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Particle Being absorbed

  1. Oct 29, 2012 #1
    Particle Being "absorbed"

    I've heard this many times before, but what does it mean when a particle is "absorbed"? Does it mean its energy is used up or is it actually absorbed. For instance, I've heard that on the way out of the core of the Sun, a photon is absorbed by a particle multiple times and re-emitted. So, also, what does it mean that a particle is "re-emitted"?
     
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  3. Oct 29, 2012 #2

    Drakkith

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    Re: Particle Being "absorbed"

    For photons, absorption means that they interact with and transfer their energy completely to something else, such as a proton, an atom, whatever. They are now GONE. Forever. Re-emission of a photon is...confusing. Your specific example of photons in the Suns core is simply the wrong way of explaining it. Once absorbed each photon is now gone, and it's energy is transferred to whatever particle absorbed it. Then that particle ends up emitting another photon (typically more than one) that is of lower energy than the one that is absorbed.

    However re-emission can mean a few other things too. For example atoms and molecules can absorb certain wavelengths of photons and re-emit that exact wavelength right back out. There are also materials such as the "glow in the dark" toys that absorb light and then slowly "re-emit" it back out, thus the glow effect.
     
  4. Oct 29, 2012 #3
    Re: Particle Being "absorbed"

    Well usually this would be the kind of language used in atomic physics. Say a photon collides with an atom (really the electrons orbiting said atom). If it is of the appropriate energy, it is destroyed and its kinetic energy used to promote an electron to a higher energy level within the atom. However, the atom is then said to be in an "excited state", and its natural tendency is to relax back to a lower energy state, which is to say that the promoted electron will fall back down to where it started (or some other cascade of electrons dropping to fill the now available lower energy states will occur). During this relaxation the extra energy is "re-emitted" in the form of photons. This is the very basics of atom-photon interactions so there is loads of info about it everywhere: http://en.wikipedia.org/wiki/Excited_state#Atomic_excitation

    You can imagine this happening many times throughout the plasma of the sun as a photon tries to travel its way from the core to the surface. Although since it is a plasma there are also lots of interactions between the photons and free charged particles, various forms of scattering and such.
     
  5. Oct 29, 2012 #4

    Drakkith

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    Re: Particle Being "absorbed"

    Kurros, just to be clear, you are using one specific form of absorption/emission, and then talking about a plasma which pretty much doesn't consist of atoms, but of free nuclei and electrons. In this case atomic excitations cannot occur.
     
  6. Oct 29, 2012 #5
    Re: Particle Being "absorbed"

    I haven't checked, but I was assuming that for most of the sun the plasma consisted of only partially ionised atoms, in which case transitions are still occurring, or that at least not all atoms in the plasma are fully ionised all the time.
     
  7. Oct 29, 2012 #6

    Drakkith

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    Re: Particle Being "absorbed"

    I thought the opposite, that the vast majority of the plasma was completely ionized.
     
  8. Oct 29, 2012 #7
    Re: Particle Being "absorbed"

    http://en.wikipedia.org/wiki/Degree_of_ionization#Physics_usage

    Ok you probably win, although annoyingly there is no citation for the claim in the wikipedia article, and I am having a hard time getting google to find me something more comprehensive.
     
  9. Oct 30, 2012 #8

    mfb

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    Re: Particle Being "absorbed"

    It depends on the region.

    If kT >> binding energy, most atoms are fully ionized, if kT << binding energy, just a small fraction is. Binding energy is ~13 eV for hydrogen and ~50eV for helium (ground state).

    In the core, with kT ~ 1keV, nearly all light nuclei are ionized.
    At the surface, with kT ~ 1/2 eV, just a small fraction is ionized.
     
  10. Oct 30, 2012 #9
    Re: Particle Being "absorbed"

    when a photon gets absorbed,it means it is destroyed.when it is emitted ,it means a photon has been created(annihilation and creation operator).It does not mean any more in quantum theory of radiation.
     
  11. Oct 31, 2012 #10
    Re: Particle Being "absorbed"

    So if the photon is destroyed when it is absorbed by an electron, does this mean that the electron - i.e. the electron in general - carries within itself the information necessary to create photons?
     
  12. Oct 31, 2012 #11

    Drakkith

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    Re: Particle Being "absorbed"

    What do you mean by "information"?
     
  13. Oct 31, 2012 #12
    Re: Particle Being "absorbed"

    Well the photon is a thing, and since the assumption is that the photon is destroyed when an electron absorbs a photon - then the question arises that when an electron emits a photon, then where is the information on how to create the photon?
     
  14. Oct 31, 2012 #13
    Re: Particle Being "absorbed"

    It is embedded in the laws of the universe that describe how fundamental particles interact with each other.
     
  15. Oct 31, 2012 #14
    Re: Particle Being "absorbed"

    So are the Laws of the Universe embedded in the electron?
     
  16. Oct 31, 2012 #15

    Drakkith

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    Re: Particle Being "absorbed"

    Let's put it this way, the photon gives energy to the electron, which can then emit another photon using some or all of that energy. It works via the rules of nature, which everything must follow, but I wouldn't necessarily call it "imbedded" in the electron any more than the rules for a lever are imbedded in a see-saw.
     
  17. Oct 31, 2012 #16
    Re: Particle Being "absorbed"

    Isn't that a problematic example - for the rules of the lever in a see-saw are a result of the fundamental particles in the see-saw?

    Anyway, so is the suggestion that it is energy that has the information of how to create the subsequently emitted photon?
     
  18. Oct 31, 2012 #17
    Re: Particle Being "absorbed"

    Are you looking for something more specific? Like why does the emitted photon have a particular wavelength? This is just due to conservation of energy in whatever situation is occurring, although this is just one of the said rules that the electron+photon system has to follow. No one knows why these rules exist, it is just how these things have been observed to behave. It is like asking where does the moon get the information about how it is supposed to orbit the earth.
     
  19. Oct 31, 2012 #18
    Re: Particle Being "absorbed"

    No, I am just asking as a consequence of noting, that if the photon is destroyed when it is absorbed by an electron, then how does the electron produce (i.e. emit) a photon?

    I'm not asking how a tally of energy is kept, but the even more basic question of simply:

    How is a photon created by an electron?
     
  20. Oct 31, 2012 #19
    Re: Particle Being "absorbed"

    The assumption that the photon is destroyed when it is absorbed by an electron, raises a far deeper observation than simply the question of how does an electron produce a photon?

    That is...

    According to chance, it is far more likely that we live in a universe that works using simple fundamental mechanisms, than one that works by complex fundamental mechanisms.

    On that basis, for an electron to have the capability to destroy a photon, and for an electron to have the capability to create a photon, is far more complex than say for the electron to have the capability to absorb a photon without destroying the photon, and for the electron to have the capability to emit a photon without having to create it?
     
  21. Oct 31, 2012 #20

    Drakkith

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    Re: Particle Being "absorbed"

    I think you are looking too far into it. Also, assuming the electron "has information" is...confusing. The absorption of a photon by an electron or other object is simpy the transfer of energy. Upon emission it is a loss of energy from the emitting object. If you want to call that information, then feel free.

    It simply does. Classically an EM wave is produced from charged particles through various means, such as acceleration. The quantum description is a bit more complicated but the end result is the same. The electron emits a photon and loses energy. There really isn't a "deeper" understanding than this. (As far as I know)
     
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