The Long Fight for Gravity
Galileo, Kepler and Newton challenged the ruling elite of Europe, fighting for science over religion.
Long after Man discovered gravity, he used his mastery to reach another world. Those who went found a dusty, barren landscape, scarred with craters and scattered with boulders. The Moon was not, as the Church once taught, a perfect sphere, smooth and uncorrupted by forces of nature. Instead, much like the Earth, it was shaped by universal laws of physics and chemistry.
Four hundred years ago such an idea would have been heretical. The Church believed in a perfect universe. Celestial bodies moved in a divine realm, and obeyed their own mysterious laws. At the centre of it all lay the Earth, placing the Church, the ultimate authority in the medieval world, at the heart of the universe.
At the time our vision of the night sky was limited to that which eyes alone could see. The Sun and Moon were clear enough, and formed symbols of the day and night. Planets, like stars, appeared as nothing more than distant points of light. The regular cycle of movements, of the rising and setting Sun, and the waxing and waning Moon, could be explained by picturing the Earth at the centre of the Universe.
Broadly this model was accepted, by priests and scholars alike, until about 1600, when an invention, the telescope, gave a new vision of the heavens. One of the first was built by Galileo Galilei, an Italian professor. What he saw was startling, and contradicted the official version of reality. He went public with his discoveries, speaking and writing for large crowds. Armed with nothing more than a primitive telescope, Galileo was daring to claim he could see the heavens better than God himself.
Naturally the Pope disagreed. The Church dismissed Galileo’s observations of Moons around Jupiter or spots on the Sun as errors caused by defective lenses. God’s work was perfect — and a mere mortal like Galileo had no standing to challenge him. In defiance, and increasingly in danger, the professor persisted. He could not ignore the evidence of his eyes: the Sun, he argued, and not the Earth, lay at the centre of creation.
Galileo certainly was persuasive. As a professor in Padua he attracted large crowds to his public lectures. He spoke, and wrote, for everyone, not just the scholars. He used Italian instead of Latin, making scientific knowledge available to the common people. He had an eye for politics and flattery too — after discovering four moons around Jupiter, he named them after the children of the Tuscan Grand Duke, ensuring his support against the Pope.
But Galileo also had a scepticism of authority. He was unafraid of questioning old masters, which, at the price of making enemies, helped him make scientific breakthroughs. His speeches often poured scorn on rivals — or on the authorities.
In the Middle Ages there was no bigger authority in Europe than the Church. Galileo didn’t hold back. When he saw errors in scripture, or teachings that contradicted his own eyes, he spoke up. He did not see this as a heretical act. He was a Catholic, and felt that the Church should embrace reality instead of denying it.
In letters to the Grand Duchess of Florence he argued that his scientific discoveries were not contradicted by the Bible. In any case, he wrote, the Bible guides faith and morals, and does not aim to provide scientific truth. Nothing was wrong with pointing out errors of thought.
Nevertheless, Galileo’s work was a major challenge to the established order. He subscribed to the Copernican model, the idea that the planets and stars moved around the Sun in perfect circular orbits. It was a controversial idea, as science and religion had long put the Earth at the centre of the universe. To deny that seemed to be denying humanity a special place in creation.
These arguments landed him in trouble with the Inquisition. They ordered him to abandon his work, and to cease teaching it. For a while Galileo obeyed, but in the 1620s, following the election of Pope Urban VIII, he started his studies once again. The new pope was, at first anyway, receptive to new ways of thinking and he asked Galileo to write a book explaining his ideas.
Though Galileo leapt at the opportunity, the project was a disaster. Galileo presented his ideas in the form of an argument between two characters. The side of the church was voiced by a character he named Simplicio — with all the connotations about intelligence such a name implies. The Pope was gravely insulted, feeling that Galileo was calling him stupid.
The result was even more trouble for the professor. The Inquisition again summoned him for questioning. He was placed on trial in Rome, and accused of heresy and disobedience. Faced with the overwhelming power of the Pope, Galileo felt forced to admit defeat. He renounced his theories, declared his belief that the Sun revolved around the Earth, and accepted the punishment of house arrest.
Although it was his arguments with religion that ultimately doomed Galileo, his ideas did have glaring scientific problems. The discovery of four moons had helped boost Copernicus’ ideas, but astronomers ran into problems trying to apply them to the rest of the Solar System.
