Einstein hoped that Mach's Principle would be incorporated into his field equations, which would require that spacetime cannot exist without matter.

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]