Through a mental experiment, the mechanics' theory, about of the relative motion, is reexamined because the vacuum, frame of the movement, has a physical reality, which allows test the absolute motion, by the Doppler Effect induced by means of a decelerator of photons, affecting an electromagnetic wave in the direction of the motion of an inertial system. This Doppler does not exist in the nature because the source of the electromagnetic wave is in rest respect to system. But, in each cycle, due to which the wave propagates inside the decelerator, the speed of the system combined with the speed of the wave, respect to vacuum, provokes the induced Doppler. The result is that an observer, inside an inertial system, fully insulated from the outside, may determine whether his system is in rest or in motion and measure its speed, in absolute terms.