The Principle of Equivalence also leads to the remarkable theory that a clock near a very massive object will run more slowly than it would out free away from everything. It means that if we put a clock in the basement and read it from the roof, it will appear to run slow. On the Earth the effect is quite small (but again, it is necessary to know about it to make GPS work). We can measure the effect on the Sun, and on very compact stars too. It is greatest near black holes - objects so massive that light cannot escape at all.