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Theoretical Photon density

  1. May 5, 2013 #1
    Hi yall, I have a few questions in regards to EM radiation and photon density, or energy density; not sure what exactly the correct term would be here.

    (1) Anyway, first off, excluding the possibility that a black hole would form, is there anything that would limit EM radiation energy density to a certain amount? Because as far as I know, the Pauli Exclusion Principle doesnt apply to photons. I would think a black hole would eventually form(?) but besides that, is there anything that puts an upper limit on how many photons you can cram into a "box" so to speak? Or should I say is there a limit to energy density assuming the energy is only EM radiation?

    Take for instance a laser with a 1 meter aperture/diameter beam, would there ever come a point where you could not possibly increase the energy of the beam due to physical effects? Ignoring engineering difficulties and black holes of course..

    (2) Also, what are the physical effects of very high radiation pressure? Like for example you have a box at which inside is full of EM radiation at Planck energy density, would it "feel" like air pressure, trying to burst apart? Or would the photons be fully "content" to stay at this density? Obviously "feel" and "content" do not apply to non-living things but thats the best wording I could think of.

    Thanks so much.
     
  2. jcsd
  3. May 5, 2013 #2

    mfb

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    Apart from the possible formation of black holes, there is no known limit on the photon density - if the energy density gets too high, you start producing electrons and positrons (or even heavier particles), but that should be fine.

    It would vaporize any container, with a shockwave expanding at nearly the speed of light.
     
  4. May 5, 2013 #3
    Thanks.

    Okay it would vaporize the container, let put me put the question this way. If you were somehow put on the surface of a planet with no atmosphere, or heck just in the vacuum of space period, and you were very near to a nuclear fusion explosion, or really any source of a huge amount of EM radiation being released all at once. Remember, this is in a vacuum, aside from the fact that you would vaporize very quickly, would you feel any type of force pushing on you? Obviously photons have a very small amount of momentum, but magnified to a huge extreme, would it feel similar to a air pressure wave? Ignoring being vaporized for a second, would there be any momentum transfer to speak of?

    Thanks and sorry for the dumb questions.

    And Ive heard of the laser propulsion drive concepts but IIRC they use the vaporization of the surface of the pusher plate/propellant to get the thrust. Im strictly asking about a photons ability to transfer momentum at very high energies/density. Just wondering, even though its meaningless when confronted with the fact that vaporization will occur long before any momentum transfer.
     
  5. May 6, 2013 #4

    mfb

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    The light would have a significant pressure, sure. The evaporating material expands outwards (and not inwards) as a result.
    Pressure is just intensity divided by c. I found 2*1022W/cm2 as record intensity http://www.laserfocusworld.com/articles/print/volume-44/issue-4/world-news/ultrafast-lasers-michigan-group-achieves-laser-intensity-record.html [Broken], this corresponds to a pressure of 6.7*1017 Pa - more than in the core of our sun. This pressure lasts for an extremely short time, however.
     
    Last edited by a moderator: May 6, 2017
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