Kant , Truth?

Kant said that Space and time aren't a priori concepts , because they are caracterised with singularity , and every spaces that may exist in any world is a part of the Space , which makes it one , same for time , every times that may exist in any world is part of one time , all synchronized , Which leads him to say that Time and Space are a priori form of intuitions , because time is an "Inner sense" he claims ,and space is an "outer sense" he adds .
What made me wonder about that ,is that , what he said is analogous to what classical physics say about that , especially Newton , he also believed in the absolute time and space(not sure about space) . But , Einstein said that time is relative , traveling near the speed of light reduce the time , if my terms are right , it slows time. So Time is relative.
So my question is , If Kant tried to answer Hume scepticism about the objective knowledge ,and what involves from scientific concepts " Time and Space".
Firstly, is my comparaison right , it is , from my part irrational to compare different ideas from different eras , and comparing it also to what science says?
Secondly , Are synthetical and analytical truths are primordial for understanding what the world is alike and what it seems to use ?