Science-based what-if questions

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SDK
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Re: Penetrate Earth

Postby SDK » Mon Nov 06, 2017 4:04 pm UTC

p1t1o wrote:Fire it at the ground at just over escape velocity, and it should exit the other side of the Earth going slightly slower, fulfilling your "through the Earth" requirement. There would be a colossal explosion for sure but I feel like the Earth would remain [largely] intact - since only a fraction of the hole's kinetic energy would be transferred to the Earth at all.

Assuming you catch it on the other side, right? Because if you don't, all that's going to happen is the Earth will be gone and the black hole will be slightly larger.

Why would a black hole move through the Earth, though? Are you just assuming it's so small and moving fast enough that it, what, sucks material up to get it out of the way or something? I'm not too sure why you think this doesn't just destroy the Earth outright and instead punches through.
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Re: Penetrate Earth

Postby measure » Mon Nov 06, 2017 4:18 pm UTC

SDK wrote:Why would a black hole move through the Earth, though? Are you just assuming it's so small and moving fast enough that it, what, sucks material up to get it out of the way or something? I'm not too sure why you think this doesn't just destroy the Earth outright and instead punches through.

As I understand it, the BH has low enough mass that it's gravitational field is negligible outside of it's immediate vicinity, and it's kinetic energy can be pretty small as well.

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Re: Science-based what-if questions

Postby gmalivuk » Mon Nov 06, 2017 4:27 pm UTC

SDK wrote:
p1t1o wrote:Fire it at the ground at just over escape velocity, and it should exit the other side of the Earth going slightly slower, fulfilling your "through the Earth" requirement. There would be a colossal explosion for sure but I feel like the Earth would remain [largely] intact - since only a fraction of the hole's kinetic energy would be transferred to the Earth at all.

Assuming you catch it on the other side, right? Because if you don't, all that's going to happen is the Earth will be gone and the black hole will be slightly larger.
If you start out above escape velocity, it'll still have close to escape velocity when it leaves, which means it will...escape. No "catching" needed.

Why would a black hole move through the Earth, though? Are you just assuming it's so small and moving fast enough that it, what, sucks material up to get it out of the way or something? I'm not too sure why you think this doesn't just destroy the Earth outright and instead punches through.
It's a small black hole, for whatever definition of "small" is needed to make this the outcome.

(Honestly, even if it's got the mass of Earth, it would take some time to destroy the planet because just moving that much stuff takes a while. If it's moving fast enough, even such a massive BH would mostly just pass through the planet without causing much widespread damage.)
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Re: Science-based what-if questions

Postby SDK » Mon Nov 06, 2017 4:32 pm UTC

We can't make it too small though - it is going to be absorbing material on its journey through the Earth, and any material it does absorb will slow it down equal to that material's momentum. So it's got to be big enough that it can actually have enough speed to reach the other side.

And you do need it to absorb material, because if you don't, you have an impact. I'm not sure that's even possible (to avoid an impact by absorbing material as you approach), but if you don't avoid impact, all your kinetic energy is lost to the resulting explosions.

Or is it just the case that black holes cannot impact anything in the traditional sense since explosions can't leave the black hole anyway? Is that what I'm missing?
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Re: Science-based what-if questions

Postby andykhang » Mon Nov 06, 2017 4:38 pm UTC

Above escape velocity with a black hole, and having enough force to destroy the earth otherwise?...Yeah, this is too destructive for my taste. If the bullet is definitively going to go relativistic, could the design of the bullet be different to not need any of that?

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Re: Science-based what-if questions

Postby gmalivuk » Mon Nov 06, 2017 4:44 pm UTC

SDK wrote:We can't make it too small though - it is going to be absorbing material on its journey through the Earth, and any material it does absorb will slow it down equal to that material's momentum. So it's got to be big enough that it can actually have enough speed to reach the other side.

And you do need it to absorb material, because if you don't, you have an impact. I'm not sure that's even possible (to avoid an impact by absorbing material as you approach), but if you don't avoid impact, all your kinetic energy is lost to the resulting explosions.

Or is it just the case that black holes cannot impact anything in the traditional sense since explosions can't leave the black hole anyway? Is that what I'm missing?
I think mainly what you're missing is how much denser a small black hole is compared to anything familiar.

A 1 cm2 hole through the Earth contains about 7000 tonnes of material. A black hole that size has a radius of 10-20 meters. Its gravity at a distance of 5mm is 0.01g, so it won't actually be pulling in anything that far away. It won't even pull in everything it affects with stronger gravity than Earth's, because at that size other forces greatly outweigh gravity. But even if we assume it sucks in everything from the region where it's gravity is higher than 1g, that's only 1/100th of the mass of the black hole itself, so it won't have a significant effect on its speed. Start it off about 1% higher than escape velocity and it'll have about escape velocity when it comes out the other side.
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Re: Science-based what-if questions

Postby SDK » Mon Nov 06, 2017 5:45 pm UTC

Ok, sure, that makes sense. It's hard to rationalize how something that weighs so little can still have a lot of gravity immediately next to it. Thanks.
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Re: Penetrate Earth

Postby Sableagle » Mon Nov 06, 2017 6:17 pm UTC

andykhang wrote:So, I have asked for a Matter-Antimatter Anti-Structure Sniper Rifle earlier in this thread. Now, I wanted to test this, and to do that, I need a target, a huuuuge target. So, I decided to aim it down, lock and loaded, and it's time to penetrate Earth. Supposed I aimed it straight down, and used the same indestructable bullet last time, what speed do I need to achieve to do that?


1) Define "penetrate the Earth" as "get a bullet from the Earth's atmosphere into the Earth's mantle" for purposes of this test.
2) Take your vibranium rifle and ammo to Iceland.
3) Follow the mid-Atlantic rift until you find an active spot.
4) Set up your rifle on a really big tripod that can straddle the rift.
5) Retreat.
6) Remotely fire the rifle.
7) .....
8) .....
9) .....
10) .....
11) .....
12) You now have a successfully tested indestructible rifle that isn't encased in basalt and isn't too hot to hold. Congratulations!
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Re: Science-based what-if questions

Postby Eebster the Great » Mon Nov 06, 2017 9:36 pm UTC

If your bullet is small enough, you can shoot it through the Earth with no problem.

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Re: Science-based what-if questions

Postby gmalivuk » Mon Nov 06, 2017 9:54 pm UTC

It also needs to have no electric charge.
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Re: Science-based what-if questions

Postby Eebster the Great » Mon Nov 06, 2017 10:25 pm UTC

Yeah, if your bullet is not a neutrino, you may run into problems. Maybe you could find something in the dark sector that works.

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Re: Science-based what-if questions

Postby andykhang » Tue Nov 07, 2017 9:35 am UTC

How about I put a spin in it? A really fast spin. This being the indestructable bullet and all, I don't really have to worry about centripedal force tear itself apart.

Edit: Also, making a bullet drill-like.

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Re: Science-based what-if questions

Postby gmalivuk » Tue Nov 07, 2017 1:15 pm UTC

If you're talking about the black hole, spinning it won't make an important difference.

If you're not talking about the black hole, you'd need something to keep it spinning for it to be drill-like, because otherwise it will lose its angular momentum too quickly to do any good.
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Re: Science-based what-if questions

Postby andykhang » Tue Nov 07, 2017 3:46 pm UTC

Even when the spin itself is relativistic?


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