3
$\begingroup$

So in my universe ship use thrust or spin to create gravity. But what if I could “contain” (assume I can contain it) a black hole to use it for gravity. This way ships can drift and still have gravity. Are there any issues with the idea and what is the most effective ship shape to apply this?

$\endgroup$
4
  • $\begingroup$ Ohh didn’t think of that but space is weightless so would it be okay? $\endgroup$
    – 11Bravo
    Dec 18, 2020 at 19:08
  • $\begingroup$ Are you sure you want the 'hard science' tag? That says you want citations and equations. You're not going to find to many published, peer-reviewed papers on this topic. $\endgroup$
    – user535733
    Dec 18, 2020 at 19:32
  • 3
    $\begingroup$ "hard science" and "captive black hole" do not go together. Not yet, at least. It will be thousands upon thousands of years of research before I will even be able to prove to you why its impossible. $\endgroup$
    – PcMan
    Dec 18, 2020 at 19:34
  • $\begingroup$ In Dragon's Egg by Forward, the Cheela use such. However, they're only about 1mm tall and made of electron degenerate matter, so without it they'd balloon back up to human size and die in agony. $\endgroup$
    – John O
    Dec 18, 2020 at 20:32

2 Answers 2

1
$\begingroup$

Everyone in the ship will die of cramps, circulation problems.

The force of a gravity field around some mass follows a gradient. That's because gravity is proportional to the masses involved, but also inversely proportional to the square of the distance.

In laysman terms, if you are standing your feet are pulled down with more force than your head. This generates a stretching force. On Earth that force is ridiculously small because you are less then three meters tall, and more than six million meters away from the planet's barycenter.

But if you are very close to a black hole? Say you are 1m away from the black hole, and your feet feels a pull of 1G. Your waist will feel a pull of only 0.25G. That means your legs are being stretched constantly with a force of 0.75G. In a few minutes you don't have blood circulating in your legs anymore.


That said, let's get to the hard science of it. As always, our lord and saviour Randall Munroe has already written about it. Someone once asked him if The Little Prince's asteroid is inhabitable (this link is also the source for the images below). Here is a graphic that describes the gravity sensation, which is equivalent to the scenario I propose above:

Gravity gradient on the Little Prince's planet

The black hole in your ship would be just like that planet. We have a new problem now - a few meters higher, and your escape velocity will be quite low (whe nyou double the distance, gravity gets 4x weaker, remember?).

So anything close to the black hole will be stuck to it, but a few meters away and they escape its gravity. This is troublesome because either your ship is really small and tight, or you need a big set of such black holes. On top of that, you might escape its orbit just by running.

(...) the weird thing about escape velocity is that it doesn't matter which direction you're going. If you go faster than the escape speed, as long as you don't actually go toward the planet, you'll escape. That means you might be able to leave our asteroid by running horizontally and jumping off the end of a ramp.

If you didn't go fast enough to escape the planet, you'd go into orbit around it. Your orbital speed would be roughly 3 meters per second, which is a typical jogging speed.

An orbit around the Little Prince's planet

Again, you have a black hole and not a planet, but it doesn't matter.


Last but not least - the only way to avoid the effects above would be if the black hole would have a surface gravity already close to Earth's. But such a black hole would have the mass of 1.5 trillion suns, and would be almost one light year wide. It might be a little difficult to assemble a ship around it, let alone move it around (you can find out more using this calculator). For comparison, the largest black hole known is TON 618, with a mass of only 66 billion suns.

Consider Cadence's comment:

(...) if you can build a ship around a stellar body and move it, you could just... use a planet. Might I suggest Venus? I'm sure the atmosphere will calm down some if you move it away from the Sun first.

$\endgroup$
8
  • 1
    $\begingroup$ To your last point - if you can build a ship around a stellar body and move it, you could just... use a planet. Might I suggest Venus? I'm sure the atmosphere will calm down some if you move it away from the Sun first. $\endgroup$
    – Cadence
    Dec 18, 2020 at 19:43
  • 1
    $\begingroup$ Your last point doesn't seem right. Why would you need a black hole 1 light year across just to get gravity similar to earth's? Black holes are (a lot) more dense than the earth, not less. Although maybe I'm not understanding the surface gravity bit, where is the black hole's surface? I thought its surface is inside the event horizon and gravity would always be >c m/s^2 there. $\endgroup$
    – Curiosity
    Dec 18, 2020 at 20:03
  • 3
    $\begingroup$ The gravity gradient would be pretty bad on a tiny ship a few meters across, but what would something larger? Say a 2km diameter sphere with a micro black hole in the centre of such mass that gravity is 9m/s^2 on the surface of that sphere. I guess the gradient wouldn't be too bad then? $\endgroup$
    – Curiosity
    Dec 18, 2020 at 20:08
  • 1
    $\begingroup$ @Curiosity my point exactly! Although with BH's that small you'd be more worried about it evaporating so quickly it explosively evaporates anything nearby as well. $\endgroup$
    – Demigan
    Dec 18, 2020 at 20:10
  • 2
    $\begingroup$ I'm sorry but this answer is almost entirely wrong. Little Prince is irrelevant since BHs have no surface. You have to build one, and you can put it at any height. For the gravity difference across a 2m tall person to be 1%, they just need to be about 400m away from the mass. To get 1 gee at that distance you need a BH of 2×10^16 kg, which would have a radius of less than a nanometer. Escape velocity (at 400m) would be around 300 km/h. The so-called surface gravity of a BH is not an actual acceleration, as Wikipedia explains. It's not what you'd feel "standing on a BH" if that were possible. $\endgroup$
    – benrg
    Dec 19, 2020 at 1:33
2
$\begingroup$

Let's assume that you are using some kind of micro-black hole, with the convenient gravity approximating Earth's surface outside of the containment chamber.

That means, of course, that your micro-black hole has a pretty good fraction of the mass of the Earth, a mass likely to be far greater than your spaceship.

  • You will need to expend fuel equivalent to moving a planet in order to move your ship. You thought that your fuels bills used to be large! Now they are (ha ha) astronomical.

  • Your ship will not be welcome near inhabited systems, as it's enormous mass will perturb the orbits of the inhabitants.

  • Other ships would prefer to not rendezvous, as your gravity well would make any exchange of cargo or passengers phenomenally dangerous and expensive. Other ships would have to orbit you and communicate by shuttles - equivalent to takeoff and re-entry.

  • Your shape will now likely be spherical, as your ship can keep an atmosphere without an outer skin. You might need a barrier wall to keep your (massive, planet-moving) engines from heating up your atmosphere.

$\endgroup$
3
  • 2
    $\begingroup$ note the hard-science tag, please $\endgroup$
    – L.Dutch
    Dec 18, 2020 at 19:18
  • 2
    $\begingroup$ You don't need an Earth-Mass black hole to have Earth gravity in your spacecraft, unless your spacecraft is Earth-sized, and all your living quarters are one Earth-radius from the black hole. That said, even with the smaller masses you can do this with, it's an insane prospect. $\endgroup$
    – notovny
    Dec 18, 2020 at 19:26
  • $\begingroup$ Don't forget that micro-black holes also emit Hawking radiation and that the smaller it is, the more radiation it emits. $\endgroup$ Dec 18, 2020 at 21:54

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .