How gravity was explained before Newton – 6/3/2023 – Science

The story of the apple falling on the head of English physicist Isaac Newton (1643-1727) may be nothing more than a legend. But it is agreed that 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.

Obviously, things fell apart before Newton. But how did people face the phenomenon? What was the explanation, until the 17th century, for what we now call gravity.

Many years after Newton, the German physicist Albert Einstein (1879-1955) would say that “gravity is the first thing we don’t think about”. After all, it seems natural the idea that a thrown stone falls, in the same way as an unpicked fruit from the tree, or that a silly stumble is a harbinger of a fall.

In the book “Why Things Fall? A History of Gravity”, published by Zahar in 2009, astronomers Alexandre Cherman and Bruno Rainho Mendonça start from the observation that gravity, without a doubt, “is special”.

“If it weren’t, how to explain that the two greatest geniuses of science, Isaac Newton and Albert Einstein, dedicated themselves to it? And not only that: they were raised to this genius condition precisely because they glimpsed part of its secrets?”, writes Cherman.

According to him, the importance of gravity lies in two factors: it is universal, “to use a word dear to Newton”, and general, “to use a term dear to Einstein”.

But how was it explained before?

To answer this question, one has to go back in the history of science to Aristotle (384 BC – 322 BC). The Greek sage is considered one of the most influential thinkers in Western history – and much of the very logic of scientific thinking is due to his prerogatives.

“He divided the phenomena a little from the elements, and understood that there was a natural tendency of the object that belonged to a certain element to return to the position of that element”, explains to BBC News Brasil, physicist Rodrigo Panosso Macedo, postdoctoral researcher at the Niels Bohr Institute at the University of Copenhagen, Denmark.

“So, if an object was made of earth, its natural tendency would be to fall back towards the earth, which is why it would fall. An object made of gaseous air would have a natural tendency to fall back into the air, which is why it would rise.”

In the book in which he is co-author, Bruno Rainho Mendonça goes back a little further in time and mentions some references to the understanding of the phenomenon by Hindu scholars, even before Aristotle.

A pictorial representation possibly from the 8th century BC indicates that these 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 record also made in Ancient India can be found in the work of a Hindu sage called Kanada, who lived in the 6th century BC”, he writes.

“It was he who founded the philosophical school of Vaisheshika.”

Rainho Mendonça explains that Kanada associated “the weight” with the fall, understanding 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.”

The astronomer agrees, however, that the zero point in the concept of gravity must be attributed to Aristotle, “because despite his work in this area not representing the current reality, the knowledge disseminated in him lasted for many centuries after his death”.

“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”, says physicist, philosopher and historian José Luiz Goldfarb, professor of history of science at PUC-SP, to BBC News Brasil.

“[Ele] explained the fall of bodies by the idea that the Earth was the center of the Universe, and heavy bodies tended to occupy their natural place in this center.”

In other words, “it’s saying that things fall when they are loose, as they tend to occupy their natural place at the center of the Universe, the Earth”, analyzes Goldfarb.

Etymologically, it is interesting to note that the word gravity derives from the Latin gravis —it has the same origin as the word “grave”, therefore. 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, the term “gravidade” already appears from the 13th century —but the variations “gravitar” and “gravitação” only appear in the 18th century, indicating the repercussion of Newtonian physics on terminologies.

In a text signed by Cherman in the book “Why Things Fall?”, there is a digression about the Sanskrit term for gravity: gurutvaakarshan.

“Note the beginning of the word: ‘guru’. It is precisely the term used to designate the respected spiritual masters and religious leaders of Hinduism”, he says.

“And, in a twist, it also results in the Greek ‘barus’ (heavy), origin of the word ‘baritone’ (deep voice)”, adds the astronomer.

In a chapter written by Rainho Mendonça in the same book, he explains that the use of the Latin term gravis to designate the phenomenon of gravity began in the 8th century, with translations of scientific treatises from the Arab world to Europe.

“And this is how the term that is the object of our study arises: gravity”, says the researcher.

“And in the context that interests us, because when referring to objects of great weight, the Latin translations used the word whose root is the adjective gravis, grave, which means ‘heavy’.”

“It is not possible to specify the first time this term was used”, observes the author.

