History of gravitational theory
In physics, theories of gravitation postulate mechanisms of interaction governing the movements of bodies with mass. There have been numerous theories of gravitation since ancient times. The first extant sources discussing such theories are found in ancient Greek philosophy. This work was furthered through the Middle Ages by Indian, Islamic, and European scientists, before gaining great strides during the Renaissance and Scientific Revolution—culminating in the formulation of Newton's law of gravity. This was superseded by Albert Einstein's theory of relativity in the early 20th century.
Greek philosopher Aristotle found that objects immersed in a medium tend to fall at speeds proportional to their weight. Vitruvius understood that objects fall based on their specific gravity. In the 6th century AD, Byzantine Alexandrian scholar John Philoponus modified the Aristotelian concept of gravity with the theory of impetus. In the 7th century, Indian astronomer Brahmagupta spoke of gravity as an attractive force. In the 14th century, European philosophers Jean Buridan and Albert of Saxony—who were influenced by Islamic scholars Ibn Sina and Abu'l-Barakat respectively—developed the theory of impetus and linked it to the acceleration and mass of objects. Albert also developed a law of proportion regarding the relationship between the speed of an object in free fall and the time elapsed.
Italians of the 16th century found that objects in free fall tend to accelerate equally. In 1632, Galileo Galilei put forth the basic principle of relativity. The existence of the gravitational constant was explored by various researchers from the mid-17th century, helping Isaac Newton formulate his law of universal gravitation. Newton's classical mechanics were superseded in the early 20th century, when Einstein developed the special and general theories of relativity. An elemental force carrier of gravity is hypothesized in quantum gravity approaches such as string theory, in a potentially unified theory of everything.
Antiquity
Classical antiquity
Heraclitus, Anaxagoras, Empedocles and Leucippus
The pre-Socratic Greek philosopher Heraclitus of the Ionian School used the word logos to describe a kind of law which keeps the cosmos in harmony, moving all objects, including the stars, winds, and waves. Anaxagoras, another Ionian philosopher, introduced the concept of nous as an ordering force.In the cosmogony of the Greek philosopher Empedocles, there were two opposing fundamental cosmic forces of "attraction" and "repulsion", which Empedocles personified as "Love" and "Strife".
The ancient atomist Leucippus proposed the cosmos was created when a large group of atoms came together and swirled as a vortex. The smaller atoms became the celestial bodies of the cosmos. The larger atoms in the center came together as a membrane from which the Earth was formed.
Aristotle
In the 4th century BC, Greek philosopher Aristotle taught that there is no effect or motion without a cause. The cause of the downward natural motion of heavy bodies, such as the classical elements of earth and water, was related to their nature, which caused them to move downward toward the center of the universe. For this reason Aristotle supported a spherical Earth, since "every portion of earth has weight until it reaches the centre, and the jostling of parts greater and smaller would bring about not a waved surface, but rather compression and convergence of part and part until the centre is reached". On the other hand, light bodies such as the element fire and air, were moved by their nature upward toward the celestial sphere of the Moon. Astronomical objects near the fixed stars are composed of aether, whose natural motion is circular. Beyond them is the prime mover, the final cause of all motion in the cosmos. In his Physics, Aristotle correctly asserted that objects immersed in a medium tend to fall at speeds proportional to their weight and inversely proportional to the density of the medium.Strato of Lampsacus, Epicurus and Aristarchus of Samos
Greek philosopher Strato of Lampsacus rejected the Aristotelian belief of "natural places" in exchange for a mechanical view in which objects do not gain weight as they fall, instead arguing that the greater impact was due to an increase in speed.Epicurus viewed weight as an inherent property of atoms which influences their movement. These atoms move downward in constant free fall within an infinite vacuum without friction at equal speed, regardless of their mass. On the other hand, upward motion is due to atomic collisions. Epicureans deviated from older atomist theories like that of Democritus by proposing the idea that atoms may randomly deviate from their expected course.
