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Blackbody in inertial frame?

  1. Apr 3, 2013 #1
    I just confused myself with this idea, need someone to fix it please!

    Say a spherical black body temperature T is moving with velocity v in some direction through some medium with a lower temperature. In the sphere's rest frame this emission is isotropic, however, in the rest frame of the medium, the net emission is blue-shifted in the direction of the sphere's motion and redshifted in the opposite direction. The observer then deduces the blackbody is not emitting isotropically, with more energy being emitted forward than backwards.

    eg. for every photon emitted forward with momentum hf1 there will be one emitted backwards with momentum hf2, where f1>f2 due to the relative Doppler shifting. The stationary observer should then deduce there is a net impulse on the sphere acting to slow it down.

    Where does this impulse come from, and where is its Newton's-3rd-law pair?
  2. jcsd
  3. Apr 3, 2013 #2


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    This is a very nice question with an interesting answer.
    The momentum of the sphere decreases in the same rate as its mass - as it loses energy, it gets lighter. The velocity stays the same.
  4. Apr 4, 2013 #3

    Jano L.

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    I agree with mfb regarding the effect of the body's own radiation. But there is also the radiation due to other bodies at the lower temperature. This will be red-shifted in the frame of the body and should slow down the body to rest.
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