Gravity has been around as long as the Universe itself. But human consciousness has recognized it only for a minuscule period of time. Though humans may have recognized gravity as soon as our species evolved a million years ago, we have records of it dating back to only a few thousands years.
Here we look at how humans have understood and defined gravity through history, starting with the most recent definitions.
Einstein (1915): Sitting in his patent office at Bern in 1907, Albert Einstein imagined how a house painter would experience gravity if he fell off a roof. Such thought experiments made him come out with the General Theory of Relativity. He theorizes that gravity is an effect resulting from the curvature created by mass and energy in our four-dimensional universe called “Spacetime”. Einstein proposed that mass curves spacetime, and that free-falling objects due to gravity are actually moving along locally straight paths in curved spacetime called geodesics. Over the last century, general relativity’s predictions have been repeatedly verified by modern scientists. One of the best proofs till date is the variation in Mercury’s orbit around the sun which cannot be explained by Newtonian theory of gravity.
Newton (1687): In his 1687 work Principia Mathematica, Newton discusses gravity as an universal force that exists between heavenly objects. He writes, “I deduced that the forces which keep the planets in their orbs must (be) reciprocally as the squares of their distances from the centers about which they revolve: and compared the force requisite to keep the Moon in her Orb with the force of gravity at the surface of the Earth; and found the answer pretty nearly.” He also gave his famous gravitational law in this work. Newton’s laws of gravity have helped us progress so much in science over the centuries in many fields.
Galileo & Kepler (16th century): Galileo, who died the year Newton was born, did experiments to show two different balls hit the ground at the same time no matter what their masses were. He also calculated the acceleration because of gravity to be constant at around 10 m/s². Kepler was a contemporary of Galileo and he described the motion of planets around the Sun improving on Copernicus’s ideas. His work suggests he must have known about a force keeping the planets moving around the Sun. In schools today, they teach children only until Galileo & Kepler when learning about gravity.
But the history of human grasp of gravity goes further back in time:
Al-Biruni (10th century): Al-Biruni was an Iranian polymath and famously called as the father of “Indology”. In a brief span of time, Al-Biruni translated an enormous volume of Indian manuscripts from Sanskrit to Arabic. He wrote an extensive commentary on Indian astronomy in the Taḥqīq mā li-l-Hind, which is mostly the translation of Aryabhatta’s work. In this he claims to have “resolved the matter of Earth’s rotation” which was proposed by Aryabhatta. He also made commentary on Archimedes’ theory of the center of gravity criticizing him for denying gravity affecting three dimensional heavenly bodies.
Brahmagupta (7th century): Brahmagupta is probably the first to use the word gravitational force to describe the force of gravity. In his book called Brahmastupasiddhanta, he says “all heavy things are attracted towards the center of the earth.” And called this as “Guru-thv-akarshanam”. Guru means mass in Sanskrit and Akarshanam meaning force of attraction. Al-Biruni in his works mentions Brahmagupta and also his statements on gravity. In fact, the word Gravity comes from Latin Gravis which is Cognate with Sanskrit Guru, meaning mass.
Aristotle & Archimedes (300 BCE): Aristotle gave one of the earliest explanation of gravity in the western world by putting forth the idea that objects moved toward their “natural place”. Archimedes who was born a century later than Aristotle, discovered fundamental theorems concerning the center of gravity of plane figures. His most famous theorem gives the weight of a body immersed in a liquid, called Archimedes’ principle.
Kanada (600 BCE): We recognize Kanada as one of the earliest natural scientists and philosophers. He founded the Vaisheshika school of Hindu philosophy, which is one of the six major schools of Hinduism. Vaisheshika school believes in understanding the universe through perception and inference, making it very similar to modern science.
Kanada hypothesized that all objects in the physical universe are reducible to paramāṇu or molecules, which are made of Anu or atoms. An incredible hypothesis 2600 years ago. He also said that the differences we see in matter around us result from the function of these Paramanus, their number, and their spatial arrangements.
Among the many aspects of nature he discusses in his Vaisheshika Sutra, he talks about gravity too. Kanada calls Gravity as “Guruthva”, which probably made Brahmagupta later call gravitational force as Guruthv-akarshanam”. (Again both referring to mass as Guru in Sanskrit and recognizing the fact that Gravity is resultant of mass).
In just one beautiful Slokha, Kanada describes gravity as:
He means to say the upward movement (Utkshepanam) of a body (on Earth) results from the difference between the force exerted (Prayatna) and gravity experienced (Guruthva). Which not only means gravity is viewed as a force similar to one exerted by action, but also says a positive difference in them will make an object move upwards.
Kannad also described many concepts in laws of motion like force, momentum, inertia of objects, and a reaction that’s always opposite to applied force (similar to Newton’s third law of motion).
At present (2021): Gravity is still a subject of research and new theories are coming out every few years. One of the mainstream theories at present to describe gravity is the Quantum Field Theory. This approach repurposed Einstein’s General Theory of Relativity to explain gravity at very short distances. But it still fails at distances as small as Plank’s length.
I believe gravity is not yet completely understood by humans though we have been trying for thousands of years. But in future, we may be able to not just completely understand it, but manipulate it. Even nullify gravity!