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Why does light/energy propagate?

  1. Sep 30, 2004 #1
    Is there a concesus as to "why" light propagates. What force(s) are involved in the process? If this is not known or understood, are there any current theories?

    Is this a reasonable question?..........Thanks in advance for all valid replies.
  2. jcsd
  3. Oct 1, 2004 #2


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    I'm not sure I understand your question, so let me re-phrase and give some views:

    "What is light?" Electro-magnetic radiation, i.e. {insert a detailed description here}. Photons, i.e. {insert a detailed description here}

    "What theories etc are consistent with the results of observations and experiments into the nature of light?" Maxwell's equations, Special Relativity, Quantum Mechanics, ... {insert detailed descriptions here}

    "Are there any results which are inconsistent with these theories?" No.

    "Within these theories, how does light travel from one place to another?" {insert discussion here}

    "Why does light move from one place to another in the way described in answer to the previous question?"
    A: because that's what 'propogation' is within these theories
    B: this question cannot be answered within the scope of the theories which account for all relevant observations and experiments
    C: {ΓΈ}

    Does this help? Would you like more detail on any of these?
  4. Oct 1, 2004 #3


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    Is force5 just looking for a layperson's hand waving' explanation of electro-magnetic radiation?
    If so will this do?

    1. You are probably familiar with magnetic forces, fridge magnets etc.
    2. You are probably familiar with electrostatic forces, such as when you vigorously brush or comb dry hair with a nylon brush/comb and find it all sticks up. Each hair strand has been charged with an electric charge by friction and each strand repels each other strand so they all stick up on end.
    3. Now a moving electric charge produces a magnetic force, as in an electro-magnet found in an electric motor.
    4. And a changing magnetic field produces an electric current in a conductor (wire) as in a dynamo.
    5. These two forces, the electric force and the magnetic force are two different manifestations of the same thing, an electro-magnetic force.
    6. Now if an electric charge not only moves but accelerates it produces a changing magnetic field, and a changing magnetic field produces a changing electric field which produces a changing magnetic field and so on.
    7. In this way the energy that has gone into vibrating the original charge now propagates across space, even empty 'vacuum' such as that between the Sun and ourselves.
    8. This is called electro-magnetic radiation and depending on wavelength, that is its frequency of vibration, the radiation may be radio waves, microwaves, infrared, light, ultra-violet, X-ray or gamma ray radiation.

    In this way light and all the other forms of e-m radiation propagate across space and time.

    Last edited: Oct 1, 2004
  5. Oct 2, 2004 #4
    Sorry if question is not phased properly

    Thanks for responding;

    Based on entropy, I assume EMR is pulled out of the producing system. And I am trying to find a connection between DE and propagation. If dark energy is responsible for expansion, as some have speculated, what would be the relationship between energy producing systems and dark energy.

    Again, I'm sorry for not phasing thinks very well. Sometimes I have a problem putting my thoughs into words. (English; not my favorite subject)
  6. Oct 2, 2004 #5


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    Leave dark energy and entropy aside. They have nothing to do with EM propogation[re: the Maxwell equations]. EM propogation is constrained by permitivity and permissibility of space.
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