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Average number of photons in a cubic meter

  1. Jan 13, 2016 #1
    It seems to me that light travels (in a room) from everywhere, to everywhere else: A complicated, messy, interconnected network of photons of varying wavelengths which somehow avoid ever colliding with each other. This is what allows me to see things, and other people to see other things without our "lines of sight" ever getting in the way of each other.

    Add to this the fact that infared (heat) photons are also traveling through this space (from every angle, to every angle), as well as radio station signals, WiFi signals, cell phone signals, cosmic rays, cosmic microwave background photons...

    It seems like the number of photons in a cubic meter must be inconceivable.

    One auxiliary question: At what scale do we not see a complicated network of photons constantly coming in from everywhere, to everywhere, and start to see gaps? Also, why do photons seem to be able to share space and never ever collide? Do they, and we just don't see it?
  2. jcsd
  3. Jan 13, 2016 #2


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    At what scale? Stellar scales are equivalent to point sources. At what point is it literally blinding? Different calculation.
  4. Jan 14, 2016 #3
    At the scale of a cubic meter. Does it not make sense to ask how many photons are in a cubic meter on Earth in a normal room?
  5. Jan 14, 2016 #4


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    In quantum electrodynamics (QED), two photons cannot interact "directly" with each other, unless they have at least enough combined energy to produce the mass of an electron-positron pair. This energy is in the gamma-ray range, far above microwaves, radio and visible light. There are "indirect", higher-order (lower-probability) effects, but to observe and study them for e.g. visible light, you need to use very intense laser beams. This is the area of "non-linear optics."
  6. Jan 14, 2016 #5
    The number of photons would be tied to the frequency of the photons and the power level. For instance, at a wavelength of one meter, you would have exactly one photon of 300 mhz radiation which happens to have a wavelength of 1 meter. So if you have a wavelength of 1 micron, an IR wavelength, then you would just go 1 million cubed which is 1E18 photons of that power level. But you could have a trillion times that many photons if you pumped in a trillion times the intensity.

    So the number is highly variable, photons don't mind being crowded together, they are quite sociable:)
  7. Jan 14, 2016 #6
    I always thought that intererence patterns of light were the result of photons colliding. Since light has wavelike qualities it should behave the same as any other wave.
    Let's see what Nikon has to say.
  8. Jan 14, 2016 #7
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