Bluff your way in the second law of Thermodynamics

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The aim of this article is to analyze the relation between the second law of thermodynamics and the arrow of time. For this purpose, a number of different aspects in this so-called arrow of time are distinguished, in particular those of time-(a)symmetry and of (ir)reversibility. Next I review versions of the second law in the work of Carnot, Clausius, Kelvin, Planck, Gibbs, Carath\'eodory and Lieb and Yngvason, and investigate their connection with these aspects of the arrow of time. It is shown that this connection varies a great deal along with these formulations of the second law. According to the famous formulation by Planck, the second law expresses the irreversibility of natural processes. But in many other formulations irreversibility or even time-asymmetry plays no role. I therefore argue for the position that the second law has nothing to do with the arrow of time.