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# General Relativity

General relativity is about gravity or the attraction that any object has for any other object. We think of gravity as what keeps us on the ground, The earth's gravity is a major factor in our lives. But gravity is universal. Every object attracts every other object. The prior Newtonian theory of gravity was a bit of a mystery. It required action at a distance. Einstein solved the mystery by showing that gravity warps space and time. It does so in a way that is called local. Gravity propagates through space at the speed of light.

Special relativity is based on the simplifying assumption that the laws of physics are the same in any inertial frame of reference. General relativity is based on the simplifying assumption that gravity and acceleration are indistinguishable. If your ship in deep space accelerates at just the right rate you will feel the same force pushing you towards the floor that you feel on earth.

Special relativity suggests that time and space measurements transform so that one cannot detect absolute motion. Philosophically (but not mathematically) special relativity denies the existence of an absolute frame of reference. General relativity suggests that mass warps space and time to appear just as they do to someone under uniform acceleration in deep space.

General relativity implies that space can be warped by a massive object to the point space turns in on itself. An object that produces a gravitational field this strong is called a black hole. External objects can fall into a black hole but nothing can escape. Black holes come in all sizes. The creation of a black hole depends on both the mass of an object and how small it is. The force of Gravity falls off with the square of the distance. Thus in theory any mass no matter how small could create a black hole if it was packed into a small enough space. This is part of the problem in combining relativity and quantum mechanics. At small enough distances the quantum mechanical uncertainty in density can be large enough to allow for the creation of black holes.