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Exciting a photon

  1. Jan 22, 2013 #1
    Is it possible to excite a photon? Or bring it to a higher electronvolt?
  2. jcsd
  3. Jan 22, 2013 #2

    Simon Bridge

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    In the case of massive particles, what would it mean to "excite" one?
    I mean in detail - not just to give it more energy.

    Then try to see how that could relate to photons.
  4. Jan 23, 2013 #3
    The words "excite", or "bring to higher level" suggests the existence of levels in the first place, that is, you need a quantized degree of freedom. One way to create this situation for photons is to place them inside a cavity where only certain fequencies/energy levels are allowed. In this case, photons can occupy higher or lower levels yes, and you can talk about excitation.
  5. Jan 23, 2013 #4

    Simon Bridge

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    @Zargon: interesting ... so what happens to the photons at the cavity walls? Isn't there a chance of being absorbed by the wall? Or were you thinking of some other way to restrict the allowed frequencies?

    Lets say you have a photon in some well-defined quantum state in such a cavity.
    How would you excite it to the next state? Wouldn't you have to annihilate it and introduce a new photon?
  6. Jan 23, 2013 #5
    if you add a magnetic field to the well would the photon get excited and move to a higher energy state ?
  7. Jan 23, 2013 #6
    The simple answer is NO. A photon is created due to some event such as a particle interaction, etc., and immediately starts traveling at c with a well defined energy given by E=fh. That's about all you're gonna do with that particluar photon. If you want a photon with a higher energy, you're gonna have to create another photon somehow.
  8. Jan 23, 2013 #7
    Now you may say, what about when a light ray hits a surface and refracts or changes direction, doesn't its energy change? Or what happens when it hits a prism? The answer to that is that the abovementioned effects are due to the incident photons of white light hitting an object which absorbs, destroys, and re-creates or re-radiates a new photon at a different (or perhaps the same) energy.
  9. Jan 23, 2013 #8


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    Well, you could introduce a blueshift by running towards the source of the light...that's about all I can think up.
  10. Jan 23, 2013 #9
    That's an interesting question, we can constrain the paths and energies of protons and electrons,etc. in particle accelerators with magnetic fields, would the same be true for photons? My guess is no, but I'm willing to be pursuaded...
    Last edited: Jan 23, 2013
  11. Jan 23, 2013 #10
    I mean, it seems as though gravity can alter the direction and energy of a photon, so maybe I was wrong with my earlier statements.
  12. Jan 23, 2013 #11
    gravity alters spacetime the photon is still moving in a straight line with the same energy level just that straight line is spacetime curved.
  13. Jan 23, 2013 #12

    Yeah but...if you shine a flashlight down a well the energy of the light increases the closer you get to the center of the Earth.
  14. Jan 23, 2013 #13
    Yeah your right I forgot the energy gained from blueshift.
  15. Jan 23, 2013 #14
    "..if you shine a flashlight down a well the energy of the light increases the closer you get to the center of the Earth....
    yes! some details below....

    just change the size of the cavity or the potential...

    classical analogy: change the fixed points of a vibrating string and it has a different resonant frequency....

    another way: drop a photon into a gravity well:


    1: A hydrogen atom is lowered into a deep gravity well. Then a photon of visible light is dropped onto the atom, which becomes ionized, although visible light does not normally ionize hydrogen. That happened because the field that keeps the atom together weakened as the atom was lowered.

    PeterDonis: No, it happened because the photon was blueshifted as it dropped into the gravity well. A visible light photon emitted locally, at the same altitude as the atom, won't ionize it, so the field of the atom is not "weakened" at all according to local measurements. The difference that lowering the atom gently means that it is at rest deeper inside the well, so it "sees" the blueshift of the photon. To see why that's important, consider an alternate experiment where you let both a hydrogen atom and a visible light photon free-fall into the gravity well, in such a way that they meet up somewhere much deeper into the well than where you released them (you time the release of the atom and the photon from your much higher altitude to ensure this). Will the photon ionize the atom? No, because the atom is not at rest in the field; it is falling inward at a high speed, so there is a large Doppler redshift when it absorbs the photon that cancels the gravitational blueshift.
  16. Jan 23, 2013 #15

    Simon Bridge

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    That just changes the normal modes - it does not change the energy-level of the photons already in it.
    I guess I could be wrong - see post #4 for questions arising from the concept, and posts #6&7 for clarification.[1]

    The magnetic field is photons. So this question is talking about photon-photon interactions, or, what we used to think of as a photon interacting with a free field.

    iirc the Feynman Diagrams sum to zero.

