# Re: Black hole question

From: ronkean@juno.com
Date: Tue Feb 15 2000 - 12:06:20 MST

On Tue, 15 Feb 2000 02:47:32 -0800 James Swayze <swayzej@earthlink.net>
writes:

> I had a thought once. What if two black holes began ever so slowly
> to collide.

Two black holes would not necessaarily collide 'slowly', since the
gravitational field near the event horizon is very strong and they would
therefore be accelerating towards one another. Most likely they would
have built up considerable speed by they time they drew close enough to
each other for their event horizons to touch. But the actual rate of
acceleration would depend on the mass of the black holes - the larger the
masses of the two black holes may be, the smaller the acceleration at the
time when the horizons touch, because of inertia. If two very large
black holes could somehow be placed right next to one another, with zero
relative initial velocity, then they would appear to collide slowly, as
they began to merge. Smaller black holes would merge more rapidly than
larger ones, as the gravitational force is larger at the event horizon of
a small black hole than it is at the horizon of a large black hole.

> What effect might one observe as the two event horizons just touch?
> Assume the mass of both for sake of argument is exactly equal. Would
particles
> be pulled in two? This would be difficult for plank sized items, no?

The tidal force at the event horizon is typically very strong, but its
strength depends completely on the size of the black hole (it's stronger
for smaller black holes). A formerly compact material object would be
stretched out like a thin strand of spaghetti and most likely would be
pulled apart into small pieces, somewhat like what happened to the comet
which collided with Jupiter a few years ago. My guess is that the tidal
force at the event horizon of a typical black hole would be nowhere near
strong enough to pull apart an elementary particle such as proton.
Smaller black holes have a stronger tidal force at the event horizon than
larger black holes, due to the laws of physics which describe tidal
forces. Inside a black hole, near the singularity, there is no limit on
how great the tidal force may be, assuming that the singularity is
infinitely small and has a finite mass, and that the classical law of
gravity applies.

Or would
> they have to
> fall to one side or the other of the resultant hump left in space
> between the two dents made in space time? What would determine to which
side a
> particle falls? Perhaps this is a foolish thought after all I'm not an
expert
> just an inquiring mind. Still though, any thoughts?
>
> James
> --
> "Quod de futuris non est determinata omnino veritas"

Just as water runs downhill, a particle would fall into whichever black
hole pulls more strongly on the particle.

Ron Kean

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