2023-06-17 09:13:44
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The story of the apple falling on the head of the English physicist Isaac Newton (1643-1727) is anecdotal.
But it is accepted that what has become known as the law of universal gravitation, the principle that explains why things fall, was formulated by him in the work “Philosophiae Naturalis Principia Mathematica”, in 1687.
Although, obviously, things were already falling apart before Newton. How then did those who devoted themselves to reflection explain this phenomenon? What explanation had, until the 17th century, what we now call gravity?
Many years following Newton, the physicist Albert Einstein (1879-1955) said that “gravity is the first thing you don’t think regarding”. Because it seems natural to us that the idea that a thrown stone falls, that a fruit that is not removed from the tree also falls and, well, that a stupid stumble is a harbinger of a fall .
In the book “Why things fall? A history of gravity”, published by Zahar editions in 2009, astronomers Alexandre Cherman and Bruno Rainho Mendonça start from the observation that gravity is undeniably “special”.
“If this were not so, how can we explain that the two greatest geniuses of science, Isaac Newton and Albert Einstein, devoted themselves to it? And not only: they were raised to this status of genius precisely because that they had glimpsed some of his secrets,” Cherman wrote.
From Greece to India
According to the astronomer, the importance of gravity is due to two factors: it is universal, “to use a term dear to Newton”, and common, “to use a term dear to Einstein”.
Universal and common. How was it explained then?
If we have to go back in the history of science, let’s go back to Aristotle (384 BC – 322 BC) because the Greek scientist is considered one of the most important thinkers. influential in Western history, and much of the very logic of scientific thought is due to its questioning.
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We tend not to think regarding gravity because it seems natural to us for a thrown stone to fall, or for a fruit that is not picked from the tree to fall.
“He separated the phenomena of the elements a bit and understood that the object belonging to a certain element had a natural tendency to fall back on the position of this element,” physicist Rodrigo Panosso Macedo, a postdoctoral researcher at the Institute, explains to the BBC. Niels Bohr Institute at the University of Copenhagen, Denmark.
“Thus, if an object were made of earth, its natural tendency would be to fall back towards the earth, and it would therefore fall. An object made of gaseous air would have a natural tendency to fall back into the air, and therefore to rise “.
In the book he co-authored, the astronomer Mendonça goes back a little further in time and cites some references to the understanding of the phenomenon by Hindu scholars even before Aristotle.
A pictorial representation probably dating from the 8th century BC reveals that Hindu philosophers already believed that gravitation held the solar system together and that the Sun, as the most massive star, should occupy the central position in the model.
“Another interesting testimony to ancient India is found in the works of a Hindu sage named Kanada, who lived in the 6th century BC. “It was he who founded the philosophical school of Vaisheshika”.
Rainho Mendonça explains that Kanada associated the “weight” with the fall, considering the former as the cause of the phenomenon. “The Hindu sage’s intuition was on the right track, but there was still a long way to go in conceptual terms.”
Site naturel
The astronomer recognizes, however, that the zero point of the concept of gravity must be attributed to Aristotle, “because although his work on this subject does not represent today’s reality, the knowledge they disseminated has endured for many centuries following his death.
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Aristotle’s influence in the field of knowledge spread throughout the West.
“Until modernity, with the new research and theories developed in the Renaissance (…), Aristotelian physics predominated in many centers of study of antiquity and the Middle Ages,” the BBC told the BBC. physicist, philosopher and historian José Luiz Goldfarb, professor of the history of science at the Pontifical Catholic University of São Paulo (PUC-SP).
“He explains the fall of bodies by the idea that the Earth is the center of the Universe and that heavy bodies tend to occupy their natural place in this center.
In other words, Goldfarb points out that this idea amounts to “saying that things fall when they are loose, since they tend to occupy their natural place at the center of the Universe, the Earth”.
Etymologically, it is interesting to note that the word gravity derives from the Latin “gravis”; it therefore has the same origin as the word tomb (grave in English). Its semantic field ranges from “heavy” to “important”, passing through meanings such as “powerful”.
According to the “Etymological Dictionary of the Portuguese Language”, by the philologist and lexicographer Antônio Geraldo da Cunha (1924-1999), the term “gravity” appears as early as the 13th century, but the variants “gravitar” and “gravitation” only appear in the 18th century, which indicates a consequence of Newtonian physics on terminologies.
In a text signed by Cherman in “Why do things fall?”, there is a digression on the Sanskrit term for gravity: “gurutvaakarshan”. Note the beginning of the word: “guru”. This is precisely the term used to refer to the respected spiritual masters and religious leaders of Hinduism,” he explains.
Moreover, it is derived from the Greek “barus” (heavy), the origin of the word “baritone” (deep voice)”, adds the astronomer.
In a chapter of the same work, Rainho Mendonça explains that the use of the Latin term “gravis” to designate the phenomenon of gravity began in the 8th century, with the translations of scientific treatises from the Arab world to Europe.
