edu

 
Newsgroups: talk.origins