Greek astronomer Aristarchus of Samos theorized Earth's rotation around its own axis, as well as Earth's orbit around the Sun in a heliocentric cosmology. Seleucus of Seleucia supported his cosmology and also described gravitational effects of the Moon on the tidal range.
Archimedes
The 3rd-century BC Greek physicist Archimedes discovered the centre of mass of a triangle. He also postulated that if the centres of gravity of two equal weights was not the same, it would be located in the middle of the line that joins them, a result he used to prove the law of the lever and to extend his equilibrium analysis to floating bodies. In On Floating Bodies, Archimedes claimed that for any object submerged in a fluid there is an equivalent upward buoyant force to the weight of the fluid displaced by the object's volume. The fluids described by Archimedes are not self-gravitating, since he assumes that "any fluid at rest is the surface of a sphere whose centre is the same as that of the Earth".Hipparchus of Nicaea, Lucretius and Vitruvius
Greek astronomer Hipparchus of Nicaea also rejected Aristotelian physics and followed Strato in adopting some form of theory of impetus to explain motion. The poem De rerum natura by Lucretius asserts that more massive bodies fall faster in a medium because the latter resists less, but in a vacuum fall with equal speed. Roman engineer and architect Vitruvius contends in his De architectura that gravity is not dependent on a substance's weight but rather on its 'nature' :If the quicksilver is poured into a vessel, and a stone weighing one hundred pounds is laid upon it, the stone swims on the surface, and cannot depress the liquid, nor break through, nor separate it. If we remove the hundred pound weight, and put on a scruple of gold, it will not swim, but will sink to the bottom of its own accord. Hence, it is undeniable that the gravity of a substance depends not on the amount of its weight, but on its nature.
Plutarch, Pliny the Elder, and Claudius Ptolemy
Greek philosopher Plutarch attested the existence of Roman astronomers who rejected Aristotelian physics, "even contemplating theories of inertia and universal gravitation". In his work De facie in orbe lunae, he suggested that gravitational attraction was not unique to the Earth, but applied to other bodies such as the Sun and the Moon, which were held to attract the parts of which they are made. His conception of gravity as a tendency of parts to unite with their whole coincides with Nicolaus Copernicus’s account of gravity. He also proposed a thought experiment in which a heavy object falling through a tunnel in the Earth might either stop at the center or overshoot and oscillate, anticipating the medieval theory of impetus.The gravitational effects of the Moon on the tides were noticed by Pliny the Elder in his Naturalis Historia and Claudius Ptolemy in his Tetrabiblos.
Byzantine era
John Philoponus
In the 6th century AD, the Byzantine Alexandrian scholar John Philoponus proposed the theory of impetus, which modifies Aristotle's theory that "continuation of motion depends on continued action of a force" by incorporating a causative force which diminishes over time. In his commentary on Aristotle's Physics that "if one lets fall simultaneously from the same height two bodies differing greatly in weight, one will find that the ratio of the times of their motion does not correspond to the ratios of their weights, but the difference in time is a very small one".Indian subcontinent
Brahmagupta
was the first Indian scholar to describe gravity as an attractive force:The earth on all its sides is the same; all people on the earth stand upright, and all heavy things fall down to the earth by a law of nature, for it is the nature of the earth to attract and to keep things, as it is the nature of water to flow... If a thing wants to go deeper down than the earth, let it try. The earth is the only low thing, and seeds always return to it, in whatever direction you may throw them away, and never rise upwards from the earth.
Bhāskara II
, another Indian mathematician and astronomer, describes gravity as an inherent attractive property of Earth in the section "" of his treatise :The property of attraction is inherent in the Earth. By this property the Earth attracts any unsupported heavy thing towards it: The thing appears to be falling but it is in a state of being drawn to Earth.... It is manifest from this that... people situated at distances of a fourth part of the circumference from us or in the opposite hemisphere, cannot by any means fall downwards .