Re:What If Planet 9 Is a Primordial Black Hole?
by ltbarcly3 on Thursday September 26, 2019 @09:39AM
(#21080769)
> Let's assume for simplicity that the radius of the Earth is 5000 km, and the black hole is moving at 1000 km/s.
You are assuming the stuff near the surface is being pulled up, but the stuff under that isn't being pulled up, so rocks are 'flying away from the earth and towards the black hole'. In reality the only net force you can 'feel' from gravity is due to tidal forces, so this would have an effect more like 'squeezing' the earth and the energy would be spread throughout the entire interior. For an earth-mass black hole, however, the difference in acceleration between something 1000km away and something 2000km away is still stupidly high (about 30g), so your assumption is correct!
But generally, this is my point. A slow moving black hole is going to rip the earth apart, a fast moving black hole is 'less dangerous' because it just punches a hole and keeps going, with almost no interaction with the earth, even though that is counter-intuitive. Yes, things would get pulled away from the surface of the earth, but they would only feel that pull upward for a very, very short amount of time, not long enough to be substantial. A small black hole, say the mass of a meteor or something, might go through the earth and not even be noticed, it's far less dangerous than a meteor of the same mass would have been.
A fast moving black hole isn't 'nearby' very long, it would go from far away to far away in microseconds.
However, as the mass gets large enough, the dynamical friction on the black hole is so large that it leaves behind significant kinetic energy by melting a hole through the earth, no matter how fast it is going. https://en.wikipedia.org/wiki/Dynamical_friction
If you neglect dynamical friction, as far as I know an earth mass black hole could potentially go right through the earth and we wouldn't have to worry so much, but it depends on the specific path it takes since it could still disrupt the Earth's orbit.
You are assuming the stuff near the surface is being pulled up, but the stuff under that isn't being pulled up, so rocks are 'flying away from the earth and towards the black hole'. In reality the only net force you can 'feel' from gravity is due to tidal forces, so this would have an effect more like 'squeezing' the earth and the energy would be spread throughout the entire interior. For an earth-mass black hole, however, the difference in acceleration between something 1000km away and something 2000km away is still stupidly high (about 30g), so your assumption is correct!
But generally, this is my point. A slow moving black hole is going to rip the earth apart, a fast moving black hole is 'less dangerous' because it just punches a hole and keeps going, with almost no interaction with the earth, even though that is counter-intuitive. Yes, things would get pulled away from the surface of the earth, but they would only feel that pull upward for a very, very short amount of time, not long enough to be substantial. A small black hole, say the mass of a meteor or something, might go through the earth and not even be noticed, it's far less dangerous than a meteor of the same mass would have been.
A fast moving black hole isn't 'nearby' very long, it would go from far away to far away in microseconds.
However, as the mass gets large enough, the dynamical friction on the black hole is so large that it leaves behind significant kinetic energy by melting a hole through the earth, no matter how fast it is going. https://en.wikipedia.org/wiki/Dynamical_friction
If you neglect dynamical friction, as far as I know an earth mass black hole could potentially go right through the earth and we wouldn't have to worry so much, but it depends on the specific path it takes since it could still disrupt the Earth's orbit.