It’s summertime, the season of lawn sports. As anyone following the World Cup, or Wimbledon, or the PGA Tour can tell you, there’s magic in the trajectory of a humble sphere, be it kicked, slammed, or driven.

Sport gives us a vision of world where human agency goes head to head — or toe to toe — with gravity, in some cases seeming to defy it entirely.

It’s easy to forget that the gravity that obtains at Centre Court, calling Rafael Nadal’s impossibly curving forehand to the manicured grass, is the same force at work in our summer gardens, making each shovelful of mulch seem heavier than the last.

In fact, gravity is one of the great unsolved mysteries of our existence. For as long as human beings have looked up at the night sky, we’ve speculated on the nature of the forces that organize the cosmos. The ancient Greeks, working from the observation that the stars and planets seemed to circle the heavens, concluded that the earth was the center of the universe. This narcissistic worldview dovetailed nicely with the religious doctrine of mankind as the highest creation of the divine: of course the planets revolved around us; we were the most important creatures in the firmament!

But as astronomical observations became more precise, the model of celestial spheres humming along in divinely guided circular orbits was at odds with what people were seeing through their telescopes. It was all well and good to think of planetary orbits as perfect circles, but they were actually messier than that, more like ellipses.

Sir Isaac Newton’s Principia, published in 1687, formulated the laws of motion and gravitation that were the final nail in the coffin of the Ptolemaic worldview. The sun was firmly established as the center of our little corner of the cosmos. We now lived in a “solar system.” Using Newton’s equations, the motions of the planets could be predicted with astonishing accuracy.

And Newton’s insights didn’t begin and end with the heavens. According to his theories, the force that governed the movement of the planets around the sun was the same one that caused his famous apple to plummet earthward. For the first time in history, there were universal laws of motion, equations that were equally useful in describing the orbit of Jupiter and the arc of a cannonball.

Newton’s work inaugurated a feverish era of discovery and invention that led directly to the Industrial Revolution. Everyone agreed that his equations worked, and worked brilliantly. But there was still an unsolved mystery: how, exactly, did gravity work?

Newton’s equations depended on an invisible force exerted by one body on another. For instance, the sun exerted a gravitation “pull” on earth, which was why earth circled the sun, just as earth exerted a gravitational “pull” on a brick or a feather. Bigger things exerted a bigger pull, which made intuitive sense. For more than two hundred years, scientists satisfied themselves that there was a thing called “gravity” that worked by…well, no one really knew. But so what? Newton’s equations perfectly described our reality.

That is, until they didn’t. One of the first problems that frustrated strict Newtonians was the very slightly eccentric orbit of Mercury. By the late 19th century, astronomy had advanced to the point where extremely accurate measurements of the planets were possible. As it turned out, Mercury didn’t behave exactly the way Newton’s equations dictated it should. Various explanations were advanced for the discrepancy, but it wasn’t until 1916, when Albert Einstein unveiled his General Theory of Relativity, that the scientific community realized how limiting Newton’s idea of gravity truly was.

Einstein had come to the conclusion in 1905 that the speed of light was a fundamental, universal constant. This was one of the main tenets of his Special Theory of Relativity. But it contradicted Newton’s idea that the force of gravity acted instantaneously over infinite distances. For Newton’s equations to work at the interplanetary scale, the sun’s gravitational “pull” would have to be felt on earth at the instant it was exerted.

So Einstein proposed a thought experiment: what if one day the sun were suddenly to vanish? According to Newton, the instant the sun disappeared, its gravitational pull would vanish with it, and the earth would fly off its orbit and into deep space. But to Einstein, that would be impossible! It takes light almost eight minutes to travel from the sun to the earth. And if the speed of light is truly a universal limit, that would mean that the sun’s gravitational pull had somehow traveled faster than light.

Working from this insight, Einstein formulated an entirely new view of gravity. Gravity wasn’t the invisible “pull” that one body exerted on another. Instead, it was a distortion in the fabric of what he called “space-time.”

To visualize what he meant, imagine a bowling ball at the center of a trampoline. In this example, the bowling ball is the sun, and the rubbery surface of the trampoline represents the fabric of time and space. If you were to take a marble and roll it towards the center of the trampoline, the marble would soon start to describe circles around the bowling ball — not because the bowling ball was exerting a “pull” on it, but instead because the heavy ball, by virtue of its mass, had distorted the fabric. The marble would simply be rolling along the path of least resistance. Just so, the earth’s orbit is the path of least resistance through space that has been heavily warped by the sun.

If the sun were suddenly deleted, we wouldn’t know about it until we felt the tumult of a monstrous gravity wave — about eight minutes later.

Einstein’s revolutionary idea was that planets and stars — in fact, any astronomical body with mass, even black holes, which his theory predicted — exert influence on each other by changing the shape of the space that surrounds them. This in turn, alters the path of everything that impinges on that warped space — even light.

When English astronomers confirmed, during a solar eclipse in 1919, that light indeed bent around stars in just the way that Einstein’s theory predicted, the world sat up and took notice.

We’re still living in the era of General Relativity, although scientists have been working hard for nearly a hundred years to find a grand theory that will once again unify the subatomic realm with the cosmological, as Newton’s Principia did for two happy centuries.

But gravity is still poorly understood. Even though it seems to behave in virtually all of the ways Einstein predicted, we still can’t observe how it propagates, the way we can track photons or electrons. In practical terms, the manipulation of gravity is still a pipe dream.

In other words, don’t look for anti-gravity boots any time soon. For the foreseeable future, to glimpse a world where space and time can be bent with a flick of a foot, we’ll just have to content ourselves with watching Ronaldo.