Gravity
In 628 CE, the Indian mathematician and astronomer Brahmagupta proposed that gravity is an attractive force drawing objects to Earth. He used the term gurutvākarshana to describe this pull. Earlier thinkers in Ancient Greece held different views. Aristotle believed each classical element had a natural place. Earth sat at the center of the universe while water, air, fire, and aether formed concentric shells around it. He thought heavier objects fell faster than lighter ones. This idea persisted for centuries until experiments proved otherwise.
Two centuries after Archimedes discovered the center of gravity of a triangle, Roman engineer Vitruvius argued in his work De architectura that gravity depended on nature rather than weight. In the 6th century CE, Byzantine scholar John Philoponus introduced the theory of impetus. He suggested motion continued due to a causative force diminishing over time. This modified Aristotle's claim that motion required constant action by a force. The concept shifted how scholars understood falling bodies across the ancient world.
Isaac Newton published his groundbreaking book Philosophiæ Naturalis Principia Mathematica in 1687. It unified terrestrial acceleration with celestial mechanics under one universal law. Before this, scientists like Christopher Wren, Robert Hooke, and Edmund Halley knew Kepler's third law implied an inverse square relationship if orbits were circular. Orbits were actually elliptical. Halley urged Newton to solve the problem. Newton sent a manuscript titled De motu corporum in gyrum to Halley in 1684. That text provided physical justification for planetary motion laws.
Newton claimed gravity operated according to solid matter quantity and propagated at distances inversely proportional to their squares. His formulation equated inertial mass with gravitational mass. Pendulum experiments verified this equivalence as best possible at the time. The second aspect involved the inverse square of distance. Ismaël Bullialdus proposed this around 1640. Newton made quantitative analysis around 1665 considering Moon orbit timing and Earth fall rates. He could not publish results then because proving Earth's gravity acted as if all mass concentrated at its center took twenty years. Henry Cavendish eventually measured the gravitational constant value in 1797.
Albert Einstein developed general relativity in 1915 to explain Mercury's orbital eccentricity. Astronomers noticed the planet's perihelion increased by about 42.98 arcseconds per century. No undiscovered celestial body explained this discrepancy. Einstein's theory accurately modeled the orbit instead. It brought together the principle of relativity and non-Euclidean geometry into physics theories of gravity. Special relativity had already shown light speed fixed independent of reference frames. This eliminated need for luminiferous aether.
In 1919, British astrophysicist Arthur Eddington confirmed predicted light deflection during that year's solar eclipse. Expeditions to Sobral and Principe studied starlight yielding values near Einstein's prediction. The experiment made Einstein famous almost overnight. In 1959, American physicists Robert Pound and Glen Rebka used gamma rays down a 74-foot tower to confirm gravitational time dilation. They observed frequency shifts supporting slower time within strong gravitational fields. These findings established spacetime curvature as the mechanism replacing force-based models.
Scientists detected the first black hole in 1971 within galaxy Cygnus. Known as Cygnus X-1, it emitted bursts of x-rays consuming a smaller companion star. This discovery confirmed another general relativity prediction since light cannot escape sufficiently large compact objects. Frame dragging ideas gained confirmation through Gravity Probe B results released in 2011. Rotating massive objects twist spacetime around them as theory suggested.
The LIGO observatory detected faint gravitational waves on the 14th of September 2015. Waves originated from a black hole merger occurring 1.3 billion light-years away. This observation validated theoretical predictions about energy transport via radiation. Researchers received Nobel Prizes for indirect evidence found in 1973 regarding Hulse-Taylor binary systems and direct detection in 2015. In October 2017, LIGO and Virgo interferometers received signals two seconds before optical telescopes saw corresponding events from sources roughly 130 million light-years distant. This confirmed gravity wave speed matched light speed exactly.
Gravitational attraction between primordial hydrogen clouds and dark matter clumps caused gas to coalesce during early universe history. Hydrogen eventually condensed and fused forming stars. At larger scales this process created galaxies and clusters making gravity the primary driver for large-scale structures. About five-sixths of total universal mass consists of dark matter interacting only through gravity without electromagnetic interactions. Dark matter halos attract hydrogen leading to star and galaxy formation.
During star formation, gravitational attraction competes with thermal pressure inside hydrogen gas clouds. Rising density increases temperature allowing additional condensation when gas radiates energy. Low-mass regions produce brown dwarfs or gas giants while higher masses reach pressures enabling nuclear fusion creating stars. Massive stars burn helium and high atomic number elements producing iron cores before becoming unstable supernovae. Results include neutron stars balancing degeneracy pressure or black holes where gravity operates alone preventing even light escape.
General relativity remains ultimately incompatible with quantum mechanics despite successful predictions at large scales. Einstein described gravity as smooth continuous spacetime distortion while quantum mechanics holds forces arise from discrete particle exchanges known as quanta. This contradiction vexes physicists because other fundamental forces reconciled decades ago within quantum frameworks. Researchers now search for theories uniting both gravity and quantum mechanics under broader mathematical structures.
One path describes gravity via quantum field theory exchanging virtual gravitons similar to photon exchange in electromagnetism. This approach reproduces general relativity in classical limits but fails at short distances near Planck length requiring more complete quantum gravity theories. Alternative models face challenges explaining dark matter nature and matching cosmological data regarding dark energy. Some observations remain inconsistent with standard cosmology leading astrophysicists to study modified Newtonian dynamics alongside continued testing of existing frameworks.
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Common questions
Who proposed that gravity is an attractive force drawing objects to Earth in 628 CE?
The Indian mathematician and astronomer Brahmagupta proposed that gravity is an attractive force drawing objects to Earth in 628 CE. He used the term gurutvākarshana to describe this pull.
When did Isaac Newton publish his book Philosophiæ Naturalis Principia Mathematica?
Isaac Newton published his groundbreaking book Philosophiæ Naturalis Principia Mathematica in 1687. It unified terrestrial acceleration with celestial mechanics under one universal law.
What year did Albert Einstein develop general relativity to explain Mercury's orbital eccentricity?
Albert Einstein developed general relativity in 1915 to explain Mercury's orbital eccentricity. Astronomers noticed the planet's perihelion increased by about 42.98 arcseconds per century before his theory accurately modeled the orbit.
On what date did the LIGO observatory detect faint gravitational waves from a black hole merger?
The LIGO observatory detected faint gravitational waves on the 14th of September 2015. Waves originated from a black hole merger occurring 1.3 billion light-years away.
How much of total universal mass consists of dark matter interacting only through gravity?
About five-sixths of total universal mass consists of dark matter interacting only through gravity without electromagnetic interactions. Dark matter halos attract hydrogen leading to star and galaxy formation.