Einstein's Equation - The Action Principle, the Principle of Least Action

Hilbert was inspired to think about gravity and curved space time. He came up with a different way to derive what we now know as einstein's equation. Heavy objects act more naturally classical than light ones do, he says. And the things t we come across in our everyday classical lives are generally very, very heavy. Their wave packet spreading is not very noticeable. But when those degrees of freedom become very, very massive, then you can show that the rate at which the wave function spreads out is much, much smaller.