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Mind experiment

  1. Apr 9, 2010 #1
    If you had an extremely sensitive scale (use your imagination) and shone a beam of light upon it, could you measure the force that the photons were exerting on the scale? would there even be a force?
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  3. Apr 9, 2010 #2


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    There would be. It's pretty tiny though. I don't know if it has been measured in a lab. (I don't remember how tiny it is, so I don't know if it's likely that it can be measured or not).

    Oh, and the term is "thought experiment". I expected this thread to be about something paranormal.
  4. Apr 9, 2010 #3
    haha, my bad, no paranormal activity going on here :D
  5. Apr 9, 2010 #4


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    You can find more about this question if you will search for "radiation pressure". For example in wikipedia - http://en.wikipedia.org/wiki/Radiation_pressure" [Broken]
    In wikipedia it is said that this pressure is proved experimentally in 1900.

    EDIT: To give perspective of scale I once tried to calculate what it is compared to gravity. It came out that Sun's radiation pressure against Earth is of 15 orders lower compared to gravitational attraction by the Sun.
    Last edited by a moderator: May 4, 2017
  6. Apr 9, 2010 #5
    Gedanken did that experiment and it showed up as light bulb above his head.
  7. Apr 9, 2010 #6
    wow, thats fascinating, so basically all the photons coming from the sun are still exert only 1/15th the power of its gravity upon the earth. So would different wavelengths therefore produce different radiation pressures? Like, if i fired gamma rays or radio waves at my "infinitely sensitive" balance it would show different pressures?
    Last edited by a moderator: May 4, 2017
  8. Apr 9, 2010 #7


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    He said 15 orders (of magnitude). So it's 0.000000000000001, not 1/15. (Assuming that his calculation and his memory are both correct).

    I remember doing an excercise once involving an astronaut with a flashlight. The question was, if he switches on his flashlight, and the batteries never run out, how long will it take until he has accelerated to a speed of 1 m/s. Unfortunately I don't remember the answer. (I think it was a hundred years or so, but I might remember that completely wrong).

    Yes, a gamma photon has a larger momentum than a photon with a wavelength in the visible range.
  9. Apr 9, 2010 #8

  10. Apr 10, 2010 #9
    Yes isn't it how they propose to power the next generation of space crafts with "Solar Sails"? They say the sails have to be dozens if not hundreds of miles long (perhaps even thousands) but they would absorb solar radiation and thus be 'pushed' to pretty high speeds.

    Other ideas I thought included blasting some sort of laser form earth into those sails to "push" the space craft to extremely high speeds, so as such I would think that electromagnetic radiation does exert force or pressure or whatever you want to call it.

    Furthermore, I was just reading something unrelated about how they use lasers to cool particles to near absolute zero. The lasers redirect the flow of particles to go into a single location so that means the light is 'pushing' against those particles that means it IS exerting a force on them of some sort.

    What I simply can't grasp is HOW is EM radiation exerting force if it has no mass (supposedly)?
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