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.

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 law that keeps the cosmos in balance, moving objects like stars, winds, and waves. Anaxagoras (c. 500 – c. 428 BC), another Ionian philosopher, introduced the idea of nous ('cosmic mind') as a force that orders things.

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

The ancient atomist Leucippus (5th century BC) said the cosmos began when a large group of atoms came together and swirled in a vortex. The smaller atoms became the stars and planets. The larger atoms in the middle formed the Earth.

Aristotle

In the 4th century BC, Greek philosopher Aristotle taught that things happen for a reason. He said that heavy things like earth and water move down because of their nature (gravity), moving toward the center of the (geocentric) universe. He believed Earth is spherical because every part of Earth pulls down until it reaches the center. Light things like fire and air move up toward the celestial sphere of the Moon. Astronomical objects near the fixed stars are made of aether, which moves in circles. Beyond them is the prime mover, the reason for all motion in the cosmos. Aristotle said that objects fall faster if they are heavier, and slower if the air is thick.

Strato of Lampsacus, Epicurus and Aristarchus of Samos

Greek philosopher Strato of Lampsacus (c. 335 – c. 269 BC) did not believe objects gain weight as they fall. He said they hit harder because they fall faster.

Epicurus (c. 341 – 270 BC) thought weight was a natural part of atoms. These atoms fall down at the same speed in empty space, without any friction. Upward motion happens when atoms bump into each other. Epicureans said atoms can sometimes randomly deviate from their path.

Greek astronomer Aristarchus of Samos (c. 310 – c. 230 BC) thought Earth's rotation on its axis and Earth's orbit around the Sun in a heliocentric system. Seleucus of Seleucia (c. 190 – c. 150 BC) agreed 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) found the centre of mass of a triangle. He said that if the centres of gravity of two equal weights were in different places, the center would be in the middle of the line joining them. He used this to explain the law of the lever and how things float. In On Floating Bodies, Archimedes said that any object in a fluid feels an upward buoyant force equal to the weight of the fluid it pushes aside.

Hipparchus of Nicaea, Lucretius and Vitruvius

Greek astronomer Hipparchus of Nicaea (c. 190 – c. 120 BC) did not agree with Aristotelian physics and followed Strato in thinking about motion. The poem De rerum natura by Lucretius (c. 99 – c. 55 BC) says bigger objects fall faster in air because air slows them less, but in a vacuum all objects fall at the same speed. Roman engineer and architect Vitruvius (c. 85 – c. 15 BC) said in his De architectura that gravity depends on what a thing is made of, not just its weight (cf. specific gravity):

If quicksilver is poured into a container, and a heavy stone is placed on it, the stone floats. But if a small piece of gold is placed on it, the gold sinks. This shows that gravity depends on what a thing is made of, not just how heavy it is. (translated from the original Latin by W. Newton)

Plutarch, Pliny the Elder, and Claudius Ptolemy

Greek philosopher Plutarch (c. 46 – c. 120 AD) wrote that some Roman astronomers did not agree with Aristotelian physics, and thought about inertia and universal gravitation. He suggested that gravity was not just for Earth, but for the Sun and the Moon as well. He also imagined what would happen if a heavy object fell through a tunnel in the Earth.

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 suggested a new idea about motion. He said that motion continues because of a force that gets weaker over time. He also said that if two objects of different weights are dropped from the same height, they will land almost at the same time.

Indian subcontinent

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

Brahmagupta (c. 598 – c. 668 AD) was an Indian scholar who first said that gravity is a force that pulls things toward the Earth. He said heavy objects fall because the Earth naturally pulls them, like how water flows downhill.

Bhāskara II (c. 1114 – c. 1185) was another Indian mathematician and astronomer who also talked about gravity. He wrote that the Earth has a special power to pull heavy things toward it, which 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 the Moon affected tides in the sea. Around 850, Abu Ma'shar al-Balkhi studied the tides and the Moon. He noticed high tides happened even when the Moon was not seen. Abu Ma'shar suggested the Moon and the sea might be connected in a special way. His ideas were later translated into Latin and helped European scholars.

Ibn Sina

In the 11th century, the Persian scholar Ibn Sina (also known as Avicenna) agreed with an idea that when something moves, it gets a push from what is moving it. Ibn Sina wrote about this in his book The Book of Healing. He thought this push stayed with the object until something like air pushed back. 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 have weight and a pulling force, like the Earth. He disagreed with earlier thinkers who thought only Earth had these qualities. Later scholars used these ideas to study how to measure the weight of different objects.

Abu'l-Barakāt al-Baghdādī

In the 12th century, Ibn Malka al-Baghdadi took Ibn Sina's ideas about motion and changed them. He said that when something moves another thing, it gives it a push that gets weaker as the moving thing moves farther away. This was an early step in learning how forces 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. Instead, they 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.

During this time, scholars like Domingo de Soto talked 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 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. Experiments by people like Luca Ghini began to show that all objects fall at the same rate, no matter their weight.

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, scientist Simon Stevin showed that objects fall at the same speed, no matter how big they are. He dropped two lead balls from a tall tower and they hit the ground together. This proved that bigger objects do not fall faster than smaller ones.

Galileo Galilei

Galileo used math to show that objects 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 still, they would move toward each other.

Giovanni Borelli

In 1666, Giovanni Alfonso Borelli explained how things keep 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 they cannot fall any further.

Mechanical explanations

Main article: Mechanical explanations of gravitation

See also: Aether theories

In 1644, René Descartes thought that space is never empty, and that moving matter could push things together. Later scientists used this idea, but these theories 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 shared 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 found a new planet called Neptune by how it changed the orbit of Uranus. This showed 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 the pull between charged particles was stronger than their push. This idea matched 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.