Over the years since Einstein's publication of general relativity, a number of exacting tests have been carried out to test observational predictions of the theory. Some of the tests are really only tests of the principle of equivalence, while others are true tests of the full theory.

A direct test of the principle of equivalence involves the measurement of the attraction of two different objects by some third body. A class of such experiments are called Eotvos experiments, after the person who devised the original experiment around the turn of the 20th century. The most accurate recent versions of the experiment were carried out by a group at Princeton University in the 1960s and a group at Moscow University in the 1970s. Their findings indicate that the principle of equivalence is accurate to one part in 10^11..

The equivalence principle we have discussed applies strictly to objects that are so small that we can ignore the differences from one side to the other in the gravitational field they feel. We can treat them as point objects. However, there is a stronger form of the principle of equivalence that says that it also applies to objects with substantial gravitational binding energy, such as planets or stars. This has been tested by closely measuring the motion of the Moon (Fig. 8.6). A series of mirrors have been left on the Moon by the Apollo astronauts. Laser signals can be sent from Earth, bounced off these small mirrors, and then detected as very weak return signals. By timing the round trip we can measure the distance to the Moon very accurately, to within a few centimeters. These studies have indicated that the Earth and Moon fall towards the Sun with the same acceleration to within seven parts in 10^12.

The 2.7 m telescope of the McDonald Observatory, Texas, has been used to fire a laser beam at a reflector on the Moon, then they detect the weak return. By timing the round trip, the distance to the Moon is very accurately determined.

 • Orbiting bodies

 • Bending electromagnetic radiation

 • Gravitational redshift

 • Gravitational radiation

 • Competing theories