For more than two centuries, scientists and students alike saw gravity through the eyes of Newton. His law of universal gravitation worked remarkably well, predicting the motion of the planets with extreme accuracy and even assisting in the discovery of a new planet, Neptune.However, by the beginning of the 1900s, scientists had discovered a small flaw in Newton’s theory. The flaw had to do with the orbit of Mercury, which was moving around the Sun in a way Newton had not predicted. Stepping in to help was the German born physicist Albert Einstein (1879 AD-1955 AD) who, in 1915 corrected Newton’s errors in a theory he called general relativity. But this new theory was not simply a small revision of Newton’s work. It completely rewrote the way we look at gravity and even changed our understanding of the very structure of the universe.
Einstein’s theory of general relativity is based on the idea that we live in a four dimensional universe called space-time. It tells us that all matter shapes space-time by curving it, so to speak, and that these curves determine how matter moves through space-time. In other words, the orbits of the planets are not caused by a direct attraction to the Sun, they simply follow straight lines in curved space.
This new way of looking at gravity was incredible to say the least. Yet even more astounding, were some of the strange things general relativity implied. Black holes, gravity waves, and even the slowing down of time inside a gravitational field were all predictions of this remarkable theory.
Yet the prediction that first gave general relativity fame and credibility was the idea that a very large object, like a star, should be able to bend space-time so much that it could change the path of a light beam passing near it. This odd phenomena of the bending of light was observed just a few years later in 1919. Overnight, Einstein became a celebrity and soon general relativity had replaced Newton’s law of universal gravitation as the official explanation of gravity. And that is where we stand today.As for Newton’s law, it is still in wide use. We know it is not 100% accurate yet it is a very, very close approximation. And it has the distinct advantage of being much easier to understand than general relativity so it continues to be the theory that is taught to grade school students today.
For more on the theory of general relativity see What is Gravity? Pages 5, 6 and 7. For more on gravity's effects on time see Small Scale Effects Page 11. For more on how gravity bends light, black holes holes and gravity waves see Large Scale Effects Pages 8, 9 and 10.
