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What Exactually is a Photon?

  1. Aug 31, 2009 #1
    I've been wondering this for a while now, and I've looked into it but I'm still confused.

    I understand that photons are emitted when an electron jumps energy levels, or (correct me if I'm wrong) when an electron and positron collide (annihilate) with each other.

    But these don't answer my question, is a photon just a a pure form of energy from the electromagnetic force put into a particle-wave form, or do scientist just simply not know yet?
  2. jcsd
  3. Sep 1, 2009 #2

    Vanadium 50

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    I don't think you will be satisfied with the answer. "Pure form of energy" is more of a science fiction concept than a scientific one. Science can say "a photon is the thing that behaves in this way (and then list its properties)", but it sounds like you will find that answer unsatisfactory.

    Note that no matter what you are talking about, you can eventually get to this point. What is water? What is air? "Yes, but what is it really" is a question that ultimately has only the answer, "the thing with these properties".
  4. Sep 1, 2009 #3
    Hi, besides the two contexts you pointed out in the OP where photon concept arises, I would add the photon as an elemetary mode of the EM field inside a cavity.

    Observe that when an atom decays inside a ressonant cavity in a way that its photon propagates along the cavity axis, at the moment of decay you may have a photon in atomic sense but still not in cavity sense. I hope I have helped to make things more complete.

    Best wishes

  5. Sep 1, 2009 #4
    Quantum mechanics says that the world is pervaded by an electromagnetic field, and it is excitations of this field that are photons. so if you imagine a trampoline, then the field is a trampoline, and when a charge jumps on the trampoline, then this causes the whole trampoline to displace as the disturbance is transmitted through the stitching of the trampoline - this disturbance is a photon.
  6. Sep 1, 2009 #5
    So would you say that resonance is possible for the photon?

    I am not sure such a simplistic explanation is possible.

    Could we go in for a move qm type of an explantation, something on the lines of

    The photon is that piece of the universe which when non accurate observations are done wrt a particle hypothesis behaves as a particle and when more accuracy is sought tends to behave as a wave :devil:
  7. Sep 1, 2009 #6


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    A photon is the particle that mediates the electro-magnetic force!
  8. Sep 1, 2009 #7
    Thank you everybody for your answers, you sorta helped me understand what a photon is a bit, but I'm assuming science can never truly answer my question, since my question was focused more on the "why/ how does it really form" of what a photon is.

    It's a shame that science (at least modern day science) will never be able to tell us the "whys" of live but can tell everything else. I myself am a devoted wanna-be-physicist, and I hope to put my research into the theories of "why" and "what".

    But thank you everybody for your input, there's still so much more I need to learn about physics before I can even start thinking of a plausible theory.
  9. Sep 1, 2009 #8
    I beg to differ: science can answer quite a lot of 'why's. For example, it can answer why the sky is blue, why water is transparent, etc. In your case, the question of why the photon forms, and how it actually forms is well known (has been known for decades). The answer, unfortunately, must be given in terms of more advanced concepts which you may/may not have an intuitive understanding of, and the answers provided in the previous posts may have appeared mystifiying, as a result. But, I assure you, the 'why' the 'what' and 'how' of a photon is well known.
  10. Sep 2, 2009 #9
    What is a photon?

    First understand the harmonic oscillator and what happens when it is quantized.

    Next understand what happens when a chain of many coupled harmonic oscillators is quantized. Understand what normal modes are, what a Fock space is and what people mean
    when they say quasi particle phonon.

    Next note that a field can be understood as infinite many coupled harmonic oscillators.
  11. Sep 2, 2009 #10
    Thanks, I might try looking into some books over light and quantum particles then, any good suggestions?
  12. Sep 2, 2009 #11
    A photon is the carrier of the electromagnetic force. Anything involving electrodynamics from radio waves to visible light to gamma rays is carried by the photon.

    A photon is both a wave and a particle. In Thomas Young's Experiment he discovered that light behaves differently when being observed. Well that was the electron but it's the same principle....

    His experiment consisted of two horizontal slits that allowed light to pass through them, and based on the resulting pattern of light distribution you can tell whether it's a wave or a particle.

    The photons go through the two slits and you'd expect to see two bands if it were a particle. But Physicists found that it behaves like a wave and creates a pattern expected of waves. This is interesting because it doesn't act like a wave when there's only one slit, it acts like a particle.

    When the photon shoots out, it breaks itself in two before reaching the slits so it can go through both of them. In reality, all of these possibilities occurs, the photon goes through BOTH slits, the left one, the right one and none at all. All of these possibilities are superposition with each other.

    Even weirder, when they tried to observe which slit the photon went through, it behaved like a particle! The very act of observing eliminated the other possibilities.

    Sigh.... gotta love the uncertainty principle.....
  13. Sep 2, 2009 #12
    I don't know why people continue to speak of photons traveling through space, either through the slits or on any other occasion. Does anyone here know of evidence for that? As far as I know, the photon is the quantized behavior that is observed only at exactly two times: the instant when the electromagnetic wave is emitted, and the instant when the electromagnetic wave is absorbed. If there is any evidence that photons exist during the trip, after emission and before absorption, please point it out to me.
  14. Sep 2, 2009 #13
    Ok, so a photon is like Gertrude Stein’s rose - it’s a photon.

    It is easy to accept the Bohr model of an atom, or a more advanced similar, though more sophisticated model which comes to much the same concept. So there is no problem with seeing that “excited” atoms emit photons of a particular energy/wavelength.

    Planck’s “black body” theory derives a distribution of energy levels, and hence the spectrum, of em radiation from a hot object, though everyone keeps quiet about where the photons actually come from. But it is clearly not a single wavelenth phenomenon.

