Why can’t we go below absolute zero? Why is it the limit?

Question: Why can't we go below absolute zero? Why is it the limit?
- Keywords:
  absolute zero,
  temperature
Asked by monika97 to Arttu, Ceri on 24 Jun 2011.
- Arttu Rajantie answered on 24 Jun 2011:
  
  At absolute zero, all the particles of the system are in their ground state. That is the state of the lowest energy, and therefore it cannot get any colder than that.
  
  Nevertheless, it is actually possible to have negative temperatures, at least in some sense, see .

Question: Why can't we go below absolute zero? Why is it the limit?