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As stated above,
Newton explained why objects fall towards the earth by postulating an
instantaneous force of attraction between all objects. This works really well
to a point, but it leaves at least unexplained one startling fact: all
objects fall towards the earth at the same rate. This is a complete mystery
in Newton's gravity theory. In order to explain this simple fact, Einstein
had to completely overturn the Newtonian view of the Universe. In particular,
Einstein taught us that that gravity (as an instantaneous force of attraction
between two objects) is a myth. Morever, space is not merely an inert stage
on which physical phenomena occur. Space is a dynamical entity: it has a
shape and structure that is determined by the matter it contains. Objects
fall towards the earth at the same rate because they are all trying to follow
the (same) straightest possible trajectory in the curved space surrounding
the Earth. (Actually, I should be talking not just about space, but space and
time together, but it is easier to visualize the geometry of space, rather
than spacetime .) A simple sheet, or membrane, like a stretched piece of
spandex, can provide a very accurate two dimensional representation of how
gravity works, as shown on the next transparency.
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