Revised 24 May 2019

In a low entro­py system the energy is free in the sense that it is avail­able for producing mech­anical work, whereas in a high entropy sys­tem the energy is said to be bound. Lord Kel­vin first developed the prin­ciple of the degradation of energy which states energy is continual­ly becoming unavail­­able for work because all natural processes dissi­pate energy to heat. Within the isolated sys­tem of the Uni­verse there is a con­tinu­­­ous and irrev­­ocable degra­dation of free into bound energy. 

The change in entropy for reversible and irrevers­ible processes is formalized by the Second Law of Thermodynamics, the law of entro­py, which states:

When all systems taking part in a process are included, the entro­py S of the total system either remains constant or increases. 

In mathematical terms: 

ΔS (Universe) 0

An open system may exchange both energy and matter with the outside, whereas a closed system exchanges only energy and not matter with the outside. Although the entropy of an iso­lated system remains either con­stant or increases, the entropy of open sub-systems may decrease. Life forms are an example of low entropy systems which, in order to retain the form of low entro­py, need to contin­ual­ly take in energy and matter in the form of food and con­sume more than one unit of food in order to gain one unit in weight. During periods of increas­­e in size and num­bers, the entropy of living organisms decreases while the entropy of the combined system, which includes their life sup­port systems, simul­taneous­ly increases. Contrary to being a viol­ation to the law of entropy, life forms com­prise sys­tems that hasten the increase of entropy in the uni­verse (Schrödinger, 1967, pp. 73-7).