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Why do particles radiate outward?

  1. Mar 25, 2005 #1
    Forgive my naive question - I am not a scientist.

    Why does radiation radiate?

    Particle a sits there giving off radiation - ok but why? Why does the radiation move away from the source instead of simply sitting there or indeed radiating inward as opposed to out?

    I ask this because we see this effect and accept it but cannot yet describe the mechanism for Gravity - can it not just be the opposite and equal reaction to radiation from mass and directed inward?

    Forgive me if this isa stupid question.
  2. jcsd
  3. Mar 25, 2005 #2


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    The only stupid question is the question not asked.

    Radiation is a photon (unless you're talking about alpha or beta radiation). By definition, it moves at the speed of light. It's got to go somehere.
  4. Mar 25, 2005 #3


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    Radiation can't move towards the source as it starts at the source and is therefore as close as it get to the source so any movement must be away from the source (thoguh for a spatially extended source some of the radiation emmited on the outer edges will generally move towards the centre).

    If radaiion doesn't move at all it stays at the source and we don't detect it, infact as it doesn't move and due to the various conservation laws it's exactly as if no radiation was emitted at all.
  5. Mar 25, 2005 #4
    Radiation is simply self inducing electric and magnetic fields. A changing electric field induces a magnetics field, this magnetic field induces an electric field, and so on.
  6. Mar 25, 2005 #5


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    re: Why do particles radiate outward?

    Because if they tried to radiate inward they'd bump into each other. :biggrin:

    Sorry... there are enough people giving serious answers...
  7. Mar 25, 2005 #6


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    The best treatment on classical radiation theory is found in J.D.Jackson's book (any of the 3 editions) and in Born & Wolf's masterpiece.


    P.S.I'm not a scientist either...:wink:
  8. Mar 25, 2005 #7


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    I hope this isn't just a plug for somebody's bizarre gravity theory.

    Anyway, causality is a good reason for radiation radiating outwards. It would be non-causal for a bunch of points in space with no prior conneciton to suddenly "decide" to radiate inwards.
  9. Mar 26, 2005 #8
    This is a pretty simplistic answer, but it seems to be what the guy is looking for:

    Radiation radiates outward because what we cann 'radiation' is by definition the stuff that goes outward. If it didn't go outward, we couldn't call it radiation. We limit radiation to this by definition.
  10. Mar 29, 2005 #9
    Your replies

    Thanks for your kind replies - sorry so late in replying was busy over Easter

    Sorry if the question was simplistic - I know by definiition radiation is outward - I was just wondering aloud about the equal and opposite reaction that we see in all other physics - I push something it pushes back - Photons are ejected - do the "push" off against the particle they radiate from - was probably more what I was after

    No not a theory of Gravity - bizarre or otherwise :bugeye: - just a 1st post from a new momber who is always curious :smile:
  11. Mar 29, 2005 #10


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    Sure. When an excited atom emits a photon, it recoils very slightly so that momentum is conserved. The effect is very tiny because the energy involved is very small compared to the energy-equivalent of the mass of the atom.

    Even in classical electrodynamics, radiation from accelerated charges is always accompanied by what can be called a "reaction force," which either causes the accelerated charge to lose energy and momentum, or gets transmitted to whatever external agent is accelerating the charge. If you could grab onto a charge and shake it by hand fast enough to produce measurable radiation, you'd feel the reaction force against your hand, sort of like when you're pushing something through a viscous fluid.
  12. Mar 30, 2005 #11
    Thanks - that was helpful.

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