What is Gravity?
Slide 4 of 22
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.