RE: Solar sailing vs. laws of physics ?

From: Jeff Davis (jrd1415@yahoo.com)
Date: Thu Jul 03 2003 - 04:30:16 MDT

  • Next message: Jeff Davis: "Radiation is your friend"

    --- Spike <spike66@comcast.net> wrote:
    >
    > Subject: Solar sailing vs. laws of physics ?
    >
    > The reflected photon would have the same
    > frequency as the pre-reflection photon
    > only if the light source and the reflector
    > are stationary with respect to each other.

    Is this actually the case? Has it been
    observed experimentally with stationary
    versus moving mirrors? Certainly we've
    heard of radiation that has been Doppler-shifted
    red or blue from radiation sources moving
    towards or away--respectively--from the
    observer. Is a moving reflector equivalent to a
    moving emitter? What are the differences between
    these two radiation-'manifesting' processes? And what
    are the differences between the reflection(1) we
    associate with mirrors (metallic and dielectric), and
    the reflection(2) we associate with, say, the moon,
    which seems more rightly a case of absorption and
    re-emission?

    More fun.

    Consider the solar sail when first unfurled. If it
    were in a conventional stable orbit, with no radial
    velocity relative to the sun, would there then be no
    force arising from 'radiation pressure'?

    That doesn't seems reasonable. I've just finished
    reading a few web pages on radiation pressure. It
    seems a fairly established phenomenon, with both
    heavyweight theoretical
    foundation--Maxwell--http://www.wikipedia.org/wiki/Radiation_pressure
    and experimental confirmation.
    http://www.wikipedia.org/wiki/Nichols_radiometer
    A Nichols radiometer is the apparatus used by Nichols
    and Hull in 1901 for the measurement of radiation
    pressure.
    And nowhere is motion of the illuminated area
    mentioned as a factor.

    So, will an observer traveling with the solar sail
    observe an alteration in frequency of the incoming and
    outgoing radiation? Does starlight from a receding or
    approaching star change frequency when reflected in an
    earthbound mirror?
     
    > If the reflector is moving away from the
    > light source, there is clearly gravitational
    > potential energy coming from somewhere.

    Yesiree. Everything suggests a pressure, a force over
    an area, a transfer of and increase in momentum, and a
    consequent inescapable increase in energy, some
    combination of potential:out of the gravity well, and
    kenetic--increase in velocity. The question remains,
    where's the energy coming from?

    It seems logical to look to the incident radiation for
    the source of the energy. I would be looking for less
    energy coming 'out' in the reflected radiation, than
    went in, originally. And I would expect the degree to
    which the reflection is 'perfect', as with a
    finely-polished mirror, to correlate with low
    absorption/heat generation, and high ratio conversion
    to velocity parallel to the radiation.
      
    > The reflected photons are red shifted by the
    > receding reflecting surface. Photon's
    > momentum is transferred to the sail, no
    > energy is lost, no conundrum or paradox, no
    > physically illegal procedures.
    >
    > spike
    >
     
    Also, from the original article, we have this quote:

    "There may also be evidence to support Gold's theory,
    in the form of a quirky device called a Crookes
    radiometer. It consists of four paddles attached to
    the arms of a rotor, inside a vacuum jar. Each paddle
    is silvered on one side and coated with a black
    absorber on the other. When placed in sunlight, the
    rotor spins. If the theory of solar sailing is right,
    the rotor should spin with the reflecting silver
    surfaces moving away from the light. But it actually
    spins the other way..."

    Now, I've seen one of these gadgets, and the little
    vanes do indeed turn away from the blackened side and
    in the direction of the silvered side. Clearly the
    'radiation pressure' on the silvered side, with a high
    degree of reflection, is no match for the competing
    force generated, presumably, from the absorption,
    heating, and reemission from the blackened side.

    Hmmmm.

    Would a better solar sail result if the inward-facing
    side were blackened and the outward side silvered?

    Best, Jeff Davis

       "Everything's hard till you know how to do it."
                               Ray Charles

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