| Themes > Science > Physics > Cryogenics > Temperature > Is a negative Kelvin temperature possible? > Systems at infinite temperature |
Systems at infinite temperature are used to attain the lowest temperatures possible. The technique is that of adiabatic demagnetization. To get a rough idea of the subject, let us look at magnetism due to electron spin. Each electron can be seen as a tiny magnet, except that it can have only two orientations in an external magnetic field, namely parallel to the field and antiparallel to it. Without a field, these "electron magnets" are randomly oriented. Energy of both states is equal, so it is equally probable to find each orientation. If an external field is applied, the energy of the parallel state is lowered and energy of the antiparallel state goes up. This phenomenon can be used to cool samples of suitable materials (specific magnetic properties are required). A sample is cooled down to the temperature of liquid helium. Still submerged in liquid helium, a magnetic field is applied to it. At the very first moment, the number of atoms in the parallel equals that in the antiparallel state. But these states are no more energetically equivalent in the presence of an external magnetic field. Our formula says that if two states differ in energy but are equally populated, then the temperature is infinite. This is an example for the previously stated assertion that one does not need infinite energy to get infinite temperature. In fact, the energy of the sample does not rise, on the contrary! Because it is at infinite temperature, the system will now lose energy as antiparallel states change to parallel. The energy is transferred as heat from the sample to the helium bath. Its temperature changes from infinity to that of the helium bath. Now the sample is insulated from the helium bath and the external magnetic field is reduced to zero. The energy that was transferred to the helium bath cannot go back to the sample. Hence in this step, while the populations of the antiparallel and the parallel states are gradually equalized, the sample cools down. This is how some of the lowest available temperatures are attained. |
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