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B Using laser to overcome conservation of etendue

  1. Jun 24, 2017 #1
    I was watching a video where the author (Isaac Arthur) says it's possible to use mirrors and lens around a star to focus the energy to boil a planet on the other side of the galaxy.
    At first it sounds reasonable, surely you have enough energy, but I have doubts that mirrors would be able to do that. On the other hand, solar panels and a laser should be.
    I was wondering if it would be possible to use mirrors to charge the laser directly? It seems the laser would violate the law of conservation of etendue, and thus violate the 2nd law of thermodynamics, i.e. heat its target higher than the temperature of the source star.
    Where is the flaw in such a design? Does a laser need cooling to operate? Or would it actually work?
  2. jcsd
  3. Jun 24, 2017 #2


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    Staff: Mentor

    You're right, you cannot do it with the setup shown/implied in the video. Your lens/mirror would need to be placed closer to the planet than to the original star. This is because the image of the star would need to be much smaller than the size of the star itself, something that can only be accomplished if you place your optical system closer to the planet than to the star.

    I think diffraction would limit the effective range of a laser to much less than the multi-kiloparsec range.
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