Gravitation, or
gravity, is one of the four
fundamental interactions of nature (along with the
strong force,
electromagnetism and the
weak force), in which
objects with
mass attract one another.
Does Gravity Travel at the Speed of Light?,
UCR Mathematics. 1998. Retrieved 3 July 2008
In everyday life, gravitation is most familiar as the agent that gives
weight to objects with mass and causes them to fall to the ground when dropped. Gravitation causes dispersed matter to coalesce, thus accounting for the existence of the
Earth, the
Sun, and most of the macroscopic objects in the
universe. Gravitation is responsible for keeping the Earth and the other planets in their
orbits around the Sun; for keeping the
Moon in its orbit around the Earth; for the formation of
tides; for natural
convection, by which fluid flow occurs under the influence of a density gradient and gravity; for heating the interiors of forming stars and planets to very high temperatures; and for various other phenomena observed on Earth.
Modern
physics describes gravitation using the
general theory of relativity, in which gravitation is a consequence of the curvature of
spacetime which governs the motion of inertial objects. The simpler
Newton's law of universal gravitation provides an accurate approximation for most calculations.
History of gravitational theory
Scientific revolution
Modern work on gravitational theory began with the work of
Galileo Galilei in the late 16th and early 17th centuries. In his famous (though possibly apocryphal) experiment dropping balls from the
Tower of Pisa, and later with careful measurements of balls rolling down
inclines, Galileo showed that gravitation accelerates all objects at the same rate. This was a major departure from
Aristotle's belief that heavier objects accelerate faster.
Galileo (1638),
Two New Sciences,
First Day Salviati speaks: "If this were what Aristotle meant you would burden him with another error which would amount to a falsehood; because, since there is no such sheer height available on earth, it is clear that Aristotle could not have made the experiment; yet he wishes to give us the impression of his having performed it when he speaks of such an effect as one which we see." Galileo correctly postulated air resistance as the reason that lighter objects may fall more slowly in an atmosphere. Galileo's work set the stage for the formulation of Newton's theory of gravity.
Newton's theory of gravitation
In 1687, English mathematician Sir
Isaac Newton published
Principia, which hypothesizes the
inverse-square law of universal gravitation. In his own words, “I deduced that the forces which keep the planets in their orbs must
be reciprocally as the squares of their distances from the centers about which they revolve: and thereby compared the force requisite to keep the Moon in her Orb with the force of gravity at the surface of the Earth; and found them answer pretty nearly.”*{{cite book
| first= Subrahmanyan
| last= Chandrasekhar
| authorlink= Subrahmanyan Chandrasekhar
| title= Newton's Principia for the common reader
| year= 2003
| publisher= Oxford University Press
| location= Oxford}} (pp.1–2). The quotation comes from a memorandum thought to have been written about 1714. As early as 1645
Ismaël Bullialdus had argued that any force exerted by the Sun on distant objects would have to follow an inverse-square law. However, he also dismissed the idea that any such force did exist. See, for example,