Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Understanding photon

  1. Feb 27, 2012 #1
    i'm trying to understand light and photons but some odd questions popping in my head.
    Hope i frame the questions right and please try to understand
    Debroglie told that matter can exist as waves and it does - its proved that electrons do have wave nature and so do many other small particles

    i think about light as a beam of photons and like electrons, protons , neutrons it also has a wave nature. But this is what confuses me : electrons , protons have wave nature but their wave nature do not contain electric and magnetic fields

    Its only with these photons that have this special wave nature consisting of electric and magnetic fields ?

    Also why is only light energy released ( i.e. photons) when electrons jump?
     
    Last edited by a moderator: Mar 1, 2012
  2. jcsd
  3. Mar 1, 2012 #2
    Each kind of particle has its own associated field. Photons have the electromagnetic field, electrons have the electron field, graviton (if they exist) have the gravitational field, etc.
     
  4. Mar 2, 2012 #3
    books and references tell that every particle has a wave nature -
    But here in case of light, oscillating electric and magnetic waves have a particle nature - i.e. photon - which confuses me a bit

    Am i looking it the wrong way. Should i view light as particle(photon) which has wave-nature as electric and magnetic waves
     
  5. Mar 4, 2012 #4
    All quantum objects have both what you call a "wave nature" and a "particle nature". Or you can think of quantum objecs as neither particles nor waves, but which have some properties similar to classical particles, and some properties similar to classical waves.
     
  6. Mar 6, 2012 #5
    I asked pretty much the same question of a physics professor at my local university:

    "Why, of all the particles and their associated waves, is it only the photon that has waves with a physical quantity that is measurable in units (electric field strength, magnetic field strength)? Why are we told that the waves associated with other particles do not represent a varying physical quantity with units?"

    His answer was that the photon was not unique in this respect - all the force carrying bosons have the same property, while the other particles don't.

    I don't know if this is true, but it seemed a small step forward for me. I'd be interested to know.
     
  7. Mar 6, 2012 #6
    Yes, this is roughly correct. Boson fields have values which are real numbers, but fermion fields have values which are Grassman numbers. Since as far as we can tell space (and thus any classical measuring instrument we use) is based on real numbers, we cannot directly detect the Grassman number-valued fermion fields like the electron field.
     
  8. Mar 7, 2012 #7

    I havent yet studied about bosons or fermion fields. So i dont understand what you are saying. Could you please tell me what you understood from your professor. Could be a step forward for me as well
     
  9. Mar 7, 2012 #8

    DrDu

    User Avatar
    Science Advisor

    The field operator destroys or creates particles. For charged particles like the electron there exists a superselection rule which states that all expectation values of operators which change particle number (or better charge) have to vanish exactly. So basically a charged particle can have no associated measurable field.
     
  10. Mar 7, 2012 #9
    I'm not entirely sure myself, but I'll have a go!

    I understood it to mean that particles like photons are (speaking loosely) force carriers i.e. they are the particles that are exchanged between, say, two electrons, which cause the electrons to influence each other through (in this case) the electromagnetic field. (These force carriers are also known as gauge bosons).

    There are other gauge bosons, such as gluons, which 'transmit' the strong force between quarks, W- and Z-bosons, which transmit the weak force etc. When we talk about waves associated with these bosons, we are talking about waves appearing as variations in fields e.g. variations in the electromagnetic field. The amplitude of these waves is something that is a measurable quantity, such as the electric field strength or the magnetic field strength.

    The particles that these bosons work on (loose talk again) are called fermions e.g electrons, quarks or even composite particles (such as protons, which are composed of quarks). These particles are also associated with waves, and, again, they have their own fields. But as lugita15 pointed out, the waves in the fields associated with the fermions (the particles that get acted upon by the force-carrying bosons, if you like) don't have a real physical property (in the sense of electric field strength).

    Whether a particle is a boson or a fermion depends upon ints quantum mechanical spin: bosons have spin that is in integer values; fermions have spin that is always half an integer value.

    You might then reasonably ask what are the physical quantities that the waves associated with the other force-carrying bosons are. I don't know yet, I'm afraid - I'm still learning this myself. But I'm convinced it'll be something I haven't heard of before.

    I'm sure there are folk out there who can correct any of this.
     
  11. Mar 7, 2012 #10
    I think the problem with weak and strong force is that they have a very short range, so they can't live in the "classical" world, they only have meaning in particle physic.
    On the contrary electromagnetic field has a long range and can show himself even in the "large distance" physics of our everiday experience.

    Ilm
     
  12. Mar 7, 2012 #11
    That makes sense.
     
  13. Aug 14, 2012 #12
    "Its only with these photons that have this special wave nature consisting of electric and magnetic fields ?"
    By definition photons are associated with the electro-magnetic fields. There is nothing spacial about photon, in this sense.
    Photons are the quanta of EM field. The speciality is that, in classical domain you see photon as EM field (wave nature), where as for electron field (Dirac field) you see particle nature in classical domain. This is because of wave length match with the classical domain.
    More pacifically, when you see something, you see it by light reflection, so you never see a photon, which behaves as a particle, because your signal (light) is itself a particle whose wave length is comparable to the particle (detecting photon) which you are trying to see. So it natural that you see photon's wave nature (EM wave).
    Where as when you see an electron by light, electrons wave length is much smaller than the photon's wave length, and that is why you see the particle nature of an electron.
    If you still want to see the particle nature of a photon you need to see it by another signal whose wave length is much smaller than the photon's wave length which is to be detected.
     
    Last edited: Aug 14, 2012
  14. Aug 14, 2012 #13
    Actually, the description of the em field by using E and B fields, or equivalently, by A potential is a classical one, corresponding to a coherent state in which the number of photons is left unspecified. If one goes over to a single photon description, then the classical fields have no definite value. You may think of this as an expression of the Heisenberg Uncertainty Principle for a coherent state of the em field.
     
  15. Aug 14, 2012 #14
    "Also why is only light energy released ( i.e. photons) when electrons jump?"
    This particular question is very interesting.
    Why is light released when electron jump? perfect one!!!
    Possible Ans:
    There are many possibilities. But why photon? more likely because other possibilities are energetically not favourable, because the typical energy release by an electron jump is of the order of few electron volt, which is not enough to get any other particles. The other restriction will come from momentum conservation, all kind of charge conservation etc.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook