Re: [asa] geocentricity

Yes, indeed. My point was not that any particular frame of reference is
preferred. Any one will do. Yet we need to recognize that more than one
possible reference frame exists. There's one in which the earth is at rest,
one in which the sun is at rest, one in which their center of mass is at
rest. We can choose whichever is most convenient for our calculations. In
any of them, the Milky Way galaxy or the cosmic microwave background are in
motion as well. In other words, the very concept of "stationary" is
relative.

The "must" I included was not to indicate a particular preferred reference
frame, but rather that the other bodies in the universe need to be
considered no matter which frame is used.

Randy

----- Original Message -----
From: "George Murphy" <gmurphy@raex.com>
To: "Randy Isaac" <randyisaac@comcast.net>; <asa@calvin.edu>
Sent: Friday, July 06, 2007 9:10 AM
Subject: 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/

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Received on Fri Jul 6 09:25:55 2007