Re: Black Holes and tidal forces

["John S. Denker" <jsd@MONMOUTH.COM>]

Tina Fanetti wrote:
> okay so how would I explain that to people who don't
> know what a gradient is...the change?

More specifically, the change per unit distance.


Near a small black hole, the gravitational acceleration is huge and
non-uniform.  You'll get shredded by tidal stresses long before you get
close to the horizon.

In constrast, near a large black hole, the gravitational acceleration is
huge but uniform.  The acceleration is (nearly) the same at your nose
and at your toes.  You can fight it by going into orbit, or you can
ignore it and drop through the horizon without being shredded at that
time.  (You'll get shredded later.)

To visualize this, imagine the following bag-a-bug system:  We set up a
large funnel leading to a powerful suction fan.  A mosquito (which has a
certain top speed) is flying in the flowing air in the funnel. There
will be a point of no return (actually a surface made of such points)
where the local flow speed equals the mosquito's top speed.  If the
mosquito goes inside this surface (the "horizon") it can never fly back
out.  But in general, the mosquito won't notice anything special when
crossing the horizon.  The mosquito doesn't get shredded at that point;
it will get shredded later when it gets to a region where there is a
large nose-to-toes difference in force.

To repeat:  A large but uniform gravitational field won't shred you if
you "go with the flow".  You only get shredded if/when there is a large
nose-to-toes difference in the gravitational field;  your nose goes one
way and your toes go another way.