Ancient world The nature and mechanism of gravity were explored by a wide range of ancient scholars. In
Ancient Greece,
Aristotle believed that each of the
classical elements had a
natural place in the universe which it tends to move toward - earth at the center of the universe (the center of the Earth, which was known to be spherical); then water, air, fire, and aether in concentric shells from inner to outer. He also thought that the speed of a falling object should increase with its weight, a conclusion that was later shown to be false. While Aristotle's view was widely accepted throughout Ancient Greece, there were other thinkers such as
Plutarch who correctly predicted that the attraction of gravity was not unique to the Earth. Although he did not understand gravity as a force, the ancient Greek philosopher
Archimedes discovered the
center of gravity of a triangle. He postulated that if two equal weights did not have the same center of gravity, the center of gravity of the two weights together would be in the middle of the line that joins their centers of gravity. Two centuries later, the Roman engineer and architect
Vitruvius contended in his
De architectura that gravity is not dependent on a substance's weight but rather on its "nature". In the 6th century CE, 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 that diminishes over time. In 628 CE, the
Indian mathematician and astronomer
Brahmagupta proposed the idea that gravity is an attractive force that draws objects to the Earth and used the term
gurutvākarṣaṇ to describe it. In the ancient
Middle East, gravity was a topic of fierce debate. The
Persian intellectual
Al-Biruni believed that the force of gravity was not unique to the Earth, and he correctly assumed that other
heavenly bodies should exert a gravitational attraction as well. In contrast,
Al-Khazini held the same position as Aristotle that all matter in the
Universe is attracted to the center of the Earth. , where according to legend Galileo performed an experiment about the speed of falling objects
Scientific Revolution In the mid-16th century, various European scientists experimentally disproved the
Aristotelian notion that heavier objects
fall at a faster rate. In particular, the
Spanish Dominican priest
Domingo de Soto wrote in 1551 that bodies in free fall uniformly accelerate. With the 1586
Delft tower experiment, the
Flemish physicist
Simon Stevin observed that two cannonballs of differing sizes and weights fell at the same rate when dropped from a tower. In the late 16th century,
Galileo Galilei's careful measurements of balls rolling down
inclines allowed him to firmly establish that gravitational acceleration is the same for all objects. Galileo postulated that
air resistance is the reason that objects with a low density and high
surface area fall more slowly in an atmosphere. In his 1638 work
Two New Sciences, Galileo proved that the distance traveled by a falling object is proportional to the
square of the time elapsed. His method was a form of graphical numerical integration since concepts of algebra and calculus were unknown at the time. This was later confirmed by Italian scientists
Jesuits Grimaldi and
Riccioli between 1640 and 1650. They also calculated the magnitude of
the Earth's gravity by measuring the oscillations of a pendulum. Galileo also broke with incorrect ideas of Aristotelian philosophy by regarding
inertia as persistence of motion, not a tendency to come to rest. By considering that the laws of physics appear identical on a moving ship to those on land, Galileo developed the concepts of
reference frame and the
principle of relativity. These concepts would become central to Newton's mechanics, only to be transformed in Einstein's theory of gravity, the general theory of relativity. In last quarter of the 16th century
Tycho Brahe created accurate tools for
astrometry, providing careful observations of the planets. His assistant and successor,
Johannes Kepler analyzed these data into three empirical laws of planetary motion. These laws were central to the development of a theory of gravity a hundred years later. In his 1609 book
Astronomia nova Kepler described gravity as a mutual attraction, claiming that if the Earth and Moon were not held apart by some force they would come together. He recognized that mechanical forces cause action, creating a kind of celestial machine. On the other hand Kepler viewed the force of the Sun on the planets as magnetic and acting tangential to their orbits and he assumed with Aristotle that inertia meant objects tend to come to rest. In 1666,
Giovanni Alfonso Borelli avoided the key problems that limited Kepler. By Borelli's time the concept of inertia had its modern meaning as the tendency of objects to remain in uniform motion and he viewed the Sun as just another heavenly body. Borelli developed the idea of mechanical equilibrium, a balance between inertia and gravity. Newton cited Borelli's influence on his theory. In a communication to the Royal Society in 1666 and his 1674 Gresham lecture,
An Attempt to prove the Annual Motion of the Earth, Hooke took the important step of combining related hypothesis and then forming predictions based on the hypothesis. He wrote: Hooke was an important communicator who helped reformulate the scientific enterprise. He was one of the first professional scientists and worked as the then-new
