In an earlier post we saw that Newton developed three significant theories, any one of which would probably have earned him a Nobel Prize but for two small details: The Nobel Prize didn’t exist when Newton was alive and the rules of the Nobel Prize prevent posthumous awards. In any event, his theory of gravity governed the cosmos for over three centuries. The story of Newton developing his theory while sitting under an apple tree and having an apple fall and hit him on the head is probably apocryphal, but there is little doubt that he was inspired by watching apples fall.
Newton’s theory of gravity simply says that any two bodies attract each other with a force that is proportional to their two masses and inversely proportional to the square of the distance between them. As millions of high school physics students know, this formula is expressed as
F = (G x m1 x m2)/r2
Here F represents the gravitational force, G is the gravitational constant which is necessary to make the force of gravity somewhat congruent with other natural forces such as magnetism, m represents the masses of the two objects in question and r is the distance between the two objects. This formula explained why planets remain in orbit around the sun, why apples drop from trees and even why astronauts feel weightlessness in space. It coalesced all the concepts of Kepler, Copernicus and Galileo about the solar system. It seemed complete, though even Newton himself felt it wasn’t the final answer. He said “I seem to myself to have been a little boy playing on the seashore and diverting myself now and then in finding a smoother pebble or prettier shell than ordinary, whilst the great ocean of truth lay undiscovered before me.”
The problem with Newton’s theory is it doesn’t explain how gravity works. We say that it is an attraction between two bodies, a force that pulls them together, but how is that force transmitted through space? For example, consider a golfer addressing a golf ball on the tee. He swings the club, which strikes the ball and sends it into a graceful arc, up, up, until gravity, that relentless force that cannot be defied, pulls it back to earth. At least that’s how it works in my mind when I swing a club. Reality is uglier. If we consider the source of the forces acting on the golf ball, it initially moves due to the impact of the club head. That impact is the product of the momentum of the club head, which is caused by the synchronous contraction of the golfer’s muscles, which in turn is caused by complex chemical reactions that contract and relax the muscles. Those chemical reactions are caused by the conversion of food the golfer ingests. All of this is complex but completely understood by science.
On the other hand, what is this mysterious force of gravity? All we know about it is that it can be measured by Newton’s equation, but that doesn’t tell us how it arises. We generally pass that off as a natural property of mass. But why should mass have this property? And, more importantly, how is it transmitted through space?
Einstein’s special theory of relativity led him to the conclusion that gravity is caused by a deformation of the very fabric of spacetime. This deformation causes bodies to follow the curvature of spacetime. This notion sounds crazy and indeed Einstein himself doubted himself. Yet he could not bring himself to abandon the theory. He spent eight years, from 1907 to 1915, trying to refine this vague notion of curvature of spacetime into a formal theory explainable by a mathematical formulation.
In the next post we’ll try to describe this whole curvature of spacetime and how it creates gravity concept.