Abnormal sequence of tenses in subordinate clauses?

Here is a type of statements that always puzzles me. For example, here is a sentence:

Einstein discovered that gravity is equivalent to a curvature of the four dimentional space-time.

My English grammar book (for instance the "A pratical English Grammar" by Thomson & Martinet published by Oxford Press) would say the above sentence is grammatically wrong because Einstein discovered that in the past that gravity WAS equivalent to..... In other words the subordinate clause should have tense consistent with the main verb. But the fact that what Einstein discovered is a timeless truth and using past tense make the sentence reads absolutely absurd. Should we use IS or WAS in this case?