Date: 1999 - 2000
I read a quote from Sir Fred Hoyle that said that because
of the theory of relativity that it was just as true to say that the earth went around the sun as to say that the sun went around the earth.
Is this true? Was the Copernican Revolution a mistake?
Well, no. The Earth does not go around the Sun, nor the Sun around
the Earth, if you want to be very careful: they both revolve around
their center of mass. Now, the Sun is much heavier than the Earth, so
that the center of mass of the Earth-Sun system is almost at the
center of the Sun. So the "orbit" of the Sun is just a little wobble
which perhaps is undetectable, while the orbit of the Earth surely
looks like it is centered on the Sun.
The larger issue here is what relativity means. Relativity does not
mean everything looks the same from all viewpoints: it merely means
that everything looks the same from all viewpoints that have no
physical reason to be different. Stated that way, it's kind of
obvious, isn't it? If you really want to see a planet-Sun system
where it doesn't matter where you stand on the Sun or the planet, then
you need a system where both have the same mass. See, because the Sun
has a much larger mass, then its viewpoint has good reason to be
different from that of the Earth. If you had a double Sun system, or
double planet, and both were exactly the same mass, then I expect you
find it easy to believe that both objects would revolve around a point
midway between them (their center of mass) instead of one arbitrarily
around the other.
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Update: June 2012