Hacker News Reader

Comment by Recurecur | Hacker News Reader

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.

View on HN

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.

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.

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.

Another point of using black hole is that you decide its output (by deciding how much matter falls in) which greatly helps building dyson sphere. You can't modify Sun output.

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.

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.

Reply on news.ycombinator.com