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B Where do photons *Actually* come from?

  1. Aug 16, 2016 #1
    1. When an electron drops from one orbit to a lower orbit, it secretes a photon.

    2. Any body with a temperature above absolute zero secretes black body radiation.

    Black body radiation can't possibly be caused by electrons dropping orbits, because there are only so many electron orbits, and bodies can emit black body radiation for a really long time. There aren't enough orbits to sustain those durations.

    So where does this perpetual stream of photons come from? I get that the black body photons take thermal energy away from their source atom or molecule, so energy is still conserved. What I'm asking is, from hence exactly do black body photons issue? From the electrons? From the nucleus? From the atom as a holistic unit?
  2. jcsd
  3. Aug 16, 2016 #2
    Do the actual physical thermal vibrations of charged particles "jiggle" the EM field around them? As if to say, if the particles could ever reach absolute zero, they'd stop dancing on the water and the pond would be still?
  4. Aug 16, 2016 #3


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    It's not a question of "orbits," but rather of the number of atoms. At low temperatures, thermal emission is mostly due to vibrations (and rotations in the case of a gas). As the vibrations relax, there is emission of light, mostly in the infra-red.

    As bonded atoms move with respect to one another, this changes the charge distribution (as the electrons are negative and the nuclei positive), which is what allows a coupling to the EM field.

    Not exactly. You need quantum mechanics to understand what is going on; you don't get to a state where everything stands still.
  5. Aug 16, 2016 #4
    Can you tell me what "a coupling to the EM field" is? I keep coming across this notion of "coupling" but don't know what it is
  6. Aug 17, 2016 #5


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    In the simplest terms, it means there is an interaction between the EM field and charged particles. Even if there is no EM wave present, there is still what we call the EM vacuum, and for example an isolated atom (in the middle of nowhere) is interacting with that vacuum. This is what leads to spontaneous emission: an excited atom will, without the presence of any EM wave, emit a photon, because the atom is interacting with this vacuum.
  7. Aug 17, 2016 #6
    Black body emission comes from all known emission processes, including from electrons dropping to lower energy levels. Black body emission isn't a specific physical process, but it tells what the statistical result of all physical processes together must be. You still have all the normal emission processes going on: spontaneous emission, stimulated emission, annihilation, etc-- which add up to the black body emission.
  8. Aug 17, 2016 #7


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    It's worth while pointing out that the individual energy levels of the H atom that we are first taught about only apply for isolated atoms (in a gas). In a dense body the energy levels become a continuum (energy bands) and any frequency can be absorbed or emitted. The electrons in the atoms of a solid are interacting with the charges of all the other atoms.
  9. Aug 17, 2016 #8
    A good approximation to a black body (which is an idealization) is a cavity with a hole in it such that any light entering the hole has the opportunity to scatter off internal surfaces many many times before being absorbed. Each time a photon is scattered, it can gain or lose energy, so after many scatters, it approaches the ideal curve, regardless of the emission spectrum of the material (which might have many spikes and gaps). The best black body is a black hole. The photons presumably came from somewhere in the history of the formation of the black hole or in the accretion disc, but undergo countless interactions with parts of the black hole so when they finally escape, we have no idea what process generated them.
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