History of gravitational theory

Theories about gravity help us understand how objects with mass move and interact. People have been thinking about gravity for thousands of years. Early ideas came from ancient Greek thinkers like Aristotle, who noticed that heavier objects fall faster. Over time, many scientists added to our understanding.

In the 1600s, Galileo Galilei showed that all objects fall at the same rate if you ignore things like air resistance. Later, Isaac Newton created a clear law explaining how every mass attracts every other mass. This became known as Newton’s law of gravity.

In the early 1900s, Albert Einstein changed our view again with his theory of relativity. Instead of simple attraction, he showed that massive objects actually change the shape of space and time around them, which guides how other objects move. Today, scientists are still working to combine Einstein’s ideas with tiny particles to create a fuller picture of gravity.

Antiquity

See also: Archimedes' principle, Aristotelian physics, Epicureanism, and Principle of inertia

Classical antiquity

Heraclitus, Anaxagoras, Empedocles and Leucippus

The pre-Socratic Greek philosopher Heraclitus (c. 535 – c. 475 BC) of the Ionian School used the word logos ('word') to describe a kind of law which keeps the cosmos in harmony, moving all objects, including the stars, winds, and waves. Anaxagoras (c. 500 – c. 428 BC), another Ionian philosopher, introduced the concept of nous ('cosmic mind') as an ordering force.

In the cosmogony of the Greek philosopher Empedocles (c. 494 – c. 434/443 BC), there were two opposing fundamental cosmic forces of "attraction" and "repulsion", which Empedocles personified as "Love" and "Strife" (Philotes and Neikos).

The ancient atomist Leucippus (5th century BC) 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 (gravity), which caused them to move downward toward the center of the (geocentric) 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 (levity) upward toward the celestial sphere of the Moon (see sublunary sphere). 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 (c. 335 – c. 269 BC) 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 (c. 341 – 270 BC) 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 (c. 460 – c. 370 BC) by proposing the idea that atoms may randomly deviate from their expected course.

Greek astronomer Aristarchus of Samos (c. 310 – c. 230 BC) theorized Earth's rotation around its own axis, as well as Earth's orbit around the Sun in a heliocentric cosmology. Seleucus of Seleucia (c. 190 – c. 150 BC) supported his cosmology and also described gravitational effects of the Moon on the tidal range.

Archimedes

The 3rd-century BC Greek physicist Archimedes (c. 287 – c. 212 BC) 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 (c. 190 – c. 120 BC) 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 (c. 99 – c. 55 BC) 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 (c. 85 – c. 15 BC) contends in his De architectura that gravity is not dependent on a substance's weight but rather on its 'nature' (cf. specific gravity):

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. (translated from the original Latin by W. Newton)

Plutarch, Pliny the Elder, and Claudius Ptolemy

Greek philosopher Plutarch (c. 46 – c. 120 AD) 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 (23–79 AD) in his Naturalis Historia and Claudius Ptolemy (c. 100 – c. 170 AD) 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

See also: History of science and technology in the Indian subcontinent

Brahmagupta (c. 598 – c. 668 AD) was the first Indian scholar to describe gravity as a force that pulls things toward the Earth. He explained that heavy objects fall down because the Earth naturally attracts them, like how water flows.

Bhāskara II (c. 1114 – c. 1185), another Indian mathematician and astronomer, also talked about gravity. He wrote that the Earth has a special power to pull heavy things toward it, and this is why objects fall down.

Islamic world

See also: Physics in the medieval Islamic world and Astronomy in the medieval Islamic world

Abu Ma'shar

Ancient Greeks like Posidonius thought that tides in the sea were influenced by the Moon. Around 850, Abu Ma'shar al-Balkhi studied the tides and the Moon's position. He noticed that high tides happened even when the Moon was not visible in the sky. Abu Ma'shar suggested that the Moon and the sea might have a special connection that pulled them together. His ideas were later translated into Latin and became an important theory for European scholars.

