[avoid3d]

The reason that negative temperatures are hard to explain is that the definition of temperature in the sense used by physicists is not very accessible.

It has to do with how much a certain property of a system (object) changes when you add a certain quantity of heat energy.

In certain edge cases adding energy makes this quantity change in the opposite to usual way and therefore the temperature is negative.

[omegant]

I´ve been reading the negative temperature link, I understand that you reduce entropy reducing energy, till a point, where you start "holding" the molecules down adding energy (I don´t know if that makes sense).

But still is there a way that you could start affecting the forces inside the atom this way?. Or then we are talking about something different?.

[avoid3d]

This is really on the edge if where I don't know what I'm talking about anymore but I would think not.

The forces holding atoms together and the energies associated with changing those forces are far larger than the amounts that are presented in this study.