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The History of Gravity

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By Andrew Zimmerman Jones

One of the most pervasive behaviors that we experience, it's no wonder that even the earliest scientists tried to understand why objects fall toward the ground. The Greek philosopher Aristotle gave one of the earliest and most comprehensive attempts at a scientific explanation of this behavior by putting forth the idea that objects moved toward their "natural place."

This natural place for the element of Earth was in the center of the Earth (which was, of course, the center of the universe in Aristotle's geocentric model of the universe). Surrounding the Earth was a concentric sphere that was the natural realm of water, surrounded by the natural realm of air, and then the natural realm of fire above that. Thus, Earth sinks in water, water sinks in the air, and flames rise above air. Everything gravitates toward its natural place in Aristotle's model, and it comes across as fairly consistent with our intuitive understanding and basic observations about how the world works.

Aristotle further believed that objects fall at a speed that is proportional to their weight. In other words, if you took a wooden object and a metal object of the same size and dropped them both, the heavier metal object would fall at a proportionally faster speed.

Galileo and Motion

Aristotle's philosophy about motion toward a substance's natural place held sway for about 2,000 years, until the time of Galileo Galilei. Galileo conducted experiments rolling objects of different weights down inclined planes (not dropping them off the Tower of Pisa, despite the popular apocryphal stories to this effect), and found that they fell with the same acceleration rate regardless of their weight.

In addition to the empirical evidence, Galileo also constructed a theoretical thought experiment to support this conclusion. Here is how the modern philosopher describes Galileo's approach in his 2013 book Intuition Pumps and Other Tools for Thinking:

"Some thought experiments are analyzable as rigorous arguments, often of the form reductio ad absurdum, in which one takes one's opponents' premises and derives a formal contradiction (an absurd result), showing that they can't all be right. One of my favorites is the proof attributed to Galileo that heavy things don't fall faster than lighter things (when friction is negligible). If they did, he argued, then since heavy stone A would fall faster than light stone B, if we tied B to A, stone B would act as a drag, slowing A down. But A tied to B is heavier than A alone, so the two together should also fall faster than A by itself. We have concluded that tying B to A would make something that fell both faster and slower than A by itself, which is a contradiction."

Newton Introduces Gravity

The major contribution developed by Sir Isaac Newton was to recognize that this falling motion observed on Earth was the same behavior of motion that the Moon and other objects experience, which holds them in place within relation to each other. (This insight from Newton was built upon the work of Galileo, but also by embracing the heliocentric model and Copernican principle, which had been developed by Nicholas Copernicus prior to Galileo's work.)

Newton's development of the law of universal gravitation, more often called the law of gravity, brought these two concepts together in the form of a mathematical formula that seemed to apply to determine the force of attraction between any two objects with mass. Together with Newton's laws of motion, it created a formal system of gravity and motion that would guide scientific understanding unchallenged for over two centuries.

Einstein Redefines Gravity

The next major step in our understanding of gravity comes from Albert Einstein, in the form of his general theory of relativity, which describes the relationship between matter and motion through the basic explanation that objects with mass actually bend the very fabric of space and time (collectively called spacetime). This changes the path of objects in a way that is in accord with our understanding of gravity. Therefore, the current understanding of gravity is that it is a result of objects following the shortest path through spacetime, modified by the warping of nearby massive objects. In the majority of cases that we run into, this is in complete agreement with Newton's classical law of gravity. There are some cases which require the more refined understanding of general relativity to fit the data to the required level of precision.

The Search for Quantum Gravity

However, there are some cases where not even general relativity can quite give us meaningful results. Specifically, there are cases where general relativity is incompatible with the understanding of quantum physics.

One of the best known of these examples is along the boundary of a black hole, where the smooth fabric of spacetime is incompatible with the granularity of energy required by quantum physics. This was theoretically resolved by the physicist Stephen Hawking, in an explanation that predicted black holes radiate energy in the form of Hawking radiation.

What is needed, however, is a comprehensive theory of gravity that can fully incorporate quantum physics. Such a theory of quantum gravity would be needed in order to resolve these questions. Physicists have many candidates for such a theory, the most popular of which is string theory, but none which yield sufficient experimental evidence (or even sufficient experimental predictions) to be verified and broadly accepted as a correct description of physical reality.

Gravity-Related Mysteries

In addition to the need for a quantum theory of gravity, there are two experimentally-driven mysteries related to gravity that still need to be resolved. Scientists have found that for our current understanding of gravity to apply to the universe, there must be an unseen attractive force (called dark matter) that helps hold galaxies together and an unseen repulsive force (called dark energy) that pushes distant galaxies apart at faster rates.