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Energy and momentum flux in the sun and Newton's 3rd law.

  1. Oct 23, 2009 #1
    Newton's Third Law of Motion:

    III. For every action there is an equal and opposite reaction.

    Photons in the sun on average travel outward, electrons and ions on average recoil inwards? Does Newton's third law come into play here?

    Thank you for help with my confusion.
  2. jcsd
  3. Oct 24, 2009 #2


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    Electrons and ions on average recoil inwards? Actually, the Sun is a net producer of charged particles; this stream of particles is called the solar wind.

    It's true that Newton's third law (and its corollary, the conservation of momentum) apply to the fusion reacts that produce photons, but it's not as if those photons are streaming out into space. The gamma photons produced enter the convection zone, where energy transfer is mostly via convection. The photons we see are black-body photons produced very near the surface of the Sun, not the gamma photons produced by fusion reactions.
  4. Oct 25, 2009 #3

    You're right to suspect that the ions and electrons throwing off photons from the Sun's photosphere experience a recoil from Newton's 3rd law. However they're very rapidly bumping into other ions and electrons before they travel very far in the photosphere, so any initially inwards vector is soon randomised.

    The photons - and, to a lesser extent, the solar wind - do cause an inwards pressure in reaction to their departure, but that's overwhelmed by gravity's much greater hold on them. The Sun would need to be much, much hotter and brighter for light pressure to match gravity in the photosphere.
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