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One Can Use any Point as the Center...

In article <3sc148$8nn@netnews.upenn.edu>,
 on 22 Jun 1995 15:10:00 GMT,
 Kevin Sterner <sterner@sel.hep.upenn.edu> writes:
>In article <3sapbs$nts@dingo.cc.uq.oz.au>, davidcs@psych.psy.uq.oz.au (David Smyth) writes:
>
>> I just know the physicists are going to tell me what I just said is
>> incorrect - and they are right according to General Relativity.  The
>> pendulum would rotate due to gravitational effects of the rest of the
>> rotating universe if the earth was stationary.  My apologies to
>> the Swami (even if I still think he did fall out of a very tall tree).
>
>Nope, it can't work that way.  You were right the first time.  If you were
>at the center of a rotating sphere of matter, there would be no force
>causing you to begin rotating.

No-one said you would begin rotating.  What happens is that you observe
centrifugal forces and coriolis forces, which means that there is a
force outwards from the center, and that things which move in your frame
experience extra deflection forces depending on which way round they are
going.

Of course, one would normally attribute this to having a rotating frame
of reference, but the point is that rotation is NOT absolute; it is a
locally defined effect which can be simulated by gravitational forces in
the same way that acceleration can be simulated, only usually much
weaker (since gravitational sources typically move at much less than c).
Since it isn't absolute, we can adopt viewpoints where one person's
rotation is another's gravitational effect, and it seems plausible
(from Mach's ideas) that what we normally define as rotation can be
taken to be relative to some sort of average of the whole universe.

This means that we can choose to describe the universe taking any
point as the center, but it's just somewhat more convenient to choose
a point which doesn't have too much acceleration or rotation relative
to other local landmarks, as that makes the calculations simpler for
the things we can see around us.

Jonathan Scott
jonathan_scott@vnet.ibm.com  or  jscott@winvmc.vnet.ibm.com
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