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Fundamental Process of an electron absorbing a photon

  1. Sep 24, 2006 #1
    I was thinkng of this question and I found it best expressed in a clear manner in another site. I quote the question and waiting for an asnwer:
    "Is it a fundamental property of electrons that they can absorb photons?
    Geometry requires that two points define a straight line. Geometry does
    not prove this; it requires this. Does physics require that electrons can
    absorb photons, or can physics prove this. Can the system be explained in
    terms of the building blocks that combine to form electrons. A tank
    destroys buildings. To explain how, we must define what the tank does to
    the bricks and then define how this affects the entire building. Why do
    we assume that the reaction between an electron and a photon can be
    explained at the level of the entire electron. I want to know how does an
    electron absorb a photon at the level of the actual reaction; at the level
    of the bricks. Can we define the precise interaction between a photon and
    some part of an electron at the moment that they merge. Entropy and
    energy have no place here. Such terms are used to define the likelyhood
    of an event occuring; not in the mechanism? If we had a magical
    microscope through which we could view the process yet not affect the
    process, what would we see?
    A thoughtful answer deserves my most sincere thanks."

    Another question in the same realm how does a discharge electron give its kinetic energy to the atom colliding with it and excite it to an excited state similar to what happens in a fluorescent lamp?
    Last edited: Sep 24, 2006
  2. jcsd
  3. Sep 24, 2006 #2
    Abu Abdallah,

    To answer your title question is easy.

    An electron is a charged particles.
    A photon is an electromagnetic wave.

    When a charged particle is exposed to electromagnetic wave, it is set into motion by the electric field of the wave. The magnetic component of the electromagnetic wave can also modify the motion of the charge. This is simple classical electromagnetism.

    When the intensity, dimension and time involved brings the physics in the quantum domain, this process takes a new aspect, from quantum mechanics. But the main aspect doesn't change: the electron can absorb energy from the electromagnetic wave. But, since now this can only go by quanta, either no absorption or total absorption of the photon will take place. This will occur at random. The physics takes a statistical aspect while preserving the classical laws in the average.

    For the details of your message, I cannot answer yet because it was difficult for me to find a precise question that I could answer.

    I would however advise you to avoid analogy in physics Analogy can be helpful for teaching but not really for understanding. For the electron/photon system the best analogy (to answer your question) is that of a boat in an harbor. When waves come from the see, the boat receives energy from the waves. Note that for the quantum aspect, it will be difficult to find any analogy.

    I would also invite you to learn about the scientific method. Developments in physics have mainly started with the emmergence of the scientific method. Therefore, in physics, every knowledge must be based on experimental data and must be verified experimentally. Everything that cannot be verified experimentally, at least in principle, has no meaning for a physicist. In the last centuries in physics, new theories have been introduced mainly to correct and improve previous theories. The new theories have then been able to correct disagrement with older theories and often to predict new results and stimulate the invention of new experiments to make further checks. The interaction between photons and electrons has been verified in the greatest details you could imagine.

    Altough I am not a specialist in the electron-photon interaction, I have worked in plasma physics in the past. Plasmas are completely ionised gases. Therefore, plasma physicists are very familiar with the interaction of charged particles and electromagnetic fields, but mostly in the domain of classical physics.

    For example, the absorption of electromagnetic waves launched by RF antennas into a plasma is very well know in plasma physics, it is a daily reality in laboratories. I could cite many different applications of this classical EM-wave-electron interaction, like plasma heating and many different plasma diagnostics.

    There is however at least one example I know in plasma physics where the quantum aspects come into play: it is the so-called "bremsthralung". In hot plasma, colliding electrons do emit (or create as opposed to "absordb"!) X-ray photons, and because of that they slow down. These X-rays are of course emitted as photons and these photons are detected/counted individually in the laboratories because this give a measure of the temperature. This phenoma is very well known and used as a plasma diagnostic tool. Other physicist have the same familiarity with the photon absorptions by electrons. These are all experimental fact described successfully by physical theories.

    Best regards,


    About why and how.
    In science and particularly in physics answering why and how only lead to new whys and hows. Why does an apple fall: because the earth attracts it. But why does the earth attracts if: some description in general relativity but now answer.
    I think the main discovery of science is about asking the right questions questions that can be verified experimentally. But since our human experience will always be limited, our knowledge will also be limited.
    Since the last centuries the number of answers has grown impressively. And so did the questions too. Is this freedom not marvelous?
    Last edited: Sep 24, 2006
  4. Sep 24, 2006 #3
    I think that your point of view has a lot of supporters specially between expermentalists. It may seem fair that whatever cannot be put in an experiment nowadays shouldn't be questioned. But I believe that asking questions for better understanding always has a benefit. New 'good' questions that may lead to new theories refine our understanding of the universe and expose us more to the greatness of God in fabricating his creatures. Even if this hows and whys are not accessible to our measurement today, a day may come when the tools necessary to examine them have been developed. I think that the truth is always accessible to human beings. Of course, we will not be able to understand everything, but the level that we undestand correctly will be always, I hope, accessible to us. If this is not the case, why God has invited us to look into his heavens and and the earth and whatever exists between them to discover his greatness ? * Not only that, but the proper answer to these questions ,I think, may lead to new technologies.

