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I understand that Black Holes, even the small ones, are very dense and heavy. Even a micro black hole will probably weigh as much as a skyscraper, a few million tonnes.

I then saw a starship propulsion idea known as the Kugelblitz Drive, using energy made by matter dropped into the black hole for thrust (this energy is Hawking Radiation). To even store the small black holes, parabolic reflectors are needed. They reflect the Hawking Radiation and seemingly keep the black hole in stasis, preventing it from destroying the starship itself.

Basically, the black hole is trapped and levitated inside the reflectors just like how magnetic objects can levitate under sufficiently-strong magnetic fields.

Assuming black holes can even be created in the first place, and the parabolic reflectors made to store the black hole can withstand all the radiation without issue, would the weight of the parabolic reflector and the black hole be just that of the reflectors, or both? If it is the latter, would magnetic or electric fields help to make the stored black hole feel weightless?

An extra thread might help: Is there a practical method of storing a kugelblitz?

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  • $\begingroup$ The notions of the last paragraph come out of left field and seem to be pulled from thin air with zero relation to anything stated prior to it. $\endgroup$
    – DKNguyen
    Nov 18, 2020 at 14:23
  • $\begingroup$ The part about magnetic and electric fields? Oh yeah, I think black holes can be stored that way as well. $\endgroup$
    – CYCLOPSCORE
    Nov 18, 2020 at 14:26
  • $\begingroup$ Those have nothing to do with mass, but you implicitly talk about them as if it were common understanding that they do. $\endgroup$
    – DKNguyen
    Nov 18, 2020 at 14:27
  • $\begingroup$ Okay, made it clearer. Does not make it massless, but has a sensation of it, such that when the mirror and BH is lifted, only the mass mirror can be felt. $\endgroup$
    – CYCLOPSCORE
    Nov 18, 2020 at 14:29
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    $\begingroup$ This question isn't an exact duplicate, but it likely has a relevant answer (and it's mine). Basically, the gravitational mass of even a bullet-sized black hole is so great that the Kugelbitz drive won't work. The amount of energy used to keep the BH from sucking in the ship is more than the ship needs to move in the first place. $\endgroup$
    – JBH
    Nov 18, 2020 at 16:38

2 Answers 2

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I'm afraid you've misunderstood quite a lot. I believe you're referring to black hole starship, as a kugelblitz refers to a particular way to make a black hole rather than anything else.

Mass/density

Black holes can have any mass or any density, however these are inversely connected — the lower the mass, the denser they get. The sort of black hole generally considered for a propulsion system is of the "extremely light and dense" variety, specifically 606,000 metric tones, with a radius of 0.9 attometers (significantly smaller than an atom, and somewhat smaller than a proton).

Thrust/Hawking radiation/mirrors

The thrust in a black hole drive does not come from dropping matter or energy into the black hole, but from the Hawking radiation coming out of it; as Hawking increases as the mass decreases (and this radiation causes the mass to decrease further), the reason to add mass/energy to the black hole is to reduce the power output of the black hole. However, every unit of mass/energy you drop into the black hole is mass you'd have to take with you when you launch, so you probably wouldn't want to do that if you can avoid it — and worse, you'd need to put in about 160 petawatts (roughly the total power use of a K1 civilisation) to keep the black hole in a steady state. If you could do this with matter, it would be a mere 1.78 kg/s, however you're aiming for a target smaller than a proton which is as bright as a noon-day sun even from a distance of 6000 km, and that Hawking radiation will be pushing against whatever matter you try to send into the black hole in exactly the same way and for exactly the same reasons that you might want to use Hawking radiation for ship propulsion in the first place.

The parabolic mirror in the design is to direct the Hawking radiation and make it useful, and is analogous to the engine bell of a normal rocket. Mirrors do not provide any "stasis" or similar benefit.

Mass/weight

Simplifying a bit, "weight" is dependent on your environment, and if you're isolated in deep space or in freefall like all satellites around Earth, you don't have any; however, "mass" is an intrinsic property, it remains the same wherever you are and whatever you're doing, and it is what gives you inertia and makes it hard to accelerate.

The black hole (and the ship itself) will produce their own gravity just by being… well, massive, in both the technical and the normal senses of the words, but this isn't important: the mass of each, and not their weight in each other's gravity fields, is what you need to care about. Unfortunately, no amount of shenanigans with magnetic or electric fields can help with that.

Extra info

Something I forgot to add in the original answer: Hawking radiation has a characteristic temperature directly related to the mass of the black hole. In this case, 606,000 tons means a temperature of 2e14 K, which is significantly hotter than the 3e8 K in the cores of my favourite kind of supernova, the pair-instability collapse in which photons have enough energy to transform into positron-electron pairs when they hit electrons.

I don't know how to account for gravitation redshift in this situation (I'm a software engineer not a physicist), but (1) that's the only saving grace you might have, and (2) lowering the temperature 6 orders of magnitude will only reduce you from "antimatter is being made inside my mirror" to "nuclear fusion is happening inside my mirror" wherever any given photon bounces off.

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  • $\begingroup$ I thought the existence hawking radiation causes ta black hole to 'shed' mass over time (in the absence of fresh mass falling in) and that the smaller the back hole the quicker this process. So a sealed,tiny black hole would have a very short lifespan. Isn't the end of black hole supposed to be a violent event?. $\endgroup$
    – Mon
    Nov 18, 2020 at 23:44
  • $\begingroup$ Not bad a response. Though, tell me, if the reflectors only reflect this radiation and nothing more, how does the black hole not collapse and destroy its container? $\endgroup$
    – CYCLOPSCORE
    Nov 19, 2020 at 0:01
  • $\begingroup$ @Mon correct; for the example mass, this would give the black hole a 3.5-year lifespan, but it would accelerate to 10% the speed of light in 20 days (assuming 100% conversion of energy into kinetic energy, 200 days if 10% efficiency), so if you have the ability to make these in the first place, it's probably better to let go of it when you're at target speed rather than feed in more mass along the journey. $\endgroup$
    – BenRW
    Nov 20, 2020 at 8:55
  • $\begingroup$ @CYCLOPSCORE Black holes are always as collapsed as it is possible to be, no further collapse is possible. The bigger problem is "how does the Hawking radiation not evaporate the mirror?" — something I forgot to add to my answer was that Hawking radiation has a characteristic temperature related to the black hole mass, which for this example is hot enough to cause positron-electron pairs to form on whatever the light shines on. $\endgroup$
    – BenRW
    Nov 20, 2020 at 9:04
  • $\begingroup$ Excellent answer. However, I'm missing the bit about how you would steer a BH Starship, if you were able to solve the feeding problem: Without acceleration, the starship would be orbiting the BH and vice versa. To get thrust, the starship would use movable mirrors to halt the rotation and point itself in the target direction, surfing on the radiation pressure from the BH. The black hole would then be tugged along by the gravity of the starship. That's the consequence of the BH reacting to gravity only. $\endgroup$
    – cmaster - reinstate monica
    Nov 20, 2020 at 9:51
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You are confusing being massless with being weightless.

You state about the mirror

They reflect the Hawking Radiation and seemingly keep the black hole in stasis

This means the black would not have any acceleration, thus would have no weight in whatever gravitational field you are considering it to be placed.

But being weightless doesn't make it massless.

To make a comparison, when I go indoor skydiving, the flow of air keeps me more or less static with respect to the floor, despite having nothing solid to support me. But I still have mass, as anybody who tried to direct me can tell you.

Same goes for the part about electric of magnetic fields: they do not make anything massless, not even a black hole.

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