Story of Gravity

We are so familiar with the natural events that we have witnessed every moment since the day we are born, so it becomes unthinkable to examine the laws of nature which are in the background of those events. The best example of this is gravity. As the Norwegian writer Jostein Gaarder expresses in Sofie's World, a baby at the breakfast table is not amazed so much if she sees her father fly away from the table. Because everything is already a surprise for that baby, everything is new and interesting. However, if the mother turns back and sees her husband in the air, she will scream from amazement. Because she has not seen anything that is contrary to gravity until that day. Gravitation is such a natural phenomenon that we have got used to. However, the brave and intelligent people who search for what is the force that attracts objects to the ground are rarely encountered in history.

Ancient Solar System Models

Today we know that the active force which is responsible of the movement of celestial bodies is gravity. The movement of the planets, the satellites and the stars and also the fall of a released tiny pebble is the result of the same force, gravity. But it was not so easy for mankind to establish this connection.

The adventure of understanding gravitational force for us began with the efforts of understanding the motion of celestial bodies. The perception based on the the Earth-centered universe that ancient Greek thinkers have shaped, has been regarded as decisive enough to require no discussion over the years. According to this model, all the stars and planets revolve around the Earth, and the Earth is at the center of the universe without moving.

This idea, which found itself in the Almagest of Ptolemy of Alexandria (90-168 AD), lasted for almost seven centuries, until El-Battani (850-929) in South Anatolia. This curious man has observed the Moon, the planets and the locations of the stars very precisely. Al-Battani, as a result of his observations, pledged serious doubts about Ptolemy's model of the Sun-centered universe. In the following century, Ibn al-Haytham wrote Doubts Concerning Ptolemy, which shows the systematic errors in Ptolemy's model. According to Ibn al-Haytham, the Greeks could not do better, although they knew that this model was faulty.

In the 13th century, in Iran, Nasreddin Al-Tusi (1201-1274) built the largest observatory in the history. Moreover, it was a time that the Mongol invasions had ruined all the civilizations, this smart man had convinced the Mongol Khan Hulagu that he would be able to prophet the future with this observatory.

In this extraordinary astronomy center, famed for the name Maraga Observatory, researchers from China, far from Egypt, were employed. Stellar catalogs were created, Motions of the planets were recorded in great detail, and post universe models were developed. However, the Earth-centered model of the universe was still alive, although it was dying.

New beginnings with Copernicus Period

At the beginning of the 16th century, Nicolaus Copernicus (1473-1543) in Northern Italy put forward a proposition that no one would dare. He took the Earth from the center of the universe and put the Sun instead. But he waited until the last years of his life to announce this idea. This was a complete arrogance for many, especially the church. The vanity and the terrible ignorance of mankind about the universe, did not like to be taken from the center of the universe. How could the world be a mean place rotating around the Sun like ordinary planets? However, natural events such as the movement of the planets, day and night, were much better explained by the solar-centered model. Strange behaviors of Mars, Jupiter, and other celestial bodies, which were moving back and forth, were more easily understood with the Sun-centered model.

In fact, Copernicus was highly influenced by Muslim scholars who lived long before him. In particular, Al-Battani's research that referred to the moon, the positions of planets and stars was groundbreaking and pioneered the model of the Sun-centered universe. In the ninth century, the works of ancient Greek thinkers and Indian scholars were translated into Arabic with great enthusiasm. For example, researchers such as Al-Biruni and Hamadani had the opportunity to discuss and compare the model of the universe of Indian mathematicians like Brahmagupta with the model of Ptolemy. In addition to the curiosity of this ancient work, Muslim scientists had an incredible observation and mathematical genius. So much so that Al-Battani calculated 365 days a year, 5 hours 46 minutes and 24 seconds. It was only a few seconds different from today's measurement. Moreover, he did not neglect to use the observations and measurements of Greeks in his works. It was a great synthesis. Unfortunately, in the following centuries the heritage of these names has not been surmounted and the effort to understand nature has waited for another civilization. 

As for Copernicus... He did not ignore the works of Al-Battani, Al-Tusi and Ibn-i Shatir. He was also fortunate to have such successors as Galileo and Kepler. Because the new sun-centered universe model would find buyers in future thanks to them.

