 At the end of the second segment on General Relativity, I pointed out that Einstein's prediction that accelerating masses can create gravitational waves had yet to be discovered. As soon as I released the book, they were discovered. So I have added this chapter to the video book to go into gravitational waves. What they are. What is a ripple in space-time? What creates them? What can create a gravitational wave large enough for us to detect? And how did we actually detect it? Now we used interferometers, a Michelson interferometer, to actually detect the first gravitational wave. I'll end by discussing the impact on cosmology that the discovery of gravitational waves means. But first, what is a ripple in space-time?