    Regarding the scientific method, I invite you if you are interested to have a look at one of the very early founders of the scientific method : Ibn Al-Haytham
    Thanks a lot for your reply.

    Seeking a better understanding:
    Abu Abd Allah

    * See quran- (29) verse 20, (10) verse 101, (7) verse 185
  5. Sep 24, 2006 #4
    Abu Abdallah,

    I don't think experimentalist are the main supporters of this point of view. Actually they play one special role in the scientific process: they concentrate on the experimental part, which also require a lot of creativity. It is true that theoreticians tend apparently to be more imaginative. But looking more closely, their imagination cannot escape the experimental facts that they must fit in the theories.

    This does not mean that questions should be limited to current technical capacities. The Bell's inequalities are a nice example. The Bell inequalities, when they were published, offered for the first time the possiblity to check quantum mechanics against classical views of physics. It took sometimes before it was accessible to experience. In the first years, some took it as science-fiction. Other examples, from "big science" are even more striking.

    However, the history of physics has shown that only reasonnable steps can be successful. The history of physics contains no record of theories totally disconnected from previous theories. On the contrary. Take quantum physics for example: it is striking to see how much it relies on classical physics, still it is maybe the boldest step in the history of physics. It makes no sense, indeed, to make theoretical proposals or models that would be totally out of reach of experience. Not only because of the experimental challenge. Actually, I believe, it is doubful that any physicist would be able to imagine theories that would be very far from current experimental reality. (altough one or two centuries could conceivable) The reason is clear: valid theories should anyway be connected to past theories and experimental data and therefore new theories are always extensions of previous knowledge. Therefore it is reasonnable to believe that the experimental steps needed to check a new theory should also be limited.

    Best regards,



    Personally I have been educated as a Christian and I have turned to a tolerant atheist.
    My personal feeling is that science invites us to forget about God and religion, for experimental reasons.
    However, it maybe possible that mankind cannot live in a decent way without religion. But how could this theory be tested? What is our experimental background in this respect?
    I do hope people can live in peace and love by nature.
    Last edited: Sep 25, 2006
  6. Sep 25, 2006 #5
    Like all good questions, the answer to this is yes and no. It depends on what you regard as an electron. One can describe an electron rather well using the Dirac equation, with or without an electromagnetic potential. In this set-up, no it is not a fundamental property because the interaction is put in by hand.

    However, if you take the point of view that the electron is a representation of the group local U(1), then it is a fundamental property. One finds that the Dirac equation (or the Lagrangian it is derived from) is not invariant under the symmert group, and one has to postulate a new particle to restore its symmetry. This new particle is the photon, and its interaction with the electron is given. So the requirement of local U(1) derives the fundamental property in the question.
  7. Sep 25, 2006 #6


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    Before things go into that much deeper, please keep in mind that the discussion in here must only be confined to strictly physics discussion. Whether it is relevant or not, and whether you like it or not, there shall be no references to any religious connotations, similarities, evidence, agreement, contradictions, history, etc.. etc.. Not only is this the wrong subforum for such discussions, but I will also point out specifically to the PF Guidelines that everyone has agreed to.

    If you cannot carry a physics discussion without invoking religious context, then you have come to the wrong forum. This rule is not negotiable.

  8. Sep 25, 2006 #7
    I thought that a photon required an harmonic oscillator to be created or annihilated because a photon has a frequency. This would make me think that a free electron could not absorb a photon unless it were embedded in an harmonic oscillator ( like an atom or something).
  9. Sep 25, 2006 #8
    I agree actionintegral, but could you elaborate on the nature of the harmonic motion for the ground state electron of the h-atom.
  10. Sep 25, 2006 #9
    No! I can't! I start with the Schroedinger equation for the harmonic oscillator, and I see that it's energies are quantized. To me that is the origin of the photon. Then I take that concept and suppose that there is something about the structure of the hydrogen atom that is approximated by an harmonic oscillator.
  11. Sep 25, 2006 #10
    1) It's true. A free electron cannot absorb or emit a photon because spin is not conserved.

    Photon (spin 1) + Electron (spin 1/2) = Electron (spin 1/2) + ...?

    The only answer to the ...? part is that this must be a photon as well. Photons can be scattered by electrons, but "absorption" cannot occur.

    2) A hydrogen atom is not approximated by a harmonic oscillator. The hydrogen atom is approximated by a central potential. One origin of the photon is the electric dipole interaction, which is an oscillation of the electron's wavefunction between the quantized energy levels of this central potential.

