6 - Irreversible processes

Summary

Lindblad's theorem

Although the fundamental laws of physics explain a great deal of the material world, they often seem to contradict our immediate experience. For instance, as physicists we believe that all elementary interactions are reversible, but as people we are immersed in irreversibility. The collision of two atoms may well be reversible and predictable, but for us time flows in a definite direction and life is sometimes unpredictable. “What then is time? If no one asks me, I know what it is. If I wish to explain it to him who asks, I do not know. ” (Attributed to Augustine.) Nevertheless, as we try to explain in this section, one can include irreversibility, the true arrow of time, into quantum mechanics in a concise and elegant form put on solid mathematical ground in Lindblad's theorem (Lindblad, 1976). Having then understood the basics of irreversible quantum dynamics, life may not be quite the same any more.

Irreversibility

A beam splitter is a reversible optical instrument: one could collect the outgoing light beams and bring them together again in a second beam splitter where they constructively interfere to re-establish the light in its original state. But now imagine a light beam is scattered many times. For example, light entering biological tissue is interacting with each of the cells along its path. Some of the light is scattered and only a part of the light continues in the original direction.