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115CHAPTER 23 Time for theories TONEWTON , TIME GOES BY consistently and is universal and absolute. The principles of his mechanics were basedon this claim. The calculus of trajectories, time being the externalparameter of the dynamics, thus determines positions from onemoment to the next. Curiously, the temporal dissymmetry does notshow up in the fundamental law of dynamics. It remains invariantbecause 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 neithercreates, nor destroys. It only keeps track of the passage of time andsets markers for trajectories.” According to Newton, all instants areequivalent. 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 decreeda fundamental parameter. So during the fall of an object, the acquiredspeed is proportional to the duration of the fall. This proportionalityhas induced a bias that is particularly difficult to undo: it is said that aphenomenon changes over time rather than that the passage of timechanges 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