Michio Kaku on Gravity

“If light takes the path with the least time between two points, and light beams bend under the influence of gravity, then the shortest distance between two points is a curved line. Einstein was shocked by this conclusion: If light could be observed traveling in a curved line, it would mean that space itself is curved.”

“Einstein independently discovered Riemann's original program, to give a purely geometric explanation to the concept of "force." …To Riemann, the bending and warping of space causes the appearance of a force. Thus forces do not really exist; what is actually happening is that space itself is being bent out of shape. The problem with Riemann's approach... was that he had no idea specifically how gravity or electricity and magnetism caused the warping of space. ...Here Einstein succeeded where Riemann failed.”