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      CommentAuthorTrim
    • CommentTimeApr 2nd 2018
     
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      CommentAuthorTrim
    • CommentTimeApr 2nd 2018
     
    Earth's stable temperature past suggests other planets could also sustain life.

    https://phys.org/news/2018-04-earth-stable-temperature-planets-sustain.html
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      CommentAuthorAngus
    • CommentTimeApr 2nd 2018
     
    Well, just a minute: the existence of life on Earth suggests by itself other planets could also sustain life.
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      CommentAuthorTrim
    • CommentTimeApr 4th 2018
     
    Magnetospheric Protection from Galactic Cosmic Radiation for long space missions.

    https://www.nextbigfuture.com/2018/04/magnetospheric-protection-from-galactic-cosmic-radiation-for-long-space-missions.html
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      CommentAuthorTrim
    • CommentTimeApr 4th 2018
     
    Machs Effect interstellar propellentless propulsion mission proposal at NASA NIAC.

    https://www.nextbigfuture.com/2017/09/machs-effect-interstellar-propellentless-propulsion-mission-proposal-at-nasa-niac.html

    Just to help Andrew's and Als bromance along.
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      CommentAuthorTrim
    • CommentTimeApr 4th 2018
     
    Is Rocket Lab the new SpaceX? The Electron VS Falcon.

    https://www.youtube.com/watch?v=C0STPK3g9c8
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      CommentAuthorAngus
    • CommentTimeApr 5th 2018 edited
     
    Weird. You'd need a three-dimensional soliton for that I guess, since the beam is unguided. Is such a thing possible? Some sort of nonnlinear self-focussing maybe. You'd think that conservation of lateral momentum would prevent a free beam and a light wave from constraining each other. Seems weird to me.
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    One further advantage, which I don’t yet see mentioned, is the likelihood that such a low-divergence system lends itself more readily to the Q-amplification scheme of Bae, whereby the available motive power of the beam upon the target may be multiplied manyfold by consecutive reflections from both source and target mirrors.
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      CommentAuthorAngus
    • CommentTimeApr 5th 2018
     
    That one already sits at worse than weird.
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    Yabut provably works. There's a lab demo up on YT.
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      CommentAuthorAngus
    • CommentTimeApr 5th 2018
     
    There are two conditions:
    (a)If the gain medium is not between the mirrors, then it is a laser shining on a resonator, and it is no different from a laser shining on a mirror. I don't think that is what was proposed

    (b) If the gain medium is between the mirrors it is an ordinary Fabry-Perot interferometer laser cavity and of course there is a force on both mirrors. The problem is to keep them aligned to the accuracy required for resonance. To achieve this you need the kind of suspension and pointing accuracy of the LIGO experiment. There has been a short range interferometer demonstration in space that this can be done, but doing it on the scale of mirror you require for propulsion seems a very long shot to me.

    The other problem is coherence time. If the propagation time around the cavity is longer than the coherence time of the laser transition, you won't be able to establish coherent oscillation. That will affect your beam quality, which you need for the collimation.

    https://www.kth.se/polopoly_fs/1.638908!/ADOPT_lecture_M_Pollnau.pdf
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    https://www.nasa.gov/directorates/spacetech/niac/2018_Phase_I_Phase_II/PROCSIMA

    All clear on the Q-amplification idea. I imagine the relative parallel alignment of the mirrors over a path length of many AU is, as you mention, practically impossible. I believe eLISA is expected to work with a path length of 1 million Km, which is only about 0.01 AU.

    Just to be clear, it was I who mentioned this idea. It's not in the actual proposal (see link here).
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    Next we will learn that it is possible to use the beam for braking, by reflecting it not back to the origin, but rather reflecting it forwards in the direction of travel.
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    I don't know if you're using sarcasm....
    ....but in fact that idea has been mentioned in the comments in respect of a sort of TWT variant.
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      CommentAuthorAngus
    • CommentTimeApr 5th 2018
     
    Well sure. You just take the incoming beam from earth and amplify it with your on-board laser, powered by the beam from earth, and squirt it out the front at a higher intensity. Simples.
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    I KNOW that's sarcastic because it's nonsense. The TWT braking idea is to transfer momentum from the light beam to the slower particle beam.
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      CommentAuthorAngus
    • CommentTimeApr 5th 2018 edited
     
    That might work. In other words, you use the energy of the incoming light to power a rocket facing front.

    ETA: In a highly abstract sense of "work", of course.
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    Well in theory yes, but whether a) it can be made to work within a reasonable mass budget, and b) whether the resultant braking force is large enough to be useful - are unknowns at this point. But generally there has to be some sort of braking effect because of momentum conservation.

    Limbach in the latest post describes the beam tightening as akin to optical tweezers.