5 ms·

It's a beautiful excerpt, thanks for sharing. Though I can't help but feel that the author's allegory fits a nebula better than "plain" space.

by pmilot 7y ago

kijin 7y ago
Even a nebula is absurdly empty. A particularly dense nebula might have ~10k particles per cubic centimeter floating around in it. Even that is vastly more empty than the vacuum we create inside particle accelerators and such.
- john_minsk 7y ago
  Wow. Really? I would have thought that our artificial vacuum is pretty empty... Any good read on the topic?
  - kijin 7y ago
    I'm not an expert in this area at all, so I pulled that comparison out of a few different articles and wikipedia. According to [1], the pressure inside the LHC is somewhere between 10^-11 and 10^-10 mbar. That's between 10^-9 and 10^-8 Pascals. CERN also says that it's "a vacuum almost as rarefied as that found on the surface of the Moon." According to [2], pressure on the surface of the moon is between 4x10^-11 and 8x10^-10 Pa. The upper limit is quite close to 10^-9 Pa, so let's say that CERN can manage 10^-9 Pa on a good day. According to [3], the inside of a nebula is 100-10k times more dense than general interstellar medium. (The "10k particles per cubic cm" figure also comes from the same page.) According to [2], interstellar medium in the Milky Way averages between 10^-15 and 10^-14 Pa. So the pressure inside a nebula should be between 10^-13 and 10^-10 Pa. Another comparison using the number of particles per cubic cm [3] yields a figure between 10^-14 and 10^-12 Pa. The difference is probably due to temperature, but anyway both results are significantly less than the LHC. We can probably make better vacuums on a smaller scale, though. [1] https://home.cern/science/engineering/vacuum-empty-interstellar-space https://home.cern/science/engineering/vacuum-empty-interstel... [2] https://en.wikipedia.org/wiki/Orders_of_magnitude_(pressure) https://en.wikipedia.org/wiki/Orders_of_magnitude_(pressure) [3] https://physics.stackexchange.com/questions/26326/how-dense-are-nebulae https://physics.stackexchange.com/questions/26326/how-dense-...

It's a beautiful excerpt, thanks for sharing. Though I can't help but feel that the author's allegory fits a nebula better than "plain" space.

by pmilot 7y ago

It's a beautiful excerpt, thanks for sharing. Though I can't help but feel that the author's allegory fits a nebula better than "plain" space.

kijin 7y ago

Even a nebula is absurdly empty. A particularly dense nebula might have ~10k particles per cubic centimeter floating around in it. Even that is vastly more empty than the vacuum we create inside particle accelerators and such.

john_minsk 7y ago
Wow. Really? I would have thought that our artificial vacuum is pretty empty... Any good read on the topic?
- kijin 7y ago
  I'm not an expert in this area at all, so I pulled that comparison out of a few different articles and wikipedia. According to [1], the pressure inside the LHC is somewhere between 10^-11 and 10^-10 mbar. That's between 10^-9 and 10^-8 Pascals. CERN also says that it's "a vacuum almost as rarefied as that found on the surface of the Moon." According to [2], pressure on the surface of the moon is between 4x10^-11 and 8x10^-10 Pa. The upper limit is quite close to 10^-9 Pa, so let's say that CERN can manage 10^-9 Pa on a good day. According to [3], the inside of a nebula is 100-10k times more dense than general interstellar medium. (The "10k particles per cubic cm" figure also comes from the same page.) According to [2], interstellar medium in the Milky Way averages between 10^-15 and 10^-14 Pa. So the pressure inside a nebula should be between 10^-13 and 10^-10 Pa. Another comparison using the number of particles per cubic cm [3] yields a figure between 10^-14 and 10^-12 Pa. The difference is probably due to temperature, but anyway both results are significantly less than the LHC. We can probably make better vacuums on a smaller scale, though. [1] https://home.cern/science/engineering/vacuum-empty-interstellar-space https://home.cern/science/engineering/vacuum-empty-interstel... [2] https://en.wikipedia.org/wiki/Orders_of_magnitude_(pressure) https://en.wikipedia.org/wiki/Orders_of_magnitude_(pressure) [3] https://physics.stackexchange.com/questions/26326/how-dense-are-nebulae https://physics.stackexchange.com/questions/26326/how-dense-...