    But what price the energy/wavelength of a photon emitted by an accelerated elctron? (if it does! Feynman has quite a bit to say about this).

    Or by an electron moving harmonically in an antenna. Why should there be any connection with the excitation frequency in the antenna and the wavelength of the emitted photons, which is what always seems to be tacitly assumed in the literature?

    Or am I completely missing the point please?
  15. Sep 3, 2009 #14
    A photon is a form of electromagnetic radiation. It is theoretically massless and is the force carrier particle for electromagnetism... meaning that it is the reason that electromagnetism can be observed as a force at all.

    Also a photon is the "particle" responsible for visible light. I have particle in quotations because light exhibits a strange phenomenon of duality in that it has characteristics of both particles and waves.

    Don't feel bad if you don't understand stuff of this nature... even Feynman said he didn't understand QED... a theory which he himself pioneered!
  16. Sep 3, 2009 #15

    I was not here for that, but it was the first post and I found that question so cute...

    I'm not sure any one will like my answer but, since it seems different from yours, I thought I should give it.

    This is a pragmatic or instrumentalist answer : ask an experimentalist working with photons.

    He will answer not with a description of the objective properties of an element of reality, but probably (if not, let's imagine he will) in terms of what the photon can do.

    I think that if you can make a list of all the things a photon can do, you will be close to my definition of a photon (even if I don't know this list). This point of view is remakably well cristalize in a famous quote by Jammer. In my memory, it is something like that : "A physical entity does not do what it does because it is what it is, but is what it is because it does what it does". Read it 2 or 3 times, there really is a lot of juce in that sentence.

    I think this way of understand the nature of things, like photons, electron, etc., is the way that fit the most with the formalism of quantum mechanics. The most eloquent text on this point of view is, in my opinion, the magistral book of Reichenbach on the philosophy of quantum mechanics. It's free from right, somewhere on the internet.

    Good luck in your quest for a definition of what things are!



    PS : English is not my primary language, so forgive my mistakes.
  17. Sep 3, 2009 #16
    Haha, it's ok your English is really good. Thanks for letting me know of that quote, it seems like more quantum physicist know what the quantum particles do, not what they are. My question is what is a photon, which appears to be something that modern day physicist can't seem to figure out.

    I hope one day to figure this out on my own, my goal in life is to know what everything in this universe and why did it get there. Maybe that's every bodies, if it is then lets all try to figure this out!
  18. Sep 3, 2009 #17
    You seem to be asking for direct evidence where is it precluded, and unwilling to accept indirect evidence. If you're demanding an answer to this duality question, you're chances of getting one are as vanishingly small here, as on any of the 1000's of previous threads discussing the issue.
    Last edited: Sep 3, 2009
  19. Sep 4, 2009 #18
    With all respect, I think you did'nt understand my answer. For me (at this moment in my psychological evolution), the photon is what he do. Nothing more, nothing less. I understand your question. And I will take a little more time to help you understand my answer.

    Feynman, in his thesis supervised by J. H. Wheeler, reformulate the Maxwell theory in terms of action-at-a-distance. It is equivalent in its prediction to Maxwell theory in which waves of light travels, but in the former there is nothing traveling. Acting here on a source make shining there the screen, nothing traveling between them.

    Now, the question is not which of the wave picture or action at a distance picture is the right description of reality (it is an unfalsifiable question, i.e. a non-scientific one). It is only for you to realize that a physical theory describing (classical waves, for my purpose) perfectly the effect of lights on apparatus do not need the existence of a thing called photon.

    With this in mind, you must know that the same situation appear everywhere in physics. Quantum mechanics, for example, can not in its actual formulation, state aboute the nature of things, about what they really are. This is why some people say it is incomplete.

    Asking your cute and profound question about photon is equivalent to asking for a completion of quantum mechanics, in such a way it can finally be able to state about the nature of reality or of the things it describe. Today, QM can only give you information about what things do. It is really important that you understand that. It means that, today, any question (like yours) about what things are is metaphysics and not falsifiable. This question cannot be answered by today's science, since our scientific theories are, at the moment and by definition, unable to answer this type of question.*

    It is now a philosophical concerns to state about the role of science : should it describe the nature of things stating what they are (even if its not falsifiable at the moment) or should it describe how nature work, how things interact and how to link different observations and manipulations? I don't know...


    *I advise you that there is some personal opinion in these last statement and I'm not sure everyone will agree with that for all physical theories. I'm as extremist as Reichenbach, who explain very well (in his philosophy of QM) why even classical physics (considered as a science) can only state about what things do, not what they actually are.
  20. Sep 4, 2009 #19
    I feel what you're describing is the same thing (similar to what you said in a previous post of yours). It would seem that things ARE what they do. If describing an apple; it is red to our eyes because of the wavelengths of light it absorbs and reflects. It is smooth because of it's molecular structure (which obviously can be broken down to atomic structure, etc.) An apple is an apple because of how it interacts with the forces around it, right down to it's most elementary make-up.

    I don't think science intends to differentiate between the two, as I think the two statements you're saying are actually just one. Because your description of "what they are" is vague and undefined... you would realize that if you tried to define what it is "to be", that it would fall in accordance with the last statement of "how they work".

    It therefore is an unnecessary argument and will only convolute scientific thought.
  21. Sep 5, 2009 #20
    There is a difference. If you describe the properties of things with quantum mechanics, you violate relativity. I mean, if the equations describe the properties of something that really exist in space and time, a photon propagating between to events, then Bell showed you violate fundamental laws of relativity since then there is an action at a distance so that cause and effect can not be uniquely defined.

    However, there is no problem considering equations giving information about what things do.

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