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Thread: Dyson spheres

  1. - Top - End - #151
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    Default Re: Dyson spheres

    Quote Originally Posted by Chronos View Post
    No, but it does mean that there's a lot less of that waste energy. Every heat engine has a hot reservoir and a cold reservoir. The greater the difference in temperature, the more efficient the heat engine: The fraction of energy that's wasted is equal to the ratio of the temperatures.

    Stellar-mass black holes have a temperature of around a millionth of a kelvin. With temperatures that low, you can make an extremely efficient heat engine. Let's say your civilization has some process that produces what would be waste heat, at a temperature of around 300 K (like, say, keeping their bodies alive, if they're much like us). To us, 300 K heat is completely useless, because we don't have good cold sinks to make use of it. But if you're using that 300 K heat with a black hole, you'd have only a few parts per billion wasted, and you could turn the rest of it into useful work. Do whatever it is you do with that useful work, get waste heat out of that process... and again reclaim almost all of that waste heat. End result, you get a billion times more use out of your energy. That's a much bigger improvement than, say, the improvement in going from fusion energy sources to total-conversion energy sources.
    I think there's a few steps missing here. A heat engine needs to touch both the hot source and the cold sink to produce work. In the case of a black hole it seems more like we'd just be dumping energy into a hole without actually getting anything out of it. I mean sure if you can affix a rod to an event horizon and the other end to the surface of a star you could make one hell of a thermoelectric generator. I'm just not sure how we'd solve the gravity problem.
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  2. - Top - End - #152
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    Default Re: Dyson spheres

    I expect the 'cold source' would be long, thin fins of aluminum or something that are facing the event horizon. Sort of like the heat sinks we use in computers, except that we're radiating the heat away instead of convecting it.
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  3. - Top - End - #153
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    Default Re: Dyson spheres

    Quote Originally Posted by Lord Torath View Post
    I expect the 'cold source' would be long, thin fins of aluminum or something that are facing the event horizon. Sort of like the heat sinks we use in computers, except that we're radiating the heat away instead of convecting it.
    What would be the difference of just radiating into space? I mean the Voyager probe has a plutonium fueled thermoelectric generator that radiates heat into space. I just don't see how putting Voyager next to a black hole could increase its power output or efficiency.

    The generator here would be generating electricity because the fins are cold, not because the black hole is cold. And the fins would lose heat because they radiate into space, where that radiation ultimately goes hardly seems to matter no?
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  4. - Top - End - #154
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    Default Re: Dyson spheres

    The black hole is colder than space. When a radiator radiates into space, space radiates back, at around 3 K. The black hole radiates at less than a millionth of that temperature.

    And yeah, there are a lot of engineering issues, here. Give ten billion people a few thousand years to work on them, and I'm sure someone would come up with something.
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  5. - Top - End - #155
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    Default Re: Dyson spheres

    But black holes are in space, put the voyager next to a black hole and it's still receiving CMBR.
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  6. - Top - End - #156
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    Quote Originally Posted by Mastikator View Post
    But black holes are in space, put the voyager next to a black hole and it's still receiving CMBR.
    Well, not from some directions...

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    Quote Originally Posted by NichG View Post
    Well, not from some directions...
    Okay let's say you magically hover a *dysonsphere over the event horizon of **Sagittarius A*, and also manage to redirect incoming CMBR around the dysonsphere. Now you are able to go from harnessing maybe 50% of the energy of the star to 50.001% because that last 3 K.

    Even if you can technology away gravity the juice doesn't seem worth the squeeze.

    *to get the most bang for your buck you'll want something that is very hot so that it can cool down the most.
    **A supermassive black hole is both colder and has an enormous event horizon, letting you put larger objects close to it.
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  8. - Top - End - #158
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    Quote Originally Posted by Chronos View Post
    The black hole is colder than space. When a radiator radiates into space, space radiates back, at around 3 K. The black hole radiates at less than a millionth of that temperature.

    And yeah, there are a lot of engineering issues, here. Give ten billion people a few thousand years to work on them, and I'm sure someone would come up with something.
    There are engineering issues, but more importantly there is the physics issue that you are bottlenecked by the surface area of the black hole, and a forth power of temperature. The only plausable time this could be useful is when you are trying to exploit a hot end in the region of a few kelvin, and then you are talking about milliwatts per square meter of energy consumption by the black hole. An entire stellar mass black hole could only be the cold end for an engine in the single digit MW region. When all the stars have gone out, and you don't have matter to spare by dumping it into the black hole, and the background radiation is your last source of energy, this is what you end up doing, but before then there is absolutely no reason to.

