One of the most remarkable features of our world is that if you hold a stone in your hand and release it, it falls to the ground. Today this seems completely obvious to us. However, as we now know, the Moon remaining in orbit around the Earth and the planets revolving around the Sun are also caused by the same gravitational force. Interestingly, in ancient times scholars did not understand this connection.
In the 4th century BC, Aristotle attempted to explain how bodies move on Earth and in the universe in general. His ideas on this subject were presented in detail in his work called “Physics.”
Aristotle’s concept of nature
According to Aristotle, some things are natural, while others exist for different reasons. Natural things include elements such as earth, fire, air, and water. Aristotle believed that every body has its own principle of motion and rest.
In his view, every object has a “natural place.” For example, a stone falls toward the Earth because its natural place is the center of the Earth. Fire, on the other hand, moves upward. The natural motion of celestial bodies was believed to be circular.
Aristotle also believed that for an object to move at a constant speed, a force must continuously act upon it. In other words, continuous force was required to maintain motion.
Natural and unnatural motion
Aristotle divided motion into two types:
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Natural motion – the motion of an object toward its natural place.
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Unnatural motion – motion caused by an external force.
For example, when a stone is thrown upward, its motion is considered unnatural because it is caused by an external force. However, when this force is exhausted, the stone falls back toward the Earth and resumes its natural motion.
This idea was accepted for nearly two thousand years and remained one of the fundamental theories of physics until the 16th century.
The idea that heavier objects fall faster
Aristotle believed that heavier objects fall faster than lighter ones. At first glance, this idea seems reasonable. For example, if you drop a feather in one hand and a brick in the other at the same time, the brick will reach the ground first.
However, this effect is actually caused by air resistance. Aristotle, however, explained it as a result of the weight of the objects.
Despite being incorrect, his theory dominated scientific thinking in Europe for nearly two thousand years. During the Middle Ages, the Christian Church adopted Aristotle’s ideas about physics, which made it extremely difficult for scientists to challenge them.
Kepler and Galileo: the beginning of a new era
In the 16th and 17th centuries, scholars began to re-examine Aristotle’s ideas. Two scientists who played particularly important roles in this process were Johannes Kepler and Galileo Galilei.
Kepler formulated two important laws of planetary motion:
Planets move around the Sun in elliptical orbits, with the Sun located at one of the foci of the ellipse.
A line connecting a planet to the Sun sweeps out equal areas in equal intervals of time.
These laws showed for the first time that the motion of celestial bodies is not necessarily circular.
Galileo and the motion of falling bodies
Galileo also disproved another idea of Aristotle. He showed that a body moving at a constant speed does not require a continuous force to maintain that motion. If no force acts upon an object, it can continue moving.
Through experiments, Galileo discovered that bodies falling toward the Earth accelerate continuously. He also determined that the distance traveled by a falling object is proportional to the square of time.
To demonstrate the flaw in Aristotle’s theory, Galileo proposed a famous thought experiment. He reasoned that if heavier objects fall faster, what would happen if a heavy stone and a light stone were tied together? The lighter stone should slow down the heavier one. However, the combined object would be heavier and therefore, according to Aristotle’s theory, should fall faster. This contradiction shows that the theory is inconsistent.
The pendulum and gravity
Galileo also studied the motion of the pendulum. He discovered that the motion of a pendulum consists of vertical and horizontal components. The gravitational force acts in the vertical direction.
His observations showed that the period of a pendulum depends mainly on the length of the pendulum, not on the weight of the bob.
Descartes’ theory of gravity
In the 17th century, the French philosopher René Descartes proposed a different theory of gravity. He believed that there was no empty space in the universe and that all space was filled with a substance called ether.
According to Descartes, as planets move through this ether, they create vortices. These vortices keep the planets in orbit around the Sun.
The Earth was also thought to be surrounded by such an ether vortex, and heavy objects moved toward the center of the Earth due to this flow.
Although this theory was initially considered convincing, it was later criticized because it did not fully agree with observations.
Newton’s revolutionary theory
In 1687, Isaac Newton published his famous work “Principia.” In it, he introduced the law of universal gravitation.
According to Newton, any two masses in the universe attract each other according to the following rule:
The gravitational force is proportional to the product of their masses and inversely proportional to the square of the distance between them.
This law explained:
the falling of a stone to the Earth
the Moon’s orbit around the Earth
the motion of the planets around the Sun
within the same physical principle.
Newton also showed that Kepler’s laws of planetary motion follow directly from the law of universal gravitation.
The paradox of Newton’s theory
Although Newton’s theory was extremely successful, it raised an important question:
How can two bodies attract each other across empty space?
Newton did not attempt to answer this question. He stated that his theory explained how gravity works, but not why it works that way.
Nevertheless, since his theory best matched observations, it was eventually accepted by the scientific community.
Conclusion
From Aristotle’s ideas about natural motion, through the discoveries of Galileo and Kepler, to Descartes’ vortex theory and finally Newton’s law of universal gravitation, humanity has traveled a long path in understanding gravity.
This development demonstrates an essential characteristic of science:
theories evolve over time, improving and being replaced by more accurate models as new discoveries are made.