edit: refer to [this link](https://en.wikipedia.org/wiki/Negative_temperature) for further explanation
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Temperature is defined with energy and entropy.
On normal matter atoms are in various states: Low energy, high energy etc. You add energy into the system and atoms go to higher energy state and the entropy increases since now there are more possible states to be in.
This type of system has positive temperature (entropy increases with added energy).
A bit more exotic system can have thing called “population inversion”. In this state all (or at least most) of the atoms are in exited high energy state where they can’t go higher with added energy.
Adding more energy to this system will just put more atoms into that same exited state. This reduces entropy because now there are less possible states to be in (all are in the same exited state).
This type of system has negative energy (entropy decreases with added energy).
(note: As whole entropy increases because the energy came from somewhere else where entropy increased. But we don’t care what happens outside)
The negative temperature matter has all of its atoms in exited state. Positive energy matter has atoms in all sort of states: some of them in low energy, some of them in high energy.
When exited atoms interact with other atoms the exited atoms will easily give some of their energy away (exited states are not stable).
So energy flows from the negative energy matter into the positive energy matter.
This is why negative matter has effectively “infinite temperature”. No matter how hot a positive temperature matter is the energy will always flow from negative temperature to positive temperature matter.
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