But, for him, the emergence of the first European universities, where Latin was the official language, contributed to the dissemination of the new nomenclature. “[… nas] universities of Bologna, Paris, Oxford, among others, which used most of those works [árabes] translated.”

Although Aristotelian thought prevailed, especially in the Western world, and the Middle Ages entered history as the “dark age” in terms of the evolution of knowledge, it is undeniable that there were scientific advances in the 2,000 years that separate Aristotle and Newton .

“Today, historians of science are able to detect thinkers from Antiquity and the Middle Ages who have already elaborated ideas that were closer to Newtonian theory than to Aristotelian physics, even though the Greek philosopher’s theory prevailed”, says Goldfarb.

The book “Why Things Fall?” offers an overview of this scenario. Rainho Mendonça cites, for example, the research of the Arab philosopher Abu Yusuf al-Kindi (801-873).

“In your treaty About Rays (Solar)he declared that the stars would exert a force on objects and on people.”

“This force would be associated with the radiation from the stars, which would propagate in a straight line through space and influence things on Earth”, says the astronomer.

A little later, the philosopher of Jewish origin Solomon Ibn Gabirol (1021-1058) also addressed the theme, “with a simple but incipient reasoning”, as Rainho Mendonça ponders.

His contribution was to the notion of inertia.

“According to him, extensive and heavy substances would be more immobile than lighter ones”, he explains.

The Iranian philosopher and astronomer Abd al-Rahman al-Khazini (1077-1155) presented the idea that heavy falling bodies always moved towards the center of the planet.

“However, even more interesting was his proposition that the thiql (Arabic term that many authors translate as ‘gravity’) of the bodies depended on their distances from the center of the Earth”, he adds.

Although many theories have emerged in that time, one idea has prevailed – and in some ways it is very close to the concept of inertia.

According to physicist Fábio Raia, a professor at Universidade Presbiteriana Mackenzie, “the most widespread theory […] it was the momentum theory […]which said that the continuous movement of a body is given by the continuous action of force”.

“When that ceased, the body would return to its state of natural movement”, he tells BBC News Brasil.

Rainho Mendonça emphasizes, at this point, 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 launcher to the projectile, remaining in it even after the end of contact. Over time, such ‘force’ would spontaneously dissipate , causing the movement to end”, he explains.

In the case of falling objects, however, Philoponus already understood that this force was caused by something that today is defined as gravity.

“According to this idea, the Earth exerted an attraction on objects, which pulled them towards its center”, explains to BBC News Brasil the philosopher Andrey Albuquerque Mendonça, professor at the Escola Superior de Propaganda e Marketing de São Paulo (ESPM-SP ).

He recalls, however, that there were dissenting 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 argued that objects fell due to an internal force pushing them downwards, but he could not explain what caused this force.”

It is worth remembering that both Leonardo da Vinci (1452-1519) and Galileo Galilei (1564-1642) studied falling objects. According to Albuquerque Mendonça, the first “proposed that the speed of the fall depended on the object’s density and air resistance”, while the second “determined that all objects fell with the same acceleration, regardless of their weight”.

Neither of them, however, managed to arrive at a universal law for the phenomenon.

Newton’s breakthrough was genius because he managed, certainly with the knowledge accumulated by his predecessors, not only to understand a universal and fundamental force, but also to make it an explainable phenomenon.

It was a true scientific revolution.

“He incorporated new cosmological conceptions into his theories, moving away from the Aristotelian universe”, summarizes Goldfarb.

“Thus the fall to the natural place was no longer thought of, but the conception of the attraction between bodies, the law of gravitation, emerged: matter attracts matter in the direct ratio of the masses and by the inverse of the square of the distance between the bodies.”

According to him, that was when people stopped “thinking about tendencies to occupy the natural place” and began to “understand the falling movements of bodies as the result of the action of the force that the Earth exerts on the bodies”.

“We can conclude that the mechanics introduced by Newton involved profound alterations in the way in which the modern world began to conceive the cosmos, bodies and the laws that govern their movements”, he says.

“Newton’s theory was an important milestone in the history of science and is considered one of the greatest intellectual achievements of humanity”, defines Albuquerque Mendonça.