  However, Einstein was mistaken. In 1917, the Dutch astronomer Willem  de Sitter published another solution to Einstein’s field equations  containing the cosmological constant. De Sitter’s solution showed  that Einstein’s solution is not a unique solution of his field  equations. In addition, since de Sitter’s universe is empty it  provided a direct counter-example to Einstein’s hope that Mach’s  Principle had been successfully incorporated into his   theory.[80] 