I don't use the term artificial gravity because, the gravity from a black hole is real.

If you have harnessed and are able to control a black hole would you be able to use it as portable gravity device?

I don't really have the physics and the math to to figure it out. But it would seem that if you are in a low gravity environment, you could place a black hole under the floor, and have gravity. Presumably by changing the distance between the floor and the black hole you could adjust to 1 gravity or partial gravity.

It would also seem to follow that if you were in a high gravity environment (like a ship under high acceleration) you could place the black hole over the ceiling and counter the force of gravity you are being subjected to.

  • $\begingroup$ I can appreciate that I didn't have enough knowledge to realize how wrong this question is. There are some fantastic answers, with PearsonArtPhoto & AlanSE providing sufficiently simple explanations explain why it is a bad question. I would hate to loose those answers on the site. Not sure if close and keep is an option, or if the question can be edited to keep it in scope, but would appreciate either. $\endgroup$ – James Jenkins Jul 20 '13 at 9:38

This question is out of scope, not in the simple sense of just this site. You need to realize that critical details that your question depends on are literally unknown at this point in history. That's because small black holes (which are easily implied by your question) involve quantum gravity - basically the area of physics where we know for a fact current theories don't work completely, and that there's new stuff to be discovered.

Also, as a matter of scope, it's not something we can do on the scale of society today. The reason is that small black holes are not naturally occurring. The only natural way we know of black holes forming are from collapse of stars, and only stars much larger than our sun. Obviously, that's not very small. Could "we" make small black holes? Yes, although the pronoun "we" is used extremely liberally here. Some advanced civilization could, this is almost certain. There is a size lower limit, which wasn't even known until people like Hawking. The black hole radiates faster and faster with lower mass. That means you need above a certain size to have it stable. Estimates I've seen range from the mass of Mt. Everest to the mass of the moon.

To address the "portable gravity device" I need to refer you to Gauss' law for gravity. This relates the mass within a volume to the gravitational flux over the surface. The gravitational flux is the gravity times the area. Gravity is a parameter we set due to the requirement that we create Earth gravity (which might be an absurd requirement by the time we're capable of making such a thing, for the record).

I imagine that you would use a rigid sphere with the black hole in the inside. The principle is then that you could stand on the outside of the sphere. Let's say that you're using the smallest stable black hole. Because of Gauss's law of gravity, the mass of the black hole needed is proportional to the area. This constant of mass divided by area is the same as for the Earth. But since surface area to volume ratio goes up with smaller size, you need higher density. This is the utility that black holes can provide. They are subatomic, and effectively points in space, allowing your structure to envelop any given density (provided you meet the stability requirement). With this setup, a black hole would allow you to create a sort of "micro-planet". You would have to have something to hold in the atmosphere still, but this could be tethered to the ground and wouldn't be particularly exotic.

So you have a sphere with a black hole in the middle tugging. That means the sphere has to have material strength to resist that tug. The requirement is almost identical to that of rotating artificial gravity habitats actually. It's the difference between the strength requirement for a spherical pressure vessel versus a tube, which is a factor of 2. Practically, this could be up to 1 km or so with fairly normal materials. It's a similar problem to building skyscrapers. Your surface has some load, which is your habitat, including people and all their things.

Is this a stable construction? No. Your structural sphere exerts no pull on the black hole because of the shell theorem. In reality, it's not perfectly symmetric, which means that the black hole will always be looking for some weak point in your mass distribution and will use that to eat your habitat. Naturally you would engineer around this, and it wouldn't even be particularly difficult. It could be done with gravitational "tugs" in the interior. Basically, large lead balls that you lower and raise according to how the black hole is moving. I should mention that this is only one of the several methods that are possible. Electromagnetic forces would be preferable. However, I know that some people caution about small black hole's values of angular momentum and charge values, because they might like to expel them quickly. But we can't say for sure. These is an open research area.

