热力学第二定律确保(通过统计概率)当两个不同温度的物体相互接触并与宇宙的其余部分隔离时,它们将演化为热力学平衡,最后两个物体的温度大致相同<ref name="Bennett87">{{cite journal | last = Bennett | first = Charles H. | title = Demons, Engines, and the Second Law | journal = Scientific American | volume = 257 | issue = 5 | pages = 108–116 | date = November 1987 | url = https://ecee.colorado.edu/~ecen5555/SourceMaterial/DemonsEnginesAndSecondLaw87.pdf | doi = 10.1038/scientificamerican1187-108 | access-date = November 13, 2014| bibcode = 1987SciAm.257e.108B}}</ref>。第二定律也表示为这样的结论:在一个孤立的系统中,熵永远不会减少。