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Comment by lmilcin | Hacker News Reader

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 order of up to 40% of mass-energy equivalent of the falling matter. With BH of the mass of Earth the sphere would be pretty small and easy to make.

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"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.

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

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).

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...

Regardless, having a compact object nearby to play with would be fun!

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 what effectively is compound interest.

I really wonder what will the societal implications of such a path being possible are going to be.

There are plenty of people that will be willing to push a pause button right now to be able to wake up in a world where you could buy a ticket to mars, add to that the quite likely possibility of becoming a millionaire if not a billionaire if you put enough money into any inflation beating investment and you get quite the incentives package.

Sure you might never wake up because the world is going to end in a nuclear holocaust or your icebox will experience a kernel panic 17 years into your suspended animation but overall this quite likely be a much lower possibility than dying prematurely today.

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.

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.

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.

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.

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.

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