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Photon Absorption

  1. Jan 5, 2005 #1
    Hi everyone,

    I was wondering (while in the shower of all places) whether photon absorption by an electron is a physical process or a mathematical consequence. That is, does an electron literally "eat up" the photon at the quantum level or is it just some consequence of the math that we give a physical comparison to (by using the word "absorption")?

    Thanks in advance.
  2. jcsd
  3. Jan 5, 2005 #2


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    That's an interesting question.Inclined towards phylosophy,but not too much.First of all,it must be said that this process is very physical.It really happens,we know it and we don't need to much of an experiment to prove it.But being a part of ohyisics,we cannot describe with words only,i.e. qualitatively.We need a mathematical model,an abstractization,to give us,if possible,numbers which can be confirmed/infirmed by our genial experimentalists.Whether we chose classical (electromagnetic) theories to describe it,or we chose quantum (QED) theories,it's for us to decide,depending of the accuracy we wish to achieve.
    So saying that "the electron eats up the photon" is not really a consequence of math.Since we can see that the photon gets "eaten up" by something,physical reasoning tells us:"Hey,that electron ate the photon and by the looks of his green-purple face,he'll regurgitate it pretty soon"... :tongue2:

  4. Jan 5, 2005 #3
    I hadn't noticed how I'd worded the question until I came and read your response. I tend to shy away from the philosophical side of things (as I find it counterproductive to debate semantics), so on that note I apologize for the obscurity.

    I suppose what I was really getting at, and your initial response was bang on to my previous wording, was where does the photon go, exactly? Obviously its kinetic energy is transferred into the electron, but physically where does it go and how does it disappear? Is it analogous to a particle decay (physically) or again is it just something that we know happens (but can't compare to every day life) because of the math? Or is this a passé way of describing photon exchange entirely? And while I'm at it, do all the field particles exchange in the same way? That is, does an photon-to-electron exchange occur in more or less the same manner as, say, a gluon-quark exchange? Obviously the gluon depends on color and what have you, but is the process similar?

    I have Griffith's book on particle physics but haven't started reading it yet. Still muddling through Shankar's QM first...but perhaps that is where I should be looking... :blushing:
    Last edited: Jan 5, 2005
  5. Jan 5, 2005 #4

    Maybe this will help.

    Photons and electrons are both point particles...they can't really be said to exist in the sense of occupying space. The photon cannot even be said to have mass, in the sense of a rest mass, that is a mass that is present when the particle is not moving. The entire mass of a photon is represented by its kinetic energy, that is the energy of motion.

    Photon capture by an electron results in the electron entering a higher energy state, and these states are quantized, that is they can only take on certain discrete values. Electron orbitals in atoms are such states. When an electron in an atom takes on a photon, it enters a higher orbital. Note that because of Heisenberg uncertainty, we cannot know exactly where in an orbital an electron is. Instead, we must think of the electron as occupying a cloud-like shape limited to certain regions around the nucleus of the atom.

    So, if this model is correct, it is not really meaningful to ask where the photon is after it has been absorbed. A photon exists only as energy, and the energy goes entirely into boosting the electron into a higher orbital. There is no leftover mass to add to the mass of the electron.

    Be well,

  6. Jan 5, 2005 #5


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    Hi Richard

    I'm glad to see you're still around. Just one important correction here: a photon is not a point particle. Actually it's not a particle, because it is not localised. Think of a photon as defining the essence of a piece of spacetime rather than as some stuff 'in' something.

  7. Jan 5, 2005 #6
    Hi Kea.

    Yeah, I guess a photon is more on the wave side of wave-particle duality. Still, it has particulate charachter, too.

    I have been thinking of this question of absorption for a while, and trying to figure out how it works out dimensionally. Photons move very fast, and are essentially linear, one dimension. Electrons usually are moving much slower, maybe around .2c under a few volts pressure. Electrons are also three plus one dimensional, at least. They do occupy space and time. So we could think of the photon as a small fast moving object encountering a slower, larger object. How do they interact, exactly? What do we have to work with?

