General Relativity

Today I started reading about this titular subject, and with a massive scientific irony, is possibly one of the easiest concepts to understand in physics, modern or classical. Ever wonder why the Earth pulls us down? Newton says because two masses attract each other... but why? I mean, we know why two magnets attract each other: they're constantly sending photons back and forth "telling" the other to move closer (or further apart). But there is no such chatter in Newtonian gravity. So how do we know which way is down?

Well many people know about "great circles". If you're looking at a map, the path of the plane traveling in a straight line appears an arc of a circle. That's because even though the plane is traveling in a straight line in three dimensions, the projection of the path of the plane into two dimensions is curved. The idea behind general relativity is that objects are always traveling in a straight line in four dimensions (the "spacetime continuum"), but when you project that line into three dimensions (aka, the universe that we observe) you get a curve!

Now you're probably wondering how mass is involved. Well, recall what I've been doing for the past couple weeks: calculating curvature. A straight line has no curvature. A slow tilt has small curvature. A sharp turn has high curvature. Well, with gravity and general relativity, mass is the curvature. In free space, nothing around you, you go in a straight line. Near a planet, you curve into it. Near a black hole, you curve into it faster.