In 1945, Albert Einstein wrote a group of soldiers stationed in the Pacific Theater of World War II, explaining that space is a four-dimensional continuum and that reconciled that with his newer scientific theories. The sale of that unpublished letter was covered in media outlets worldwide. It sold to a private collector. The full description can be seen here: http://raabcollection.com/albert-einstein-autograph/einstein-letter-relativity-spacetime