(urth) "been teaching literature for over 35 years"

Gerry Quinn gerry at bindweed.com
Mon Sep 9 07:42:28 PDT 2013

From: Jeffery Wilson
On 9/9/2013 8:56 AM, Gerry Quinn wrote:
> From: Jeffery Wilson

>>> > It doesn't matter (in engineering terms) whether stuff that reaches 
>>> > the
>>> > event horizon goes somewhere else or not (that's kind of the 
>>> > definition
>>> > of an event horizon).  But an accretion disk outside the Schwarzschild
>>> > radius is just standard dynamics for any super-condensed object.
>>> What force acts on the matter to prevent it from following the usual
>>> swarm of elliptical orbits instead of the disk?
>> In a word, friction.  The matter would be gaseous in this case and
>> particles could not swirl in independent orbits.  But even if it were
>> composed of solid particles, they would soon collide, which would (1)
>> heat the colliding particles (2) reduce their energy, and (3) set the
>> colliding particles on closer trajectories than they were before.

>But all the stuff inside the sun's already colliding and touching.

>How is the axis and irection of rotation of the disk picked?

The Sun is already rotating, so its matter has angular momentum. Any 
difference in angular momentum gets magnified as an object shrinks (e.g. 
spinning skater pulling her arms in spins faster).  And angular momentum is 
conserved - there's nowhere for it to go except into the black hole, and 
even then it's one of the three kinds of 'hair' a black hole can have - i.e. 
a black hole's angular momentum can still interact with its surroundings, as 
can its electric charge and gravitational field.  A rotating object has a 
defined axis as well as direction.

This has happened before as the solar system was formed: note how all the 
planets and asteroids are in a plane, rotating in the same direction (bar 
the odd outlier that had some heavy collisions that sent it on an unusual 
trajectory)?  And the sun and planets also tend to rotate on their own axes 
in the same direction.  The solar system has a specific axis of rotation, 
because collisions early on resulted in a single axis dominating.

Another example: water draining out of a bathtub picks a direction, and 
sticks with it.

- Gerry Quinn

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