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Basic Photon Question

  1. Jan 26, 2006 #1
    Is the photon that hits my eye when I look up at the Sun the same photon that left the Sun at that angle?

    More clearly, do photons travel great distances singly or do they simply bump into other photons and transmit the light in a sort of chain fashion?

    If they make the 8 minute journey alone, wouldn't a great number of them hit other particles, or even each other along the way? If that happened, would their trip be over?

  2. jcsd
  3. Jan 27, 2006 #2


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    Most probably not.

    They would travel great distances alone, if they weren't interacting with the matter particles between your retina and the system that has produced them. They hardly bump into other photons and when they do, they elastically scatter one off another (see Delbrueck scattering in QED).


  4. Jan 27, 2006 #3

    Meir Achuz

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    1. I would say yes.

    2. Scattering off other photons is completely negligible and unobservable at that intensity.

    3. Only a tiny fraction gets stopped in that way. Otherwise, the sun would not be bright.
  5. Jan 27, 2006 #4
    The photon that left the sun, even if it encountered no obstacles en route to the Earth, will have been absorbed by particles in the atmosphere which re-emitted another photon. Which was probably absorbed by another particle in the atmosphere that re-emitted another photon. And so on and so forth, until one made it to your eyeball.

    Then the absorbtion and emission of photons happened again, through the watery film on the outside, through the cornea, through the lens, through the goop on inside your eye, through the blood vessels in front of the retina...

    And finally, a photon will be emitted that makes contact with a photoreceptor in your retina.

    There's zillions of photons along that chain. And it's a good thing, too, because your lens depends on it for its focusing properties.
  6. Jan 27, 2006 #5


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    Well, one has to be careful here. You can indeed view things that way, but remember that in order for us to have a clear image of the sun, it is important that coherence is conserved in all these presented absorption-emission processes. And now I leave it up to you if the absorption and coherent re-emission of a photon is the "same" photon, or not...
  7. Jan 27, 2006 #6
    Dear jhe1984,

    A photon is not a "particle". It is a "quantum" of energy field. It has, though, both the behavior of wave and of particle.

    So, photons do interact both with atomic particles and with fields of energy.

    But there is not "a specific photon" that originates from the sun to hit your eye. Your eye is seeing quantums (packets) of energy, which were generated in the Sun and after interacting with the atoms of atmosphere of earth and with the magnetic/electric field of earth they end up in your eye, producing "vision".

    It is quite a journey, which actually has also non "visible states" from humans, like the journey of photons within the Sun.

    And finally, Sun does radiates a wide spectrum of electromagnetic frequencies, from ultraviolet to infrared and humans can only see a small portion of this spectrum (visible spectrum: 400-760 nm). And the "vision" that we have is affected by the atmoshere of earth, therefore during dawn and during sunset we do not see "white" sun light, but we see the colours of subset and dawn.

    Also, the blue color of earth's atmosphere means that the original "sun light" is filtered going through the atmosphere.

    So the electromagnetic energy that is radiated from the sun interacts in various ways with particles and with other fields that are in its path. Photons are the points which show the trajectory of the energy packets, which also describe the status of these packets of energy.

  8. Jan 27, 2006 #7

    Perhaps it's a distinction without a difference, but I'd consider them equivalent photons, rather than the same photon.

    But I'm not wedded to this perspective, so please correct me if I'm getting this wrong.
  9. Jan 27, 2006 #8
    Thanks all.
  10. Jan 28, 2006 #9
    A photon is not a "particle". It is a "quantum" of energy field. It has, though, both the behavior of wave and of particle.

    So just what is the difference between a 'quantum' and an 'elementary particle'?
  11. Jan 30, 2006 #10
    When someone says "quantum," think "a certain amount."

    So a "quantum of energy" means a certain amount of energy. A photon is sometimes referred to as a "packet" of energy, to give a more easily visualized mental image.

    Whereas an "elementary particle" or "fundamental particle" is a bit of matter that is not made up of anything smaller -- it is what other things are made of.
  12. Jan 30, 2006 #11


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    There is only one important difference between a photon and "an elementary matter particle", and that is that a photon is a boson, and a matter particle usually a fermion.

    Photons are the quanta of the electromagnetic field,
    electrons are the quanta of the electron field (a Dirac spinor field)
    muons are the quanta of the muon field (also a Dirac spinor field)
    up quarks are the quanta of the up-quark field (also a Dirac spinor field)
    gluons are the quanta of the gluon field (very similar to the electromagnetic field)
  13. Jan 31, 2006 #12
    So a "quantum of energy" means a certain amount of energy.

    Gribbin (Q is for Quantum) defines a quantum as the smallest possible amount, are you saying this is not the professionally accepted usage?

    There is only one important difference between a photon and "an elementary matter particle", and that is that a photon is a boson, and a matter particle usually a fermion.

