What is the Theory of Relativity?
Question
Energy equals mass times the speed of light squared. This is known as
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What is the Theory of Relativity?

Hey, I’m Khalid and today with About.com we are going to explore Einstein’s Theory of Relativity, using lessons from the About.com guide site.What is commonly referred to as Albert Einstein’s Theory of Relativity actually encompasses two different concepts of relativity by Einstein, special relativity and general relativity.  These two theories enhanced the classical views of relativity, and explained the physics of objects either very large or very small traveling near the speed of light which classical physics could not explain.Special relativity was outlined in Einstein’s 1905 paper “On the Electrodynamics of Moving Bodies” and explained how to measure motion between objects that are moving at constant speeds in a constant direction relative to one another.  This was based on two postulates; the principle of relativity, which states that the laws of physics are the same for all non-accelerating observers, and the principle of constancy of the speed of light, which states that the speed of light in a vacuum has a definite velocity.  The breakthrough concept here is that the speed of light is constant regardless of the speed of the observer or the speed of the source. From this discovery came the consolidation of the measurements space and time into the one four-dimensional measurement of space-time.  This in turn presented time-dilation, which basically states that, for object traveling near the speed of light, time can actually speed up and slow down in respect to how fast an object travels. Also proposed in this special relativity is Einstein’s famous E=mc2, energy equals mass times the speed of light squared, know as mass-energy equivalence.  This states that energy and mass can neither be created nor destroyed and that there is a connection between energy and matter.  This theory is applied mainly to elementary particles and is used in studies such as atomic physics, nuclear physics, and quantum mechanics.General relativity was published in Einstein’s Field Equations in1915 and explains how to measure motion between two objects that are accelerating relative to one another, and applies to cosmological and astrophysical physics.  In this theory, Einstein expanded his theory of special relativity to include the effects of gravity on space-time.  He also presented the principle of covariance, which established that the laws for physics must apply uniformly in all systems.At first, both studies of relativity were understood as theoretical, but eventually gained practical applications.  Special relativity was the first of the two theories to gain application throughout the scientific community.Thanks for watching.  For more information, please visit About.com