  9. - Top - End - #159
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    Actually I wonder how much energy you could extract by intentionally blueshifting the CMB to be your 'hot end', since hanging out deep in a black hole means the entire rest of the universe is blueshifted with respect to you.

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    Default Re: Dyson spheres

    Quote Originally Posted by Mastikator View Post
    **A supermassive black hole is both colder and has an enormous event horizon, letting you put larger objects close to it.
    But isn't the photon sphere - the limit of how close you can have a stable orbit around a black hole - also dependent on the diameter of the event horizon? My understanding is that the limit for light orbiting a black hole is 1.5 times the diameter of the event horizon, which means you still have the black hole taking up the same percentage of your sky.

    You do still get the colder surface, although -as pointed out - it's such a minuscule difference in temperature that unless your heat source is in the low single-digit Kelvin range, you're not going to see a huge improvement in efficiency.
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  11. - Top - End - #161
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    Default Re: Dyson spheres

    As for those low single digit kelvin energy sources though, couldn't you theoretically use the hole to extract energy from the cosmic microwave background? That's probably as close to producing energy out of nowwhere as anybody's ever gonna get.
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    Default Re: Dyson spheres

    Quote Originally Posted by Lord Torath View Post
    But isn't the photon sphere - the limit of how close you can have a stable orbit around a black hole - also dependent on the diameter of the event horizon? My understanding is that the limit for light orbiting a black hole is 1.5 times the diameter of the event horizon, which means you still have the black hole taking up the same percentage of your sky.

    You do still get the colder surface, although -as pointed out - it's such a minuscule difference in temperature that unless your heat source is in the low single-digit Kelvin range, you're not going to see a huge improvement in efficiency.
    That's how close light can orbit the black hole. You need to much farther out if you want to construct your heat engine out of matter. The innermost stable circular orbit is at 3 times the Schwarzschild radius of a non-rotating black hole, there half of your field of view would be the black hole.
    However since you probably don't want a tiny heat engine but rather something like an actual dysonsphere to make it worth while to undertake this project you'll instead have to deal with the Roache limit which would probably put you well outside, so far that the black hole may only take up a tiny bit of your field of view.

    The funny thing here is that even if we somehow solve for gravity, we're really talking about getting energy from 3 degrees kelvin. That is how much colder a black hole is compared to the CMBR. You have to technology/magic away both gravity and entropy and what you get out of it is 3 degrees worth of energy. Gravity and entropy are not things you can technology away, not either of them and certainly not both of them.
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  13. - Top - End - #163
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    Quote Originally Posted by Bohandas View Post
    BTW, does anyone know why sci-fi programs always depict sealed buildings like this as being dome shaped? Is it just because it looks cool or is it inspired by some real proposal?
    I think architects have always liked domes, and they are (in some senses1) efficient.

    If you ignore the need for radiation shielding, enclosing the entire habitat in one dome is a fairly efficient use of resources.

    Another thing is that certain architectural norms don't really apply. On Earth, certain rooms are considered to "need" to have windows. Sprawling buildings tend for form rings and have lots of wings rather than a block shape.

    Once a transparent dome was drawn a few times, it became a visual shorthand. Anything more realistic is not going to look nearly as impressive. A big blocks just looks like the artist was lazy, a cave doesn't let you see much at all.

    1The sense that it's not efficient are that it requires a lot of scaffolding, and that people tend not to be prepared to use curved spaces efficiently. Perhaps the endlessly enthusiastic sci artist/writer of the early 20th century assumes those problems will be "solved".
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  14. - Top - End - #164
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    You could use inflatable domes. Trouble is, fabric is the best material for those, structurally, and those tend to be opaque. Maybe you could use fibers and some sort of coating that are both transparent with similar refractive index. If the coating permeates all the way through the fabric, it could optically function much as a solid sheet.

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    Default Re: Dyson spheres

    The biggest problem with the "transparent dome" is that it would need to be really thick.

    On Earth there's 14 pounds per square inch (10 ,000 Kg / m2) of atmosphere to absorb ionizing radiation from the sun.

    Astronauts are partially protected by the ionosphere and by the fact that they only spend a small fraction of their lives in space.

    Serious moon and mars proposals involve meters of rock on top of any habitat.

    A dome creating a thick shell of water would work, but water is expected to be pretty scarce most places.
    The thing is the Azurites don't use a single color; they use a single hue. The use light blue, dark blue, black, white, glossy blue, off-white with a bluish tint. They sky's the limit, as long as it's blue.

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