The Order of Time is divided into four sections, covering the theory of relativity, space-time, loop quantum gravity, and thermodynamics. The first section, The Crumbling of Time, opens with Rovelli explaining time, which is considered as a fourth dimension in space-time. He then discusses Ludwig Boltzmann 's concept of entropy, which never decreases, and its role in thermodynamics and Bayesian probability theory, which Rovelli described as "the only equation of fundamental physics that knows any difference between past and future", and concluded that the direction of time is lost. In the second part, The World without Time, Rovelli writes that events constitute the universe instead of particles, and introduces the concept of quanta of time in loop quantum gravity. The final section, The Sources of Time, proposes that the apparent flow of time is due to the inability to observe all the microscopic details of the world.