History of gravitational theory
In the 5th century BC, Leucippus proposed that a large group of atoms swirled as a vortex to create the cosmos. Smaller atoms became celestial bodies while larger ones formed Earth at the center. Heraclitus used the word logos to describe a law keeping all objects in harmony. Anaxagoras introduced nous as an ordering force for the universe. Empedocles personified two opposing cosmic forces as Love and Strife. Aristotle taught that heavy bodies move downward toward the center of the geocentric universe due to their nature. He asserted that objects immersed in a medium fall at speeds proportional to their weight. Vitruvius argued that gravity depends on specific gravity rather than total weight. Plutarch attested that Roman astronomers contemplated theories of inertia and universal gravitation.
John Philoponus modified Aristotelian concepts in the 6th century AD with his theory of impetus. He stated that if one lets fall simultaneously from the same height two bodies differing greatly in weight, the difference in time is very small. Brahmagupta described gravity as an attractive force in the 7th century. He wrote that the earth attracts any unsupported heavy thing towards it by its nature. Ibn Sina published his own theory of impetus in The Book of Healing during the 11th century. He viewed inclination as persistent requiring external forces such as air resistance to dissipate it. Al-Biruni proposed that heavenly bodies have mass just like Earth in the 12th century. Abu'l-Barakat al-Baghdadi adopted and modified Ibn Sina's theory on projectile motion around 1150.
Leonardo da Vinci recorded drawings showing the acceleration of falling objects between 1452 and 1519. He noted that a water-pouring vase moving transversally produces a right triangle composed of falling material. Simon Stevin dropped two lead balls from the Nieuwe Kerk in Delft in 1585. From the sound of impacts he deduced that the balls had fallen at the same speed. Galileo hypothesized in a 1604 letter to Paolo Sarpi that distance is proportional to the square of time elapsed. Grimaldi and Riccioli confirmed this relation between 1640 and 1650 using pendulum oscillations. Domingo de Soto theorized uniform free fall acceleration in his 1551 book Physicorum Aristotelis quaestiones. Jean Buridan attributed motion to an impetus varying according to velocity and mass.
Robert Hooke wrote to Isaac Newton in 1679 about orbital motion as a combination of tangential inertial motion and central force. Halley visited Newton in January 1684 asking what trajectory an inverse square force would produce. Newton sent De motu corporum in gyrum by November 1684 deriving Kepler's laws mathematically. Philosophiæ Naturalis Principia Mathematica appeared in 1687 with Halley's support. Gottfried Wilhelm Leibniz complained that gravity relied on invisible action at a distance. Voltaire countered these concerns in 1738 writing his own book for French readers. The great inequality of Jupiter and Saturn troubled astronomers until Laplace resolved it in 1784. Le Verrier predicted Neptune's position in 1846 which Galle spotted the same night. Mercury's orbit could not be accounted for entirely under Newtonian gravity by the late 19th century.
Albert Einstein published papers establishing special relativity in 1905 claiming mass and energy are equivalent. He described free fall experiencing no gravitational field as the happiest thought of his life in 1907. General relativity fused three dimensions of space and one dimension of time into spacetime fabric. Einstein and David Hilbert discovered field equations relating matter presence to spacetime curvature. Arthur Eddington observed gravitational lensing around a solar eclipse in May 1919 matching Einstein's equations. Edwin Hubble observed the universe appears to be expanding in 1929. Paul Dirac developed the hypothesis that gravitation should slowly decrease over cosmic history by the 1930s. Alan Guth and Alexei Starobinsky proposed cosmic inflation driven by negative pressure in 1980.
The Schwarzschild solution describes spacetime surrounding a spherically symmetrical non-rotating massive object. The Robertson-Walker metric predicts the expansion of the universe from 1922 and 1924. Irwin Shapiro identified time delay of light passing close to a massive object in 1964. Binary pulsars such as PSR 1913+16 provided indirect confirmation of gravitational radiation. LIGO experiments directly detected gravitational waves from two colliding black holes in 2015. Neutron star mergers since detected in 2017 may also create detectable amounts of gravitational radiation. Dark energy composed around 68.3% of the early universe was found in 2013. Dark matter comprised 26.8% of the early universe according to 2013 findings.
General relativity cannot be the complete theory because it is incompatible with quantum mechanics. Attractive force arises due to exchange of virtual gravitons in quantum field theory frameworks. This reproduces general relativity only at linearized level and Planck length scales. Jacobus Kapteyn proposed dark matter existence in 1922 moving stars faster than gravity alone accounts for. Max Abraham developed an alternative model where speed of light depends on gravitational field strength in 1914. Gunnar Nordström attempted unifying gravity and electromagnetism in five-dimensional gravitation during 1914. Oskar Klein gave physical interpretation to prototypical string theory in 1921 as potential theory of everything.
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Common questions
Who proposed that atoms swirled as a vortex to create the cosmos in the 5th century BC?
Leucippus proposed that a large group of atoms swirled as a vortex to create the cosmos. Smaller atoms became celestial bodies while larger ones formed Earth at the center.
When did Galileo hypothesize that distance is proportional to the square of time elapsed?
Galileo hypothesized in a letter dated the 1st of January 1604 that distance is proportional to the square of time elapsed. Grimaldi and Riccioli confirmed this relation between the 1st of January 1640 and the 31st of December 1650 using pendulum oscillations.
What year did Newton publish Philosophiæ Naturalis Principia Mathematica with Halley's support?
Philosophiæ Naturalis Principia Mathematica appeared in the year 1687 with Halley's support. Robert Hooke wrote to Isaac Newton about orbital motion on the 1st of November 1679, and Halley visited Newton in January 1684 asking what trajectory an inverse square force would produce.
How did Albert Einstein describe free fall experiencing no gravitational field in 1907?
Albert Einstein described free fall experiencing no gravitational field as the happiest thought of his life in 1907. He published papers establishing special relativity in 1905 claiming mass and energy are equivalent.
When did LIGO experiments directly detect gravitational waves from two colliding black holes?
LIGO experiments directly detected gravitational waves from two colliding black holes in the year 2015. Neutron star mergers since detected in 2017 may also create detectable amounts of gravitational radiation.