“This is how the term that is the subject of our study appeared: gravity”, explains the researcher. And in the context that interests us, because to designate objects of significant weight, the Latin translations used the word whose root is the adjective “gravis”, grave, which means “heavy”.
“It is not possible to determine the first time that this term was used”, specifies the author. For him, the emergence of the first European universities, where Latin was then the official language, contributed to the spread of the new nomenclature. “In the universities of Bologna, Paris, Oxford, among others, which used most of these translated works (in Arabic).
Advancements
Although Aristotelian thought predominated, particularly in the Western world, and the Middle Ages were dubbed the “Dark Ages” in terms of the evolution of knowledge, it is undeniable that scientific advances were made in over the 2,000 years that elapsed between Aristotle and Newton.
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Newton was preceded by many scientists around the world who attempted to explain why objects fall.
“Today, historians of science are able to detect thinkers from Antiquity and the Middle Ages who were already developing ideas closer to Newtonian theory than to Aristotelian physics, although officially the Greek philosopher’s theory has prevailed,” says Goldfarb.
The book “Why Things Fall Down” provides insight into this scenario. The astronomer Mendonça cites, for example, the research of the Arab philosopher Abu Yusuf al-Kindi (801-873). In his treatise “On (Solar) Rays”, he asserts that the stars exert a force on objects and people.
“This force would be associated with radiation from the stars, which would propagate in a straight line through space and influence things on Earth,” explains the astronomer.
A little later, the philosopher of Jewish origin Solomon Ibn Gabirol (1021-1058) also approached the subject, “with a simple but nascent reasoning”, as Rainho Mendonça points out.
Its contribution is the notion of inertia. “According to him, large and heavy substances would be more immobile than lighter substances,” he explains.
The Iranian philosopher and astronomer Abd al-Rahman al-Khazini (1077-1155) put forward the idea that falling heavy bodies always move towards the center of the planet. But what is even more interesting is his proposition that the “thiql” (Arabic term that many authors translate as “gravity”) of bodies depends on their distance from the center of the Earth,” he adds. -he.
driving forces
Although there were many theories at that time, one idea prevailed which, in a way, is very close to the concept of inertia. As physicist Fábio Raia, a professor at the Presbyterian University of Mackenzie in Brazil, explains to the BBC, “the most widespread theory (…) was the momentum theory (…), according to which the continuous motion of a body is due to the action of a force”.
“When this ceases, the body returns to its natural state of movement,” he explains.
The astronomer Mendonça emphasizes, in this respect, the fundamental role of the Alexandrian philosopher Iohannes Philoponus (490-570).
“According to him, when thrown, a body receives a kind of driving force, which would be transferred from the thrower to the projectile, and which would remain in itself even following the end of the contact. In time, this “force” would dissipate spontaneously , resulting in the end of the movement,” he explains.
In the case of a falling object, Philoponus had already understood that this force was caused by what is now called gravity.
Photo credit, FERDINAND SCHMUTZER / BIBLIOTHEQUE NATIONALE D’AUTRICHE
Many years following Newton, the physicist Albert Einstein (1879-1955) will say that “gravity is the first thing we don’t think regarding”
“According to this idea, the Earth exerted an attraction on objects, drawing them towards its center,” philosopher Andrey Albuquerque Mendonça, a professor at the Escola Superior de Publicidade e Marketing de São Paulo (ESPM-SP), told the BBC. .
The philosopher recalls, however, that there have been dissonant voices, such as that of the French philosopher and theologian Jean Buridan (1301-1358), who “proposed an alternative theory to explain the fall of objects”.
“He maintained that the objects fell because of an internal force that pushed them down, but he might not explain the cause of this force”.
Leonardo da Vinci (1452-1519) and Galileo (1564-1642) both studied falling objects. As Albuquerque Mendonça indicates, the first “proposed that the speed of fall depends on the density of the object and the resistance of the air”, while the second “determined that all objects fall with the same acceleration, regardless of their weight”.
None of them, however, succeeded in finding a universal law to explain this phenomenon. Newton’s breakthrough was brilliant because he succeeded, certainly with the knowledge accumulated by his predecessors, not only in understanding a universal force and fundamental, but also to transform it into an explainable phenomenon.
This is a real scientific revolution. “He integrated new cosmological concepts into his theories, moving away from the Aristotelian universe,” summarizes Mr. Goldfarb.
“Thus, he no longer thought of the fall towards the natural place, but the concept of attraction between bodies, the law of gravitation, appeared: matter attracts matter in direct proportion to the masses and by the inverse of the square of the distance between the bodies”.
According to the professor, it was at this point that people stopped “thinking regarding tendencies to occupy the natural place” and began to “understand the falling movements of bodies as the result of ‘action of the force that the Earth exerts on bodies’.
“We can conclude that the mechanics introduced by Newton involved profound changes in the way the modern world began to conceive of the cosmos, bodies and the laws that govern their movements,” he concludes.
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