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What is normal to most people in winter has so far been impossible in physics: a minus temperature. On the Celsius scale minus temperatures are only surprising in summer. On the absolute temperature scale, which is used by physicists and is also called the Kelvin scale, it is not possible to go below zero. Physicists at the Ludwig-Maximilians University Munich and the Max Planck Institute of Quantum Optics in Garching have now created an atomic gas in the laboratory that nonetheless has negative Kelvin values (“Negative Absolute Temperature for Motional Degrees of Freedom”).

1272312397623C042AEECA4F51C1209FThese negative absolute temperatures have several apparently absurd consequences: although the atoms in the gas attract each other and give rise to a negative pressure, the gas does not collapse – a behaviour that is also postulated for dark energy in cosmology. What is more, atoms in this “negative temperature” are more hot than any set of atoms in infinite positive temperatures! Ans this does end here: supposedly impossible heat engines such as a combustion engine with a thermodynamic efficiency of over 100% can also be realised with the help of negative absolute temperatures. (source: 1, 2, 3, 4, 5)

Many times Science sets limits which for many years to come we think as insurmountable. But we must not forget that as we arbitrarily set axioms and principles, we also set these limits that cannot supposedly be overcome. How many people have been ridiculed because they thought thermal machines with efficiency larger than 100% can actually be built? As in the case of imaginary numbers (we once upon a time thought as “true” that the square of a number cannot be negative – then we changed the… definition!) the case of negative temperature shows clearly that everything is in our mind…

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