    Veltman (Facts and Mysteries in Elementary Particle Physics) Includes the photon and graviton in his list of 81 elementary particles.
  14. Jan 31, 2006 #13


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    Yes, it is correct. In fact, elementary particles, in quantum field theory, are seen as bookkeeping devices for the quantum states a FIELD can be in. What is taken (often) as fundamental, is the field, you know, like the electromagnetic field. Quantum field theory takes it that to each different kind of "particle" corresponds in fact a different field.
    But those fields have to follow the laws of quantum theory, not of ordinary classical theory, and then it turns out that those fields can only appear in certain states (or quantum superpositions thereof) - this is the result of applying the rules of quantum theory to fields, and is similar to using the rules of quantum theory to, say, a hydrogen atom: the hydrogen atom can only appear in specific states (ground state, excited states...) or superpositions thereof. Well, in a similar way, a field can also appear only in a ground state, and several excited states. But a field is a way more complicated thing than an atom, and there are miriads of possible excited states. Nevertheless, one can easily find a classification scheme for these excited states: each excited state of a quantum field corresponds to a certain number of multiples of "basic units" of energy and momentum and eventually some other quantity such as spin. These basic units (in the accounting scheme of the possible excited states of the quantum field) are called "a particle of the field" with given energy and momentum.
    If you apply this to, say, the electromagnetic field, you will find that the excited states of the electromagnetic field correspond to a certain number of different multiplicities of basic units, called "photons", and we thus have photons of different energies and momentum (= direction) and spin (polarization) which build up the possible excited states of the quantum electromagnetic field.
    Electrons are seen in (about) the same way, for a different field, the electron field (instead of the electromagnetic field), etc...

    Yes, a photon is seen as an elementary particle. I was referring to a concept the OP seemed to have, hence my quotes around "elementary matter particle".
  15. Feb 1, 2006 #14

    As an amatuer sticking to classical particle physics, I have made occassional visits to PF since 1989, usually to obtain answers to a particular question.
    Your reply is the first time I have seen an explanation of 'quantum' that can be clearly understood by an untrained amateur, the text books just do not explain 'quantum' with the same clarity.
    Now I can point out that the same (mathematical) statement can be made in classical theory. What QT explains using waves and quantum, classical theory can explain using force fields. Where QT uses multiples of a single particle (photon) classical theory can use condensations of a single force field (graviton). The end result is the same in that a number of 'condensations' is the classical equivalent to a number of quantums.
    There is still a fundamental difference between QT and classical in that QT expects the Higgs particle to be the ultimate particle, whereas there is no end to the classical condensing cycle, you just need more powerful machines (the graviton being the only true elementary particle or force field).
    This of course, is the wrong forum for either new theory or classical physics, so my submission must end here; I just wanted you to know how much I appreciate your reply.
  16. Feb 1, 2006 #15


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    Yes, I know that there are people trying to make such a thing work, they are called "local realists", because they think (rightly or wrongly) that classical, local field theory with nonlinearities can solve many issues that are also addressed by quantum field theory. What is already known is that, if such a field theory is going to respect locality, it will NOT be able to be entirely empirically equivalent to quantum field theory, because of a property of quantum field theory (every quantum theory) which is NOT possible to find agreement with in a classical field theory, and that is quantum entanglement. Bell's theorem indicates clearly that both are not compatible. Up to experiment to decide - up to now, there are serious indications that entanglement seems to exist, but it is also correct that there is no exclusive experimental proof, and there is still a possibility to explain the results without entanglement. At least, in principle, because to my knowledge, there DOESN'T EXIST yet an all-comprehensive local-realist working theory. There exist at best, some specific local realist models for certain branches of physics, such as stochastic electrodynamics.
    So I don't worry, because we are talking about two different theories (classical field theory and quantum field theory) which are, at least in principle, experimentally distinguishable, so one day or another, this issue will be adressed for good, once we have (from the local realists) a complete theory which can make as many predictions as can quantum theory, and once we will have performed the relevant experiments that distinguish both.

    In the mean time, and on this forum, we take quantum theory to be the correct one, as a working hypothesis, because that's the topic of this forum.

    I think you're misunderstanding the standard model here. First of all, know that the standard model is only that, a model. In the standard model, the particles all have a different, fundamental, field associated to them ; electrons, for instance, are just as fundamental as the elusive Higgs. The Higgs field is just something that was mathematically needed to make the whole model work, and so is another quantum field, to which one associates another quantum, or particle. It is certainly not more fundamental than the electron, for instance. It might even be LESS fundamental, or simply not exist. That will then only say that the *specific model* which we call the standard model, has a problem. That does not necessarily mean that the whole structure of quantum field theory has a problem.

    To give you an equivalent: take it, say, that we have a model which states Coulomb's law as the force law between charged particles. And now, observe that Coulomb's law is not exactly followed (namely, because of magnetic effects). Does that invalidate immediately the Newtonian framework of particles, and forces working on them ? I don't think so ; I think it only indicates that there is something wrong with Coulomb's law.
  17. Feb 2, 2006 #16

    For years PF subscribers have told me that I am a nutcase with nothing to say. Now I know that I am a ‘local realist’ and more importantly, the key problem that local realists must deal with; I do not think it is impossibility.

    Where I disagree with you is in your statement that a local realist theory should be about prediction. In my view QT deals quite successfully with prediction, local realist should concentrate on ‘what, how and why’. I do not think that a theory that explains what a fundamental particle is should necessarily be expected to predict what it will do, although it would be nice if it did so.

    Local realist should concentrate on explaining why particles and atoms are the way they are, and why they have their particular quantities. Perhaps one day we will have a forum of our own to do just that. Until then many thanks for clarifying the issues involved.
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