Galileo did not merely overturn Aristotle’s geocentric cosmology. He also challenged Aristotelian physics. According to Aristotle, objects move only when pushed or pulled by something else, so moving objects should quickly come to rest when left to themselves. However, Galileo discovered that the reason objects usually come to rest is because the force of friction slows down their motion. Without friction, he correctly inferred, an object in motion will remain in motion forever. (Isaac Newton later formalized this fact in his first law of motion, as we’ll see in the next chapter.)
Galileo challenged the Aristotelian theory of gravity too. Aristotle had taught that material objects are comprised of four elements: earth, air, fire, and water. Each of these elements has its rightful place in a natural hierarchy: earth belongs at the bottom, water on top of that, air above water, and fire above air. The reason rocks and other heavy objects fall to the ground, according to Aristotle, is that they are made of earth and are seeking their rightful place at the bottom of the hierarchy. The reason fire rises is that it is seeking its rightful place above the air. And the reason feathers fall more slowly than rocks is that they are comprised of only a little earth, mixed with air; so feathers feel somewhat ambivalent about where they should go. The more earth something contains, Aristotle said, the faster it falls. That’s why, as everyone knows, heavy objects always fall faster than light ones—or do they?
Galileo discovered that the reason some objects fall more slowly than others has to do with air resistance. When air resistance is negligible, all objects fall at the same rate, regardless of weight. While teaching at the University of Pisa, Galileo famously demonstrated this fact to his students and colleagues by dropping things from the top of the Leaning Tower. Such demonstrations provoked the ire of some of Galileo’s colleagues, who embraced Aristotelian philosophy and were chagrined to be proven wrong in front of their students. As one of the students later recalled:
[T]o the great discomfort of all the philosophers, through experiences and sound demonstrations and arguments, a great many conclusions of Aristotle himself on the subject of motion were shown by him to be false which up to that time had been held as most clear and indubitable… he showing this by repeated experiments made from the height of the Leaning Tower of Pisa in the presence of other professors and all the students.Quoted in Stillman Drake, Galileo at Work: His Scientific Biography (Chicago: University of Chicago Press, 1978), 19-20
Much to the embarrassment of the scholarly community, which had accepted Aristotelian philosophy almost unanimously, Galileo published his findings in widely read pamphlets and letters. Intense controversy quickly surrounded Galileo’s work, and in 1615 he penned a famous letter to the Grand Duchess of Tuscany, expressing concern that the “academic philosophers” were trying to incite church officials to take action against him:
Showing a greater fondness for their own opinions than for truth, they sought to deny and disprove the new things which, if they had cared to look for themselves, their own senses would have demonstrated to them. To this end they hurled various charges and published numerous writings filled with vain arguments, and they made the grave mistake of sprinkling these with passages taken from places in the Bible which they had failed to understand properly, and which were ill suited to their purposes.
His fears were realized the next year when the Roman Inquisition (the court system of the Roman Catholic Church, which held legal authority at the time) ruled that heliocentrism was heretical and ordered Galileo to abandon it.
Fortunately, Galileo’s friend and supporter Pope Urban VIII took office a few years later. Galileo continued his work, leading to the publication in 1632 of his most influential book: the Dialogue Concerning the Two Chief World Systems. The book was published in vernacular Italian (rather than scholarly Latin) and written in the style of a dialogue between fictional characters, making it easily readable and accessible to the general public. Less fortunately, Galileo was a man of little social tact, and he portrayed the Pope’s views through the mouth of a character named Simplicio, which means “simpleton” or “fool.” He was tried by the Roman Inquisition the following year and sentenced to house arrest for the remainder of his life.
The attack against Galileo was unjust and inexcusable, but it had little to do with the relationship between faith and science. As Galileo noted in his letter to the Duchess, the conflict was born out of human arrogance, not faith. The lesson of this tragic story is not that science and faith are enemies to each other, but that arrogance is an enemy to both. In fact, the persecution of Galileo was mild in comparison to the sufferings many non-scientists endured at the hands of both Catholic and Protestant Christians. The history of the church is marred by horrible evils committed by many who falsely professed themselves to be followers of Christ—people who obviously did not have faith in Jesus’ words about loving one’s enemies! Kepler’s own mother was accused of witchcraft, and she might have been burned at the stake were it not for a lengthy legal defense written by her highly-esteemed son. She was released after a year in prison, but others were not so fortunate. Some Christians were even killed by other professing Christians as factions waged war against each other over politics and theology.
Jesus warned us that such things would happen. Many would vainly call him “Lord” while devouring their fellow Christians like ravenous wolves (Matthew 7:15-23). Some would even convince themselves that murder is a service to God (John 16:2). These tragedies serve as warnings to us, lest we become arrogant and forget the most important lessons Jesus taught us. Even sincere followers of Christ occasionally stumble in their faith and do bad things. Consider for instance the numerous failings of the apostle Peter (e.g. John 18:10-11, Galatians 2:11-12). Perhaps some of the Christians who persecuted Galileo were not evil men, but merely sinful—like you and me. Either way, here’s the crucial point: neither the wrongs committed by sincere Christians nor the evils done by godless hypocrites show that there is any conflict between science and genuine Christian faith.