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What if Planet 9 is a Primordial Black Hole?

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Re: What if Planet 9 is a Primordial Black Hole?

#91

Post by xingyzt »

geoffmunn wrote: post #81
Sharlin wrote: post #78

Earlier quoted context omitted.

There's no material that you could make the box or shell out of that could withstand the gravitational forces near the black hole. Even the gravity differential (tidal forces) is strong enough to rip anything apart (the so-called spaghettification). And if you made the shell large enough (planet-diameter), it would collapse into a spherical body just from its self-gravity, never mind the additional gravity of the PBH…

But given that it's so small, what would the gravitational forces be if you made a shell with a 100km radius? Or pick a number - surely at some point it might be practical to build something.

Size of the object doesn’t matter. Gravitational force depends on its mass and how far away you are from the black hole. Standing on a shell ~6300 km above a black hole with the mass of the Earth is equivalent to standing on the Earth itself.

Re: What if Planet 9 is a Primordial Black Hole?

#92

Post by wcoenen »

lmilcin wrote: post #79

If this was true, this would be extremely fortunate for our civilization if we can figure out not to kill ourselves in next couple hundred years. Black holes offer about the most effective process to extract energy from matter, orders of magnitude more efficient than fusion and it is not picky about what kind of matter you put in. You can put a Dyson sphere/swarm around small black hole and extract something on the o…

"Small" Dyson sphere is relative. The tidal forces at 50km from an earth-mass black hole would still be pretty significant, so it probably couldn't be smaller than that.

E.g. to pick something on the scale of a human, the difference in gravitational acceleration (being GM/(r^2)) between two points at 50000 and 50002 meters from such a black hole would be about 1.6 G.

Re: What if Planet 9 is a Primordial Black Hole?

#93

Post by pas »

wcoenen wrote: post #92
lmilcin wrote: post #79

If this was true, this would be extremely fortunate for our civilization if we can figure out not to kill ourselves in next couple hundred years. Black holes offer about the most effective process to extract energy from matter, orders of magnitude more efficient than fusion and it is not picky about what kind of matter you put in. You can put a Dyson sphere/swarm around small black hole and extract something on the o…

"Small" Dyson sphere is relative. The tidal forces at 50km from an earth-mass black hole would still be pretty significant, so it probably couldn't be smaller than that. E.g. to pick something on the scale of a human, the difference in gravitational acceleration (being GM/(r^2)) between two points at 50000 and 50002 meters from such a black hole would be about 1.6 G.

What does that do? I mean you can walk in 1G just fine, so probably we could go there, take a walk/sit there in 1.6G, and leave, right?

That means that if we put a 100km wide structure around it, we can service it easily(?) even without robots.

Re: What if Planet 9 is a Primordial Black Hole?

#94

Post by bodhiandpysics1 »

onlyrealcuzzo wrote: post #23
Konohamaru wrote: post #7

Earlier quoted context omitted.

This is not scary. Yes the event horizon will expand the more matter falls into its gravity well, but otherwise a black hole acts like a normal planet orbiting the Sun.

Is there a theoretical limit on the minimum mass of a black hole? It's counter-intuitive to me that something could get dense enough to form a black hole and not have more mass than the Sun, or at least enough mass to steal Pluto.

It's a little complicated... Hawking radiation establishes the minimum size of stable black hole in our universe. If the temperature of the event horizon is higher than the temperature of the CMB, than a black hole will lose mass by Hawking radiation, which raises is temperature further and so on. Using a knowledge of expansion of the universe (and thus the cooling of the CMB) plus the basic math of hawking radiation, you can pretty easilly calculate the minimum size of a black hole that you should see.

Re: What if Planet 9 is a Primordial Black Hole?

#95

Post by wcoenen »

pas wrote: post #93
wcoenen wrote: post #92

Earlier quoted context omitted.

"Small" Dyson sphere is relative. The tidal forces at 50km from an earth-mass black hole would still be pretty significant, so it probably couldn't be smaller than that. E.g. to pick something on the scale of a human, the difference in gravitational acceleration (being GM/(r^2)) between two points at 50000 and 50002 meters from such a black hole would be about 1.6 G.

What does that do? I mean you can walk in 1G just fine, so probably we could go there, take a walk/sit there in 1.6G, and leave, right? That means that if we put a 100km wide structure around it, we can service it easily(?) even without robots.