Gelileo Galilei

The coup of Copernicus to the perception of the universe was just beginning. Next was Galileo Galilei (1564-1642). Galileo kept watching celestial objects with his simple telescope. In particular, observations of Venus and Jupiter had wonderful results. According to Galileo's observations, Venus had phases just like Moon. It was a very powerful argument that Venus was rotating around the Sun. Because if it had been turning around the Earth, its appearance would have always been the same, or it had to make a different circular motion on an axis on its own orbit. That would be an absurd explanation. Because there was no need for the planet to turn around a point where there was nothing. The real blow to the old universe picture came from Jupiter. Galileo looked at the telescope and saw four new celestial bodies around Jupiter. These celestial bodies sometimes approached Jupiter, sometimes disappeared behind Jupiter. This could only mean one thing: the celestial bodies near Jupiter, they could be nothing but satellites revolving around Jupiter. This was a completely contrary discovery to the perception of the Earth-centered universe, which attributed a strange holiness to the Earth, saying that all celestial bodies must rotate around the Earth. Not long ago, Galileo's telescope was proclaimed as an evil tool that bends and twists reality. 

In the same year, a Danish observer Tycho Brahe (1546-1601) began to doubt the earth-centered universe, which was supposed to be true for that day, due to some strangeness in the movement of the planets. He proposed his own system in the light of the observations he made over the years. In Brahe's system, there was Earth again in the center, and the Sun was rotating around the Earth. But this time the planets were spinning around the Sun.

Kepler and the Meeting of Observational Data with Mathematics

In short, Brahe thought the entire Solar System was a separate system that revolved around the Earth. The perception that the Earth was the center of the universe was so strong that Brahe never doubted it. Brahe's model of the universe was still violently rejected. Brahe could not have created a mathematical background to support his own universe system. He needed a talented mathematician to prove his model. At the end of the 1500's, Johannes Kepler (1571-1630) was on the stage with his book named Mysterium Cosmographicum. Brahe thought Kepler had the talent he was looking for. Kepler was very influenced by Plato's ideas and thought that the universe was full of perfect geometric movements. Kepler considered the tremendous amount of data from Brahe's observations about the movements of the planets and began searching for a perfect geometric infrastructure that defined the planetary movements.

Even though it took a long time, Kepler could not adapt his data to a pattern of flawless circular movements. Long after Brahe's death, Kepler noticed that it was an ellipse rather than a geometric circle he was looking for. Including Brahe and Copernicus, people had been thinking for thousands of years that celestial objects are floating in orbits of perfect circles. So it was hard to believe Kepler's imperfect elliptical orbit idea. Kepler has made another boldness! In his new model of the universe, he took the Earth from the center of the universe and replaced it with the Sun. Moreover, he obtained a mathematically perfect background with equations that define the motion of planets, their orbits, radii, and rotation periods around the Sun. Kepler explained all the finds in three basic laws and reached formulas for calculating the planets' orbits and rotation periods.

What is Gravity Actually?

All this is still far from explaining the basic reason for the movement of the celestial bodies. In addition to these efforts that only interested in the identification of the motion of objects in space, Galileo was also looking for ways to describe the motion of objects on Earth. Perhaps there could be a way to describe the movements of objects altogether. Galileo set out on the prospect as Copernicus's universe model proposed that the Earth was spinning around its own axis. If the Earth was rotating around its axis, then the object released from a higher place would have fallen somewhere differently than the projection of the place where it was dropped. Because the world keeps rotating around its axis when the object falls. Galileo calculated that the body he left at the Tower of Pisa should be at least a few hundred meters away. But he could not even observe any slight deviations in his experiments. This showed that he knew very little about the motion of objects.

It was time to pluck his arms. He realized that he needed to make very precise measurements when observing, so he had to combine mathematics and experimentation. He could not make a real measurement because the objects he dropped were falling very quickly. So he decided to use inclined planes to slow down the free fall motion. He studied objects moving downward through planes with different slopes. He had got a very interesting result. Whether the slope was very steep or very horizontal, when the time interval doubled, the path taken by the objects increased fourfold. Moreover, the change of the mass of the objects did not affect the motion. Galileo discovered that the motion of all objects on earth would be the same if there had been no air friction, with a foresight far beyond the time of Galileo. The acceleration of the falling objects was not bound to their masses.

