7 ms·

"supersonic" relative to the sound speed at normal density I assume. With no air, there would be no sound! Edit: corrected pressure to density

• Robotbeat 9y ago
  https://mobile.twitter.com/elonmusk/status/901975117797851137 https://mobile.twitter.com/elonmusk/status/90197511779785113... @elonmusk: "No such thing as true vacuum" BTW, speed of sound doesn't change (to first order) with changing pressure (edit: nor does it change (to first order) with density, assuming temperature is the same). Mostly effected by temperature. Hyperloop uses a partial vacuum, of course. No true vacuum exists.
  • _Codemonkeyism 9y ago
    "No true vacuum exist." How high is density of atoms in space? Could there be a 1mm x 1mm space without atoms?
    • vlahmot 9y ago
      My understanding is that there would still be quantum fluctuatons causing particles to pop in and out of existence.
      • _Codemonkeyism 9y ago
        Would this be protons? Can vacuum contain photons and still be a vacuum? Not sure :-)
        • xhrpost 9y ago
          https://en.wikipedia.org/wiki/Quantum_fluctuation#cite_note-1 https://en.wikipedia.org/wiki/Quantum_fluctuation#cite_note-...
    • mikeash 9y ago
      According to Wikipedia: "Plasma with a number density of less than one hydrogen atom per cubic metre and a temperature of millions of kelvins in the space between galaxies accounts for most of the baryonic (ordinary) matter in outer space; local concentrations have condensed into stars and galaxies." - https://en.wikipedia.org/wiki/Outer_space https://en.wikipedia.org/wiki/Outer_space I knew that intergalactic space could see densities of one atom per some human-relevant volume of space, but I had no idea that actually accounted for most of what there is!