“In Einsteinian relativity space and time are relative but the speed of light is constant.”
- The speed of light is constant only in complete vacuum according to Einstein.
Anyhow, Einstein was very much of brilliant mathematician and theorist and didn't bother much about the concrete aspects of the physical world as long as they didn't interfere with his theories - like they do in the case of the behavior of subatomic particles.
Irritatingly enough, they continue to elude our attempts at trapping them into a given mathematical definition of the freedom behind their choices of actions.
On the contrary, Einstein was quite aware that experimental results of subatomic particles contradicted his theory of relativity. This distressed him greatly. That is why he spent the last 30 years of he life trying to reconcile relativity and quantum mechanics. They can't both be true. And perhaps they are both false. At any rate he failed to find the unified field theory he was looking for.
This contradiction between relativity and quantum mechanics still lies at the heart of physics. Whether string theory or some of the other related theories have resolved the contradiction is not yet clear. I guess we will have to wait and see if physicists have discovered the Theory of Everything.