Letter to the Editor: About gravity …

I will contribute some remarks about gravity, which has been the subject of a couple of recent letters. As a physical scientist, I believe that if we can construct a mathematical model of an observed natural phenomenon, we can say that we “understand” it. There are two mathematical models of gravity. The first was developed by Isaac Newton in the seventeenth century. It is one very simple equation, which you would encounter in an elementary physics class. It works extremely well in predicting almost all observed behavior of massive objects. For example, it allows us to calculate the trajectories of spacecraft with very high precision, so we can send them to the farthest reaches of the solar system.

The other model of gravity is Einstein’s General Relativity. It is very complicated mathematically. This theory has proved to be completely accurate in predicting the results of every experimental test. Its first success was precisely accounting for a subtle effect in the behavior of the orbit of the planet Mercury. Planetary orbits are ellipses, which exhibit what is called precession: the major axis of the ellipse rotates slowly about the center of the ellipse. This was known by the nineteenth century, and the behavior of Mercury’s orbit was found to deviate from the prediction of Newtonian mechanics. The result calculated from General Relativity matched the observed behavior.

The other early test of General Relativity was in predicting the correct value for the bending of light as it passes near the sun. This had also been predicted by Newtonian mechanics, but again that value was incorrect. As in the previous case, Einstein’s theory predicted the correct result. Measurements of this effect were made in a famous “experiment” during a total solar eclipse in 1919, by Sir Arthur Eddington and collaborators, by simultaneously observing starlight passing near the occluded solar disc from two locations, one in Brazil and the other on the west coast of Africa. The results made headlines world-wide.

I will end with two amusing quotes. When Einstein was asked what his reaction would have been if Eddington’s results had not matched his theory, he said, “Then I would feel sorry for the dear Lord; the theory is correct anyway.” The second may be apocryphal. When Eddington was asked it were true that only three people understood general relativity, he reportedly responded, “Who’s the third.”

Boyd Earl
Quincy