nN 47 ‘963

PALMER PHYSICAL LABORATORY
PRINCETON UNIVERSITY
PRINCETON, NEW JERSEY

January 16, 1963

Professor Joshua Lederberg

Department of Genetics

Stanford University School of Medicine
Stanford Medical Center

300 Pasteur Drive

Palo Alto, California

Dear Professor Lederberg:

This is in reply to your letter of January 12th. While the possi-
bility of other physical constants, in addition to the gravitational
constant, changing with time has been occasionally considered in the
past, the evidence seems to rather strongly suggest that none of these
affects take place. The affects which have been considered have been
the variation in the ratios of masses of elementary particles and the
electromagnetic coupling constant. The very precise gravitational ex-
periment performed by E8tv¥s and several more recent precise experiments
set stringent limits on the variability of dimensionless numbers of this
type. I would conclude that the only physical constant which we need
to consider as variable is the gravitational constant.

You are quite right about the brightness of stars in the past being
greater, however, this would only show up on very distant galaxies where
there is already very great uncertainty concerning the absolute bright-
ness. Galaxies change their absolute luminosity for other reasons such
as the gradual depletion of gas and the resulting decrease in the rate
of formation of new bright stars.

Because of your interest in genetic questions, I wonder if I could
give you a question? It might reasonably be presumed that genetic
materials are among the oldest biological materials that we have. Is
there any evidence that they incorporate particularly stable amino acids
(that is stable from a thermal point of view)? The point to this question
is that if living forms had their origin at a time of rather high tempera-
tures, one might expect to find biological materials at this time capable
of survival under high temperature conditions.

Sincerely yours,

NAC -
RAD:mf VR - Dicke