Statistical physics goal is to describe matter's properties at macroscopic
scale from a microscopic description (atoms, molecules, etc\dots). The great
number of particles constituting a macroscopic system justifies a
probabilistic description of the system. Quantum mechanics (see
chapter ---chapmq---)  allows to describe part of systems made by a large
number of particles: Schr\"odinger equation provides the various accessible
states as well as their associated energy. Statistical physics allows to
evaluate occupation probabilities 
  
    
      
        
          P
          
            l
          
        
      
    
    {\displaystyle P_{l}}
   of a quantum state 
  
    
      
        l
      
    
    {\displaystyle l}
  . It introduces
fundamental concepts as temperature, heat\dots
Obtaining of probability 
  
    
      
        
          P
          
            l
          
        
      
    
    {\displaystyle P_{l}}
   is done using the statistical physics principle
that states that physical systems tend to go to  a state of ``maximum
disorder  [ph:physt:Reif64], [ph:physt:Diu89]. A disorder measure is given
by the statistical entropy\index{entropy}

The statistical physics principle can be enounced as: