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Robert L. Piccioni, Ph.D.

General Relativity
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Many consider Einstein’s General Theory of Relativity to be one of the greatest achievements of the human mind.

Einstein realized that someone in free fall would feel no gravity. This led him to discover the Equivalence Principal: locally, gravity is equivalent to a uniform acceleration, which greatly simplifies the mathematics.



Einstein said what we call gravity is really the effect of objects traveling on the shortest paths through space and time that is “curved” by mass and energy. These paths don’t look straight to us because we can’t perceive the true shape of spacetime. The image shows the shortest air route from Los Angeles to Moscow; it doesn’t look straight because the map distorts the true shape of Earth’s surface, making it look flat while it is really curved.

The complex math of General Relativity has only been solved in a few simple situations. In these, the curvature of spacetime is determined by a simple expression: 2M/r, where M is the mass of the gravitational source and r is distance from that source. The slide tabulates how much time is slowed down by in various cases. v


Black holes, the most exotic objects in the universe, have just two parts: a central singularity, where all the mass is concentrated; and a surrounding event horizon, which is the point of no return. Just as the horizon on Earth is not a material object but rather the limit of how far we can see, a black hole’s event horizon separates the universe of everything we can observe from the black hole’s interior, from which nothing can ever escape.