I was watching a Science Channel show on time travel (though I don't recall the title) and in this show (I hope I get this right from memory, correct me if need be) they stated without proof that 'if something happened in the past is must have always have happened that way', and when they posit something without proof I infer that the explanation must set the bar too high for cable TV but I assume it has some basis in actual science. So let's hear it please? Just how do physicists know that the past is immutable? Do note I have marked this thread Basic, so please try to keep your tensors in your pocket, sir. Note this does not say that you can't time travel to the past, it's just means that if you do, there wasn't ever some alternate timeline wherein you were not present in it, if I understand correctly. Or for those more down-to-Earth with their physics, if a particle collision produces an effect before the collision occurred, it must have happened before the detector was running: time to build a new collider!a I fully expect someone to answer, "Well, isn't it obvious? We can't change the past". I've heard about some physics experiments where effects precede their causes, and I just love that but I'll save it for another post. It may help you formulate your answer to know that I was a math major. I studied mostly analysis, but I enjoyed the standard three semester sequence course of physics once upon a time: should have never stopped doing physics, it was fun. a: Long story short, I heard about a theoretical particle collision that dropped a negative time out the math but they said the collider wasn't configurable to detect things before the collision.