Discussion in 'Photography' started by Alan Browne, Mar 16, 2013.

    Wolfgang Weisselberg, May 22, 2013
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  2. Similar to the chromatic aberration problem in lens design. You can
    never get rid of it completely. But you can use carefully placed later
    chromatic aberration to undo most of an earlier introduced chromatic
    aberration. You can do the same kind of thing with gravitational
    bending of light ray bundles, especially when the rays are very narrow
    and the gravitational fields are large and distant, so minimising the
    non-linearity of the bending across the bundle.

    Perhaps a nearer example is the bending of light rays due to
    atmospheric thermals on a hot sunny day. Easily seen through a long
    lens as the wriggling of distant straight lines. Have you ever noticed
    that if you take a short enough exposure those wriggly straight lines
    are often both wriggly and sharp at the edges. In other words despite
    the thermal bending of the light rays they've maintained a close
    approximation to parallelism.
    Chris Malcolm, May 22, 2013
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  3. Alan Browne

    Noons Guest

    (muppet newsflash): it's called
    - wait for it -
    (/muppet newsflash)
    Noons, May 23, 2013
  4. Homogenous hollow sphere (as the only object). Whereever
    you are inside, the pull is identical in each direction.
    Therefore a Dyson sphere is inherently stable, but a ringworld
    would be unstable and needs active stabilization against the
    tiniest movements.
    Hod do you propose bending no-longer-parallel light rays back
    to being parallel again without having negative gravity?
    Yep, that's why it doesn't matter usually, but in theory it
    Air --- like the glass in our lenses --- can both concentrate
    and spread light rays. How do you do that with gravity?

    Wolfgang Weisselberg, May 23, 2013
  5. Alan Browne

    PeterN Guest

    PeterN, May 23, 2013
  6. Alan Browne

    Paul J Gans Guest

    A Dyson sphere, or more properly a Dyson shell, is also
    inherently unstable just as Ringworld is. See wiki
    Paul J Gans, May 24, 2013
    Wolfgang Weisselberg, May 24, 2013
  8. I gladly admit that 'inherently stable' might be the wrong word,
    but please explain what happens in Dyson sphere, where the sun
    is off center (and sphere and sun initially not moving against
    each other) and contrast that to what happens to a ringworld
    under the same conditions.

    Wolfgang Weisselberg, May 24, 2013
  9. Alan Browne

    Paul J Gans Guest

    Same thing. The sphere is just a three dimensional version.

    The attraction of the sun does not provide a restoring force
    to center either one. When off center there is more sphere
    further away than there is sphere nearer the sun. So the
    net attraction works out to be zero.

    Don't take my word for it. Check Google or Wiki.
    Paul J Gans, May 24, 2013
  10. Interesting claim. Gravitation only works in 2 dimensions?

    For the sphere, yes. Any angle you can find that fills some
    part of the closer half will always be filled on the further
    half, too.

    For the ringworld, no.
    Thought experiment: You move one side of the ring world 1/10th
    the distance to the sun the other side has. Say the ringworld
    is 1000km from upper to lower rim. Pick a square, from the
    upper to the lower rim, and also 1000 km wide. From the corners
    of the square draw straight lines through the middle of the sun.

    Where they meet the far further ring half on the other side:
    what happens? Each km is now 10 km, so we have 10,000 km
    along the ring ... no problem, there's the ring to fill it.
    But the 1000 km height is also 10,000 km high now ... and the
    ring can only fill 1000 km, as IT IS ONLY 1000 km from rim
    to rim! A sphere fills it all (for any shape, actually and
    for any direction), that's why it's stable.

    Therefore the ringworld the closer side will accellerate towards
    the sun as soon as it's ever so slightly off center. That's why
    I called it unstable. The sphere will not, as you agreed.
    Please do.

    Wolfgang Weisselberg, May 25, 2013
  11. Alan Browne

    Alan Browne Guest

    The other problem with these constructs is that in practice they cannot
    be perfect spheres (or rings). The sun and all sorts of external
    influences on the d-sphere would upset the balance. (Not to get into
    building it in the first place).

    But the first two Ringworld books were a great read in any case.
    Alan Browne, May 26, 2013
  12. Alan Browne

    Alan Browne Guest

    Ouch. That really hurt to read.

    The "original" is the Dyson sphere.

    The ring is a subset - and of course 3-dimensional in all respects -
    whether the whole thing or any subsection of it.

    Alan Browne, May 26, 2013
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