To be useful, an astronomical model should predict the future movements of the planets. But every time astronomers tried using the Sun-centered model to do so, they failed. Something must be wrong, but no-one, Galileo included, knew what it could be. For the Church the failures were welcome news; they gleefully used them to pound Galileo and discredit his ideas.
Fortunately Galileo was not alone in challenging the orthodoxy. A few years earlier Johannes Kepler, a German astronomer, had made heroic efforts to calculate the orbit of Mars. His starting point was Copernicus’ model of circular orbits. But, like everyone else, he had found no way to predict the future movements of the red planet.
Unlike everyone else, however, Kepler was obsessed with geometry. This obsession had already led him to write one book expounding a complicated geometrical structure for the Solar System. Though completely wrong, his experience inspired him to seek geometrical answers for other astronomical problems. If the orbit of Mars didn’t fit a circle, could another shape work?
He tried many different shapes, settling for a while on an egg shaped orbit. This too consumed many months with little result. In the end he compromised, making the egg a bit more like a circle. This shape — the ellipse — turned out to be the answer to the riddle. Kepler calculated the movements of Mars perfectly, and then published his work in a book that has since become a classic of astronomy, the Astronomia Nova.
Kepler was triumphant, but if he expected the world to recognise his genius he would have been sorely disappointed. The great Galileo, perhaps distracted with his own problems, ignored his ideas. Kepler’s own university professors criticised him for mixing physics and astronomy, two subjects that until then had remained distinct. For most other astronomers, steeped in ancient ideas of heavenly perfection, abandoning circular orbits was a step too far.
Despite the lack of acclaim, Kepler’s ideas flowed into the turbulent sea of knowledge swirling across Europe. Little by little astronomers gradually abandoned the old ways and came to accept ideas that were once deemed heretical. The predictive power of Kepler’s theory of ellipses eventually became too strong to ignore.
As the idea that the Sun lay at the centre of the Solar System gained force, a key shift occurred in scientific thought. Previously the motion of the planets had been ascribed to the powers of the Gods. Ptolemy, the Greek philosopher, had believed that the planets existed in concentric shells. As the outermost shell rotated, propelled by heaven itself, it rubbed against the inner shells, pushing them into motion as well. But as these ideas were abandoned, a new question arose. What made the planets move?
For most astronomers the answer was still God. But maybe the hand of God was not revealed so easily as people had believed — perhaps his influence was more subtly manifested. For Kepler the representation of God’s power was obvious: if God had positioned the Sun at the centre of the Universe, surely that must also be centre of his power.
Equipped with divine power, the Sun could move the planets. But there was another problem: the further a planet lay from the Sun, the slower it moved. Why? For Kepler there could only be one explanation: somehow God’s power waned with distance. Planets close to the Sun could be easily moved, but those further away took more effort.
Kepler put these ideas in terms of physics. Scholars already knew of a force that could act without contact, magnetism. He concluded, therefore, that the Sun’s influence took on a form of magnetic force.
The suggestion horrified traditionalists. The idea of physics, the explanation of the natural world through logic and mathematics, was strictly limited to the Earth. Sure the Earth may have magnetic forces, it may change and it may be troubled and imperfect. But extending this idea to the planets, to the Sun? It was sacrilegious. It was revolutionary.
That revolution would have to wait. Kepler’s work held deep implications, but the world was not yet ready to realise them. In the decades that followed his death, Central Europe was swept by religious wars. Millions died, often horribly, and the forces of war dramatically reshaped the map of Europe. England, too, suffered war. The king, Charles I, was captured and beheaded at the climax of a brutal civil war. Sickness and plague periodically raged across the island, killing vast swathes of the population.
Amid all this upheaval, it’s a surprise that physics progressed at all. But crucial foundations for the future of science were being laid down, especially in England. Freed of the influence of the Pope after Henry VIII’s reformation, the political elite of England keenly supported science.
In this environment science flourished, despite the ongoing chaos and sickness. Institutions sprung up and universities embraced radical ideas. But none were quite as radical or unorthodox as Isaac Newton, a student at Trinity College Cambridge.
There’s an often-told story that, confined to his country home by plague, Newton saw an apple falling from a tree. In a moment of genius, the story goes, the apple inspired him to discover gravity. The rest is history.
