Dr. Robert Wald, the Charles A. Swift Distinguished Service Professor of Physics at the University of Chicago, discusses three major ingredients underlying general relativity—special relativity, which says that space and time comprise a single entity; the equivalence principle, which says that all bodies fall the same way in a gravitational field; and the equation that relates the curvature of spacetime to the matter content in the universe.

Dr. Heino Falcke, a professor of astroparticle physics and radio astronomy at Radboud University in Nijmegen, the Netherlands, discusses “Black Hole Horizons.” Falcke is a member of the Royal Netherlands Academy of Arts and Sciences. In 2011, he was awarded the Spinoza Prize, the top science award in the Netherlands. In 2000, he received the Ludwig Biermann Award, the German Astronomical Society award for outstanding young astronomers. In 2013, the BlackHoleCam team he co-founded received a...