Re: [asa] geocentricity

----- Original Message -----
From: "Randy Isaac" <randyisaac@comcast.net>
To: <asa@calvin.edu>
Sent: Thursday, July 05, 2007 5:21 PM
Subject: Re: [asa] geocentricity
.....................

> To the scientific question of whether the helio- or the geo- frame should
> be used, the answer should be neither. As particle physicists do, one must
> talk of the center of mass reference frame wherein the two-body system is
> at rest, though eventually the effects of other bodies must be included.
.........................................

Randy -

Please excuse me if I seem to be both pedantic & fanatical about this but
there is no "must" in regard to reference frames. Of course it's true that
the earth exerts a gravitational influence on the sun as well as vice versa,
so that what we usually call a "heliocentric" frame is (when we're doing
precise celestial mechanics) one in which the CM of the solar system is at
rest. But there is no "must" about using this frame or any other. General
relativity really is general - you can use any space-time coordinates you
wish (subject to some modest conditions about continuity &
differentiability). Calculations may be far simpler in one frame than in
another but that is a matter of practical convenience. & there's also the
question of what calculations you want to do. If you just want to calculate
orbits in the Schwarzschild geometry around a black hole, you'll probably
choose the coordinates that are now standard in introductory presentations
of general relativity, ones in which time & the radial coordinate are easily
connected with corresponding quantities in classical physics. But in these
coordinates the nature of the black hole & what happens inside the event
horizon are obscure. Another system of coordinates (Kruskal-Szekeres) makes
these properties of a black hole obvious but they would be clumsy for
orbital calculations.

The choice of space-time coordinates is entirely a matter of convention &
convenience, with no fundamental significance.

& this does not mean that "everything is relative." Motion is relative - &
that includes accelerated motion. (You can use an accelerated frame if you
wish.) But there are other things that aren't relative - space-time
interval, curvature invariants, entropy, charge, &c.

Shalom
George
http://web.raex.com/~gmurphy/

To unsubscribe, send a message to majordomo@calvin.edu with
"unsubscribe asa" (no quotes) as the body of the message.
Received on Fri Jul 6 09:11:24 2007