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View Full Version : Do blackholes bounce?

Betenoire
2004-Feb-13, 07:09 PM
Say you had a "mini" black hole (small enough not to have a particularly significant gravitational pull, and, you know, somehow not boiling off in a flash, with a ridiculously small event horizon): If it impacted on a solid, non-singularity-type object, would it deflect? Travel through (swallowing everything in its path)? If the latter, would its momentum be affected by the matter it absorbed?

Taibak
2004-Feb-13, 10:17 PM
It's a little more complicated than either of those scenarios. Let's assume for the sake of argument that we have a black hole with an event horizon with a radius of one meter that is moving directly towards the Earth. For simplicity, assume their centers of gravity are moving directly towards each other. Also for simplicity, assume the Sun's gravity is temporarily turned off.

As the black hole gets close to the planet, its gravity will start to exert tidal forces on the planet. Granted, we're talking a small black hole, but even the black hole in question here is going to be over 100 times more massive than the largest known terrestrial planet, Earth. Anyway, those tides will distort the planet into a tear-drop shape, with the distortion becoming more extreme as it gets closer. (Extreme is a relative term here. The Earth will hold together pretty well until bits of its surface are within the event horizon.) Eventually, the black hole's gravity will pull chunks of planet in. As the mass falls into the black hole, the event horizon gets bigger (it's proportional to the black hole's mass) and its gravity gets stronger. Eventually, the entire planet will be caught in the black hole's gravity and pulled in while the black hole continues on its merry way.

The important thing to remember is that black holes aren't solid objects anywhere except for the singularity at the center. Matter can pass the event horizon at will (so long as it's going *into* the hole) so there really isn't anything that could bounce. Also, as long as you're outside of the horizon, you can just use Newton's laws. Namely, the more massive object wins. If the black hole I described were to hit the Earth, the Earth's gravity would be caught in the black hole's gravity and, eventually, ripped apart by tidal forces as it got close to the horizon. Once the Earth is trapped in this object's gravity, it's only a matter of time before it gets destroyed. Until then, the black hole will drag the Earth along, simply because its gravity is stronger.

eburacum45
2004-Feb-15, 01:32 AM
If you hit the Earth just right with a 100 earth-mass, 5 metre black hole, you could probably spin the Earth round in a matter of seconds and convert it into a spray of rubble in a parabolic orbit...

a smaller black hole the mass of a mountain or small asteroid would be very small, and would have too much momentum to be slowed down much by interaction with the Earth; but if you aimed such a black hole carefully you could put it in orbit around the centre of the earth, with the orbital path beneath the surface of the planet...

eventually the whole planet would fall into this small object, but it would not happen instantly.

2004-Feb-15, 03:54 AM
...It certainly wouldn't bounce under any convetional understanding of the word. That implies it will move away from whatever it hit, and things that go beyond the event horizon don't come back.

Wingnut Ninja
2004-Feb-15, 05:27 AM
a smaller black hole the mass of a mountain or small asteroid would be very small, and would have too much momentum to be slowed down much by interaction with the Earth; but if you aimed such a black hole carefully you could put it in orbit around the centre of the earth, with the orbital path beneath the surface of the planet...

eventually the whole planet would fall into this small object, but it would not happen instantly.

But could you bounce gravity waves off of it to launch icebergs at the moon?

Betenoire
2004-Feb-16, 02:52 PM
Thank you all very much! I've been puzzling that question a while, and my knowledge of singularities ends more or less at the last page of Brief History... and some of the stuff in there I still don't understand, after slogging through it a good five times.

Edymnion
2004-Feb-16, 04:12 PM
I don't see how it could bounce. It would have to touch the body before it could bounce, and it would simply begin eating the body as soon as it came near.

Stregone
2004-Feb-16, 09:27 PM
What if you (artificialy) created a black hole so tiny its event horizon was on or below its surface? Is that even possible? (the event horizon part, not the creating part :p )

Swift
2004-Feb-16, 11:05 PM
What if you (artificialy) created a black hole so tiny its event horizon was on or below its surface? Is that even possible? (the event horizon part, not the creating part :p )
As I understand it, the event horizon IS the "surface", as much as you can say a black hole has one. "Below" that is just the singularity, which is a literal point.

Stregone
2004-Feb-16, 11:13 PM
I was under the impression that the singularity is just a creation of classical physics and not neccessarily a real thing, and that according to quantum theory blackholes do have a size(and so they would have a real surface).

Espritch
2004-Feb-16, 11:17 PM
The size of the event horizon of a black hole is a function of the amount of mass inside the black hole (the amount that has fallen past the event horizon). A small amount of mass could produce a black hole provided it was squeezed into a small enough space. But the event horizon will still be bigger than the area occupied by the mass that created it since the mass will collapse to a singularity point while the EH will extend some distance beyond it. The EH could be microscopic but it could not be infinitly tiny. The EH is just the distance from the singularity at which light cannot escape the gravity well.

Taibak
2004-Feb-17, 03:11 AM
I was under the impression that the singularity is just a creation of classical physics and not neccessarily a real thing, and that according to quantum theory blackholes do have a size(and so they would have a real surface).

I don't think this is the case. As I understand it, the singularity is a point-like object thanks to the extreme effects of gravity there. I didn't think ther was a quantum mechanical theory of gravity that was developed enough to say otherwise.

Stregone
2004-Feb-17, 03:37 AM
I was under the impression that the singularity is just a creation of classical physics and not neccessarily a real thing, and that according to quantum theory blackholes do have a size(and so they would have a real surface).

I don't think this is the case. As I understand it, the singularity is a point-like object thanks to the extreme effects of gravity there. I didn't think ther was a quantum mechanical theory of gravity that was developed enough to say otherwise.

Well my understanding is that, on the 'quantum level', classical physics treats things as if they are a uniform homogeneous 'stuff' that is infinitly divisible while quantum mechanics takes into account the size and etc. of everything.

George
2004-Feb-18, 06:15 PM
It sounds like a "PacMan" analogy is being stated. Although the tidal stress would rip the planet apart somewhat, the black hole would be a simple "gulp" once it passes the event horizon. I am unsure this is entirely corrrect.

Wouldn't the EH expand for a brief moment on the side of the mass it was "gulping"?

Wouldn't the rapid acceleration (jerk) that occurs at the gulp moment affect the EH sphere a smidgen?

Would the release of energy at the time of impact create a shutter or something in the EH sphere causing gravity waves to be sent out?

Is there a possible elasticity to the EH that would allow an oscillation around the sphere once distrubed? The "gulpee" acting like a stick hitting a metal spherical drum (gulper's EH)?

I'm just wildly guessing. I can comfortably spell GR but that's about it. :)