There is actually an extra proviso that can be used. The small black hole will emit radiation. Hopefully nice, roughly 5000 K radiation. You could reflect this off the inner walls. It would affect the growth/decay rate of the black hole, but who cares, its mass will budge very little for millions of years. That reflection would provide some lift to the surface, and it could even be used to eliminate the structural requirement. Basically, radiation could keep the surface afloat. The same principle has been noted for Dyson spheres. However, there are limited design variables. The upward pressure you can get from this is limited by the blackbody radiation law. That surface power rate is then limited by temperature. Temperature is what limits how small you can make the black hole.

So yes, this is all plausible. Whether it's practical is a different story. Even if everything goes perfectly you're still limited in the surface area to mass department. In short, if you tore apart Earth and made many micro-planets from it, you would end up with exactly the same surface area. That's not a very compelling business case. If we're so advanced that we can make black holes, then matter will probably have other uses, such as powering fusion power plants.

There is another hazard I have yet to mention of small sizes. Small size correlates with higher temperature, but in the strange world of quantum gravity, it can even make new particles at the temperature that roughly corresponds to the particle's rest mass. Photons can be made at any temperature - that's why we all give off thermal radiation. But at higher temperature, neutrinos, electrons, and others can be made. This isn't particularly very good news. If the black hole is too small, it will make an exotic zoo of particles that would make any modern particle physicist envious (it's also dangerous). It is just one more reason that small black holes present dangers. But thankfully we won't encounter them anytime soon.

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  • $\begingroup$ Wow - wondering something essentially like this question. Very thorough answer! My thought involved having many much smaller black holes "embedded" in a floor panel of some sort, basically inches apart or closer, that would provide a more "even"gravitational field to those on either side of the panel. I don't think from what I've read here, I don't think that makes it any more practical, but I'd love to hear if you have any thoughts on that modification $\endgroup$ – Code Jockey Jun 24 '15 at 20:59
  • $\begingroup$ @CodeJockey Relying on much smaller black holes creates problems. The few 100 meter range for 1g gravity will be near the limit because the radiation gets intense for smaller black holes. There's practically no way to confine them before a supernova-like explosion. You could still make planar geometry, but the spacing would have to be large. More like kilometers than centimeters. $\endgroup$ – AlanSE Jun 25 '15 at 0:05

Well, it's a neat idea, but let's take a few things in to account first.

  1. Let's make the assumption that you don't want the gravity to be more than 1% different from your feet to your head. Assuming you are 2m, that gives us a 200m difference between the location of the black hole and you.
  2. The amount of mass that would be required to give, let's say, 1g. Using this calculator, I figured the amount of mass required would be 5900 000 000 000 000 kg. That is 5.9*10^15, about 1 billionth of the Earth's mass, or about the mass of 50 Mt. Everests.
  3. The requirements to thrust become quite complex with all of that extra mass.
  4. The managing of a black hole of that size becomes a challenge.

It is an interesting idea, but unlikely to ever be done in the real world.

However, it should be noted that black holes are very efficient converters of mass to energy, and might someday be used to power spacecraft. But that's pretty far out there for right now.

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    $\begingroup$ With your mass and radius, orbital velocity would be 70 mph. Now I'm not saying there's anything wrong with that... $\endgroup$ – AlanSE Jul 20 '13 at 2:59
  • $\begingroup$ Did you have any thoughts on what might change by arranging many more much smaller black holes in a matrix, embedded in a panel? Doesn't seem like it would make it more feasible, but if it could be accomplished, it would be more practical for a spaceship like those in the movies (?) $\endgroup$ – Code Jockey Jun 24 '15 at 21:04

Cool idea. If it's massive enough to help, it seems like it would be difficult to manage. You'd have difficulty moving it about your vessel, and your propulsion system would have all that mass to accelerate.

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  • $\begingroup$ This looks like it should be a comment not an answer. $\endgroup$ – James Jenkins Jul 20 '13 at 9:40

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