    Further reading.


    [1] If I have a charged particle confined to a potential well, then change the width of the well by some means, then changing the width does not automatically change the energy eigen-state of the particle does it? Wouldn't the situation be more like making the energy of the particle uncertain - (represented as a superposition of eigenstates of the new potential) requiring a measurement of some kind to establish it?
    Last edited: Jan 23, 2013
  17. Jan 23, 2013 #16
    I think it does affect energy...for example:

  18. Jan 23, 2013 #17


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    I think you guys are making this too hard --- the best way to excite a photon is to show it a really sexy electron !
  19. Jan 23, 2013 #18

    Simon Bridge

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    Wikipedia is not that great a reference - and that quote is does not actually contradict what I've been trying to say. You also have not shown how this model takes into account the other comments and questions I have referenced. Have you done the math? [1]

    The particle-in-a-box model can be solved analytically fersure - but it is not a good model of actual physical systems. It is especially problematic for light, since you have to figure out what the box is made out of that would confine a single photon without it annihilating at the walls. When it comes to energy eigenstate transitions, you still have to figure how that would come about. i.e. what would be the physical process that changes the width of such a strange box? Not everything describable in math is physically possible.

    You can confine a photon gas in a box though.
    This uses a model where photons are constantly being annihilated and created.
    In this case, you can raise the mean and total energy of the system by changing the width of the container. But what is it that happens to individual photons?

    You could be imagining a single photon bouncing between ideal, perfectly reflecting, walls [2]. In which case, the photon is being annihilated at each wall, and then a new one is created. (Though there is some philosophical hair-splitting over this point.) It is possible to arrange for the photon thus created to be a higher frequency than the one annihilated. I would assert that this process does not well fit the concept of "exciting a photon": it kinda means that it is the same photon that has more energy like an excited electron-in-a-box is the same electron.

    For a single particle in a box, when you make the box smaller, the energy eigenstates raise in value, and so does the expectation value of a measurement of energy of the system. The particle, however, is not in a single energy eigenstate until a measurement of energy has been made. You can figure out the odds by expanding the initial eigenstate wave-function in terms of eigenstates of the final potential.

    So the process would involve two steps - making the box smaller, and then measuring the energy level. For a perfectly reflecting box of one photon (as discussed) how would you (or the system) conduct that measurement without annihilating the photon?


    [1] see this example for what happens when you change the width of a confining potential.
    The author has the potential increasing in width, and finesses the system so there is an eigenstate in the final system with the same energy as the ground state of the initial system. As an exercise, do the problem the other way around - making the box smaller.

    [2] You realize that reflection, at the photon level, is described using creation and annihilation operators right? The law of reflection is only obeyed on average and all that?
    (In fact D Simanek has a pmm puzzle using the idea of a photon bouncing between perfect reflectors.)

    [edit] @phinds +1 that! I have been resisting the pun from the start :)
    Last edited: Jan 23, 2013
  20. Jan 23, 2013 #19
    No, it are humans that are ill-informed, not nature nor the physics are bizzare.
  21. Jan 23, 2013 #20
    The "quantum profile" of the hydrogen atom can also be solved analytically, which is seemingly a miracle since 70% (is that figure right?) of the universe is hydrogen. Of course, you get to helium and above, or maybe even deuterium, and you're forced to use the Runge-kutta matlab function and solve these problem numerically. In any case, doesn't anybody think its weird how electron orbitals manifest from the Shrodinger equation? It is bizzare, solving the radial and azimuthal equations yield these weird Legendre polynomials, and we infer the quantum numbers from co-efficients in the Euler exponents. I mean, who'd of thunk?
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