<story><title>A Dyson sphere around a black hole</title><url>https://arxiv.org/abs/2106.15181</url></story><parent_chain><item><author>ardit33</author><text>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.&lt;p&gt;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&amp;#x2F;waste and materials.  The sun itself is actually very inefficient in producing energy.&lt;p&gt;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.&lt;p&gt;It makes great sci-fi stories, but that&amp;#x27;s about it. Scientifically, it just doesn&amp;#x27;t make sense.</text></item></parent_chain><comment><author>marcyb5st</author><text>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.&lt;p&gt;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.&lt;p&gt;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.&lt;p&gt;[1] &lt;a href=&quot;https:&amp;#x2F;&amp;#x2F;www.quora.com&amp;#x2F;What-would-the-estimated-weight-of-Mount-Everest-be&quot; rel=&quot;nofollow&quot;&gt;https:&amp;#x2F;&amp;#x2F;www.quora.com&amp;#x2F;What-would-the-estimated-weight-of-Mou...&lt;/a&gt; (I took the highest estimate in the first answer)</text></comment>
<story><title>A Dyson sphere around a black hole</title><url>https://arxiv.org/abs/2106.15181</url></story><parent_chain><item><author>ardit33</author><text>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.&lt;p&gt;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&amp;#x2F;waste and materials.  The sun itself is actually very inefficient in producing energy.&lt;p&gt;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.&lt;p&gt;It makes great sci-fi stories, but that&amp;#x27;s about it. Scientifically, it just doesn&amp;#x27;t make sense.</text></item></parent_chain><comment><author>healsjnr1</author><text>But what are you fusing? If you want efficiency I guess you&amp;#x27;ll be using deuterium etc, of which we only have so much.&lt;p&gt;After that your efficiency declines until... Well you end up with fusion reactors that look like the sun. You also end up fusing all the mass that we need to live our lives on and make stuff out of.&lt;p&gt;If you don&amp;#x27;t think a civilisation could ever need the amount of energy that a Dyson sphere could provide, that&amp;#x27;s fine, we don&amp;#x27;t need one.&lt;p&gt;But if you imagine a civilisation that does need the energy a Dyson sphere can provide, then they make a lot of sense.</text></comment>