Vol. 3, 10a. “On Boltzmann’s Principle and Some of Its Direct Consequences” [Zurich, 2 November 1910][1] Thermodynamics is known to be based on two principles, the energy principle (also called the 1st law) and the principle of the irreversibility of natural events (also called the 2nd law). This latter principle by states, The substance of this lat- ter principle can be expressed in the Planckian sense thus, according to Planck All natural science[2] is founded on the presumption of an unbroken entirely causal relation for all occurrences. Let us assume that Galileo had found from his pendulum experiments that the period of one oscillation of this pendulum changed in a very irregular way. Let us assume furthermore that this change could not be connected with any change in other observable relations. Then it would have been impossible for Galileo to gather his observations under a law. If all phenomena accessible to us had as irregular a character as we have just pictured in this fictitious case, people would certainly never have resorted to scientific endeavors. Which characteristic must phenomena have in order for science to be possible? To this, one might first want to reply somewhat as follows: If we put a system into a particular state, then provided this system is separated from other systems—such as by a large spatial distance—then over time the course of the states of this system is completely determined i.e., if we put two arbitrarily many equally composed isolated systems into exactly the same state and leave these systems alone, then the evolution of the phenomena in time is exactly the same for all these systems. Now, how about the unbroken entirely causal connection between events according to our knowledge today? This question has to be specified more pre- cisely before it can be solved. Let us do this right away by means of an example. Take a cube of copper of a given size. Within this cube we imagine we establish by external influences a very specific temperature distribution and then, after hav- ing enveloped it in a thermally insulating shell, we leave it be. We know that in the course of time a temperature equilibrium will then set in by the process of thermal conduction. The temperature gradient at all points of the cube will thereby prove to be “uniquely determined” by the initial state by the expression “uniquely deter- mined” we mean that we are always going to perceive the same temperature gradi- ents, no matter how often we may repeat the experiment, that is, no matter how often we may set the starting temperature distribution and then leave the cube be. Does this unique determinability of the process, this entirely causal connection of events really exist? In order to address a pertinent objection but of no interest to us [p. 1] [p. 2]
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