Newton vs. Einstein
According to Newton's theory, a planet moves around the sun because
of the gravitational force put out by the sun. According to Einstein's theory
of general relativity, the planet chooses the shortest possible path
throughout the four-dimensional world. If Einstein is right, and no gravity y
is involved, why have not the planets collided into the sun instead of
circling the sun forever, which they seem to be doing. My first guess s
would be centripetal force, but this force all involves the gravitational
pull. Can you explain this?
Einstein did not claim that gravity did not exist: Einstein's theory
is actually exactly equivalent to Newton's theory for very weak
gravitational forces of normal slow-moving bodies. What happens
is that in Einstein's theory the sun (or other heavy body) distorts
space time around it, in a way that mimics Newton's gravity.
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Update: June 2012