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Light as an EM wave

  1. Oct 26, 2009 #1
    Since light is a type of EM wave, wouldn't that mean that when you flash a beam of light on a system of charges, you're exerting electric and magnetic fields onto the system?
     
  2. jcsd
  3. Oct 26, 2009 #2
    What's a "system of charges" ?

    Fields are present, but (depending on specifics), It may be possible for nothing to be "exerted", on the "?system of charges?".

    Was my answer vague enough?
     
  4. Oct 26, 2009 #3
    Lets say you have a charged metal sphere. You send a beam of light in its direction. Wouldn't you be sending oscillating electric and magnetic fields towards it (thus, causing the charges to fiddle around)?
     
  5. Oct 26, 2009 #4

    jtbell

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    Yes. This is how a radio, TV or wireless antenna works. :smile:
     
  6. Oct 26, 2009 #5
    Wouldn't that mean that a current is generated in those devices (which, because of the oscillations, would be an alternating current)?

    Also...wouldn't this make photons electromagnetic packets of energy?
     
    Last edited: Oct 26, 2009
  7. Oct 26, 2009 #6
    Photons are packets of electromagnetic energy. Look up the photoelectric effect if you haven't. Light waves can be used to transmit information. You do cause change at the quantum level by shining light on an object. Visible light frequencies aren't high enough to produce usable alternating current, but there are electronic devices which use visible light to produce DC via a rectifier. You can use an oscillator to convert DC to AC though.
     
  8. Oct 26, 2009 #7

    jtbell

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    Yes and yes.

    Photons are quanta of the electromagnetic field, yes.
     
  9. Oct 26, 2009 #8
    Where do they come from? How is it that an accelerating electron emits them (in the sense that there is an electron "shooting" out a bunch of photons---where were the photons before they were being emitted)?
     
  10. Oct 26, 2009 #9

    Born2bwire

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    Well... in one way of thinking of it they are stored in the vacuum state. Dirac made some mention of this in saying that one could think of the vacuum state (0 photons) has infinite energy (up to debate, depends on if you consider there to be a limit to the frequency of light). Any time a photon is created, it is pulled out of the energy of the vacuum state, and when destroyed, is returned to the vacuum state energy. And since the vacuum state has infinite energy, there is an infinite possible supply of photons to be created for any electromagnetic wave when demanded.

    It's an interesting way of thinking of it, the ideas of what photons are can be pretty interesting. Not that the idea of photons are in any disagreement, photons are the quantized energy packets of the electromagnetic wave. However, there are some finer details to the idea. Do photons actually propagate through space, like we generally describe in a simplified manner, or do they simply occur only upon interaction with the fields? Read into quantum field theory and you'll find that many advocate that all particles are similar to photons. That an electron is simply the quanta of the electron matter field interacting in the same way that the photon is the quanta of the electromagnetic wave.

    Any interaction or energy transfer with the electromagnetic wave is done via photons. So where do they come from, well, where does the electromagnetic wave come from? The field and photon are inseparable.
     
  11. Oct 26, 2009 #10
    Interesting stuff...since it was mentioned that the vacuum state has energy, has there also been consideration of space-time energy (assuming space-time and the vacuum have some commonality)?
     
  12. Oct 26, 2009 #11

    Born2bwire

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    In normal quantum mechanics, no because quantum mechanics, in terms of the Schroedinger equation, is non-relativistic. However, quantum field theory does take into account special relativity, which is one of the reasons why some people talk about matter fields in quantum field theory. In field theory, we can create and annihilate particles, even particles with mass, due to special relativity, they just go into energy. So in this case, the matter fields behave like the electric/magnetic fields. That is, the photon is the energy quantum of the electric/magnetic fields. Whenever the fields interact with a system, they do so in the manifestation of a photon. Likewise, an electron (or other particle) has a matter field. Whenever the electron matter field interacts with a system it does so in the manifestation of the electron.

    Another example, the creation of a photon is just taking a photon "out" of the vacuum state. If we have an electron and positron collide, create energy, and then create new particles, this can be roughly shown as the electron and positron annihilating, creating a virtual photon (which carries the mass energy of the pair), annihilating the virtual photon, and creating the resulting particle pair (I can't remember what they can be... muons maybe?).

    What I am not too sure on is the universality of this interpretation. When you look at quantum mechanics, you will get all sorts of interpretations, like Copenhagen or many-world. But I am not sure if the same goes with field theory (or if field theory is just a really complex interpretation in itself but then are there other quantum mechanics interpretations that cover fild theory) and field theory explains a lot more processes and physics than regular quantum mechanics since it takes into account special relativity. It's really something I have never seen in my readings on the subject when even a simple quantum mechanics text like Griffiths will take the time to mention other interpretations. So I don't know if the whole "matter field" is something that is common to all quantum field theories (though it seems like the name and theory imply it by definition). Particularly since I have only seen a few authors come out and explicitly explain it as thus (Zee in his Quantum Theory in a Nutshell does so).
     
  13. Oct 26, 2009 #12
    Interesting...I know less than or about as much as the amateur when it comes to modern physics...I was (or still am) unaware of the essence of QFT (all I really knew was that it existed and it was related to quantum mechanics) and was unaware of this theory of the matter field (though, ironically, I was aware of the theoretical existence of the Higgs Boson...which is stuff that one can pick up in readings on physics for the layman). Is the Higgs Boson the energy quantum of the matter field?
     
  14. Oct 26, 2009 #13

    Born2bwire

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    There are supposed to be multiple "matter fields." Every particle is a quantization of the energy of a particular field. One of the mentors here I think referenced a few papers by Art Hobson, a guy at the University of Arkansas, that suggested teaching the matter field concept at the onset of quantum mechanical theory to help better illustrate the concept between the wave-particle "duality" of quantum mechanics. Most people can accept the photon having wave properties since light is a wave in classical electromagnetics. However, the idea that a particle with mass, like an electron, has wave properties is a harder concept.

    Hobson quotes Robert Mills as saying, "The only way to have a consistent relativistic theory is to treat all the particles of nature as the quanta of fields... Electrons and positrons are to be understood as the quanta of excitation of the electron-positron field, whose 'classical' field equation, the analog of Maxwell's equations for the EM field, turns out to be the Dirac equation, which started life as a relativistic version of the single-particle Schroediger equation."

    The relativistic version of the Schroedinger equation is the Klein-Gordon wave equation. Dirac developed a relativistic equation based on the Schroedinger equation too, called the Dirac equation. But both of these have problems (like negative energies). Apparently Weinberg also had a say on this. "Material particles can be understood as the quanta of various fields, in just the same way as the photon is the quantum of the electromagnetic field." "In its mature form, the idea of quantum field theory is that quantum fields are the basic ingredients of the universe, and the particles are just bundles of energy and momentum of the fields." Interesting, I do not recall him stating it so explicitly in his field theory text but then again, it's three volume set and I have not had the time to go through them in detail.
     
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