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CHAPTER To NEWTON, TIME GOES BY consistently and is universal and absolute. The principles of his mechanics were based on this claim. The calculus of trajectories, time being the external parameter of the dynamics, thus determines positions from one moment to the next. Curiously, the temporal dissymmetry does not show up in the fundamental law of dynamics. It remains invariant because we can use the same equation to calculate, at random, a posi- tion in the past and in the future. This is called symmetric evolution. It is how nature could undo its doings. Klein emphasized that “there- fore, in the ideal case, where there is no friction, Newtonian phenom- ena are reversible. Newton’s time is scrupulously neutral. It neither creates, nor destroys. It only keeps track of the passage of time and sets markers for trajectories.” According to Newton, all instants are 1 equivalent. Special relativity, the theory of ST, affirms that time and space are entirely physical phenomena. They are not virtual containers. How- ever, time, although quite different from the Newtonian conception, does not have the three-dimensional stature of space. Since Galileo, time has not only become a quantified magnitude, it has been decreed a fundamental parameter. So during the fall of an object, the acquired speed is proportional to the duration of the fall. This proportionality has induced a bias that is particularly difficult to undo: it is said that a phenomenon changes over time rather than that the passage of time changes during a phenomenon. It seems that exterior time — advanc- ing continuously and isometrically rather than observer time — advances more or less quickly than that of the falling object, or vice 115 Time for theories