Coldest temperature achieved on earth
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Cold in nature
Antarctica is the coldest place on earth. The lowest temperature ever recorded on earth was −89.4 °C recorded in 1983 at the Russian Vostok Station in Antarctica. This is still higher than the lowest temperatures achieved in cryogenic labs. Moving away from the earth, the coldest temperature found in nature is the Boomerang Nebula, at about one kelvin, which is cooler than the cosmic microwave background radiation.
Early cooling
In 1904 Dutch scientist Kamerlingh Onnes created a special lab in Leiden with the aim of producing liquid helium. In 1908 he managed to lower the temperature to less than one degree above absolute zero (−273.15 °C). Only in this exceptional cold will helium liquefy, the boiling point of helium being at −268.94 °C . Onnes received a Nobel Prize for his achievement.
Onnes method relied upon depressurising the subject gases, causing them to cool. This follows from the first law of thermodynamics;
<math>\partial U = \partial Q - \partial W</math>
where U = internal energy, Q = heat added to the system, W = work done by the system.
Consider a gas in a box of set volume. If the pressure in the box is greater than atmospheric pressure, then upon opening the box our gas will do work on the surrounding atmosphere to expand. As this expansion is adiabatic and the gas has done work
<math>\partial Q = 0</math>
<math>\partial W > 0</math>
<math>\Rightarrow \partial U < 0</math>
Now as the internal energy has decreased so has the temperature.
Modern cooling
Modern experiments in cooling can get very close to absolute zero. The current world record is held by a group from the Helsinki University of Technology (HUT), who have reached temperatures as low as 100 pK. The aim of their experiment was to study the characteristics of rhodium in this low temperature regime. This Group also held the previous record of 280 pK from 1993.
The current apparatus for achieving low temperatures has two stages. The first utilizes a helium dilution refrigerator to get to temperatures of millikelvins, then the next stage uses adiabatic nuclear demagnetisation to reach picokelvins.