Aside: the Kardashev scale is about energy use. One thing we know from @chdbennett & Yves Lecerf is that the energy cost of computation can be made arbitrarily low
I wonder often: to what extent is energy use a good proxy for how technologically advanced a civilization is?
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Also, TIL that Lecerf wasn't just a pioneer of reversible computing, but also a pioneer in ethnomethodology (cc
@meaningness):https://fr.wikipedia.org/wiki/Yves_Lecerf …1 reply 1 retweet 7 likesShow this thread -
Michael Nielsen Retweeted Win Wang
The intrinsic energy cost of space travel is surprisingly low: https://twitter.com/Winium/status/1471968655516835840 … Escape velocity from the solar system at the Earth is about 17 km / sec (IIRC). So for 1 kg need about 150 Megajoules, i.e., about a fortnight's calorie intake for a grown human!
Michael Nielsen added,
Win Wang @WiniumReplying to @michael_nielsen @chdbennettI've happened upon reversible computing a bit (via your QCQI book
), but I'm not sure about "where" the "waste" (if any?) of reversible computing goes in a reversible (classical) circuit?
While computing heat can be reduced, what about the energy needed to colonize space?2 replies 8 retweets 16 likesShow this thread -
Lots of things make space travel hard, notably the atmospheric barrier at Earth, & getting to LEO. But the frictionless nature of space simplifies some things once in space. Braking becomes almost as much of a problem as acceleration.
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The actual numbers are funny. Escape velocity from Earth is ~11 km/sec. Ignoring friction, that means 1 kg needs about 60 MJ. Energy density of kerosene (& lots of similar substances) is about 40-50 MJ/kg. So it's not quite enough to bring that 1kg to escape velocity.
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You think about this, and it's obvious why the main thing rocket fuel is doing is lifting rocket fuel, which is used to lift rocket fuel, which is used to lift rocket fuel... which lifts a payload.
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It's a funny coincidence: Earth's gravity well is _just_ shallow enough that chemical rockets work. Of course, other sources have much higher energy density, so you don't have the same problems . That's why Project Orion happened.
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(Note that the fuel itself doesn't need to escape. But there is a tricky tradeoff. One extreme is to simply combust all the fuel at once initially. This provides a huge initial impulse, and negates the need to carry the fuel. But it also creates huge initial acceleration...)
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Replying to @michael_nielsen
Someone I met recently claims that if you have a large enough object you can shoot it with a cannon into orbit, so no fuel (If they're small, they can't dissipate the heat, there seems to be a sweet spot between drag and heat dissipation). Would be fun to see it working.
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Replying to @ArtirKel
Yes, standard rail gun type thing. Enjoy being torn apart by the 100g initial acceleration. May be useful for certain types of things, eg getting mineral ore into space, especially in lower gravity environments (say, the moon).
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good for sending up water, probably, which you then electrolyze in space
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