Forze di marea, navicelle spaziali e principio di equivalenza

As it is written in many books, within a vehicle in free fall or in a spacecraft orbiting the Earth, everything goes as if there were no gravity, a situation commonly described as “weightlessness.” In the handbooks on relativity, a free-falling vehicle is considered as an example and a practical realization of an inertial reference system. However, this is not exactly true, because the Earth’s gravitational field is not uniform and forces are generated, called tidal forces, which affect various effects and complicated movements. In this article I study in detail these forces and effects in the case of a spacecraft orbiting the Earth. I show that some of these effects do not cancel out when the considered dimensions tend to zero, so a system in free fall cannot be considered exactly an inertial system, not even at a local level, and the principle of equivalence according to which the effects of a gravitational field are equivalent to those of a system in accelerated motion is not valid. The topic is also a significant example of a study of relative motions and of dynamics in a non-inertial reference frame.