![]() ![]() In Newton's world (first iteration) anything which could be done could be undone: for example we could collect many atoms together to make a planet, but we could then also prise these atoms apart to break the planet back to individual atoms. The existence of such black holes marked a very unusual step in the development of physics. ![]() This process is called the gravitational collapse of the star, and it results in the formation of a black hole. That is, it would get more and more dense as it compresses, till it becomes an infinite density point which forms the singularity. ![]() Computations show that if a star is more than 3 times heavier than the sun, then it would compress in a runaway fashion. When stars run out of the fuel they can burn, they start to compress under their own gravitational attraction. In fact more is true: once things fall through the horizon, they must keep moving towards the center of the hole, where they eventually get crushed at a location called the singularity. There is a boundary surface called the horizon: things can fall in through the horizon, but they cannot come out. In this theory the curvature of spacetime can be made such that we get a trapping region: i.e. A key feature is that no form of matter - particles or waves - can move faster than the speed of light. The behavior of matter in general relativity is given by a natural extension to curved spacetime of the behavior one had in flat spacetime.In this theory the effects of gravity are incorporated by making space and time curved rather than flat. Gravity is described by Einstein's theory of general relativity.The situation changed with the work of Einstein. Thus we see that with Newtonian physics, nothing is really 'trapped' by gravity, and we cannot get a black hole. But he can sit in a rocket, and the thrust provided by the rocket can lift him out to empty space, away from the star. He cannot jump up and escape the gravitational pull of the star. ![]() Consider a person standing on such a star. Even though light and other objects may not be able to fly out on their own from such dark stars, they can still be extracted out by applying a suitable force. Are such dark stars therefore black holes? If we add this assumption to our Newtonian thinking, then it would seem that nothing can fly out of a dark star. Today we have learnt that nothing can travel faster than the speed of light. Using these principles, John Michell conjectured in 1783 that sufficiently massive stars may be invisible: the gravitational pull of the body may be so strong that even light cannot escape from their surface and reach our eyes.
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