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Conservation of momentum and laser powered solar sail

  1. May 11, 2012 #1
    I was watching a show on the science channel called "Alien Encounters". Well in one of them they showed a ship that looked as if it was propelling itself by firing a laser at it's solar sail. Now I am pretty sure that from what I understand about physics, that would not be possible.

    Correct if I am wrong but when a laser is fired, the laser beam (electromagnetic waves) carry momentum, now since the laser beam carry's momentum then a reverse momentum must be created on the ship that it is firing from, only in the opposite direction. So now if you have a solar sail on that very same ship and the laser beam is absorbed or reflected from that solar sail, then the laser beam momentum is transferred to the solar sail. So now your momentum from firing the laser and the momentum absorbed from the solar sail should cancel out and thus your ship should not move at all. Is this correct?

    Don't atoms move the opposite direction that a photon is emitted from?
    Last edited: May 11, 2012
  2. jcsd
  3. May 11, 2012 #2


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    Yeah, I saw that same typically moronic show. I watch physics shows on the Science Channel when I feel the need to raise my blood pressure and scream obscenities at the TV.

    You are of course correct.

    NEVER take things seriously on those shows. They get an awful lot right, but you just never know when they are going to put it some really bone-headed statement just as though they had actually checked it with a real scientist.

    EDIT: there have been numerous threads on this forum pointing out how laughable their "physics" is much of the time.
  4. May 11, 2012 #3
    Thanks, I figured there was something wrong there ,lol . I love watching the shows but I sometimes you just catch something that seems a bit off, lol
  5. May 12, 2012 #4


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    Not entirely correct

    If the light is reflected from the sail then the ship would move - I am quite sure you can realize that as the outcome.

    Even if the beam is absorbed at the sail the ship would have a translation opposite the direction of the beam.
    The engine part of the ship would experience a recoil as the photon is emitted. The sail part would experience a recoil in the opposite direction as the photon is absorbed. Diring the brief time interval while the photon is travelling from engine to sail, the engine part will have moved ahead, and the absorption would cancel out this movement. Since the emmision and absorbtion are not instantanious, the ship has translated froward a small distance.
  6. May 12, 2012 #5


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    Can anyone elaborate on how the reflection of light causes an object to move away from the source of the light? Does the light lose momentum after reflection? Otherwise I can't understand how it works.
  7. May 13, 2012 #6
    So wait would it actually work then?

    I was just thinking about it and while it seems impossible, I don't think it is, lol.

    Here is how I think of it.

    1.Engine part creates laser beam
    2. Laser beam will have momentum and the engine will have a opposite momentum causing the ship to move backwards.
    3. laser traverses distance and meanwhile ship is moving backwards (backwards would be the actual direction you want)
    4.laser is absorbed by sail imparting it's momentum on the sail which is the ship, this stops the movement.

    conservation of momentum still holds because all momentum = 0

    So what you said "256bits", just explained step by step. Wow if that is true then that is really cool, your basically taking advantage of the speed of light delay to move your ship.

    The question I have though is theoretically, could you have 100% efficient solar panel that absorbs your laser which then turns that energy back into a laser with a 100% efficient electricity to laser generator then use this system to move your spacecraft through space. So you would just have to supply the energy in the beginning.
  8. May 13, 2012 #7


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    Nemesis, if the sail is bouncing the laser beam backwards, away from the direction you want to go, I believe it would work. This is similar to using nozzles to propel the exhaust of a rocket engine. You will feel a force opposite to the direction the exhaust is moving, no matter what direction it was initially in during combustion. I'm not understanding how 256bits explanation works however.
  9. May 13, 2012 #8

    Momentum of light changes after reflection. It transfers the momentum to the object so that the total momentum is conserved. Consider a beam of light with energy E and momentum p, perpendicular to the face of a cubical reflecting box of area A.

    [itex]E = pc[/itex]

    where c is the speed of light.

    Now when the light is incident on the cube, the momentum of photons is E/c, and when it is reflected, the momentum is -E/c. So the change in momentum of light is 2E/c. Change in momentum implies force, which is exerted on the body.


    Edit : If the beam is absorbed, the change in momentum would only be E/c since the final momentum of photons is zero. So, this force would be half of that would occur due to reflection.
  10. May 13, 2012 #9


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    Hmm. I don't understand how momentum is conserved during reflection. If the light reflects and has the same energy/momentum, and the sail has gained some, where did it come from?
  11. May 13, 2012 #10
    Too drakkith,

    The object moves away from the source of the light because light which is just electromagnetic fields can carry momentum. So if the light hits a object it can impart that momentum on it. If the light is reflected then only some of that momentum would be imparted on the object and the light (electromagnetic field) would lose some energy (drop in frequency). It would then be remitted with momentum in the opposite direction (less momentum due to less energy of emitted photon) to which it came in, there would also be momentum in the direction that it originally came in so as to cancel the momentum of the emitted light.

    I think this is about right but I am not too sure about some of it which is why I asked the question below

    edit: apparently I wrote this one too late, lol

    to others,

    Would the photon actually impart only some it's momentum when hitting the mirror or would it impart all it's momentum and then just get back some of that momentum when remitted?
    Meaning that there is a net momentum in one direction.

    Edit: yeah I am pretty sure it would impart all the momentum and the net momentum in one direction comes from the decreased energy of the emitted photon which would mean less momentum.
    Last edited: May 13, 2012
  12. May 13, 2012 #11
    Light will not have the same momentum. The magnitude of initial and final momenta may be same. In the box case, momentum after reflection is -E/c. (Momentum is a vector!)

    This change in momentum is transferred to the sailboat,

    [tex]\frac{2E}{c} = M_{boat}V[/tex]

    From wikipedia,

    All of the photon's momentum is conserved. There will not be a net momentum in any direction.
  13. May 13, 2012 #12
    ok so then is it possible for the ship to move at all. I am pretty sure it will like how 256bits said. It would just move for a bit until the light hit the sail and stopped the motion. So while momentum is conserved everywhere, the ship still moved a bit. So would the show be correct in showing a ship with a sail and laser and be correct as a viable idea even if impractical for say energy reasons?
  14. May 13, 2012 #13


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    Alright, so the momentum is now changed. Has the light lost energy? IE will the reflected light be of the same frequency as it was prior to reflection from an observer stationary to the emitting source?
  15. May 13, 2012 #14
    Considering a perfectly reflecting surface...The frequency of the emitted light will be same.

    This can be verified from the equation I wrote above,

    [itex]E = pc[/itex]

    For partially reflecting surfaces, it depends on how much fraction of the energy they reflect, and the frequency will change accordingly.
  16. May 13, 2012 #15
    I'm somewhat confused by this explanation. There can't be a net displacement with no external force acting on the light-boat system. My idea to this is, the photon cannot stay in the atom of the sail forever, it -has- to be emitted out, since atoms hate excited states. This would result in the boat moving back again to its original position. I'm not entirely sure of this though...
  17. May 13, 2012 #16


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    Then I am confused lol.
  18. May 13, 2012 #17
    I drew a picture just so to make sure everyone knows what I am trying to visualize at least

    Attached Files:

  19. May 13, 2012 #18
    Now what if the solar sail was just 100% efficient solar panel? Your ship moves and you get the energy back as long as the laser engine is 100% too, lol, something is wrong
    Last edited: May 13, 2012
  20. May 13, 2012 #19


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    What has happened is that some energy has moved from one end of the system to the other end - all internal and nothing external needed at all.

    Correct. If the energy moves back to the original end.
  21. May 13, 2012 #20


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    Yes. Moving the energy from the sail back to the engine moves the ship in the reverse direction.
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