The Physics of Building a Black Hole Powered Starship [pdf] | Hacker News Reader

The Physics of Building a Black Hole Powered Starship [pdf]

arxiv.org

44 pointsawwx25 comments

"It is now known that any prolonged human presence deeper in space would need to be behind a shield of the effective strength of two feet of lead, which would weigh 400 tonnes for a small capsule."

Uhm, the first thing that comes to my mind - regardless of whether this black hole propulsion might actually work - is the amount of lead we have available. Just building the spaceship seems to be just as much a problem as accelerating it..

Effective shielding against cosmic radiation is possibly an even greater problem in interplanetary travel than propulsion.

The solution will probably come from materials science, but at the moment I don't think there's enough effort being concentrated on this problem to solve it in my lifetime.

Would placing the spaceships crew behind its black hole engine provide adequate shielding from cosmic radiation and blue-shifted photons/particles? Killing two birds with one black hole, so to speak.

Also, wouldn't being in such proximity to a black hole negate the need for time dilation effects of speed anyway? If you can build black holes, speed is meaningless as you essentially have a stasis machine. You could travel at .1c, reach Alpha Centauri in 40+ years and never even age a day if you can get close enough to the event horizon.

How do you place the crew 'behind' the black hole? Space is 360 degrees.

Actually space is ~130,000 degrees (slightly less, disappointing I know) because it's 3 dimensional, not 2D.

However I was talking about a vehicles design here, and suggesting an engine-in-front design rather than an engine-behind. Like the humble car, an engine-in-front design may protect the driver from nasty impacts, in this case blue-shifted gamma rays and microscopic particles that have been blue-shifted to the point that your ship is in a giant particle collider. If you could hide the crew in a sort of eclipsed position behind (in relation to the direction of momentum) the event horizon would act as a shield. This would mean you only had to harden equipment against radiation, rather than find a way to protect the crew themselves.

IMO though, if you've built a black hole, you might as well be moving something big, as in either a planet, small moon, or giant colony. On these scales the need to travel fast would become redundant, your colony ship could be so big it could literally house a world. Then an organisation like NASA would essentially become a planetary bus-driver association.

> Actually space is ~130,000 degrees (slightly less, disappointing I know) because it's 3 dimensional, not 2D.

That's what I was thinking, but it came out in a 2D manner. How embarrassing. ^^;; > an engine-in-front design may protect the driver from nasty impacts, in this case blue-shifted gamma rays and microscopic particles that have been blue-shifted to the point that your ship is in a giant particle collider.

Ah, I thought that we were talking about other radiation (solar,background,gamma,etc) in space. In that case, you can't really hide 'behind' something because there isn't necessarily a single source to hide from. Even if only nearby solar radiation was a problem, if you ended up in a binary star system, you would have two sources to 'hide' from.

> IMO though, if you've built a black hole, you might as well be moving something big, as in either a planet, small moon, or giant colony.

I thought that the LHC was supposed to create miniature black holes. Though they're supposed to immediately dissipate, I don't think we've necessarily crossed the boundary of 'able to move whole planets' just because we're able to create a black hole.

Getting an asteroid-sized mass to lase gamma rays all at once is probably much harder than preserving the quantum state of a 100kg person.

The world is a big place - annual lead production is approximately 7 million metric tons.

Source: http://www.financialexpress.com/news/lead-uses-that-go-back-...

Sure, and I admit I have no idea how much lead building such a spaceship might take, but keep in mind 2 things:

1) "More than half of the lead currently used comes from recycling." (from your article) - Once the spaceship is built and has left Earth, the lead is essentially gone (for the time being), so the more of those we build, the fewer lead we will have to recycle, and it will get even more difficult to build additional ones.

2) 400 tons for a small capsule, and consider they're proposing a spaceship in which multiple people could live in autonomously, i.e. they need places to live in, but also room to grow food, process their waste, etc. So I'd guess it's much higher than you seem to have assumed.

Oh and btw, I'm fairly certain we also need much of that lead on earth - hence why we're producing so much of it in the first place, so it's not like we have some spare lead in the order of magnitude of, say, 10000s (I'm really just guessing here, though) of tons lying around collecting dust.

