The existence of irreversible processes is well known from everyday life: one cannot go backwards in time. This holds not only for living beings but also for those objects of macroscopic dimensions which we usually deal with. Everybody laughs when a film is run backwards, and not only because people then walk backwards. Just imagine a film showing a cup of coffee falling from a table, breaking into pieces, and dispersing its contents on the floor, and think of what you would see when the film ran backward. I shall not dwell on this example, since a similar one which goes back to 1874 and is due to Sir William Thomson (Lord Kelvin) is quoted in detail in Chapter 5. The strange fact is that all the fundamental laws of physics are symmetric with respect to the inversion of the time arrow; the cup which reunites itself from the broken pieces, recovers the coffee from the floor, and jumps back to the table does not violate any law of mechanics. [download]
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