# B A Noob Time Travel Question

1. Nov 4, 2018

### benorin

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.

2. Nov 4, 2018

### andrewkirk

This is a metaphysical question and like so many metaphysical questions, it boils down to the meanings of the words used.

What do we mean when we say that at time t2 it is the case that event E happened at time t1 (t1 < t2)?

What would it mean to say that that at time t2 it was the case that event E happened at time t1 (t1<t2) but at time t3 (t2<t3) it is the case that event E didn't happen at time t1? And how could we tell the difference between that and the case where at both times t2 and t3 it was the case that E happened at t1?

Many metaphysical questions dissolve to nothingness once we zoom in on the meanings of the words.

3. Nov 5, 2018

### benorin

I do not think that just because a statement is made about an event that happened in the past that it is somehow magically theoretical. You physicists speak about the future as if it were real. The past is real too. Isn't it?

With regards to the rest of what you said, suppose there were some lingering effect caused by event $E$ at time $t_1$ that we could detect at time $t_2$, would that bring this conversation to the real physics table?

4. Nov 5, 2018

### sophiecentaur

That statement fits the comment, below. (And, hey - "We Physicists" are not bad people ) What do you mean by "real"?
There is a problem with a question that has the 'basic' label when it deals with a very non-basic idea. It is that the questioner needs to take quite a lot on trust (as we all do, at some point, of course) and accept the results of complicated and sophisticated arguments. There are frequent discussions on PF about Time's Arrow and reversibility and they mostly end up with the lack of any evidence that we can make time go the other way except in very rare effects involving fundamental particles and very short times. As far as I know, there are no examples in which we could go back and kill our grandfathers.

5. Nov 5, 2018

### Staff: Mentor

It's a postulate. You can take it or leave it.

This thread is closed. As with all thread closures, if there's something more to say, you can PM any mentor to ask that it be reopened for your contribution.