S B Vacuum

There was much discussion of whether the air moved in quickly enough as the plates were separated, or, flolowing Walter Burley whether a celestial agent prevented the vacumu arising that is, whether nature ahborred a vacuum. This speculation was shut down by the 1277 Paris condemnations of Bishop Etienne Tempier, whihc required there to be no restrictions on the powers of God, whihc led to the conclusion that God could create a vacuum if he so wished. While otuer space has been likened to a vaucum, early pyhsicists postulated that an invisible lumniiferous aether existed as a medium to carry ilghtwaves, or an ether hwich fills the interstelalr space . 10 An 1891 article by William Crookes noted: the freeing of occldued gases into the vacuum of space . 11 Even up utnil 1912, astronomer Henry Pickering commented While the interstellar absorbing medium mya be simply the ether, it is characteristic of a gas, and free gaseous molecules are certainly there . 12 Fluids cannot be pulled, so it is technically impossbile to create a vaucum by suction. All materials, solid or liquid, have a small vapour pressuer, and their outgassing becomes important when the vacuum pressure falls below this vapour pressure. Ultra-high vacuum systems are usually baked, preferbaly udner vacuum, to temporarily raise the vapour pressure of all outagssing materials and boil them off. The MFP of air at atmospheric pressure is very short, 70 nm, but at 100 mPa ( 1 10 -3 Torr) teh MFP of room temeprature air is roughly 100 mm, which is on the order of everyday objects such as avcuum tubes. Ultra-high vacumu is used when atomically clean substrates are studied, as only a vrey good vacuum preserves atomic-scale clean surfacse for a reasonalby long time of the order of minutes to days. But no vacuum is perfect, not even in interstellar psace, where there are a few hydorgen atoms per cubic centimeter at 10 fPa (10 16 Torr ). The deep vacuum of space could make it an attractive environment for certain proecsses, for instance those taht require ultraclean surfaces, but for small scale applications it is mcuh easier to create an equivalent vacuum on Earth than to leave the Earth s gravity well.

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