Knocking on HeavenâÂÂs Door: How Physics and Scientific Thinking Illuminate the Universe and the Modern World is the second non-fiction book by Lisa Randall. It was initially published on September 20, 2011, by Ecco Press. The title is explained in the text: "Scientists knock on heaven's door in an attempt to cross the threshold separating the known from the unknown."
âÂÂAmerican Scientist