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B Particles in deep space query

  1. Jul 5, 2018 #1
    I hear that deepest void of intergalactic space may contain say one particle per cubic cm. I don't want to quibble the amount but let's take that as close enough for my purposes.

    Now is this figure a statistical average so that if it were correct that each cubic kilometre of deep space would contain say a quadrillion particles and if we took random samples of cubic kilometres say from a cubic light year of deep space, that we would find more or less a quadrillion particles in each of these cubic kilometres. Or is this not the right way to think of the figures given for deep empty space.

    You see I'm wondering if these quadrillion particles would have all been ejected from stars or similar object and by the time they are out there they're still moving through at close to their original ejection speed. Or are none of these particle there from galaxies because they never were able to escape the galaxy's gravity and they are in fact just left over particles from the original formation of the universe as the nearly uniform particle gas began to clump. In which case I then wonder are they sitting there as would particles of hydrogen in a 3k box. And if the latter, could we then speak about the motion of an object in deep intergalactic space relative to this thin gas.

    I hope I've put that clearly.
  2. jcsd
  3. Jul 5, 2018 #2


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    I'm not sure what your actual question means but
    that is reasonable; The mean velocity of the particles in your "thin gas" has to be 'relative to something' and that something could be your object. But it works both ways according to your choice of frame. The mean velocity of the particles, relative to their centre of mass will be zero.
    All velocities and positions are relative; there is no graph paper reference in 'space'.
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