<P> The earliest instance of gravity in the Universe, possibly in the form of quantum gravity, supergravity or a gravitational singularity, along with ordinary space and time, developed during the Planck epoch (up to 10 seconds after the birth of the Universe), possibly from a primeval state, such as a false vacuum, quantum vacuum or virtual particle, in a currently unknown manner . Attempts to develop a theory of gravity consistent with quantum mechanics, a quantum gravity theory, which would allow gravity to be united in a common mathematical framework (a theory of everything) with the other three forces of physics, are a current area of research . </P> <P> 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 gravitational acceleration is the same for all objects . This was a major departure from Aristotle's belief that heavier objects have a higher gravitational acceleration . Galileo postulated air resistance as the reason that objects with less mass may fall slower in an atmosphere . Galileo's work set the stage for the formulation of Newton's theory of gravity . </P> <P> 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 ." The equation is the following: </P> <P> F = G m 1 m 2 r 2 (\ displaystyle F = G (\ frac (m_ (1) m_ (2)) (r ^ (2))) \) </P>

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