Redefining Reality - The Intellectual Implications of Modern Science

Einstein pointed out that this line of thinking made two important
assumptions: that light moves along the shortest possible route and
that the shortest possible route is always a straight line. Einstein saw
that if we want to maintain the equivalence, we need to surrender
the idea that the shortest path between any two points is always a
straight line.
points is a straight line. But this is not true if you are restricted
to the surface of a smooth globe.
The shortest path between any two points on the equator of the
Earth when represented as a perfect sphere is along the equator,
and that is a curve.
For Einstein to make the move that solves his acceleration and
gravitation problems at the same time, he had to posit that space—
or space-time—is curved. With the help of his friend Marcel
Grossman, Einstein then worked out the math that resulted in a
theory in which gravity is thought to be the curvature of space by
mass and energy.
Newton’s picture of the universe had separate space and time. Time
force, gravity, acting between all the objects but not affecting the
underlying space.
For Einstein, in contrast, gravity is not a force between objects but
an interaction between space and objects. Matter and energy tell
space how to curve, and the curvature of space tells objects how
to move. We need to account for two basic motions that come from
gravitation: straight in for objects close by strong sources of gravity
and periodic orbits for those farther out.
To see how the curvature of space accounts for both, picture
space as a trampoline. If you put a bowling ball on the
trampoline, it sinks, creating a curved surface around it. If you
then tried to roll a tennis ball toward the bowling ball, it would
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