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Our Usual View of the Universe is a Convenient Approximation

In article <DAo7rr.5x@cix.compulink.co.uk>,
 on Sat, 24 Jun 1995 09:31:03 GMT,
 "Brian Portlock" <???@???> writes:
>In article: <CLUNE.95Jun20092538@pasta.colorado> clune@pasta.colorado
>(Thomas L. Clune) wrote:
>> I disagree.  It is entirely straightforward to take a theory like
>> general relativity and *express* it entirely in terms of a stationary
>> Earth at the center of a revolving universe.
>
>This is total drivel! Any amateur astronomer with time on their hands and
>a modest telescope (say 6"-10") can  show that the planets do *not*
>revolve around the earth simply by *looking* at them and observing their
>phases.  ...

This isn't the point.  Choosing a different viewpoint does not affect
what happens in the universe, but may make it more or less difficult
to describe what is going on.

Relative to an observer on the Earth, really crazy things appear to
happen to the planets and stars, for example due to the rotation of the
Earth and the fact that we are in a gravitational field (and hence a
non-inertial frame of reference) but we have learned to account for
these by using mathematical techniques to transform our viewpoint to a
frame in which the centre of mass of the solar system is approximately
at rest and non-rotating relative to the fixed stars.  We are hardly
even aware that we do this normally.

In a primitive Newtonian description of the universe, it seems that
acceleration and rotation (strictly speaking, angular velocity) might be
absolute, in which case there is some sort of universal frame of
reference in which everything can naturally be described, so that is the
one we should normally use.  Our mental images of the solar system tend
to be based on this idea of a Newtonian universe.

Relativity theory shows us that although accelerations and rotations can
be measured locally (for example using springs and gyroscopes), they do
not necessarily match up exactly at different points in space, because
of gravity.  We can say that one set of measurements is "right" and the
other is being affected by gravity, but in general there is no clear
distinction as to which one we should take, especially when dealing with
situations where the curvature of space is significant, such as on the
cosmological scale.

We can of course use Newtonian laws to ask "what would everything look
like if I was rotating?".  It is well known that a rotating observer
sees fictitious forces (centrifugal and coriolis forces) that make
things move in odd ways, which turn out to be perfectly logical when
seen by a non-rotating observer.

In relativistic gravity theory, we don't even have to distinguish
explicitly between rotating and non-rotating frames of reference, as if
we happen to choose a frame is rotating, these forces arise naturally as
consequences of the motion of the gravitational sources.  (However,
rotation effects caused by anything less than the whole universe are
really tiny in practice, since both the source of the potential and the
object in its field would have to be travelling at near the speed of
light for the acceleration caused by the rotation to be similar in
magnitude to the ordinary gravitational acceleration).

The conclusion is that although some viewpoints make the universe look
simpler than others, neither acceleration nor rotation is really
absolute, and our usual way of looking at the solar system and nearby
stars is just a convenient approximation to the local average motion
which makes everything approximately Newtonian, and hence easier to
describe.  Describing the universe from the point of view of the Earth,
or even worse from the point of view of a location on the rotating
surface of the Earth, is rather complicated, but astronomers have to do
it all the time in order to know where to point their telescopes.

Jonathan Scott
jonathan_scott@vnet.ibm.com
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