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Very basic question(s) about photons

  1. Oct 15, 2010 #1
    I'm a beginner at quantum mechanics and I was just wondering about:
    1) How photons can interact with charged particles when they are uncharged themselves. Also in the same regard according to my (school) textbooks light is an electromagnetic wave. But it also says that photons are uncharged and light is made of (a stream?) photons.
    2) If so, how can a polarizer (is that even the correct term?) work? I have this basic definition with me that the polarizer 'cuts' out a component of the electromagnetic field thereby forcing the beam to become 'polarized'.
    3) Can polarization be explained in terms of photon interaction?
    4) Can the wave nature and particle nature be explained in the way I see it? [Depending on the 'resolution'/depth/scale of view entities may be regarded as individual particles or 'strings' or waves depending on whether we are able to see characteristic repetitions of behavior/interaction with other entities. Sort of something like a worm under a microscope. It looks like a worm at one resolution and we can see it wriggling about (wave?) and at a higher resolution you can consider it a multiple of a fundamental unit of a 'quantized' unit (the unit being perhaps the striations on its body or perhaps the number of cells per unit volume at another resolution and so on).]
    5) I know this might sound a bit stupid, but is the 'Many Worlds' thing something like a wave having 'secondary' wavelets? [Its impossible (?) for us to experience all possibilities of an event (and if the event is represented as a dot, then the possibilities may be represented as a wave traveling in an unique direction each). Each such wave(possibility) at every point of time may undergo splitting (due to the reason that every event may have n number of possible outcomes since I don't think anyone can exactly say that an even can have only a defined number of outcomes. The 'maybe' factor always adds a few more, or reduces a few) and each of the new wave from the 'splitting' is (supposed) to be a secondary wavelet.] If this were so, wouldn't support the reasoning that the universe had no beginning or end? I know this is not physics but more like philosophy but I see no distinction between the two except in terms of the spelling and the lack of mathematics in the latter.

    I'm very interested in learning quantum physics properly from scratch. Most unfortunately the books at my disposal are very unhelpful and wikipedia even worse. And I'm sorry if these questions have been asked before.

    Regards.
     
    Last edited: Oct 15, 2010
  2. jcsd
  3. Oct 15, 2010 #2
    1) electrons interact by exchanging photons.

    2) polarization interacts with the ANGULAR MOMENTUM (spin) of the photons.

    3) see (2)

    4) I don't understand this question. My opinion is that string theory is fundamentally wrong.

    5) There are many bizarre, and sadly funny, beliefs when it comes to the philosophy of quantum mechanics. The classic "many worlds" idea is that there exists infinite parallel universes in which every possible outcome of a quantum experiment is realized, for every event; I do not consider this to be a serious idea.
    The assumptions of cosmology (perfect isotropy/homogeneity) imply a beginning to the universe by general relativity; but depending on the expansion of the universe, there is or is not an end.
     
  4. Oct 15, 2010 #3
    the divergence of the field of a photon is zero (no charge)
    but the curl is non-zero
     
  5. Oct 15, 2010 #4
    i like your fourth question. but i think there are only two options. the resolution is turned on or off. meaning that when you measure the light it turns on becoming a particle but when its off its traveling as a wave. though a third thing my be considered quantum stuff
     
  6. Oct 16, 2010 #5
    Hm I didn't imply any association with the string theory. I don't even know it properly. But if these are similar ideas then hm...
     
  7. Oct 16, 2010 #6
    You think that electromagnetic radiation is decomposable into discrete quanta called photons. What, of a photon, is quantized?
     
  8. Oct 16, 2010 #7
    This bugs me also.

    I was once told, that spin (or polarization) live in a separate Hilbert space. It is not that some physical tiny bit of a photon points in some direction. Photons don't have any width.

    However when we make an antenna from several parallel metal wires, then it is able to polarize radio frequency photons. I don't understand it. How come some device in "position" space interact with spin?

    I could imagine this if polarized photons were actually waving left and right while propagating in space. Then the polarizer would simply block a "translation" movement in one dimension. But it is not like that.

    Instead, photons can somehow transfer their polarization to electron linear motion. Doesn't this violate spin conservation?
     
  9. Oct 16, 2010 #8
    Is that even possible? I mean if you call something a 'ball', it means that you are addressing a unit of that thing. So if you call something a photon isn't it supposed to quantize something? Even 'loose' entities such as a cloud are quantized (implied by the name itself?).

    Wave left??
     
  10. Oct 16, 2010 #9
    1) How photons can interact with charged particles when they are uncharged themselves. Also in the same regard according to my (school) textbooks light is an electromagnetic wave. But it also says that photons are uncharged and light is made of (a stream?) photons.
    Answer:
    First of all, we have to understand that the photon is a bundle of energy. This energy travel from one point to another as a wave (Electromagnetic Wave), but when absorbed or emitted it is considered as a photon. The photon as a form get absorbed by electron which gives the electron more energy than it had. The photon may interact with electrons in its state (electron belongs to certain energy level in atom or molecule). This interaction can be done between wave functions, one of the photon and the other for the electron, which may lead to electron transition from upper to lower state i.e. laser stimulated emission.
    I understand the idea about being sometimes a wave and other time a particle is puzzling! But we choose to unravel this dilemma by choosing this definition: the photon travels as EM wave and absorbed and emitted as a photon, instead of saying we don’t know the exact nature of light!!

