We rarely have complete control over all the variables of a problem. When we do, we can verify that the system is governed by dynamical equations in which, at the fundamental level, as we have seen, time does not appear. However, most of the time we measure only a tiny fraction of the innumerable variables that characterize the system. For example, if we study a piece of metal at a certain temperature, we can measure its temperature, its length, its position, the speed at which the piece of iron is moving, but not the microscopic movements of each of its little atoms which, as we know, are
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