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The Question

(Submitted April 07, 2006)

General relativity explains gravitation without using the force of gravity by postulating that mass curves space-time. Why would a curved space-time accelerate me towards the earth?

The Answer

Thanks for your excellent question. This is not really an easy question to answer in a few sentences. We'll give you a brief answer here and then recommend you to better websites for more details. As you already know, matter tells spacetime how to curve and curved spacetime, then, tells matter how to move. The idea of acceleration and how it relates to gravity comes from Einstein's Equivalence Principle, which states that you cannot tell the difference between a gravitational field and an equivalent uniform acceleration. So that objects in free fall under gravity all accelerate by the same amount, or in other words, they move the same way as if there was no gravity (ie weightlessness). So what seems to be the result of gravity is really the result of curved spacetime. Since spacetime is curved around massive objects, the Earth is merely traveling along the shortest path in curved spacetime, which has the same appearance as if the Sun were pulling on the Earth due to gravity. Einstein used a simple gedanken experiment using light to illustrate his ideas. We recommend the following websites to follow this gedanken experiment and understand General Relativity better:

http://www.astronomynotes.com/relativity/s3.htm
http://en.wikipedia.org/wiki/General_relativity

Hope this helps,
Georgia & Mike
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