Royal Society's curator of experiments for 40 years. However his valuable insights remained hypotheses and some of these were incorrect. He was unable to develop a mathematical theory of gravity and work out the consequences. Halley was impressed by the manuscript and urged Newton to expand on it, and a few years later Newton published a groundbreaking book called
Philosophiæ Naturalis Principia Mathematica (
Mathematical Principles of Natural Philosophy). The revolutionary aspect of Newton's theory of gravity was the unification of Earth-bound observations of acceleration with celestial mechanics. This formulation had two important parts. First was
equating inertial mass and gravitational mass. Newton's 2nd law defines force via F=ma for inertial mass, his
law of gravitational force uses the same mass. Newton did experiments with pendulums to verify this concept as best he could. Newton's formulation was later condensed into the inverse-square law:F = G \frac{m_1 m_2}{r^2}, where is the force, and are the masses of the objects interacting, is the distance between the centers of the masses and is the
gravitational constant While is also called Newton's constant, Newton did not use this constant or formula, he only discussed proportionality. But this allowed him to come to an astounding conclusion we take for granted today: the gravity of the Earth on the Moon is the same as the gravity of the Earth on an apple:M_\text{earth} \propto a_\text{apple}R_\text{radius of earth}^2 = a_\text{moon}R_\text{lunar orbit}^2 Using the values known at the time, Newton was able to verify this form of his law. The value of was eventually
measured by
Henry Cavendish in 1797. Newton's theory of gravity ran counter to a key idea of science, both then and now: forces should not rely on instantaneous
action at a distance. Newton was well aware of this issue and his decision to continue anyway marked a shift in scientific thinking away from philosophically sound but empirically flawed models. Scientists like
Gottfried Wilhelm Leibniz complained about this aspect of the theory of gravity. The issue was not resolved until Einstein's work on relativity in the 20th century. Einstein's theory brought two other ideas with independent histories into the physical theories of gravity: the
principle of relativity and
non-Euclidean geometry. The principle of relativity, introduced by Galileo and used as a foundational principle by Newton, led to a long and fruitless search for a
luminiferous aether after
Maxwell's equations demonstrated that light propagated at a fixed speed independent of reference frame. In Newton's mechanics, velocities add: a cannon ball shot from a moving ship would travel with a trajectory which included the motion of the ship. Since light speed was fixed, it was assumed to travel in a fixed, absolute medium. Many experiments sought to reveal this medium but failed and in 1905 Einstein's
special relativity theory showed the aether was not needed. Special relativity proposed that mechanics be reformulated to use the
Lorentz transformation already applicable to light rather than the
Galilean transformation adopted by Newton. Special relativity, as in
special case, specifically did not cover gravity. Every description of gravity in any other coordinate system must transform to give no field in the free-fall case, a powerful
invariance constraint on all theories of gravity. In 1919, the British astrophysicist
Arthur Eddington was able to confirm the predicted deflection of light during
that year's solar eclipse. Eddington measured starlight deflections twice those predicted by Newtonian corpuscular theory, in accordance with the predictions of general relativity. Although Eddington's analysis was later disputed, this experiment made Einstein famous almost overnight and caused general relativity to become widely accepted in the scientific community. In 1959, American physicists
Robert Pound and
Glen Rebka performed
an experiment in which they used
gamma rays to confirm the prediction of
gravitational time dilation. By sending the rays down a 74-foot tower and measuring their frequency at the bottom, the scientists confirmed that light is
Doppler shifted as it moves towards a source of gravity. The observed shift also supports the idea that time runs more slowly in the presence of a gravitational field (many more wave crests pass in a given interval). If light moves outward from a strong source of gravity it will be observed with a
redshift. The
time delay of light passing close to a massive object was first identified by
Irwin I. Shapiro in 1964 in interplanetary spacecraft signals. In 1971, scientists made the first-ever discovery of a black hole, in the constellation
Cygnus. The black hole was detected because it was emitting bursts of
x-rays as it consumed a smaller star, and it came to be known as
Cygnus X-1. This discovery confirmed yet another prediction of general relativity, because Einstein's equations implied that light could not escape from a sufficiently large and compact object.
Frame dragging, the idea that a rotating massive object should twist spacetime around it, was confirmed by
Gravity Probe B results in 2011. In 2015, the
LIGO observatory detected faint
gravitational waves, the existence of which had been predicted by general relativity. Scientists believe that the waves emanated from a
black hole merger that occurred 1.5 billion
light-years away. ==On Earth==