Ibn Sina

In the 11th century, the Persian scholar Ibn Sina (also known as Avicenna) agreed with another thinker's idea that when something is moved, it gains a kind of push from the thing moving it. Ibn Sina wrote about this in his book The Book of Healing. He thought this push stayed with the object and only went away if something like air pushed back on it. He also talked about how objects move toward the center of the Earth because of a pulling force.

Al-Biruni

Another Persian scholar from the 11th century, Al-Biruni, said that stars and planets also have weight and a pulling force, just like the Earth. He disagreed with earlier thinkers who thought only Earth had these qualities. Later scholars built on these ideas and studied how to measure the weight of different objects using special balances and scales.

Abu'l-Barakāt al-Baghdādī

In the 12th century, Ibn Malka al-Baghdadi took Ibn Sina's ideas about how things move and changed them a bit. He said that when something moves another thing, it gives it a kind of push that gets weaker as the moving thing gets farther away. This was an early step toward understanding how forces really work.

European Renaissance

See also: Science in the Renaissance

In the 1300s, French thinker Jean Buridan and scholars at Merton College in Oxford changed how people thought about gravity. They stopped using old ideas from Aristotle and instead thought that objects move because of something called "impetus," which is like momentum. This impetus changes based on how fast something is moving and how heavy it is. Another thinker, Albert of Saxony, used these ideas to explain how things speed up when they fall and even tried to measure how speed changes over time.

During the same time, scholars like Domingo de Soto started talking about things speeding up evenly when they fall. They called this "uniform difform" motion. One big idea from this time was the "mean speed theorem," which says that something falling from rest will travel the same distance as something moving at a steady speed that is half as fast. This idea helped shape later ideas about gravity.

Artists and scientists like Leonardo da Vinci also studied falling objects and wrote about gravity. By the 1500s, thinkers such as Nicolaus Copernicus and Petrus Apianus were exploring new ways to understand how gravity works, and experiments by people like Luca Ghini began to show that all objects fall at the same rate, no matter their weight. These ideas paved the way for even greater discoveries in science.

Main article: Mean speed theorem

See also: Equations of motion § History

Scientific Revolution

See also: Scientific Revolution and Science in the Age of Enlightenment

Simon Stevin

Main article: Delft tower experiment

In 1585, Flemish scientist Simon Stevin showed that objects fall at the same speed, no matter their size. He dropped two lead balls from a tall tower in Delft, and heard them hit the ground at the same time. This proved that heavier objects do not fall faster than lighter ones.

Galileo Galilei

Galileo used math to show that objects falling down fall faster the longer they are falling. He wrote about this in 1638. He also thought that in space, with no air, all objects would fall at the same speed. Italian scientists later checked this with pendulums.

Johannes Kepler

In 1609, Johannes Kepler suggested that Earth pulls on objects, like how magnets stick together. He thought if Earth and the Moon were not moving, they would come together.

Giovanni Borelli

In 1666, Giovanni Alfonso Borelli explained how things stay moving unless something stops them, and how gravity pulls them together. Isaac Newton later used his ideas.

Evangelista Torricelli

A student of Galileo, Evangelista Torricelli, thought that things balance when their center of weight cannot fall further.

Mechanical explanations

Main article: Mechanical explanations of gravitation

See also: Aether theories

In 1644, René Descartes thought that space cannot be empty, and that matter moving in whirls could push things together. Later scientists built on this idea, but these theories had problems and were replaced by better ones.

'Weight' before Newton

Main article: Mass § Pre-Newtonian concepts

Before Newton, people used the word "weight" to mean both how much stuff something is made of and how heavy it feels. They thought these two ideas were the same.

Mass as distinct from weight

In 1686, Newton explained that mass is how much matter something has, based on its density and size.

Newton's law of universal gravitation

See also: Gauss's law for gravity

In 1687, Newton published his ideas about gravity. He showed that the force pulling planets works the same way as the force pulling things down on Earth. He explained that this force gets weaker the farther away two objects are.

Newton’s big idea was that every object pulls on every other object. The strength of this pull depends on how much stuff (mass) is in each object and how far apart they are. He called this the law of universal gravitation.

