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**Einstein's 1916 paper on General Relativity**

In 1916 Einstein expanded his Special Theory to include the effect of gravitation on the shape of space and the flow of time.

This theory, referred to as the **General Theory of Relativity**, proposed
that matter causes space to curve.

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The large ball will cause a deformation in the sheet's surface. A baseball
dropped onto the sheet will roll toward the bowling ball. Einstein theorized
that smaller masses travel toward larger masses not because they are
"attracted" by a mysterious force, but because the smaller objects
travel through space that is warped by the larger object. Physicists
illustrate this idea using **embedding diagrams**.

Contrary to appearances, an embedding diagram does not depict the three-dimensional "space" of our everyday experience. Rather it shows how a 2D slice through familiar 3D space is curved downwards when embedded in flattened hyperspace. We cannot fully envision this hyperspace; it contains seven dimensions, including one for time! Flattening it to 3D allows us to represent the curvature. Embedding diagrams can help us visualize the implications of Einstein's General Theory of Relativity.

This is a basic postulate of the Theory of General Relativity. It states that a uniform gravitational field (like that near the Earth) is equivalent to a uniform acceleration.

What this means, in effect, is that a person cannot tell the difference between (a) standing on the Earth,
feeling the effects of gravity as a downward pull and (b) standing in a very smooth elevator
that is accelerating upwards at just the right rate of exactly 32 feet per second squared.

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In both cases, a person would feel the same downward pull of gravity. Einstein asserted that these effects were actually the same. A far cry from Newton's view of gravity as a force acting at a distance!

When "generalized" to include gravitation, the equations of relativity predict that gravity, or the curvature of spacetime by matter, not only stretches or shrinks distances (depending on their direction with respect to the gravitational field) but also w ill appear to slow down or "dilate" the flow of time.

In most circumstances in the universe, such **time dilation** is miniscule, but it can become very significant when spacetime is curved by a massive object such as a black hole. For example, an observer far from a black hole would observe time passing
extremely slowly for an astronaut falling through the hole's boundary. In fact, the distant observer would never see the hapless victim actually fall in. His or her time, as measured by the observer, would appear to stand still. The slowing of time near
a very simple black hole has been simulated on supercomputers at NCSA and visualized in a computer-generated animation.

Rest assured that the next section will further illuminate your grasp of relativity -- without math overload!

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