Reality Is Not What It Seems: The Journey to Quantum Gravity

by Carlo Rovelli

But it is not only space that curves: time does too. Einstein predicts that time on Earth passes more quickly at higher altitude, and more slowly at lower altitude. This is measured, and also proves to be the case. Today we have extremely precise clocks, in many laboratories, and it is possible to measure this strange effect even for a difference in altitude of just a few centimeters. Place a watch on the floor and another on a table: the one on the floor registers less passing of time than the one on the table. Why? Because time is not universal and fixed; it is something that expands and shrinks, according to the vicinity of masses: Earth, like all masses, distorts spacetime, slowing down time in its vicinity. Only slightly—but two twins who have lived respectively at sea level and in the mountains will find that, when they meet up again, one will have aged more than the other (figure 3.8).