You can't "walk" in this 1.6G. You'd experience it while floating freely in space. And the stress on your body would be tension instead of compression. I guess it would feel like hanging upside down, except that your blood rushes to both your feet and your head.

Re: What if Planet 9 is a Primordial Black Hole?

#96

Post by juped »

pas wrote: post #93
wcoenen wrote: post #92

Earlier quoted context omitted.

"Small" Dyson sphere is relative. The tidal forces at 50km from an earth-mass black hole would still be pretty significant, so it probably couldn't be smaller than that. E.g. to pick something on the scale of a human, the difference in gravitational acceleration (being GM/(r^2)) between two points at 50000 and 50002 meters from such a black hole would be about 1.6 G.

What does that do? I mean you can walk in 1G just fine, so probably we could go there, take a walk/sit there in 1.6G, and leave, right? That means that if we put a 100km wide structure around it, we can service it easily(?) even without robots.

The difference in gravitational acceleration between two points, not the gravitational acceleration at the points.

Re: What if Planet 9 is a Primordial Black Hole?

#97

Post by hliyan »

sidcool wrote: post #71
317070 wrote: post #53

Earlier quoted context omitted.

Black holes do not have more gravity per mass than any other object. Put the other way, if the sun would suddenly change into a black hole without losing a lot of mass, the earth would still follow the exact same orbit.

That's true. My point was that if there were a solar mass object beyond Pluto, it would have significant effect on orbits of all planets. But apparently PBHs can be lighter

I would suppose so. I would expect a lot more perturbation in the kuiper belt if a solar mass object was lurking out there.

Re: What if Planet 9 is a Primordial Black Hole?

#98

Post by lmilcin »

pas wrote: post #93
wcoenen wrote: post #92

Earlier quoted context omitted.

"Small" Dyson sphere is relative. The tidal forces at 50km from an earth-mass black hole would still be pretty significant, so it probably couldn't be smaller than that. E.g. to pick something on the scale of a human, the difference in gravitational acceleration (being GM/(r^2)) between two points at 50000 and 50002 meters from such a black hole would be about 1.6 G.

What does that do? I mean you can walk in 1G just fine, so probably we could go there, take a walk/sit there in 1.6G, and leave, right? That means that if we put a 100km wide structure around it, we can service it easily(?) even without robots.

I can't imagine we will be building things like that manually.

By the time a swarm of robots will process blueprints and take raw materials and construct the structure semi-autonomously or even completely autonomously.

The structure would most certainly not admit human presence. It only has to redirect radiation to where it is useful (maybe send a starship on a laser beam, for example).

Re: What if Planet 9 is a Primordial Black Hole?

#99

Post by me_me_me »

canjobear wrote: post #40
themodelplumber wrote: post #17

Earlier quoted context omitted.

Fascinating. And kind of funny. I wonder, are there any YouTubers who walk through the math in papers like this? Asking because it seems really interesting and sometimes it's fun to learn bits here and there from applied math rather than always working up from the fundamentals.

PBS SpaceTime

Half of their stuff goes over my head. And I have to watch videos multiple times, but very interesting channel indeed.

Re: What if Planet 9 is a Primordial Black Hole?

#100

Post by Recurecur »

pas wrote: post #93
wcoenen wrote: post #92

Earlier quoted context omitted.

"Small" Dyson sphere is relative. The tidal forces at 50km from an earth-mass black hole would still be pretty significant, so it probably couldn't be smaller than that. E.g. to pick something on the scale of a human, the difference in gravitational acceleration (being GM/(r^2)) between two points at 50000 and 50002 meters from such a black hole would be about 1.6 G.

What does that do? I mean you can walk in 1G just fine, so probably we could go there, take a walk/sit there in 1.6G, and leave, right? That means that if we put a 100km wide structure around it, we can service it easily(?) even without robots.

It's not that, it's the difference in the gravity field between two points. On Earth, the gravity field is very close to uniform.

Near a small black hole, the field changes rapidly and is very strong.

It's a "pull" effect, meaning that if you were oriented with your feet towards the object as described by the GP, and 2m tall, your feet would experience 1.6 G more "gravity" than your head. It's a tidal force, literally working to pull you apart (same effect causes tides on Earth).

If you were to fall into the black hole, at some point you'd be "stringified" as that force stretched you into spaghetti shape.

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