    EDIT: Sorry, I should clarify. An atom can be approximated as a harmonic oscillator, for example in the Einstein model of a solid, but this is for the purposes of determining the thermal properties of the atom. In terms of the interaction between the electron and the hydrogen atom, it is a central potential approximation.
    Last edited: Sep 25, 2006
  12. Sep 25, 2006 #11
    2) A hydrogen atom is not approximated by a harmonic oscillator. The hydrogen atom is approximated by a central potential. One origin of the photon is the electric dipole interaction, which is an oscillation of the electron's wavefunction between the quantized energy levels of this central potential.[/QUOTE]

    Thanks - I will read up on that.
  13. Sep 25, 2006 #12


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    Even that isn't accurate even for thermal properties, since in solid, the Einstein model has been replaced with the Debye model.

  14. Sep 25, 2006 #13
    Spin is a vector quantity, so the vector [tex]\vec{J}_e +\vec{J}_\gamma[/tex] has no problem having length 1/2 (e.g. [tex]{J_z}_e =-1/2[/tex] and [tex]{J_z}_\gamma=1[/tex])

    You are confusing two issues. You are correct that an on-shell electron cannot absorb a photon (a second phton is required), but this is because of momentum conservation, not spin.
    Last edited: Sep 25, 2006
  15. Sep 25, 2006 #14
    The photon is an excitation of the EM field. The Hamiltonian for the EM field has parts which correspond (roughly) to the kinetic and potential parts of the harmonic oscillator.

    Thus using the ladder operator formalism for harmonic oscillators (but replacing the terms to match the corresponding Hamiltonian) we get creation and annihilation of photons at particular points in spacetime at particular frequencies. Q.E.D. (couldn't resist)
  16. Sep 26, 2006 #15
    Yes, I looked at the schrodinger equation for a hydrogen atom and saw no mention of harmonic oscillators. Harmonic oscillators didn't come up until they discussed the EM wave. Then they magically appeared because we chose to describe an arbitrary EM waveform by harmonic oscillator frequencies.

    I would suppose that if you chose another set of waveforms to describe your EM field, you would use a different type of "photon".
  17. Sep 26, 2006 #16

    The stationary modes of a rectangular cavity are really harmonic oscillators.
    This occurs because of how harmonic waves propagate in such cavities.
    It is clear that in the limit of very large cavities, it doesn't play any role that it is rectangular.
    Thefore it appears naturally in QED.

  18. Sep 26, 2006 #17
    Hi lalbatros,

    Please comment on my reasoning:

    There are many solutions to the wave equation. One such solutions is simple harmonic motion. These are called "photons". Any solution to the wave equation can be formed by superposition of these "photons".

    The choice of the simple harmonic motion solutions to the wave equation was arbitrary. Any complete and linearly independent set of solutions could have been chosen, giving rise to quanta of a different nature than
    SHM photons.
  19. Sep 26, 2006 #18
    Hi actionintegral,

    Linear superpositions of normal modes are not normal modes.
    This means that they are not stationary, the wave amplitude varies in time at the beating frequency(ies). By a suitable combination you could even create a field pulse. Such a pulse is always made of many photons=quanta.

  20. Sep 26, 2006 #19
    There is no a priori reason to declare normal modes as the fundamental solution of the wave equation, except for their simplicity. You could consider square-wave pulses as the fundamental solutions, and create normal modes from a linear superposition of these.

    Applying this to the schrodinger equation, aren't I free to define my "photons" as square wave pulses, and claim that your "normal mode" photons are really just superpositions of my "true" photons?
    Last edited: Sep 26, 2006
  21. Sep 26, 2006 #20
    10 characters
  22. Sep 26, 2006 #21


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    When an alectron absorbs a photon, it doesn't "eat" it like pacman would, instead, the photon's energy is absorbed by the electron which in turn jumps to a higher orbital in the atom.
  23. Sep 26, 2006 #22
    Yes, a photon is emitted or absorbed when the electron jumps between the quantized energy levels but what is responsible for the electric dipole configuration in the central potential model.
  24. Sep 27, 2006 #23
    Quantum mechanics shows that the electron does not "jump" once from one energy level to another: the wavefunction has a time-varying probability of being in the first energy level and the second energy level. This probabiltiy oscillates between the two levels whilst gradually moving from one level to the other.

    A helpful (although not exact) analogy is to think of it like dropping a bouncing ball onto a table. When you are holding it above the table, it's in the higher energy state. Then when you drop it, it bounces lower and lower until it eventually ends up in the lower energy state.

    Classicaly, it's this "bouncing" of the electron between the states that produces electromagnetic waves through the acceleration of the charge.
  25. Sep 27, 2006 #24
    No. In simple terms, electrons do not absorb photons. Electron bonds absorb photons.
  26. Sep 27, 2006 #25
    On the surface this seems to be a reasonable explanation, but what causes the electron to change directions as it oscillates back and forth and decays to the lower energy state. There must be a force acting on the electron to cause such periodic motion. Where does this force originate? Ellaborate on the electric dipole interaction mentioned earlier. What evidence do you have for this?
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