Sir Isaac Newton quantified the gravity between two objects when he formulated his three laws of motion. Yet Newton's laws assume that gravity is an innate force of an object that can act over a distance.

Albert Einstein, in his theory of special relativity, determined that the laws of physics are the same for all non-accelerating observers, and he showed that the speed of light within a vacuum is the same no matter the speed at which an observer travels. As a result, he found that space and time were interwoven into a single continuum known as space-time. Events that occur at the same time for one observer could occur at different times for another.

Soon after publishing
the special theory of relativity in 1905, Einstein started thinking about how to incorporate gravity into his new
relativistic framework. In 1907, beginning with a simple thought experiment involving an observer in free fall, he embarked on what would be
an eight-year search for a relativistic theory of gravity. After numerous
detours and false starts, his work culminated in the presentation to the Prussian Academy of Science in
November 1915 of what are now known as the Einstein field equations. These
equations specify how the geometry of space and time is influenced by whatever
matter and radiation are present, and form the core of Einstein's general theory
of relativity.

The Einstein field
equations are nonlinear and
very difficult to solve. Einstein used approximation methods in working out
initial predictions of the theory. But as early as 1916, the astrophysicist Karl
Schwarzschild found the first non-trivial exact solution to the Einstein
field equations, the so-called Schwarzschild metric. This solution laid
the groundwork for the description of the final stages of gravitational
collapse, and the objects known today as black holes. In the same year, the
first steps towards generalizing Schwarzschild's solution to electrically charged objects were taken,
which eventually resulted in the Reissner–Nordström
solution, now associated with electrically charged black holes. In 1917, Einstein
applied his theory to the universe
as a whole, initiating the field of relativistic cosmology. In line with contemporary
thinking, he assumed a static universe, adding a new parameter to his original
field equations—the cosmological constant—to reproduce that
"observation".

^{ }By 1929, however, the work of Hubble and others had shown that our universe is expanding. This is readily described by the expanding cosmological solutions found by Friedmann in 1922, which do not require a cosmological constant. Lemaître used these solutions to formulate the earliest version of the Big Bang models, in which our universe has evolved from an extremely hot and dense earlier state. Einstein later declared the cosmological constant the biggest blunder of his life.
During that period,
general relativity remained something of a curiosity among physical theories. It
was clearly superior to Newtonian gravity, being consistent with special
relativity and accounting for several effects unexplained by the Newtonian
theory. Einstein himself had shown in 1915 how his theory explained the anomalous
perihelion advance of the planet Mercury without any arbitrary parameters ("fudge factors").

^{ }Similarly, a 1919 expedition led by Eddington confirmed general relativity's prediction for the deflection of starlight by the Sun during the total solar eclipse of May 29, 1919,^{ }making Einstein instantly famous. Yet the theory entered the mainstream of theoretical physics and astrophysics only with the developments between approximately 1960 and 1975, now known as the golden age of general relativity. Physicists began to understand the concept of a black hole, and to identify quasars as one of these objects' astrophysical manifestations. Ever more precise solar system tests confirmed the theory's predictive power, and relativistic cosmology, too, became amenable to direct observational tests.
Source: wikipedia

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