http://adsabs.harvard.edu/abs/1968PASJ...20..230T http://adsabs.harvard.edu/abs/1968PASJ...20..230T A few atoms per cubic meter, but yeah, no real thing as a pe

http://adsabs.harvard.edu/abs/1968PASJ...20..230T http://adsabs.harvard.edu/abs/1968PASJ...20..230T A few atoms per cubic meter, but yeah, no real thing as a perfect vacuum.