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Reaction force of a laser

  1. Jan 17, 2010 #1
    I'm aware that photons carry momentum while technically having no mass. However I recently saw a video of a laser used to propel a small silvered object up several meters in the air just using the momentum of the light.

    So what about the reaction force? As the light is coming out of the laser is there an opposite force on the laser? If you had a powerful enough laser could you feel it pushing back like water coming out of a hose? And if so does classical physics explain this process or is light based momentum on objects in the realm of quantum?
  2. jcsd
  3. Jan 17, 2010 #2


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    Yes, but it would need to be a pretty chunky laser and they are heavy
  4. Jan 17, 2010 #3
    I think I know of the video, but it's been several years since I've seen it. Was the small silvered object disk shaped and was it spinning? And did it make a snapping or popping sound as it lifted into the air? If this is the video you are talking about then it was not the momentum of the photons that propelled it. The underside of the disk is designed to convert the energy of the pulsed laser light into propulsion by super heating the intake air and causing it to rapidly expand. So the laser is the energy source, but air is the reaction mass.
  5. Jan 17, 2010 #4


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    An industrial CO2 laser
    10.6 um = 0.12ev = 2E-20 J/photon
    Momentum = h/wavelength = 6.6E-34/10.6E-6 = 6E-29 kg m/s

    Say 10kw laser = 10E3/2E-20 photons/s = 5E23 photons/s

    Gives a momentum of = 6E-29 * 5E23 = 3E-5 kg m/s,
    about the momentum of a spider running across the floor?
  6. Jan 17, 2010 #5
    Yeah thats the one. Oh, I thought it was just the photons momentum.
  7. Jan 18, 2010 #6
    good question tho: since the photons have a momentum in the forward direction doesn't conservation of momentum mean the laser must be pushed backward a little bit?

    Possibly not. A gas laser works by reflecting light between two mirrors. As the light passes through the gas in the cavity between the mirrors, the atoms of the gas undergo stimulated emission. In this way the chemical energy of the gas is used to increase the intensity of the light. Thus, perhaps the more interesting questions are these:

    What about the momentum of the two mirrors in the laser?
    Also, how does momentum transfer work on an atomic level, ie stimulated emission?
  8. Jan 18, 2010 #7


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    Photons have momentum, so there would be a reaction force on the laser. But the force is more apparent in its effect on small objects, such as individual atoms.
    Classically, an electromagnetic field (such as a laser beam) also carries momentum, so quantum mechanics are not necessary for explaining the force exerted by a laser beam.

    The mirrors can be considered to be rigidly attached to each other and the laser housing.
    Photons transfer momentum to/from atoms upon absorption or emission. This is the basic idea behind laser cooling and trapping of atoms.

    More reading:

    See also these Scientific American articles:
    W. D. Phillips and H. J. Metcalf, Cooling and Trapping Atoms, March 1987.
    Steven Chu, Laser Trapping of Neutral Particles, February 1992.
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