Aspects Of Our Sun

摘要

The shape of the Sun touches on several issues in cosmology and solar physics, including whether Einstein's General Theory of Relativity is the correct theory of gravity, and to what extent the solar interior rotates. On page 560 of this issue, Fivian et al. (1) present compelling satellite-based observations showing that the Sun's shape is in agreement with what is expected from the rotation of its visible surface. These latest observations eliminate the possibility of a rapidly rotating core and remove one of the last remaining challenges to the validity of General Relativity from solar system studies. Gravity is still a bit mysterious. Of the four fundamental forces, it defies unification with the other three (electromagnetism, weak nuclear, and strong nuclear). Einstein's General Theory of Relativity, the currently accepted theory of gravity, has passed all tests that have been devised for it with flying colors. Early on, the only tests involved the solar system and the Sun's gravity (2, 3). The two most famous tests were the deflection of starlight and the precession of Mercury's orbit (the motion of its perihelion). Both of these effects are caused by the warping of spacetime in the vicinity of the Sun. General Relativity was able to explain the precession of 43 arc sec per century that was not accounted for by the Newtonian gravitational effects of the other planets. However, another theory of gravity, the scalar-tensor theory of Brans and Dicke (4), suggested a different value of the precession that depended on a proposed "coupling" of a mass with the mass of the universe. For a modestly small value of this coupling constant, the scalar-tensor theory predicted a smaller value of the precession than was observed. Then, in order for the scalar-tensor theory to agree with the observed precession of Mercury's orbit, the Sun needed to have an excess oblateness (i.e., the equatorial radius minus the polar radius) over and above what is expected from its observed surface rotation (with a rotation period of about 28 days).