RE: Solar sailing vs. laws of physics ?

From: Spike (spike66@comcast.net)
Date: Thu Jul 03 2003 - 21:00:26 MDT

  • Next message: Eliezer S. Yudkowsky: "Re: Solar sailing vs. laws of physics ?"

    I was in a hurry when I posted my one-liner.
    Let me deal with this in a little more detail.

    from: Jeff Davis
    Subject: RE: Solar sailing vs. laws of physics ?

    --- 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?...

    Ja.

      ...Has it been
    observed experimentally with stationary
    versus moving mirrors?...

    Ja.

      ...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?...

    Ja.

      ...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?...

    Oy. Jeff I hope I get to your question here by saying
    what you need is to remember is momentum is
    conserved, same with reflected photons as with any
    other collision. You know that the momentum of a
    photon is h*(nu)/c. If one collides with your
    motor home it will appear from the point of view
    of the driver to have the same frequency when it
    leaves as when it arrives. Of course the photon
    is slightly blue shifted from the drivers view
    compared to the cop's view. The difference between
    the frequency the cop sees and what the driver sees
    is doubled, and that is the blue shift the radar gun
    sees.

        

      

    ...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'?...

    No. Assume the sail of just the right density that
    the force due to light pressure exactly equals the
    force due to the sun's gravity. The sail hovers,
    stationary with respect to the sun, no work is done,
    no frequency shift of the reflected photons.

      

    ...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...

    Ja, these are all well understood physical phenomena.
    Thomas Gold can access many good texts on the subject.
    I looked over his site briefly and saw several areas
    which look erroneous or incomplete. Solar sails are
    not perfect reflectors. Standard theory has a factor
    which describes what fraction of the sun's energy is
    reflected.

      

    ...So, will an observer traveling with the solar sail
    observe an alteration in frequency of the incoming and
    outgoing radiation?...

    No. The astronaut on board the solar sail sees the
    same frequency incoming and outgoing. The observer
    on the sun sees a redshifted spectrum if the sail is
    moving away and blue shifted if the sail is inbound.
    Note that in practice a solar sail would actually
    orbit the sun, then orient itself at some angle, say
    pi/4 to spiral away from or spiral towards the sun.
    That way, there is no need to have the sail so thin
    that its mass per unit area is less than the light
    pressure from the sun.

      Does starlight from a receding or approaching star change frequency
    when reflected in an earthbound mirror?

    Ja, of course. Starlight is no different from
    any other kind of light.
     
    > 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?...

    The reflected photons have transferred some of their
    energy to the sail. That's why they are red shifted,
    assuming the sail is outbound. If the sail is inbound,
    the photons are blueshifted, higher frequency, higher
    energy.

    ...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...

    Exactly so, Jeff. Whats the mystery?

    ...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...

    No. Light pressure is verrrrry small compared to
    the force generated by air inside the bulb being
    heated more on the black side than on the cooler
    silvered side. If you took one of those things, pulled
    a sufficiently strong vacuum on it and shined a
    sufficiently strong light, it *would* turn the
    other way.

    This would be provable with something as simple
    as a vacuum bell jar. Drill a hole in the
    bulb, keep the light source steady and watch the
    spinner slow as the pressure in the bell jar
    drops.

      

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

    No. A solar sail doesn't have a blackened side. One
    cannot afford the weight hit of such a thing, even if
    it were useful for some reason, which it isn't.

    Gold's Carnot cycle argument comes apart when one
    realizes how very little actual energy we are extracting
    from the photons. It really isn't much, and the final
    resulting accumulated gravitational potential energy
    isn't much. The magic of the solar sail is that the sun
    is providing both the energy source and the propellant
    (the photons) so the spacecraft neednt haul the stuff
    itself.

    Again I urge you, young man, consult the established
    texts on this topic. The smart guys have figured
    this stuff out. Everything's hard till ya know how to
    do it, Jeff. We know how to do this one.

    spike



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