Or perhaps not. True, plague did ravage London in 1666, and force Newton to seek shelter in the countryside. And true, Newton did often tell a similar story as an old man. But, while Newton’s thoughts may often have turned to gravity at the time, he didn’t dedicate much time to the topic until much later.
Most of his major breakthroughs in the topic of gravity came fifteen years later, shortly after a magnificent comet sparked a new interest in astronomy. That comet was extraordinarily bright, visible even during the day. For a month astronomers tracked the comet moving closer and closer to the Sun, until, at the end of November, it disappeared.
Two weeks later another bright comet appeared. Unlike the first one, this comet moved steadily away from the Sun, before gradually fading away. Many took the two comets as a dreadful omen. Fears of death, war and famine swept the world. Religious leaders from China to the Americas called for days of fasting and penance.
Astronomers were split — were the two comets the same? Or did two comets happen, by coincidence, to appear together? Newton at first took the latter view. He, like Kepler, thought that comets travelled in straight lines through the Solar System. For a comet to turn around and head back in the direction it came from seemed impossible.
But the question stayed in Newton’s mind. The more he worked on it, and the more he examined Kepler’s work and his own thoughts on gravity, the more he started to believe the two were the same comet. Instead of travelling straight through the Solar System, perhaps the power of the Sun had curved the comet’s orbit. And, if the Sun could curve the orbit of a comet, could that explain the orbits of the planets?
Newton calculated the nature of the force that would be needed to explain the orbit of the comet. Then he did the same for the orbit of the planets — and found that the numbers matched. So far so good. He could explain the movements of planets and comets with this force, but still, the true scope of it eluded him.
His next step was to apply the same force to the Earth and Moon. Again it worked; the influence of the Earth could keep the Moon in orbit. His final move, though, would change everything. When Newton calculated how strong this force would be at the Earth’s surface, he found it matched almost exactly the known strength of gravity.
It was a breakthrough, and an astonishing revolution in physics. In a stroke Newton showed that the force that held the planets in place was the same as the one that made apples fall from trees. Gravity didn’t just apply to mortals on Earth; it extended across the Universe, and shaped the night sky.
Newton spent two years writing down his findings and theories. The end result — three books together titled Philosphiae Naturalis Principia Mathematica — was a masterpiece. In it Newton revealed not just his theory of gravity, but introduced calculus and his three laws of motion (including the famous and often misused line: ‘every action has an equal and opposite reaction’).
Unlike Galileo centuries earlier, Newton published his work in a society that welcomed the pursuit of science and was relatively free of religious restrictions. The Royal Society, with the support of the King, distributed his work widely and offered a venue for discussion. The pope had little say. England had rejected his authority under Henry VIII, and Newton was free to blast him as the antichrist.
The Principia, as it became known, formed the foundation of physics for centuries to come. But it was not — as some later claimed — inspired by a single spark of genius. Newton’s work was the culmination of a long struggle against the old order in Europe. As the man himself freely admitted, “if I have seen further, it is by standing on the shoulders of Giants”.
The scientific revolution posed a huge threat to the Church, and to the powers that dominated Europe five hundred years ago. The great awakening, led by giants like Galileo, resulted in years of warfare, death and famine. But the struggle was vital for forming the world that emerged — and laid the scene for the next great leap for humanity, the Industrial Revolution.
Galileo himself eventually found acknowledgement, even from the Church that sought to destroy him. In 1737, almost a century after his death, his body was removed from the small side chamber of the Basilica where it had been placed, and reburied in the great hall of the church. A monument, which still stands, was constructed there in his honour, placing him among the greatest heroes of Italy.
The Church was slow to acknowledge the truths revealed by science. It took two hundred years before they accepted that the Earth revolved around the Sun — and another century and a half before Pope John Paul II formally cleared Galileo of heresy in 1992.
Nonetheless, the ideas of Newton and Kepler underpinned science for more than two hundred years. Indeed, as the twentieth century dawned Victorian scholars still treated Newton as the ultimate foundation of all of physics. Only the revolutions of quantum mechanics and relativity could dislodge him from that position.
But perhaps the greatest tribute to the struggles of Newton, Kepler and Galileo came in 1968, when astronaut William Anders, heading back to Earth after circling the Moon, was asked who was piloting his spacecraft. The answer, conveyed by radio over hundreds of thousands of miles, paid homage to the long struggle to understand gravity: Newton; Isaac Newton.