    Photons have various energies, but all move at the same speed. A low energy photon cannot lift an electron into a higher orbital, so presumably it just goes on by. A higher energy photon can lift an electron into a higher orbital, and it is absorbed. What about polarization? Does it matter which orientation the light has when it hits the atom? Say we are looking at a crystal, so the atoms are all pretty much sitting targets. They are arranged in a structure which means their orbitals are all oriented the same way. So if we have an electron in a p orbital, which is shaped like a dumbell, and a polarized photon, does it matter which way the beam of light hits the crystal? If we turn the beam so it is polarized at 90 degrees to the direction of the long axis of the p orbital, will that change the probability of absorption?

    Still searching for better questions.

    So if a photon is a spacetime unit, and spacetime is the frozen river, then the photon is like an edge of something. The something would have to be in 5 or more dimensions, I guess, since we don't see the bulk of it anywhere in our usual 3+1. We just see the edge. An electron in 4d is tubular, and when it gets to the edge it is lifted, kind of like a hose laid across a curb. The curb is the edge of the lawn, we see it as a photon, and when-where the electron crosses the photon, it is lifted......

    Nah, it is too many analogies. Gotta go have a nice think.

    Be well, Kea. And keep an eye on your boots.

  8. Jan 5, 2005 #7
    Just for my own clarification...localisation means the entire wave function of the particle is "at" the particle's position? If, however, this is not the case then the particle isn't really a point "particle" at all? Is this close to the idea?

    I remember reading in a Feynman book a long time ago (I think it might have been "Surely You're Joking...") how his father once asked how a photon was emitted by an electron, that is, where did the photon come from? All he could answer with was "photon number must be conserved". This is where my thought of "resulting from the mathematics" came from as opposed to a physical interpretation.

    There's so much I don't understand yet, but I'll catch up. :approve:
  9. Jan 6, 2005 #8

    Kea should answer this since the word occurs in Kea's post. But wave functions are not localized in the sense of occuring at a point. They occur in a set of probabilities......the wave function for a p orbital describes a shape like twin dumbells sticking out on either side of the nucleus. Other orbitals are more complicated. The s orbital is simpler, just a sphere. The electron is not located anywhere exactly, just occurs as probabilities in the region of space occupied by the wave function. Altho the shape is where the electron is most likely to be found, there is a small but probable chance it could be found outside of the shape too. In theory, it could be almost anywhere. But mostly it is found inside the shape of the orbital cloud.

    What I meant by point particle is that the mass and charge of the electron do not occupy a specific region of space such as might be described by a volume or radius. A proton or a neutron do have structure that can be defined, and in fact recent work suggests that the shape of the proton is actually eliptical or possibly toroidal. It has an actual size, with a radius and a surface area.

    The electron, on the other hand, does not have a little localised chunk of matter at some place in the cloud at all. It is spread out over all the possible places where it can be found. It does have a certain wavelength and can be said to have an effect within a certain area, but the effect is not that of a solid object, but more like a swarm of virtual particles that appear and dissappear almost instantaneously throughout the wave. The presense of the charge and mass of the electron cause these virtual particles, and they occur close to a location or a point, but there is nothing measurable at the point itself.

    As Kea said, it might be best to think of these point particles as units of spacetime structure. Spacetime is four dimensional and even the string theorists try not to think about what things look like in four dimensions. I happen to be an independent self supporting researcher, so I get to think about whatever I like. And I like to give myself serious cases of the spins by trying to visualise four dimensional objects.

    Particles in quantum mechanics are described by certain characheristics called quantum numbers. One of these quantum numbers is spin. Another is mass, another is charge. In all particle interactions, the quantum numbers are conserved, that is, if you list all the numbers going into the reaction, and all the numbers coming out ot the reaction, they must total up the same. There may be a different number and kind of particles going in than there is coming out, but the quantum numbers, when totalled up, remain the same. I think that may be what you remember from Feynman.

