Things fall.
This is such a basic part of life that our very bodies are built to function in a world where things fall. When astronauts go up into space, they have to exercise hard to combat the effects of a lack of gravity, and when they come back to Earth, their heads are swollen and their legs are thin for a few days afterwards.[1. I am not making this up.]
We’ve been dealing with the fact that things fall since the moment the first homo sapien came up with a word (or particular tone of grunt, maybe) to describe falling. Longer than that – we’ve been dealing with it since the time when our distant evolutionary ancestors began to be able to develop a dim distinction between up and down.
We knew that things fell long before we knew that the stuff around us was made up of little bits called atoms, before we knew what stars were, even before we knew how to rub two sticks together to make fire.
The tendency of things to fall is discussed in some of the oldest scientific writings we have. It is fundamental to the Aristotelian model of the universe, for example, one of the longest- and most widely-held belief systems about science that we know of.
My point is, with all of this time to get used to it, and with all of the energy people have spent throughout the years thinking it through, you might think that falling would be the most well-understood part of all of physics.
It is not.
True, in many ways we’ve mastered gravity: millions of airplanes fly around the globe each day, thousands of human-made satellites hover effortlessly just beyond our atmosphere, and we’ve even sent people to the moon and back. But that doesn’t mean that we truly understand what gravity is, and why it happens. Of the four fundamental forces, gravity still stands alone. It looks just a little bit different from the rest. It doesn’t fit into the same models the way the others do. In short, it doesn’t play nice.
Without trying to provide an exhaustive history of the theory of gravity, a quick review will show just how much of a challenge this phenomenon has presented us with.
When Aristotle considered the problem of things falling more than two thousand years ago, he came up with a basic system to explain it. Everything was made up of only a few elements, he decided, and each element had its natural place in the world depending on an inner quality he called gravitas. The earth element tends to head downwards toward the center of the universe, while fire, the lightest element, tends to float up to the top of everything. This is actually one of the least helpful scientific ideas in history, but it gets mentioned often because it became very popular, influencing Western scientific thought for centuries.
By the time Galileo got started with revolutionizing the practice of science nineteen centuries later (in the 1600s), there was a scattering of facts already known about falling objects; he’s credited with phrasing them more formally and with backing them up with experimental evidence. One such fact is that everything falls at the same rate, as long as air resistance is negligible. This surprising notion contradicts both everyday experience and centuries of Aristotelian teaching, and I’ll discuss it further in this chapter.
Then, a few decades after Galileo, Isaac Newton finally found something useful to say about falling. As I’ve already mentioned, he was the one who is generally credited with discovering the law of gravity and with using it to successfully explain the motion of stars and planets, as well as more earthly objects. His formula is so accurate that it is still used often today in astronomical calculations. But although he knew how gravity worked, he had no idea why – for the next few hundred years, scientists tried to figure out the mechanism which actually caused gravity to happen. But they had litttle success.
It wasn’t until three hundred years later, in 1915, that Einstein announced his theory of General Relativity to the world, finally answering Newton’s troubling why and producing a theory with profound and far-reaching applications.
Sure, this was a hundred years ago, so it may seem like we’ve had this all figured out for a long time. But remember, people have known about falling objects for all of human history. The motions of most of the stars and planets had been charted in stupendous detail for centuries. By 1915, scientists had conquered electricity, effortlessly pumping it across huge distances to do all kinds of useful things. Darwin had published his theory of evolution some 56 years earlier. Hell, around this same time, physicists were already starting to investigate the ideas leading to quantum mechanics, which is considered a relative newcomer to the scientific world.
So our current understanding of gravity is embarrassingly recent. Even today, gravity continues to hold onto some of its secrets. We can’t seem to fit it into the larger picture of the world we’ve been developing with the rest of physics. Why did it take us so long to figure out one of the most obvious features of the world around us? What do we still have left to figure out? In this chapter, we’ll investigate this most beguiling and intriguing of all of the four forces.
Next: 3.2 – Gravity
Previous: 2.9 – Circular Motion and Orbits