The conceptual consequences of the General Theory of Relativity remain controversial for a long time.

Do space and time only exist when there is matter too?

Does the fact that space and time are involved in physical events mean that the old concept of ether returns in a new form into physics?

On that basis Einstein discusses reintroducing the concept of ether in his inaugural lecture as Extraordinary Professor at the University of Leiden on 27 October 1920.