So now, when a planet is moving on it, it's going to have an orbit which deviates from a straight
line, like this. And so Einstein's insight was to say that planets go around the sun in elliptical
orbits, because the cu
rvature of space around the sun makes it so. And that's what we call
gravity.
Compare Einstein's gravity with Newton's. In Newton's world, gravity acted instantaneously at a
distance, like some kind of ghost. Newton didn't like this, but his theory was go
od enough to
describe many phenomena. Einstein changed everything. Space became a physical entity,
deformable by mass and from the relationship between mass and energy, E equals mc squared,
also by energy. Even light, heavy energy bends space. In Einstein'
s world, space is plastic, part
of physical reality, as is time.
n 1917, Einstein applied his new theory to the universe as a whole. If we know how much
matter there is in the universe, we can calculate its geometrical shape. Hence is born modern
cosmology, the application of
general relativity to the whole universe.
