Even if apparently reversible processes in time can be easily imagined (the oscillation of a pendulum, the motion of planets, the trajectory of a marble bouncing between the sides of a billiard table), the microscopic analysis shows that, in reality, even these processes are irreversible thermodynamic processes that define a unique way of flowing of time, that is always in the same direction.
In fact, it is known that, of the energy that is given to a system to induce any transformation, an ineliminable fraction must still be dispersed in a non-recoverable way in the form of disordered energy such as, for example, heat.
The decreasing amplitude of pendular oscillations due to friction (heat generators), the heating of the ball as a result of collisions, the dissipation of kinetic energy in the form of gravitational radiation by the planet in orbit provide as many keys to define the direction in which time flows. In general, all real systems, not interacting with each other, in implementing thermodynamic transformations, tend to reach states of increasing internal disorder.
In strict terms, the laws of thermodynamics require that the real transformations of isolated systems take place in the sense of increasing entropy. Irreversibility and increase of entropy are synonyms that, in the same way that they denote the characteristics which events obey in their becoming, also contain the essential property that gives meaning to time, since the latter would lose all meaning if there were no production of events, or there were no means to determine the universal direction of flow.
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