What is general relativity? Plus asks David Tong

When physicists talk about Einstein's equation they don't usually mean the famous E=mc2, but another formula, which encapsulates the celebrated general theory of relativity, published in 1915. We asked physicist David Tong of the University of Cambridge to explain what general relativity is and how Einstein's equation expresses it. You can watch his explanation in the video below, or read the accompanying article.