Falling Apples And Ocean Tides

    Up to the 17th century, our scientific knowledge was largely influenced by the Aristotelian thought. Aristotle's worldview can be summarized by the following fundamental principles:

§  Geocentric. The Earth is stationary and is at the center of the universe or cosmos. The moon, planets and the sun revolve around the Earth, completing a revolution about every 24 hours.

§  The universe is divided into two regions. The region between the Earth and the Moon (including the Earth itself) is called the sublunar region and the region beyond the Moon is the superlunar region.

§  In the sublunar region, there are four basic elements, these being earth, water, air, and fire. 

§  Objects in the superlunar region like the sun and planets are composed of a fifth basic element called ether.

     In Aristotle’s geocentric model, the basic elements move toward their “natural place.” To accomplish this, the universe is divided into four concentric spheres. The first sphere is the Earth itself. Surrounding the Earth is a concentric sphere that is the natural realm of water, surrounded by the natural realm of air, and  then  the  natural realm  of  fire above that.  Thus,  Earth sinks in water,  water sinks in the air,  and flames rise above  air.  Everything gravitates  toward  its natural  place  in Aristotle’s worldview,  and  it comes  across as fairly consistent with our intuitive understanding and basic observations about how the world works.

     In 1608, a Dutch spectacle maker, Hans Lippershey, announced a new lens-based instrument that made distant objects appear much closer.  The instrument would later be called “telescope.” The following year, Galileo Galilei heard about the Dutch “perspective glasses” and within days had designed one of his own---without ever seeing one.  Other astronomers and natural philosophers followed suit and trained the sights of their new instrument to the heavens. The telescope enabled them to see more objects in space. Galileo was able to make out mountains and craters on the moon, as well as a ribbon of diffuse light arching across the sky — the Milky Way. He also discovered the rings of Saturn, sunspots and four of Jupiter's moons. But the most profound and startling discovery was that the Earth is not the center of the universe. The observers discovered that all planets revolve around the sun---including Earth itself. Thus Aristotle’s worldview was proven wrong!

     The geocentric model was not the only prevailing view at the time. As early as the third century BC, Aristarchus of Samos proposed heliocentrism---the idea that the Earth and the planets revolve around the Sun. But Aristarchus’ heliocentrism attracted little attention---possibly because of the loss of scientific works of the Hellenistic period.  Aristotle was so influential that his geocentric model was embraced by the Roman Catholic Church and became part of her doctrines.

     When Galileo began proclaiming that geocentrism is wrong, that the planets, including Earth revolve around the Sun, he was arrested and accused of being a heretic for opposing church’s teachings. In 1633, he was tried and condemned before the Roman Catholic Inquisition. The crime of heresy was punishable by death. But because of Galileo’s stature in the scientific community, he was cleared of charges of heresy, but was placed under house arrest and told that he should no longer publicly state his belief that Earth moved around the Sun. He was under house arrest for 8 years until his death in1642 at the age of 77. It took the Church more than 350 years before Pope John Paul II issued an apology in 1996 to rectify one of the Church’s most infamous wrongs. 

     During Newton’s time, Aristotle’s geocentric worldview was practically debunked and the heliocentric model became the accepted norm in the scientific world. But for serious thinkers, a scientific question remains: Why do planets revolve around the sun? Isaac Newton was still a young university student when he began pondering on the problem. He understood that the planets revolve around the Sun because the Sun pulls them toward itself. But why don’t they just fall in and burned up? It is because these planets are initially in sideward motion. Just like when you tie an object with a string, when you swing around while holding the other end of the string, the object orbits around you. The force that pulls the planets toward the Sun is called gravity.

     In 1665 the Great Plague epidemic hit London in which a quarter of the city's population would die. Just like what we do today, they practiced "social distancing" to contain the outbreak. Schools were closed and students were sent home. Young Isaac Newton went home to Woolsthorpe Manor, his family's estate about 60 miles northwest of Cambridge. The epidemic lasted 18 months which gave Newton ample time to work on the gravity problem in the comfort of home.

     One late afternoon he was in the garden when he saw an apple drop from a tree. There’s no evidence to suggest the fruit actually landed on his head. But it was an “aha moment” for Newton.  He came to the realization that the force that pulls the apple to the earth is the same kind of force that keeps the planets in orbit around the sun. And while the Earth is pulling the apple, the apple is also pulling the Earth but since the Earth is so massive and the apple so relatively tiny, it’s the apple that comes to the Earth and not the Earth to the apple. Just like a small boat and a large ship floating stationary on the ocean pulling each other. Eventually, it’s the boat that moves toward the ship and not the other way around. Each object in the universe pulls every other object! That’s how Newton came up with the Universal Law of Gravitation.

     The universal law of gravitation  became  one of the prime movers  in the rapid advances of science and technology.  But the law is far from perfect.  Newton’s  mathematical model fails  when the objects are too massive and/or the distances are infinitesimally small. It took more than 200 years before Albert Einstein  came  up with his Theory of Relativity  effectively  superseding  Newton’s law.  Although  the theory of  relativity  is  more precise,  it is cumbersome  and  unwieldy to apply.  Today,  engineers  and scientists  still use  Newton’s law  in  space exploration  and  when calculating  trajectories  of  ballistic missiles or passing comets.