Strange attractions, or why Newton’s universe made no sense. This is gonna get historical.
Isaac Newton mathematized gravity in his 1687 Philosophae Naturalis Principia Mathematica. He wasn’t the first to think of the natural laws he described, but he brought them all together and professed that evidence—just evidence—was enough to support gravity’s effect on the world, whether the force was understood or not. Through his mathematical concept of “attraction” he could describe everything from the orbit of the planets to the tides to falling objects. To understand just how unprecedented that was, we have to look back pretty far, to the strange earlier views of how the universe worked. We’ll see why it would take awhile for his theories to catch on and why he felt the need to go very in-depth on fluid dynamics despite the fact that the Principia was all about gravity.
In antiquity, the explanation for a physical phenomenon could be as simple as, “the Moon loves the Earth, so it stays close to it.” Something like the moon didn’t have a natural effect on everyday life, so there wasn’t a good reason to put too much thought into it.
The concept of basic elements that make up the world was codified by Aristotle in the fourth century BCE and held sway in Europe in one form or another for hundreds of years. There were five elements—earth, air, water, fire, and the aether that filled the sky (everything past the moon). Each element behaved a specific way, or had specific affinities: earth wanted to be under everything else, near more earth, water wanted to be in a sphere over earth but under air, air wanted to be above earth and water, and fire wanted to be above everything—just watch it strive to reach the sky! So objects fell because the earth in them wanted to be near the earth. Everything in the universe was formed somehow of these elements, and any behavior could be explained (if not predicted) by their interactions. Take a log: it floats on water, so it must have air in it, but it falls to the ground, so it must have earth. It uses water to grow, and if you burn it you can see the fire element escaping upwards. The elemental take on the world was modified, clarified, and contradicted over the years, but it remained the most general, inclusive “physics.”
[Halfway through the history!]
And then came Descartes. You’ve probably heard that “an object in motion tends to stay in motion” and that “every action has an equal and opposite reaction.” Those were from his 1664 text The World, which ascribed rational rules to the universe—a heliocentric one, centered around the sun as Galileo had recently professed (and The World‘s publishing was actually delayed to avoid a dicey political situation). You’re less likely to have heard of a “plenum,” though—his assertion that the universe was entirely full of matter, and that an empty vacuum was impossible in nature. This view was shared by Aristotle, actually, but Descartes was the one to work out its consequences. The reason planets orbited was because all the matter in the sky circled: great vortices, since there could be no empty space. The pressure of the circling skies pushed down any object thrown into the air, and the pressure of the moon drove the tides. Descartes didn’t quantify the world, but he imbued it with a certain regularity. His universe would continue on forever—after all, matter was always pushing, infinitely circling.
Newton’s Principia was published—like I said earlier—over 20 years later in 1687. More laws of nature had been discovered, and a vacuum had been created, but overall people largely still followed the deductive route of Descartes—working from an elegant theory to understand how the universe worked, instead of accepting evidence and trying to form ideas around it.
Newton professed that some sort of attraction pulled pieces of matter together—“I don’t know how, but look at the math!” The mentality was alien to Europe, which was used to the theory coming before the evidence. A mysterious attraction, that didn’t even have a rational cause—it sounded a lot like the “love between the Earth and the Moon” of ancient times. It felt like a step backwards.
And that’s why Newton had to write about fluid dynamics. He wanted to prove, once and for all, that Descartes’ system, full of vortices of matter, couldn’t possibly cause the effects it was said to. The system would run down, or not be able to move at all. Without another system to fall back on beyond the ancients, Newton’s mathematical universe would have to be accepted.
England accepted Newton’s ideas and celebrated him, but it took awhile for things to catch on elsewhere. Voltaire, coming to England from France, greatly admired their veneration of science—and Newton—but describes the shock of the new system: “A Frenchman who arrives in London, will find philosophy, like everything else, very much changed there.
“He had left the world a plenum[—a space completely full of matter], and he now finds it a vacuum. At Paris the universe is seen composed of vortices of subtile matter; but nothing like it is seen in London. In France, it is the pressure of the moon that causes the tides; but in England it is the sea that gravitates towards the moon; so that when you think that the moon should make it flood with us, those gentlemen fancy it should be ebb.”
Just imagine—by crossing a body of water you could go from a society that accepted theory, elegance, and rigor above all to one that was beginning to let evidence tell its own story, even without an explanation. You could go from a world entirely full of matter—full of certainty and direction—to one that was frighteningly empty, but ripe with possibility.
See? Cool, right?
P.S. Sorry this was a week late. It was MOSTLY written last week, but there were some random sources I couldn’t find.