🚀 go-pugleaf

In article <30SEP199220042116@zeus.tamu.edu> dwr2560@zeus.tamu.edu (RING, DAVID WAYNE) writes:
>baez@ucrmath.ucr.edu (john baez) writes...
>>>                                           The region of space which is
>>>inside a galaxy satisfies the same local einstein equation as the above
>>>recollapsing example, so why do galaxies live so long?
>> 
>>'Cause they're smaller.  I'm sure one can work out the math, but
>>it's bound to amount to just that.  For example, one can have
>>a fist-sized ball of iron, which will essentially never collapse
>>(quantum fluctuations aside), but a galaxy-sized ball of iron
>>would collapse mighty fast.
>
>I'm surprised at you John, you forgot the atomic repusion! :)
>
>I get that a fist-sized ball of superdust weighing in around a kilogram
>collapses in about an hour. I think that will be about the same for an
>observer in the center of your iron galaxy. But I'm depending on my
>already disproven intuition. :)

You're right, my example was fairly irrelevant, even though what I said
about the ball of iron versus BIG ball of iron is true.  For iron the point
is that gravity beats atomic repulsion for large objects.  Galaxies
aren't held up by atomic repulsion.  So - is, or is not, Dave Ring's
theory right?  Namely, say one takes a ball of "dust" (in the sense of
relativity, i.e., no repulsive forces) of a given density and radius R.
How long will it take to collapse as a function of R?

Extra credit if you solve it for rotating dustballs, so that it's more
like honest galaxies.