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Why is space cold

  1. Nov 25, 2014 #1
    I was wondering what is the reason why space is so cold ? My own answer would be that because there is no matter , or almoust no matter so almoust no atoms.

    then i thought but there is sun and lots of energy source which radiate EM energy on various frequencies , but since there is almoust no matter in space there is nothing in the way of the radiation to intercept it and heat up so the waves just fly by until they hit someting that can absorb that radiation which is matter , like planet earth and our atmosphere , is my thinking correct ?

    so the answer i came up myself is space is cold because where there is no matter also there can't be any heat because heat is energy which is normally associated with matter.
  2. jcsd
  3. Nov 25, 2014 #2
    What about the CMB? That permeates all of space.
  4. Nov 25, 2014 #3


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    Heat transfer occurs in 3 forms

    radiant --- any body above 3 degrees will lose heat by radiating it into space, which has an average temperature of 3 degrees. (this is, of course, not true way off the average, such as near a star)

    convection --- you get none of that in space

    conduction --- you get none of that in space

    SO ... basically, in deep space all you do is radiate into a 3 degree sink, which is damned cold by human standards.
  5. Nov 25, 2014 #4


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  6. Nov 25, 2014 #5


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    What about it ?

    its only a few Kelvin
  7. Nov 25, 2014 #6
    Space isn't "cold." Most of it is empty, so there is nothing which could conduct heat, or support the existence of heat without external input of energy.

    However, the ISM (interstellar medium) and WHIM (warm-hot intergalactic medium) do exist, and are generally very hot (excluding dense molecular clouds from the definition of ISM). This is kind of misleading, because as yes, individual particles can reach upwards of 107 K, the density is so obscenely low that our daily understanding of temperature is pretty much meaningless when thinking about it. For instance, if you were to stick your hand in a pot of boiling water (@ 100° C) you would get some severe burns, not a fun time. However, if you were to stick your hand into an oven which had been heated to 400° C for an equal amount of time, nothing horrible would happen at all. The air in the oven is much less dense than the water in the pot, so despite the overall greater energy of individual particles, fewer of them get a chance to transfer that energy to your skin as heat. The same logic applies to space; while individual particles in the ISM are really, really, really energetic and "hot," there are only a few atoms per cubic meter! If you were to float around in a cloud of ISM, you would radiate your energy away much faster than you could be heated up by the few atoms around you, leading to the illusion that space is inherently cold, and that its low temperature is a result of something other than the fact that most of it is empty.
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