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Poof goes the solar sail ?

  1. Jul 6, 2003 #1


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    Greetings !

    Please read this short story about solar sails' physics:

    Can the experts here provide their opinions, please !

    I personally thought that was all simple and basic stuff
    long since solved by physics. I mean, a photon has
    energy and hence momentum which it transfers to the
    sail and then it is reflected - reemmited thus again the
    momentum exchange occurs only that this time the photon
    and the sail push against each other (actualy kin'na smells
    fishy in terms of energy if I put it this way).

    But the light pressure experiment with the reflective and
    absorbing surfaces clearly draws a different picture. I've
    heard of it before but no details were given about the
    surface (or, in fact, I think it was said that a reflecting
    surface would move, which is clearly a book's or my memory's
    mistake ?).

    So, what actualy happens here ?!

    The only possible conclusion here as I see it is that
    the reflected - reemmited light "drags" the reflective
    material back, but how can that be ?!

    Is that really so complex that nobody thought about it before ?!

    Does this actually doom the whole idea of solar sails - they'll
    just evaporate if they're absorbing or produce no thrust
    if they are reflecting ?!


    Thanks ! :smile:

    Live long and prosper.
    Last edited: Jul 6, 2003
  2. jcsd
  3. Jul 6, 2003 #2
    I don't believe Mr. Gold

    Anyone wanna place any bets on this one?

    Photons will reflect off the whole mirror surface and transfer momentum and energy to the sail. And a reflecting sail will work twice as well as an absorbing one.
  4. Jul 6, 2003 #3


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    Re: I don't believe Mr. Gold

    So the experiment brought as an example in the story is a lie ?
    (SpaceDaily ussualy doesn't post false info.)
  5. Jul 6, 2003 #4
    Re: Re: I don't believe Mr. Gold

    The Crooke's radiometer has been around a long time, and versions have been tested with a high vacuum and bright light on the reflecting surface. They are the relevant experimants.
  6. Jul 6, 2003 #5


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    I wonder if he considered the doppler effect.
  7. Jul 7, 2003 #6


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    I don't know this "Thomas Gold" of Cornell but it makes you wonder what the physics department at Cornell thinks of this.

    Yes, of course, a photon (or anything else) reflecting perfectly from a "sail" maintains the same energy. However, momentum, unlike energy, is a vector quantity. If the photon has momentum m intially and reflects off the sail in the opposite direction, it imparts momentum 2m to the sail. I believe that's the way it was explained to me when I was in highschool (more years ago than I care to remember) and don't see anything in the article that contradicts that.
  8. Jul 7, 2003 #7
    I don't think conservation of momentum can be applied to massless particles such as photons. While it is true that they do have momentum associated with them, it is a different type of momentum, originating from the photons frequency.

    I guess Quantum mechanics must be able to explain this.

    I assume that when a photon strikes a reflective surface, it is absorbed and excites the atom absorbing it. That atom will return to its rest state, releasing a new photon having the same energy.

    Since conservation of energy will hold, even at the quantum level, no energy can be imparted to the mirror, and thus it can't be accelerated.

    (Maybe I'm wrong, but that seems to be the only way to make sense of the problem) [?]
  9. Jul 7, 2003 #8


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    No your wrong, a photons momentum is given by p= h/wavelength. Therfore as Planck's constant is always constant an inelastic collision results in a change of wavelength.
    Last edited: Jul 7, 2003
  10. Jul 7, 2003 #9
    I didn't deny that photons have momentum.
    I only said that I don't think the law
    of conservation of momentum,
    (which I know is supposed to be universal)
    would apply in this case.

    Once again I said I *think*
    So I might be wrong!
  11. Jul 7, 2003 #10


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    What actually happens is that from the reflectors point of view the photons momentum stays the same but from a 'stationery' point of view the phtons are redshifted and thus lose momentum.
  12. Jul 7, 2003 #11
    Ive solved the problem for once and for all...
    Here it goes...

    Momentum is always conserved yes? - NO!
    Momentum is always conserved in a collision? - YES! :smile:

    When a photon strikes a reflective surface, or any
    other surface for that matter (could be a brick wall)
    It does not undergo your ordinary everyday classical
    collision. Instead it is absorbed and anihilated by an
    atom of the surface it strikes. This excites the atom
    which will then jump back down to a more stable
    state releasing a DIFFERENT photon.

    Since this is not a collision, Momentum need not be
    conserved, and thus no energy needs to be transfered.

    So... Therefore the Mirror sail thing will NOT work!
  13. Jul 7, 2003 #12


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    No, momentum is ALWAYS conserved in some form or other, when the photon is absorbed (tho' in the case of a perfect reflector though it's specifically NOT absorbed but reflected) the electron aquires the photon's momentum. Of course in this case were talking about free electrons and collison not absorbtion.

    Rememberit is only the iew of one scientist that the solar sail won't work and from what I know I'm not convinced he is correct.
  14. Jul 7, 2003 #13
    I don't know, I think you're wrong!
    Photons are not classical particles, and
    so will not behave like pool balls which conserve
    momentum when they collide.

    Photons are quantum particles, which behave
    quite differently. Photons do not collide with
    things, which bounce them back.

    When you look at a red sheet of paper, what is
    hapenning to the photons hitting the paper??

    QM says...
    - The photons hitting the paper are absorbed,
    they will excite atoms within the paper, the
    atoms in the paper will have energy levels which
    correspond to a red frequency.

    The atoms in the paper will then drop back down
    to their ground state emitting a NEW photon
    having a red frequency.
    The same applies for all matter, including
    reflective surfaces.
  15. Jul 7, 2003 #14


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    In QM mometum is still conserved.

    Compton scattering is the QM process of the scattering of phtons by free electrons, there is no absorbtion in this. The idea of a perfect refelctor is that it doesn't absorb any of the light as opposed to it's polar opposite the perfect absorber.

    The electrons in a reflective surface are free electrons so they cannot absorb the photons, they scatter them.
  16. Jul 7, 2003 #15
    Oh yeah... he he he
    My bad (I forgot about Compton Scattering)
    I guess youre right!:smile:


    So, would your ordinary household
    mirror not be perfect reflector?
  17. Jul 7, 2003 #16


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    Actually the sail being a perfect refelector would, I believe use Thompson scattering but still no absorption.
  18. Jul 7, 2003 #17
    Whats the difference between Thompson
    and Compton scattering?
    Ive never heard of Thompson!
  19. Jul 7, 2003 #18


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    Thompson scattering is classical and elastic, Compton scattering is non-classical (i.e. QM) and inelastic.
  20. Jul 7, 2003 #19
    Then why on earth would you
    call this a classical collision?
  21. Jul 7, 2003 #20
    Ace, if the photon

    is reflected it will be from the whole surface of the mirror and impart momentum to the whole mirror. If it's absorbed it will only be absorbed by a little part of the mirror and it will heat that part up.
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