gjphw@iham1.UUCP (07/02/84)
This note is in response to a query for comments by L.C. Noll (nsc!chongo).
I suspect that the astro newsgroup will be swamped with replies to the
question soon. The original question concerned the size and shape of the sun,
and the influence the new discoveries would have for relativity.
Despite its appeal of intellectual elegance and physical symmetry, the
general theory of relativity (hereafter called Einstein theory) has only three
experimental tests to date. These are the redshift of light traveling out of
a gravitational field, the deflection of light passing a strong gravitational
source (e.g., a massive body), and the advance in the perihelion of Mercury's
orbit. Einstein theory yields a value for the perihelion advance in agreement
with observations of Mercury's behavior only if the sun is very close to being
spherical. Since 1915, one of the moons of Jupiter has also been observed to
possess an advancing perihelion (after correcting for a non-spherical
Jupiter), which is indicative that the local (to that moon) gravitational
field is not strictly inverse-radius-squared. This observation provides a
good test for competing general relativity theories.
Since shortly after the publication of Einstein theory, people have
wondered about the sun being exactly spherical. After all, Jupiter is
obviously oblate, and Newton's technique of treating a mass as located at a
point works only for spherical bodies (or very far away from irregular
bodies). The first person that I am aware of who began an observational
program to check for any aspheroidicity was Robert Dicke (who incidentally
produced a competing general relativity theory known by its authors - Brans,
Dicke, Jordan). He failed to observe any deviation from spherical in the late
1950's. A former student of Dicke's, a guy called Moffet (sp?), is continuing
the study. This uncertainty concerning the sun as spherical is the only issue
that directly touches Einstein theory.
I have vague memories of articles discussing the stability of the sun.
From the ongoing studies of other nearby stars, the sun is now considered to
be in a relatively quiet period in terms of sunspot activity and power output.
A chemist set up a large neutrino detector in an old silver mine, and has been
observing a neutrino flux of about a third of what was predicted assuming the
most popular solar energy generating cycle. To my knowledge, this discrepancy
between observation and model predictions for the solar neutrinos has not been
resolved.
There has been some discussion in technical journals dealing with the
Earth's climate that the output of the sun has been slowly changing with time
(usually increasing), but there seems to be little definitive treatment of
this issue. Certainly there are other issues that are relevant to your
question concerning the size, shape, and stability of the sun, but I am not
knowledgeable to discuss this issue further. Good luck.
--
Patrick Wyant
AT&T Bell Laboratories (Naperville, IL)
*!iham1!gjphw