The black hole is a hypothetical extremely dense and massive heavenly body generally believe to populate the universe in a sizeable degree relative to the visible stars. Everything that falls into its effective territory known as the event horizon is dragged into it and into oblivion by the extremely strong pull of its gravitation. Not even light can manage to escape, making it completely invisible, thus its name.
As early as the late18th century, a British amateur astronomer Reverend John Michel put forward the idea of these bodies with gravity very strong enough to prevent light from escaping. And in 1795 Pierre-Simon Laplace of France arrived at the same deduction independently. But having lived in the period in which scientific developments were just starting, none of them succeeded at providing proofs for their bizarre hypothesis. Albert Einstein's General Theory of Relativity published in 1916 would later provide compelling and striking theoretical backing for the existence of these invisible, yet extremely massive heavenly objects.
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Simulated view of a black hole in front of the Milky Way. The hole has 10 solar masses and is viewed from a distance of 600 km.
One of the grand deductions from Einstein's field equations is the attachment of time as the fourth dimension, making a four dimensional firmament that Einstein aptly called spacetime. He went on to show the what is now considered as the main crux of the General Theory - the alteration of spacetime geometry when enfolding massive bodies. Very dense or massive objects curve spacetime around it. And everything is constrained to traverse or get dragged into that spacetime curvature, including light. So instead of a force as viewed from the Newtonian theory, gravitation according to Einstein is spacetime warping, leaving the Newtonian gravitation as an accurate approximation only of the new theory in which the former remains nevertheless extremely accurate at predicting behaviors of such relatively non-massive entities as the planets in our solar system, but when it comes to gigantic bodies in the universe, gravitation being viewed as force fails and as spacetime curvature comes into play. Being the most massive object in the solar system, Einstein predicted and provided rigorous proofs for such curvature existing at the sun and that external light rays passing nearby will traverse such curvature, thus the bending of light.
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Warping of spacetime caused by the presence of a massive and dense body
Controversial at first, but Einstein became extremely famous when the bending of a star light passing near the sun was experimentally confirmed by Arthur Eddington of England during the 1919 total solar eclipse. Here on earth flashlights always travel on a straight line path because the earth's mass and density are insignificant to cause our own spacetime encompassing our little planet to warp, thus the no event or observation of light bending in the earthbound setting.
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Bending of a starlight near the sun
Although spacetime warping and the bending of light near the sun was some indication of the black hole grabbing light into it was not used by Einstein to argue in favor of the existence of the black hole. He reasoned that black hole was sheer product of bizarre illusions of the human mind. In fact, barely three months after the publication of the General Theory, a young German physicist, Karl Swartzchild found one of the exact solutions to Einstein's field equations whose result pointed out strongly to the existence of the black hole. The creator of the theory was all appreciation to the solution that he himself missed to obtain, but surprisingly black holes in the mind of Einstein until his death in 1955 were just sort of mathematical curiosity that simply did not exist.
Nevertheless, Einstein's theory is now the cosmologists' primary tool in unlocking the mysteries of the universe involving complex mathematical proofs for the existence of these strange heavenly bodies.
In addition, the more sophisticated scientific methods and advancement in scientific knowledge today are now gradually bringing to light more compelling evidences of these invisible and strange objects.
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A NASA concept drawing of a black hole
Among the multitude of observed facts supporting their presence are the many recurrent sightings of stars revolving around an invisible center. These are compelling evidences for the orbital motion of heavenly bodies is due to the constant gravitational grip of a much larger body. The situation is pretty much similar to our solar system with the planets revolving around the sun. Alternatively, one may observe gas (from a nearby star, for instance) that has been drawn into the black hole. The gas spirals inward, heating up to very high temperatures and emitting large amounts of radiation that can be detected from earthbound and earth-orbiting telescopes. Such observations have resulted in the general scientific consensus that-barring a breakdown in our understanding of nature-black holes do exist in our universe (Wikepedia).
Quiet remarkably, these discoveries are just subsequent results of further scientific explorations that may have not been ventured upon without the strong theoretical indications provided by the General Theory of Relativity.
