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Effect of CMB (cosmic microwave background) on objects at relativistic speeds

  1. Jul 20, 2012 #1
    A hypothetical question relating to

    1) moving a physical object at a significant % of c.

    2) interaction with the cosmic microwave background radiation

    Is it the case that doppler effect and time dilation means that the CMB is going to be physically damaging to the object?

    Further what would happen if a source of microwave energy (e.g. from a transmitter) was directed towards such an object?

    This is from a sci-fi discussion, but I just got this idea and I don't know what would happen.
  2. jcsd
  3. Jul 20, 2012 #2


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    Staff Emeritus
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    Sort of, but first off, it doesn't just have to be a significant fraction of c, it has to be ultrarelativistic, i.e., very, very close to c. Also, there are other forms of matter and radiation besides the CMB that would fry you much worse than the CMB. I think the interstellar medium (mostly hydrogen) would be the most damaging. Starlight from the forward direction would also be Doppler shifted into the gamma spectrum.
  4. Jul 20, 2012 #3
    I know that there is the possibility of some objects to interact with CMB. It is the case of high energy electrons that can react with CMB photon through inverse Compton scattering. This leads to a detectable distortion of CMB spectrum. It is a way used to detect clusters of galaxies and the anisotropies. This effect is called Sunyaev Zeldovich effect.
  5. Jul 20, 2012 #4


    Staff: Mentor

    A Doppler-shifted blackbody spectrum is itself a blackbody spectrum. So at about 150 m/s less than c (time dilation factor = 1000) the CMB will be a blackbody with a temperature around 2700 K, close to stellar surface temperatures.

    bcrowell is pretty smart, so he may be right that matter will be a probelm before the CMB. I am not sure, I would have to see a calculation, but certainly the CMB isn't the only hazard to ultrarelativistic travel.
  6. Jul 20, 2012 #5
    Interesting, then I might be on the right track: beaming microwaves in all directions could possibly act as a shield against such a hyopthetical projectile. It's off topic for this thread however I guess. Although it seems to be correct that such projectiles would initiate a nuclear reaction on impact.

    EDIT: I think i already mentioned hitting stray hydrogen atoms and space dust in that other thread.
    Last edited: Jul 20, 2012
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