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Questions RE: radiation pressure

  1. Sep 13, 2010 #1
    Hi everyone,

    I am not a physicist, so I've been looking at "layman's terms" type sources for the answers to these questions, but I'm afraid they've just left me with more questions. I'll frame the questions around the concept of solar sails:

    1. Solar sails are reflective -- since some of the momentum of light is being transferred to the solar sail, I assume the reflected light has a lower energy than the incoming light. Is the reflected light a lower frequency? Or lower amplitude (brightness?)?

    2. According to this source: http://science.howstuffworks.com/solar-sail.htm/printable" [Broken], you could power a solar sail with an on-board laser... that doesn't make sense to me. Maybe I'm just thinking too "Newtonian" about this, but wouldn't the emitting laser impart a force equal and opposite to force of the light bouncing off the solar sail? Isn't this why we don't see large fans on sail boats to create wind on calm days?

    3. If solar sails were absolutely black, would they still work (though less efficiently)? I'm ignoring the fact that they would heat up, and then radiate their own energy for the purposes of this question. I'm having trouble wrapping my head around how reflecting the light results in more momentum transferred than absorbing it completely.

    I thank you in advance for your help here.
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Sep 13, 2010 #2
    1. Slightly lower frequency. But a solar sail is just a big mirror, so since no mirror is perfect, slightly lower amplitude also.

    2. The laser is to be shot as direct thrust towards space, not towards the sails.

    3. If solar sails were absolutely black, energy would be absorbed, heat everything up, and emitted in any random direction. Specular reflection from a mirror allows perfect control of "emission" direction : all rays parallel, working "together".
  4. Sep 13, 2010 #3
    Thanks for responding, Dr Lots-o'watts!

    On #2, the article states "You might be wondering what happens when the spacecraft finds itself far from sunlight. An onboard laser could take over providing the necessary propulsion to the sails. " They say specifically "to the sails". Perhaps they just got it wrong?

    #3: I think I get it -- when the photons are absorbed, the energy is re-emitted in every which direction so the net force in any specific direction is 0, so the acceleration is also 0. Correct?

    As an extension to #3, what if we had a random black object at which a laser was pointed. This object has a large heat sink attached, so that it does not radiate any energy. The heat sink could do something productive, like convert the laser light to electrical energy or mechanical energy. Our object should still experience no acceleration, correct?
  5. Sep 14, 2010 #4
    3. Re-emitted in every direction yes, but the net force is not zero. It is still in the forward direction, but less than if there is 100% specular reflection. The situation where light is 100% absorbed is comparable to an inelastic collision, while for 100% reflection, it is comparable to an elastic collsion.

    3. ext: Energy is radiated from the heat sink! There would still be acceleration because the net direction of reflected photons is still towards the source from the the absorbing surface. However, it is not as efficient as if it were a mirror, in which case, you would get 99% or so of the photons reflected in the same direction. (you might want to see my homepage to see how close you are to the devices I work with daily)
    Last edited: Sep 14, 2010
  6. Sep 14, 2010 #5
    Thanks again for responding -- I see from your website and profile that you're definitely the guy to ask about this stuff :)

    I suppose I'll skip to where these questions are leading -- I want to put a twist on the various laser propulsion systems that have been proposed. Instead of having the laser at the source, I want the laser at the destination. For this to be possible, I need the the force from the incoming photons to be negligible in comparison to the force generated by the propulsion system (which is powered by the laser beam). The problem is, if I'm just emitting photons in the opposite direction, I've essentially created a transparent material, and I don't imagine that creates any net force at all. But, if I add a reaction mass (perhaps by means of a scoop, similar to a Bussard ramjet), I think propulsion could be achieved. I just can't figure out how much propulsion is being sapped by overcoming the momentum of the incoming photons. Does that make sense?
  7. Sep 14, 2010 #6
    I think any photon momentum from the destination laser will be dwarfed by nuclear reactions from scooped matter. No contest. A useful thermonuclear reaction should emit much more then the trigger that causes it. That's what they're trying to do at the NIF.

    In other words, many more photons should be emitted by mass converted to energy by a nuclear reaction, than the number of photons that would be used to trigger the reaction. You just have to direct the newly emitted photons towards the back with a parabollic mirror (such as a dentist chair light).

    Note that this may be interesting for a sci-fi story but I wouldn't hold my breath waiting for it to happen in our lifetimes. A sufficiently powerful interplanetary laser and a space-based thermonuclear reactor are not close to being technologically feasable. Closest thing is the NIF.
  8. Sep 14, 2010 #7
    Thanks again :)

    I agree this would be a "future" technology, mostly because of the interplanetary laser. In my design, just the scoop is similar to the Bussard ramjet -- the scooped up matter is just accelerated with the energy from the laser, rather than fused. I thought this would be more technologically feasible in the short term, albeit much less efficient.

    If you're interested, the reason I want to place the laser at the destination is because of the relativistic doppler effect. I calculated that at .8c, you'd observe the laser to have 3x the original frequency, and according to the Planck relation, you could extract 3x the energy from the beam. Meanwhile, at .8c, your vessel's relativistic mass is only 1.66x the original -- so theoretically you could achieve constant acceleration for a much longer time. Of course, the photons also have 3x the momentum that you must overcome too.

    But like you said, it's all sci-fi for now -- I just hope it's scientifically plausible sci-fi.
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