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Radiation pressure question

  1. Jan 8, 2007 #1
    Pressure = energy flux / speed of light
    Taking an energy flux of 1000 watts per meter sq (typical flux of sunlight on earths surface)

    We gain, pressure = 1,000/300,000,000 = 1/300,000 Pascals.

    If it is reflected, we gain twice that = 2/300,000 pascals

    Now my question, if I had two flat mirrors of 1m^2 face to face one meter apart and put a beam of light in between them of energy 1000j (basically a beam of sunlight for 1 second). I would get my 2/300,000 Pascal’s of force per "collision" of the light on the mirrors. Now since they are bouncing back and fourth. I would get 300,000/2 collisions per second on each mirror. That gives me one Newton of force on each mirror?

    Is that correct or have I done something wrong?

    If it is correct, why isn’t this method used to propel spacecraft? or even earth craft? that is assuming we can get near perfect mirrors (which we have already). It would also not violate any physics laws I can see. It conserves momentum. It conserves energy (since the light will be red Doppler shifted, losing its energy and giving it to the KE of the mirrors).

    So where have I gone wrong?
  2. jcsd
  3. Jan 9, 2007 #2
    If youre asking why dont we use two mirrors face to face its because as it hits mirror A you get motion in the positive direction and as it its mirror B it then negates that motion.

    If youre asking why not use mirrored materials in solar sails due to the extra oompf from reflection, we already do.

    Hope thats what you were askin
  4. Jan 10, 2007 #3
    Two independent mirrors not connected to each other! so both fly off in opposite directions.

    Anyway I think I figured it out
    Reflection requires promotion of electrons from one orbital to a higher one, then back again, this requires a finite time which is probably >> the flight time between the two mirrors. On top of that a mirror would need to be >99.9999% reflective which we can not make at the moment.
  5. Apr 26, 2007 #4
    If one mirror were on a large mass (say a large, airless moon to minimize atmospheric issues), then the problem of both mirrors traveling in opposite directions could effectively be eliminated.

    The more practical way to do this, used in a number of science fiction stories (viz. Niven & Pournelle's "The Mote in God's Eye"), is to use a high-powered laser aimed at a concave (parabolic?) mirror on the tail of a spacecraft. In trying to use the two-mirror method, even collimated laser beams spread out with distance. I'm not sure exactly what shaped mirror would be most efficient in imparting momentum to the spacecraft, but I suspect it's not the same shape that would reflect the tightest beam back toward the source.

  6. Apr 26, 2007 #5


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    This all sounds so familiar - even the 'orbital' nonsense. Maybe I've been here too long. The big error is trying to multiply the force by some sort of collision rate - just because it's coming from another mirror doesn't change the energy flux. The force is still tiny. And note as the mirrors get farther apart (even if they are ideal) the force will weaken since the flux will drop just because the volume between the mirrors is growing.
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