I assume that we'll somehow solve this one with a 'force' field of some sort in the future. Think about it. The Earth isn't surrounded by lead, but we are still protected from the radiation. I imagine that if we can harness a black hole for propulsion we can somehow use it for power too??? Then we just use some of the power to create a magnetic field that blocks/reflects/deflects the radiation.

> The Earth isn't surrounded by lead, but we are still protected from the radiation.

A thick atmosphere accomplishes roughly the same effect as a few feet of lead. A magnetic field could deflect incoming charged particles, but I'm not sure if a "shield" is feasible, due to any side effects that such a magnet might have.

I was under the impression that the Earth's magnetic field was doing the majority of the work. IIRC, the reason that Mars has no atmosphere is probably due to lack of a (or just a weakened) magnetic field, causing solar winds to strip away the atmosphere.

For 1) That's going to be true of any resource that's used in spaceship construction. If you're building interstellar ships at such a scale that you might conceivably run out of stuff to build them out of, it's a fair assumption that you have the capability to go and mine the rest of the rocky planets in the solar system.

2) That 400 tons is not going to scale up with volume. Density of lead is around 650 lbs/ft3. Let's say you have a big ship, a mile in diameter. And let's be generous and put a 4 ft shell of lead around the whole thing. 4/3pi(5280^2-5286^2) * 650 * 1/2000 = 57000 tons of lead = 52000 metric tons. A drop in the bucket, tiny compared to the amount of steel and other material that would make up the rest of the structure. (Annual steel production is around 100 times more than of lead. We won't be running out of it either.)

The bottom line is that unless you're talking about transuranics or other extremely rare elements, or are building things at comically large scales (ie: thousands of ships) amount of construction material is not going to be a limiting factor in starship design.

One thing the author doesn't address is why use lead at all? We know that Earth's magnetic field is what protects us here on the ground. If we could manufacture black holes, doesn't it seem possible that we could manufacture a magnetic field powerful enough to surround said ship and protect it?

Earth is big, so all of that flux is pretty spread out. I wouldn't want to be close to a small portable magnet able to generate that size of magnetic field.

But hey, we're talking about highly hypothetical situations, so I'm sure we could make it work.

You don't actually need to use lead. The key phrase there is "effective strength" so if you have a massive supply of energy at your fingertips, it may be feasible to divert some of it towards the creation of an artificial magnetosphere similar to Earth's own.

Of course you could also use water (obviously in much greater volume), which is a good thing to have in large quantities anyway while you're in space, and is evidently not all that uncommon, at least throughout our solar system.

How much water do you need? Are we talking a pond or the Caspian Sea in space?

Wolfram Alpha tells me you'd need roughly 11x more water than lead, so we're probably talking about a pretty large amount here.

They don't seem to mention how to contain the black hole. What holds its position steady within the spaceship?

If we made a large enough one say 1/2 earth mass, we'd get free "artificial" gravity on the spaceship.

Their proposed blackholes are small - some of their calculations use 4 × 10^11 grams [440.924 tons ~ comparable to Empire State Building which weights about 300.000 tons].

The radiation involved means that you probably couldn't shield a human well enough to be close enough to one of these black holes to feel gravity. Not to mention tidal forces.

I meant, how do you stop the black hole from hitting the spaceship, moving around, falling out the back, etc?

They touch on this briefly at the top of page 11: "The most optimistic approach is to solve requirements 2 [accelerate the black hole to keep it with the ship] and 3 [feeding the black hole with matter so that it doesn't evaporate] together by attaching particle beams to the body of the ship behind the BH and beaming in matter. This would both accelerate the SBH, since BHs “move when you push them” (see [3] p270), and add mass to the SBH, extending the lifetime."

The engineering details are left as an exercise for the reader :-)

I saw this headline and flashed back to the Gunbuster Science Lessons.

"Dimensional Wave Super String Excitation Degenerated Radius Jump Gravitational Field Super-light Speed Navigation. WARP for short."

This is really, really cool. And this is obviously how they powered Rama.

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