    2) If so, how can a polarizer (is that even the correct term?) work? I have this basic definition with me that the polarizer 'cuts' out a component of the electromagnetic field thereby forcing the beam to become 'polarized'.
    Answer:
    Ok, this can be resolved as follow: if you want to picture the incident beam as a stream of photons, each photon has state of polarization associated with its wavefunction. In this wavefunction contains all the information about the photon including the state of polarization. Thus the photons with state of polarization that matches the optical axis of the polarizer crystal passes through otherwise it get absorbed. Because of the randomness of the state of polarization the thermal light the probability of passing is 50%.

    3) Can polarization be explained in terms of photon interaction?
    Answer:
    Yes you can, but if you don’t accept the fact travelling as a wave and absorbed and emitted as a photon you will go through hellish road of complexity!

    4) Can the wave nature and particle nature be explained in the way I see it? [Depending on the 'resolution'/depth/scale of view entities may be regarded as individual particles or 'strings' or waves depending on whether we are able to see characteristic repetitions of behavior/interaction with other entities. Sort of something like, worm is under microscope. It looks like a worm at one resolution and we can see it wriggling about (wave?) and at a higher resolution you can consider it a multiple of a fundamental unit of a 'quantized' unit (the unit being perhaps the striations on its body or perhaps the number of cells per unit volume at another resolution and so on).]
    Answer:
    Question one answer answers this!

    5) I know this might sound a bit stupid, but is the 'Many Worlds' thing something like a wave having 'secondary' wavelets? [It’s impossible (?) for us to experience all possibilities of an event (and if the event is represented as a dot, then the possibilities may be represented as a wave traveling in a unique direction each). Each such wave (possibility) at every point of time may undergo splitting (due to the reason that every event may have n number of possible outcomes since I don't think anyone can exactly say that an even can have only a defined number of outcomes. The 'maybe' factor always adds a few more, or reduces a few) and each of the new wave from the 'splitting' is (supposed) to be a secondary wavelet.] If this were so, wouldn't support the reasoning that the universe had no beginning or end? I know this is not physics but more like philosophy but I see no distinction between the two except in terms of the spelling and the lack of mathematics in the latter.

    Answer:
    I don’t know how to answer this one!
     
  11. Oct 16, 2010 #10
    Well, for example, the electron field has quantized mass, charge, angular momentum and magnetic moment. A photon field has quantized angular momentum, and a dubiously quantized mass of zero.
     
  12. Oct 20, 2010 #11
    Ah! I think I understand your question 4 now. Well the answer is yes, you can think about the wave/particle duality in this way. One way to see this is to realize that in QM particles cannot move along definite paths (see Landau vol 3 section 1); this is the main physical content of the uncertainty principle. The higher the level of resolution, the more erratic the path of the particle you are looking at will become. In fact, there is somewhat of a fractal nature to quantum paths for this very reason. The length of the path depends on the scale at which you are looking. Now physically, we say that an electron actually changes it's path because the photons that we are using to watch the electron bounce off the electron and transfer some momentum to it. However, this leads us to the conclusion that there is in principle no way to determine how the electron moves when we don't watch it by bouncing photons off it!

    Suppose we are watching an electron at point A, and then we stop watching it for some time, and then latert we start watching it again and notice that the electron has traveled some finite distance to the point B. Now according to QFT, the way we understand this motion is to add the amplitude for all possible ways it can happen (the amplitude for an event is the exponential of the classical action over h-bar), including every possible path that goes all over the place and changes velocity like crazy, and also including the possibility that our original electron collided with a positron, turned into a photon, and then awhile later another electron/positron pair was formed and ended up at point B. Another possibility is that our original electron went flying off somewhere, and another electron came and ended up at point B (since electrons are identical particles, there is no way to tell them apart). These arguments amount to the statement that in relativistic quantum theory, there is no physical meaning to the temporal order of events (see Landau volume 4). Now we can connect this to your original "worm under a microscope idea" by considering instead of some characteristic length scale, a characteristic energy scale for virtual particle creation and destruction (this limits the total amount of particle/anti-particle pairs that can be created from vacuum fluctuations)
     
  13. Oct 20, 2010 #12

    fluidistic

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    Isn't it by virtual photons, according to QFT? I'm a total newbie but I think I read it yesterday in Hecht's book on Optics. If so, it would be totally different from photons, or I'm wrong?
     
  14. Oct 23, 2010 #13
    Ok. This leads me to another question. Does this absorption affect the polarizer's material in anyway? (over a significant time frame)
     
  15. Oct 23, 2010 #14
    Thanks!! Your reply sorted it out!! Would it be true that the uncertainty principle can be overcome IF a particle with a lower momentum or (lol) a smaller particle itself were used to watch the electron??
     
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