  Until now, the second law of thermodynamics is not formulated for a single
(deterministic and time-symmetric) microscopic trajectory. Here, such a
formulation is suggested by introducing information as a full-valuable physical
quantity defining (a) microscopic entropy and (b) information difference
between an observed process and its time reverse. (a)+(b) then form the second
law and imply that macroscopic irreversibility and mesoscopic stochasticity are
relative phenomena depending on the used information reference frame. The
results provide fundamental insights for further applications in microphysics
(informational engines, quantum computing, etc.).