Later, scientists tested Newton’s ideas with comets and even predicted a new planet called Neptune by how it affected the orbit of Uranus. This proved his theory was very accurate for most situations.

Modern era

See also: Alternatives to general relativity

In 1900, Hendrik Lorentz tried to explain gravity using his ether theory and Maxwell's equations. He thought that the pull between charged particles was stronger than their push. This idea matched what we call universal gravitation, where the speed of gravity equals the speed of light. Lorentz found that his calculation for Mercury's orbit didn't quite match.

In the late 1800s, Lord Kelvin wondered if everything in the universe pulsed, which might explain gravity and electric charges. But his ideas needed something called the aether, which wasn't found in experiments. This, along with Mach's principle, led to new ideas about gravity.

Albert Einstein changed everything with his theory of relativity, published in 1905 and 1915. It explained Mercury's orbit. In 1919, when Arthur Eddington saw gravitational lensing during a solar eclipse, it proved Einstein's ideas. After that, many scientists built on Einstein's work.

Einstein's field equations included something called a cosmological constant to keep the universe steady. But Edwin Hubble found in 1929 that the universe was actually expanding. Later, ideas about dark energy and dark matter came up to explain more about the universe.

In 1957, Hermann Bondi suggested that certain types of mass might fit with Einstein's ideas and Newton's laws of motion.

Early gravity theories tried to explain how planets move. Then came attempts to mix gravity with other ideas in physics. The discovery of Lorentz transformations changed everything, leading to more tests and finally to general relativity.

Einstein (1905–1912)

In 1905, Albert Einstein published papers that showed mass and energy are equivalent. In 1907, he had a big insight: someone falling freely feels no gravity, like being in space.

Einstein's work in 1912 built on this. He knew about how gravity bends light and that some basic ideas about space and time needed updating. His theories used special math to describe space and time.

Lorentz-invariant models (1905–1910)

Using ideas from Henri Poincaré, Hermann Minkowski, and Arnold Sommerfeld, scientists tried to update Newton's gravity to fit with Lorentz invariant ideas, where gravity's speed is like light's.

Abraham (1912)

Max Abraham created a different gravity model where the speed of light changed with gravity. His 1914 review was good, but his model wasn't as strong.

Nordström (1912)

Gunnar Nordström first tried to keep some basic space-time ideas but let mass change with gravity. His second try in 1913 was the first clear theory that matched relativity, kept Newton's ideas in simple cases, and worked with basic physics rules.

Einstein and Fokker (1914)

In 1914, Einstein and Fokker created a gravity theory that followed general rules strictly. They linked their work to Nordström's ideas and showed how matter and space curve together.

Between 1911 and 1915, Einstein turned his idea that gravity feels like acceleration into his full theory of general relativity. This theory combines space and time into one fabric, but it doesn't yet explain gravity at the smallest levels.

General relativity

Main article: History of general relativity

In general relativity, gravity isn't a force but comes from how space and time curve because of matter. The starting idea is that falling freely feels like moving without forces. Einstein and David Hilbert found equations that show how matter makes space curve. These equations help us understand space's shape and how things move through it.

Important results from these equations include:

• The Schwarzschild solution, describing space around a round, still object. For very small objects, this can create black holes.
• The Reissner–Nordström solution, for objects with charge, which can also create black holes.
• The Kerr solution for spinning objects, also making black holes.
• The Robertson–Walker solution, showing the universe expands.

General relativity has been proven many times. It explains:

• Why Mercury's orbit wobbles.
• How light bends around the Sun.
• That the universe expands.
• That time passes slower in stronger gravity.
• That light takes longer to pass near heavy objects.
• That space can wiggle, as seen when black holes crash.

We think events like neutron stars crashing might also make space wiggle in ways we can detect.

Quantum gravity

Main article: Quantum gravity

Later, people saw that general relativity didn't fully fit with quantum ideas. Trying to mix gravity with quantum physics, they thought of tiny particles called gravitons that carry gravity's push, like photons carry light's push. This works in simple cases but not always, especially at very small sizes.