It was Einstein's greatest idea, and one of the most audacious leaps of scientific imagination ever. Much of what physicists know or think they know about space, time and the cosmos depends on it. But General Relativity is usually brushed over in pop sci accounts, because GR is considered too GD difficult for ordinary brains. Even on this scientifically-minded program, we've given it pretty cursory treatment. But not this time. I'm devoting two whole shows to the subject with physicist Anthony Aguirre. He's taught relativity and applies it in his own cosmological research, and does a yeomanly job here of making some very alien concepts approachable.