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DanielleFong's profile
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Empire of the Future
Joined February 2008

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    1. Michael Nielsen‏ @michael_nielsen 17 Dec 2021

      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?

      4 replies 3 retweets 21 likes
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    2. Michael Nielsen‏ @michael_nielsen 17 Dec 2021

      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 likes
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    3. Michael Nielsen‏ @michael_nielsen 17 Dec 2021

      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 @Winium
      Replying to @michael_nielsen @chdbennett
      I'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 likes
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    4. Michael Nielsen‏ @michael_nielsen 17 Dec 2021

      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.

      2 replies 1 retweet 5 likes
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    5. Michael Nielsen‏ @michael_nielsen 17 Dec 2021

      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.

      2 replies 1 retweet 8 likes
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    6. Michael Nielsen‏ @michael_nielsen 17 Dec 2021

      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.

      1 reply 1 retweet 8 likes
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    7. Michael Nielsen‏ @michael_nielsen 17 Dec 2021

      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.

      2 replies 4 retweets 12 likes
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    8. Michael Nielsen‏ @michael_nielsen 17 Dec 2021

      (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...)

      2 replies 0 retweets 7 likes
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    9. José Luis Ricón Fernández de la Puente‏ @ArtirKel 17 Dec 2021
      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.

      1 reply 0 retweets 2 likes
    10. Michael Nielsen‏ @michael_nielsen 17 Dec 2021
      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).

      2 replies 0 retweets 2 likes
       🤷🏻‍♀️‏ @DanielleFong 17 Dec 2021
      Replying to @michael_nielsen @ArtirKel

      good for sending up water, probably, which you then electrolyze in space

      3:28 PM - 17 Dec 2021
      0 replies 0 retweets 0 likes

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