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We say that at temperatures very close to absolute zero (nano or pico Kelvin) there is no gas, so how do we do experiments on gases at zero Kelvin, and why is Bose-Einstein described as a gas?

1995 – Eric Cornell and Carl Wieman create the first[14] Bose-Einstein condensate, using a dilute gas of Rubidium-87 cooled to 170 nK.

2015 - Experimental physicists at the Massachusetts Institute of Technology (MIT) successfully cooled molecules in a gas of sodium-potassium to a temperature of 500 nanokelvins.

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    $\begingroup$ From en.wikipedia.org/wiki/Bose%E2%80%93Einstein_condensate "A BEC is formed by cooling a gas of extremely low density (about 100,000 times less dense than normal air) to ultra-low temperatures". $\endgroup$
    – PM 2Ring
    Apr 27, 2022 at 9:04
  • $\begingroup$ Ok, but what in general as I mentioned in the experience in 1995 & 2015 $\endgroup$ Apr 27, 2022 at 9:17
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    $\begingroup$ They call them gases, but they're more like hard vacuums with a few atoms (typically less than a million) floating in them. $\endgroup$
    – PM 2Ring
    Apr 27, 2022 at 9:20

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For the technique of cooling and trapping, see https://www2.physics.ox.ac.uk/sites/default/files/2013-01-19/andrian_pdf_71380.pdf.

By the way, trapping is not only needed to thermally isolate the system and to further cool it with lasers after the Doppler effect, but also to allow the atoms to align themselves undisturbed to a BEC. Any contact with the walls of the experimental setup results in interactions of the magnetic dipoles of the atoms of the BEC with those of the walls. Since the atoms of the walls have chaotically distributed magnetic dipoles, a BEC would never come into being.

Yet another prediction. The choice of gas that can condense into a BEC also depends on the design of the magnetic trap with its intersecting magnetic fields. In principle, the possible "crystal" structure resulting from the existing magnetic dipoles of the gas atoms must be reflected by the MOT.

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