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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?

#121

Post by arc-in-space »

lmilcin wrote: post #112
Recurecur wrote: post #105

Earlier quoted context omitted.

So, I just calculated the orbital speed at 50 km. It came out to 282,378 km/s. The speed of light is 299,792.5 km/s, so we're way into relativistic territory. Good luck having enough energy to build a structure around it! ha

Black hole with radius of 50km (escape velocity of speed of light at radius X is definition of Schwarzshild radius for nonrotating black hole) would have mass of 17 suns. We are talking about small primordial black holes, in the vicinity of 10-50 times mass of Earth. At 10 Earths, the radius is 8cm (!!!) and at 50 Earths it is still measly 44cm.

Wasn't Recurecur talking about a 1 mass-of-earth BH, with a dyson-sphere-like structure 50 km away from it, rather than a 50km black hole? I don't understand the physics here myself, just checking if you haven't missed his point.

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

#122

Post by jychang »

lmilcin wrote: post #112
Recurecur wrote: post #105

Earlier quoted context omitted.

So, I just calculated the orbital speed at 50 km. It came out to 282,378 km/s. The speed of light is 299,792.5 km/s, so we're way into relativistic territory. Good luck having enough energy to build a structure around it! ha

Black hole with radius of 50km (escape velocity of speed of light at radius X is definition of Schwarzshild radius for nonrotating black hole) would have mass of 17 suns. We are talking about small primordial black holes, in the vicinity of 10-50 times mass of Earth. At 10 Earths, the radius is 8cm (!!!) and at 50 Earths it is still measly 44cm.

You can calculate the orbital velocity of an object from your distance from the object. v=sqrt(Gm/r) where v is the velocity, G is the gravitational constant, m is the mass of the black hole, and r is the distance from the black hole. Any slower and it’ll fall in; faster and it’ll drift away.

For an object 10x the size of the earth, a satellite would need to travel at 282300m/s to orbit the object at 50km. He’s off by a factor of 1000, due to units change from m to km. This is about a thousandth of the speed of light, but about 4x the speed of the fastest manmade object.

If you weren’t at orbital velocity and were stopped instead, you’d be pulled towards the black hole at 162600g of acceleration.

10mass of the earth is a lot of mass. You get pulled towards the earth at the rate of 1g when you’re (earth’s radius) distance away from the center of earth, you’re dealing with an insane amount of force when you’re only 50km away.

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

#123

Post by jychang »

lmilcin wrote: post #113
Recurecur wrote: post #100

Earlier quoted context omitted.

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, literall…

The point of using primordial black hole is that it can be very small (in the range of centimiters for its Schwarzshild radius). You can then calculate safe distance from the black hole where the gravitation and gradient generates acceptable forces for your structure. This is of course much more than the centimiters given but still very small compared to astronomical distances you would need to encircle entire Sun. A…

The schwarzchild radius doesn’t really matter for engineering purposes. Think about the earth; when you are standing on the surface, 1 radius away from the center of mass, you experience 1g of acceleration. Imagine how much more force you experience if you were closer to the center of mass, and replaced that mass with a black hole.

It’s hard enough to build a dyson sphere at 1*earth radius, the amount of forces acting on the structure at 50km radius is insane.

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

#124

Post by me_me_me »

shifto wrote: post #107
me_me_me wrote: post #99

Earlier quoted context omitted.

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

This channel makes me feel dumb. It all goes over my head but he is explaining it like he is teaching a group of 10 year olds.

Its good to remind yourself you are not the smartest person in the room.

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

#125

Post by 127 »

themodelplumber wrote: post #17
ISL wrote: post #14

One should not miss Figure 1 of the arXiv preprint, which is, alas, absent from the final PRL. https://arxiv.org/pdf/1909.11090.pdf It is among the rarest journal-article figures in physics. If I'd been asked to referee the paper, I'd have campaigned hard to keep it in.

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.

Anton Petrov https://www.youtube.com/user/whatdamath goes through astrophysics papers quite often. The channel is pretty cool if you like space in general.

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

#126

Post by [deleted] »

dogma1138 wrote: post #109
Recurecur wrote: post #101

Earlier quoted context omitted.

Maybe ships will need laser power . It remains to be seen if near-C travel is at all feasible, due to radiation/erosion from impacting interstellar material. If not, a ship can easily carry its own fuel, it will just be a generation/hibernation ship. Or, perhaps our extropian descendants won't care about travel time. ;-) OR, maybe the black hole could power a hyperspace portal or something equally exotic... Regardles…

One thing will have to change if we ever can travel at relativistic speeds or have function cryogenics (or anything similar) will be markets and investments, just think of putting even a relatively small amount into a yielding investment even an index fund and going to “sleep” for 200 years not only would you get immediately to benefit from 200 years of advancement but you’ll also get to benefit form 200 years of wha…

[deleted]

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

#127

Post by sliken »

lovemenot wrote: post #106
Recurecur wrote: post #104

Earlier quoted context omitted.

BTW, regarding "standing" in this gravity field... If there were a structure to stand on, rather than you being in orbit, the force of gravity would be about 160 billion times the force of gravity on Earth (~1,594,000,000,000 m/s^2). That's for a 10,000,000 Earth mass black hole at 50 km.

If Planet 9 were indeed a black hole its significance would surely be that prevalence of lower mass black holes is orders of magnitude more common than currently known. This particular one may not be practical for energy generation, but it would be obviously worth looking for others of more useable mass.

Why not? There's various approaches to extracting energy from a black hole of any size, and being smaller generally makes things easier.
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