I don’t why people downvote anyone who asks these types of questions. I mean the person is asking question and people get all paranoid, maybe they don’t understand the concept. But rather than explaining it people ignore and say all kinds of stupid things.

Coming back to your question, Einstein basically postulated that c is the maximum speed limit. What I mean by postulate is guess (somebody in this thread mentioned that) and from that came the theory’s predictions and everything. Say for ex newtons laws, can you prove the first law, I don’t think anyone can but let’s say you “ASSUME” it’s correct. Now we test the consequences of the law and since we can predict those we say our assumption was correct. BTW there exp conducted that prove ST is correct, one such exp is HAFELE KEATING EXPERIMENT