Gravity

Alrighty then... time for an introduction to another topic in physics - gravity.

I am not going to start with the story of the apple falling on Newton's head. The previous statement should not have been the first!

So, Newton explored the concept of gravity and explained it as a force that attracts two bodies to each other. He also provided a mathematical relationship between this gravitational force and the distance between the bodies in question. For most non-relativistic physics, Newton's theory of gravity seems to suffice. For example, the orbits of almost all planets in the solar system seemed to follow the trajectory Newton's equations predicted. Almost! Mercury's orbit seemed to deviate slightly from Newton's predicted trajectory. There really was no explanation for this discrepancy for a while.

Then came Einstein with his theory of general relativity. In this theory, he postulated the concept of the curvature of spacetime - a concept I wish I could visualize. As per this theory, Einstein stated that the planets and stars are just following straight paths along the curvature of spacetime. So, they are not moving due to a force of attraction/repulsion, but due to inertial motion. These straight paths are called 'geodesics'. The theory of general relativity successfully accounted for the deviation of Mercury from Newton's predicted orbit.

Check out this video to get a glimpse into how gravity is a consequence of spacetime curvature - gravity in terms of space-time .