In statistical mechanics and thermodynamics, temperature is defined as follows: 1/temperature = change in entropy/change in energy for our hypothetical system of 100 atoms with only two energy states, there is an upper and lower bound of energy, and the entropy of the system equals zero at these boundary limits. Let's consider the case in negative absolute temperature, if the complete work-to-heat process with only one heat bath is possible, then the total entropy of the system and the heat bath will decrease, which goes against the second law that the entropy of a isolated system cannot decrease what is sink temperature (in thermodynamics.
Negative absolute temperatures in optical lattices as we have discussed in our previous post, there is an important debate in the scientific community regarding the existence of negative absolute temperaturesthe debate is not over, and both sides have valuable arguments, but in any case, one thing has already been proved.
In quantum thermodynamics, certain systems can achieve negative temperature that is, their temperature can be expressed as a negative quantity on the kelvin or rankine scales a system with a truly negative temperature on the kelvin scale is hotter than any system with a positive temperature.
Let's consider the case in negative absolute temperature, if the complete work-to-heat process with only one heat bath is possible, then the total entropy of the system and the heat bath will decrease, which goes against the second law that the entropy of a isolated system cannot decrease. If negative absolute temperatures are real, the requirement to achieve them is to have a system where the energy is bounded from above in our normal, macroscopic world, this is not the case for instance, kinetic energy has a minimum value when particles are at rest, but it has no maximum.
Negative temperatures and negative absolute pressures are both possible in physical systems negative temperatures arise in (for example) populations of two-state systems, which have a maximum amount of internal energy they can contain negative pressure indicates a system in tension and is rare in fluids but common in solids.
Absolute zero is unreachable, but thermodynamics does not explicitly forbid to create a system with a negative temperature it is even more tricky than the example of the speed of light, because negative temperatures are not really below absolute zero.