Sunday, September 4, 2016
Gravity: From Aristotle to Einstein, by Erik Gregersen
"This view of why objects fall reigned until the scientific revolution that began in the Renaissance. “Standing on the shoulders of giants” like Kepler and Galileo, Isaac Newton realized that the apple falling to the ground and the Moon orbiting Earth were subject to the same gravitational force. The force was proportional to the mass of the two bodies attracting each other and inversely proportional to the square of the distance between them. (That is, when two bodies are twice as far apart as they were before, the gravitational attraction is 1/[2×2], or ¼ as strong.) The force operated between everything in the universe and explained the motions of the Moon and the planets very well.
Well, almost. Newtonian gravity had its triumphs. It was used to predict the location of the then unknown planet Neptune. However, for Mercury, the closest planet to the Sun, Newton’s law was not quite as accurate in predicting the location of the planet’s perihelion (the point in its orbit where it is the closest to the Sun) as it was for the others. This point seemed to move about the Sun, and the motion vexed astronomers until Einstein introduced his theory of general relativity in 1915, in which gravity is not a force reaching out across the universe but is a bending of space-time around a massive object. The orbits of the planets and the apples falling to the ground follow the shape of space-time."