Re: [Phys-l] Clarification: A ball at the center of a planet

[David Bowman <David_Bowman@georgetowncollege.edu>]

Regarding John M's comment:

> I think the reason we all got caught up in picky minutiae is that
> the question seems like it can't possibly hold any interest
> absent the effects of minutiae.
>
> Now, however, I think we have a fairly well-defined question.
> I'm not sure what a "massive run away planet" is, but assuming
> the intent is to distinguish between a position at the center of
> a cavity in a gravitating body in free space and that same
> position minus the gravitating body, I'll choose C; no
> difference. I suspect that isn't literally true general
> relativistically, but I'll trust Hasan's implication that he
> isn't interested in such quibbles.

It is *still* true (i.e. answer c) "general relativistically".  Note Birkoff's Theorem (i.e. the one related to relativity, not ergodic theory, nor abstract algebra, nor boolean algebra).  The spacetime inside the spherically symmetric cavity at the center of the spherically symmetric planet is flat (barring the tiny effect from the presence of the ball itself when it is placed there.  But that tiny effect is also present in deep space anyway when the ball is there.  Both situations give the *same* results for the ball.

> John Mallinckrodt
> Cal Poly Pomona

David Bowman