Galileo had noticed something else in his experiments. The objects he dropped from the U-shaped sloping surfaces could reach as high as they were released initially. Even if the path had two different slopes, the altitude at which the objects were released was the same as the height at which they were last reached. We know this as the conservation of energy, but Galileo noticed something else in there. If one side of the trajectory of the object was straight, the object would keep its motion forever and continue to move. In other words, if there is no friction, the objects in motion will keep their motion as they are. This principle is known today as inertia. So he realized why a high-dropping object fell into the projection. The velocity of the object at the top of the wool is the same as the velocity of the Earth. As the object moves vertically, it keeps its horizontal velocity, so the more the Earth rotates, the more it travels horizontally.

Galileo built his way step by step to modern physics with his powerful intuition and observation ability. But the idea of creating a general theory linking the motion of celestial bodies with the motion of objects on earth was beyond even Galileo's genius. This mission was waiting for the most magnificent scientist of human history.

The First Step towards Modern Science with Newton

In the year of Galileo's death, a child named Isaac Newton (1642-1727) was born in England. Galileo's ideas spread throughout Europe like a fire despite all the repressions. Newton had a much more relaxed atmosphere than Galileo in his youth. He also had many valuable competitors and friends, such as Robert Hooke and Edmond Halley, whom he could freely discuss his ideas with. In those times, the idea of the Earth-centered universe has been almost abandoned, and the existence of a force that the Earth created towards itself had begun to be debated. According to Hooke, there had to be a force of attraction, depending on the distance between objects. Moreover, Halley had found evidence of this by measuring that the pendulums were slower than the sea level in the experiment on a high hill. These were big steps for science.

Hooke thought that all celestial bodies had the same attraction force. He was curious about Newton's views on this issue, and unfortunately he could not receive any response to his questions he asked in several letters. Halley went on to do the same thing years later and in a letter written to Newton in 1684, he asked a question about how the planets would move if the Sun pulled the planets away with a force inversely proportional to the distance. According to Newton, the answer was very simple: Of course they would move in elliptical orbits. Newton had reached this result 18 years ago, yet at the age of 23, but he was not very keen to publish his findings. He was more interested in alchemy, and he spent his time by documenting the theological manuscripts.

The Birth of Principia

Newton was back into real science with his friend Halley's insistence. As he kept working, he realized that he was about to find the key to a cosmic mystery. In 1686 he finally finished his work Principia. In his work he succeeded to find general laws that could describe the motion of all celestial bodies and all objects on earth. He also created the necessary terminology for this job. Definitions of the mass and inertia were described. It was understood that what was the source of gravity was the mass of the bodies, and that every massive object had its own gravity. Principia was  pure happiness for scientists of that day. It was the fruit of a search for 1500 years. Observations, astonishing data, positions of comets and planets, velocities of pendulums... Everything has found meaning in this book. It was a huge scientific step. The centuries-old doubts had come to light. Kepler's elliptical orbits for celestial bodies were confirmed by Newton's inverse square law. Galileo's work on the movements of Earth's objects had also found a mathematical explanation of Newton's famous F = ma formula.

In addition, the movements of celestial bodies and earthly objects are gathered under a single frame, proving that the same laws are valid everywhere, and for the first time in history, a general theory has been formed by combining different theories.

Mechanical Universe

Even now, Newton's laws are being utilized, they can easily predict the position of a planet with Newtonian mechanics, and they can pinpoint the movements of the Moon and the Sun. For example, New Horizons, launched at the beginning of 2006, changed the direction and increased its speed, using the gravitational force of Jupiter without spending fuel after the spacecraft exited the gravitational field of the Earth. In July 2015 it reached Pluto. It is an extraordinary achievement to reach a target 5 billion kilometers away with an object launched ten years ago. There exists the power of Newton's laws behind this success. The historians of science particularly addressed the progress of physics in two different periods, pre-Newtonian and post-Newtonian. The Newtonian physics ruled for many years.

The Newtonian physics ruled for many years until a name at least as magnificent as Newton took his place in the history scene. With general relativity theory shaped in 1910, Einstein changed all our knowledge about the universe. According to him, gravity was not only affecting objects, but also bending the universe! Einstein discovered that the universe was a terrific physical phenomenon, which was bent and twisted by gravity, and even the time was not absolute. After Einstein, the Newtonian era ended.

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