    I suggest you try reading Feynman's book QED, which is available in inexpensive versions from your local big box book store. It discusses all of this in much better form than I have used.

    Be well,

    and don't forget, while trying to catch up, that the path itself is the goal.

  10. Jan 6, 2005 #9
    can i add something in here?? not an answer but a new question.

    you say that protons structure can be determined? ive always wondred how an electron and electron antineutrino can be emited from a neutron during beta decay to form a proton. i understand there must be conservation of mass and charge (the neutrino and electron respectivly) but this still doesnt tell me where they came from. :|

    ive been taught that protons consist of 3 quarks uud and neutrons udd. does this mean that the 'u' quark emits the electron and neutrino? the book im reading is old and says that quarks are single point (like singularities i guess) i know this cant be true but i thought quarks and leptons were considered fundemental and this emission of a lepton from a quark seems to disprove that!! i think i rambled a bit but i hope you see my problem.

  11. Jan 6, 2005 #10
    This is a good question....meaning I don't know the answer. Maybe someone else can do a better job of explaining it. Meanwhile, my study has been that quarks are confined very strongly, and cannot (yet anyway) be seperated from each other.

    What I have read about this is that the energy confining quarks is very large, which should mean that the masses are large, but we do not see the large masses in practice. I think this is called the confinement problem. The force involved is one of the four forces which GUT's try to unify, called the strong force, and represented by force carrying particles called gluons.

    I am sure I am out of my depth here. However, I would point out that we may have to reconsider our ideas of what particles are at these energy levels to make sense of the data. Almost certainly it is not sufficient to think of particles at this scale as hard little balls of matter. So it may not be sufficient to think of the quantum numbers as actually belonging to one quark or another.....I am just speculating. It seems to me that the quantum numbers may somehow be a relationship between the quarks.

    Anyway my work is currently in trying to understand spacetime relationships. So that is my hypothesis. There may be domains of spacetime which have boundaries which interact to create the qualities we think of as matter. In which case, the better question would be, not where do the particles come from, but how is spacetime structured?

    Perhaps one of the advisors here will be able to make more sense out of this. Thanks for the stimulating ideas.

  12. Jan 9, 2005 #11
    ok so we have to consider the structure of spacetime... does string theory answer the question of how to leptons can be produced from one quark? if i undserstand string theory at all i believe it states that each fundemental particle is a vibrating string. I still cant work out how one string can suddenly make to new strings. I guess the strings properties must change seeing as we detect 'd' quarks as having almost double the mass of a down quark and -1/3 charge not +2/3 of the 'u' quark. (i even read somewhere the that the 'u' quark might be massless). could it be that when beta minus decay happens the 'd' quark in the neutron is pulled from the other 'u' and 'd' quarks but thanks to the strong bonding of quarks (confinment thanks to chromodynamics), less energy is required to make 2 new particles than to seperate the quarks. Maybe this would make the electron and electron antineutrino although i think its only a quark/antiquark pair that can be produced. Can anyone shed light on the situation?


    p.s. i duno if i should ask this here but how does the Higgs field theory relate to string theory? there doesnt seem to be a place for it to fit in.
    Last edited: Jan 9, 2005
  13. Jan 9, 2005 #12
    I doubt I could help you guys with where you are taking the thread but I can interject a few notions about photon absorbtion and emission. I think the best way to think about photon absorbtion is with the concept of 'resononace'. Whenevrer an electron 'absorbs' a photon while it is bound by a nucleus, it will only absorb photons of a very specific frequency and emit one close to that frequency. I like to imagine that the change in the EM feild due to the photon gives electron a little kick to the higher energy state much like a tuning fork would excite a string on a piano when it is tuned. So the energy in the photon translates into the higher energy state of the photon. It is interesting that you do not really have to really get bogged down in QMs to understand this, good old classical notions apply quite adequately.
    Last edited: Jan 10, 2005
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