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Is radiation pressure a thing?

  1. Sep 30, 2015 #1
    I have seen the radiometer (mill in a bulb with differently coloured leaves) explained by thermodynamics, but also this:

    where little particles are propelled by laser beam.

    So, does light beam actually transfer or cause momentum in some way? If it does, is there any chance to make a propulsion engine with this? I would like to know more about it, explained in terms of simple-ish mechanics or photons' behaviours.
  2. jcsd
  3. Sep 30, 2015 #2
    One has to account for solar radiation pressure for exact calculations of (Earth) satellite orbits.
    Google "solar sail".
  4. Sep 30, 2015 #3
    Momentum is conserved, right? That means if I shoot a beam of light in one direction, I should receive an impulse in the other direction, and no matter whether or not the beam of light lands on something. Would that be how radiation pressure can be used for a (very ineffective) kind of propulsion?
  5. Sep 30, 2015 #4
    Ya know, I've never thought about it before... light for propulsion is always presented as an outside source fired at the craft.
  6. Sep 30, 2015 #5

    Vanadium 50

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    Two ideas are getting confused here. There is such a thing as radiation pressure, but it's not what turns a Crookes radiometer. (Look and see which direction it turns)
  7. Sep 30, 2015 #6
    I never said it does.
    I am asking if a flashlight is getting pushed backward when it's on.
  8. Sep 30, 2015 #7


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    It is. Calculating the magnitude of that reaction force is a good exercise that will go a long ways towards explaining why we don't notice this effect with flashlights. On the other hand, if something with very low mass emits sufficiently energetic electromagnetic quickly enough (for example, an atomic nucleus emitting gamma radiation) there will be a measurable recoil.
  9. Sep 30, 2015 #8


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    That's correct.

    I think it's an issue of fuel. Using a ground-based or space-based laser and a vehicle-based reflector, you can get a identical effect without having to use any power on the vehicle, which means you save tons of weight since you don't have to lug around as much fuel.
  10. Oct 1, 2015 #9
  11. Oct 1, 2015 #10

    Andy Resnick

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    Two more applications- optical trapping, and applying torque using polarized light on birefringent materials.

    This is, AFAIK, the earliest demonstration:


    (I don't count Kepler's hypothesis to explain comet tails as a demonstration)
  12. Oct 1, 2015 #11


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  13. Oct 2, 2015 #12
    That light as electromagnetic wave has momentum has already been said in this thread by several poster and can be seen from the flux of Poyinting vector, force on a charge, etc. Actually I was looking for "laser" or similar "light" engine but I didn't find any, so I posted that, even because it was one of the OP choices: "I would like to know more about it, explained in terms of simple-ish mechanics or photons' behaviours."
    I can only add a link to solar sails even if it's not exactly what I was looking for:

    P.S. it's "Panofsky and Phillips", my friend! :smile:

    Last edited: Oct 2, 2015
  14. Oct 2, 2015 #13


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    Haha. My memory was working phonetically. The book is still in a box, after we moved house.

    I must avoid posting after 'reading' threads on my phone. It gives me tunnel vision.
    I guess that the 'mechanical' view of the phenomenon, involving particles is the more intuitive. One of the (not too frequent) times that photons actually seem really to help the understanding of an em phenomenon.
  15. Oct 2, 2015 #14


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    I suppose that any discussion concerning the reality of radiation pressure wouldn’t really be complete without any mention of the most depressingly vivid demonstration of its power: it’s how the H-bomb works.

    In a thermonuclear weapon, thermal radiation emitted by the walls of a container heated to millions of degrees is used to compress and heat a mixture of hydrogen isotopes until they undergo nuclear fusion.


    The H-bomb is a horrible contraption, but you have to marvel at the ingenuity of its design.
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