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A Dyson sphere around a black hole

(arxiv.org)
214 points SkyMarshal | 9 comments | 19 Aug 21 03:40 UTC | HN request time: 0.489s | source | bottom
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ardit33 ◴[19 Aug 21 08:55 UTC] No.28231727[source]▶
The whole concept of a Dyson sphere is kinda idiotic. Any civilization that is capable to build one, it is probably able to work out fusion energy very efficiently.

There is no point to go and harness energy around a star or a black hole, when you can just produce it locally with a lot less resources/waste and materials. The sun itself is actually very inefficient in producing energy.

There is no need to harness the sun million of km away, when you can recreate it in your home planet. The only way to produce a dyson like of sphere, is to tame an over-heated sun, and reflect away un-needed energy. But there is no point to build one to just harness it.

It makes great sci-fi stories, but that's about it. Scientifically, it just doesn't make sense.

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marcyb5st ◴[19 Aug 21 10:06 UTC] No.28232103[source]▶
Because to power a type 2 civilization you need around 4 * 10^26W (as the paper states). Over a year that is around 10^31 kWh worth of energy. Assuming fusion can transform 1% of the input mass in energy you need ~ 1.4 * 10^19 kg per year. To put that number into perspective mount Everest weights 2.7×10^14 kg [1] so thousands Everests worth of mass.

That is a lot of mass to extract and transport to the power stations (accelerate, decelerate). So it just makes sense to only needing to build the facilities to collect the power from existing sources (stars, black holes) without the logistics of transporting the fuel.

Moreover, the space around a star or black hole is real estate that would go unused otherwise, while asteroids, moons, ... are more likely usable by such an advanced civilization.

[1] https://www.quora.com/What-would-the-estimated-weight-of-Mou... (I took the highest estimate in the first answer)

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1. SkyMarshal ◴[19 Aug 21 14:04 UTC] No.28233961[source]▶
Any idea if there’s enough construction material in any given planetary system to construct a Dyson sphere around its star?

Seems like it would require, at minimum, several planets worth of raw material.

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2. serverholic ◴[19 Aug 21 14:25 UTC] No.28234198[source]▶
In reality you probably wouldn't see a dyson sphere but a dyson swarm instead. With that you just need millions of thin sheets of material to reflect energy.
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3. marcyb5st ◴[19 Aug 21 14:47 UTC] No.28234502[source]▶
PBS Space time on YouTube has a great video about it. For the sun cannibalizing Mercury would be enough.
4. TremendousJudge ◴[19 Aug 21 14:50 UTC] No.28234544[source]▶
>In reality

Why do people keep insisting on the Dyson concept as though it was real, even though it's just the fanciest thing we can imagine with our current understanding of physics?

The same way in the 19th century nuclear power couldn't be envisioned, there's definitiely a bunch of technology we don't know about now that an alien civilization could be using; we could even be observing this right now and we would be none the wiser. Talking about Dyson shperes, or swarms, or rings, and then concluding that there's now alien civilzations because we can't see them is just anthropocentrism, failing to consider other possiblities

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5. JohnJamesRambo ◴[19 Aug 21 15:11 UTC] No.28234845{3}[source]▶
It’s possible this is as advanced as we can get. Innovation seems to be slowing down don’t you think? At some point you have exhausted the laws of physics and this is what you’ve got to work with.

https://www.cold-takes.com/this-cant-go-on/

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6. TremendousJudge ◴[19 Aug 21 15:48 UTC] No.28235335{4}[source]▶
It's possible, but I find it extremely unlikely. The "end of physics" has been predicted several times, even in the face of obvious unanswered questions, like today's "what's the deal with dark matter and dark energy?", "can chaotic processes be understood?", "what even _is_ gravity?"

When in the past, unanswered, mystery questions like these were finally answered (for example "how come light seems to appear in discrete packets?"), the answer ended up being a door to a completely new understanding of reality, and not a little footnote question at the End of Physics

So yeah, when there are no more unanswered physics questions, I will consider that the laws of physics have been exhausted. Right now that's not the case.

7. DerekBickerton ◴[19 Aug 21 17:17 UTC] No.28236570[source]▶
> Seems like it would require, at minimum, several planets worth of raw material.

You could mine it from asteroids, which have plenty of base metals. There's already a speculative market for investors wishing to mine them. The only thing is; the technology for transporting it around doesn't exist yet. (We can land probes on them however for recon, but that's about it so far!).

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8. crubier ◴[19 Aug 21 21:41 UTC] No.28239718[source]▶
You might be overestimating the mass of asteroids. The total mass of asteroids in the asteroid belt is 3% of the mass of the moon. Kuiper belt is 2% of the earth’s mass.
9. Beldin ◴[20 Aug 21 06:26 UTC] No.28243241{3}[source]▶
A star is an abundant source of energy. Outside star power (potentially stored in chemical bonds such as in oil), a civilisation may make use of their planet's heat (geothermal power), their planet's tides, nuclear fission, or nuclear fusion.

On Earth, none of those are available in the quantities needed to provide sufficient energy for us (close to a planetary civilisation in energy needs) on the scale of millions of years.

If you want your civilisation to not deplete available energy in the next million or so years and to allow for growth of energy demands in that time, you're going to have to make use of star power.

A planet only gets so much starlight that it becomes a limiting factor in energy budget. The easiest next step is to make more use of the abundant energy source next door: your star.

There are other options, e.g. using black holes. But those are orders of magnitude more challenging: black holes are extreme stars (extremely far from any developing civilisation).

On Earth, we've seen that every leap-frog in civilisation (fire, agriculture, industrialisation, ...) increased the available energy budget by orders of magnitude. There's not that many orders of magnitude left on this planet to grow further; if we want more, we'll need to get it from elsewhere.

Alien civilisations that remain bounds to their planet's energy budget similarly must remain stuck at a certain level of energy use. They can make advances in efficiency, but their total energy budget is capped.