Philip Grossweiler: Development of the Law of Universal Gravitation

The theory of gravitational attraction states that objects are attracted to each other with a force that is proportional to the product of their masses. The development of this law spans hundreds of years, from the time it was first theorized, to the time it was refined, perfected, and proved. 

In the 4th century B.C., Aristotle believed that there was no effect on motion without a cause; therefore the cause of downward motion of heavy objects was related to their nature and that objects moved to their “natural place”.  Again a Roman engineer later theorized a similar rule, that gravity is not dependent on weight, but instead on nature. Brahmagupta formed a contradicting argument which stated that earth was spherical and attracted objects downward. Many years passed before any more major developments or proposed theories to why objects accelerate downward or anywhere.

The next notable development in the theory of the law of universal gravitation was Galileo who hypothesized that all objects accelerated equally when falling; therefore directly contradicting the theory given by Aristotle. Isaac Newton used Robert Hooke’s idea that gravitational force depends on the inverse square of the distance. He published his findings in the Principia which can be summarized as if the force of gravity pulling down on an apple reaches to the highest tree, could that force possibly reach all the way to the moon, which would then mean that the orbit of the moon around the earth would be a consequence of the Gravitational force. Using the theory of universal gravitation, Newton was able to prove the astronomical observations made by Kepler. This theory further garnered success when it was used to hypothesize the existence and position of Neptune and its orbit based solely on the movements of Uranus. Newton was able to describe gravity and its effects, but was unable to define the reason behind it.

Albert Einstein was the first to be able to determine why gravity worked the way it did. In 1905, Albert Einstein developed his “Special Theory of Relativity”. The “Special Theory of Relativity” showed how Newton’s three laws of motion were correct until the velocity approached the speed of light. In 1915, Albert Einstein published his “General Theory of Relativity” which proposed a new theory about gravity and again showed how Newton’s theory was correct, this time about Gravitational Energy, until there is a presence of very strong gravitational fields. In the “General Theory of Relativity”, he describes gravity as being more than just a force, and that instead is a consequence of mass’s influence in space. Newton and Einstein both believed that space had three dimensions, but where Newton believed that space was not able to be influenced, Einstein proposed that mass can warp, bend, push, or pull space, and that gravity was the outcome of mass existing in space. Today, it is widely accepted that Einstein’s theories are correct, but are not necessary unless dealing with speeds approaching the speed of light or extremely large gravitational fields; in all other cases, Newton’s laws will suffice.