Book excerpt: What we call time in physics is not a real entity in the universe. The real entities in the universe are matter and energy. Energy gives motion to matter or allows the transformation of matter, giving us the illusion of passing time or more precisely a perception of an "arrow of time". The mistake about using time as a real entity in physics came first in the 18th century when L. Euler, in 1752, states that acceleration of a body is the time derivative of velocity. Instead, we will use d'Alembert definition of acceleration deduced by experimental results: (phi*dt)=dv, where dt is not time differential as in differential calculation but only a small constant of "duration", and phi the acceleration. Einstein theory was to think that time is a real entity in the universe and then space-time coordinates can describe the universe.