Q&A: Going Back in Time
Going Back in Time
Question
In the “Tannaim” series you argued that going back in time is logically possible if we define it over two separate axes. That immediately raised a question for me. First, if today is Thursday and I go back to Monday on axis T1, there would still be something strange here. For example: on Monday I was at the bank and spoke with the clerk. On Thursday I returned to that same situation that took place at the bank on Monday. Is the clerk I’m speaking with (again) the same clerk as on Thursday? Meaning, is it the same person or a different person? If it’s a different person, then I didn’t go back in time but moved to a different timeline.
Second, there may even be something contradictory here: for example, suppose people were hiking on a trek in Europe on Monday. I wasn’t there then (I was in Israel), and now (on Thursday) I go back in time to Sunday (on axis T1 of course), catch a flight to that trek, and meet those people on Monday. In what sense did I go “back in time”? After all, the original event that happened on that Monday was that only those people were there. But on the new Monday that I returned to, there was a different event, one in which I was also present. So I didn’t return to the same event, but to a different one. It follows that I didn’t go back in time but moved to a parallel timeline.
Besides that, the grandfather paradox shows that acting backward in time is impossible, so to claim that I went back and simply changed the situation by flying to the trek is a problematic claim.
I’d be happy to hear your thoughts on these comments.
Answer
A. I didn’t understand question A. You started formulating a “either way” argument but stopped after the first horn. So what exactly is the question? It’s the same clerk, that’s all.
B. You’re ignoring the fact that there are two axes here. Going back in time happens only on one axis. The second keeps progressing normally. In the new time, you have not returned to the same point on the second time axis, and therefore there can be a difference in the situation.
Beyond that, you assume that going back in time means being in the same situation. But you can go back in time and the situation will change as a result of your return. That is assuming that time is not something fictional, but has existence in its own right, independent of the events that occur along it. Therefore it is possible that I return to the same time even though the situation at that moment is now different.
The grandfather paradox is a completely different question. I was talking about the definition of going back in time. Without two axes, the concept of “going back in time” is contradictory, so there is no point discussing whether it is possible or not. Once you adopt two axes, the discussion becomes meaningful. Part of that discussion will deal with the grandfather paradox. By the way, I now think that if you continue developing the description of the two axes, you’ll discover that this paradox also disappears. If you went back and killed your grandfather, then you were not born. Therefore, when you arrive again at Thursday, you really will not exist. But that is a different point once you take the two axes into account.
In short, when there are two time axes, a description of any situation is a function of two variables: f(T,t). Identity has to hold only if you are at the same T and the same t. If one of them is different, there is no problem with the situation being different even if the other is identical. f(T1,t) is not necessarily equal to f(T2,t), and the same applies with a change in t.
Discussion on Answer
Here you’re getting into questions of continuity of identity. If when you meet me tomorrow, have you met the same person I was today? My assumption is yes. There is no contradiction between the claim that Reuven met me today and the claim that Shimon will meet me tomorrow. And it’s the same me.
In a world built on two time axes, the person exists and acts across both of them. His state is defined as a function of both, f(T,t). Therefore, if it’s the same f, it’s the same person, even if you meet him at a different t or a different T.
And the same goes for the world. The function F of the two times describes the same world even if you change t or T.
I understand that there’s no problem asserting what you said, but in my opinion there is a problem with the claim: Shimon meets Haim on Sunday and Avi meets Haim on Monday, while the t-axis is the same. After all, if it’s the same Haim, then he has experiences that he does not feel (Sunday-Haim does not feel what Monday-Haim feels, and vice versa). And more than that — if Shimon causes Haim to be in a different situation on Monday, then when he arrives tomorrow (Monday) he will not experience what Haim experienced on Monday with Avi. In other words, it smells like a contradiction, because it comes out that “Haim experienced pain, for example, and also did not experience pain.”
It just feels to me that the solution of parallel universes lets us give a satisfying explanation to all these difficulties. What do you think?
Parallel universes are only a name. I don’t see how that differs from my proposal. Just as I do not experience today what I will experience tomorrow, and yet I am still the same person. When I get to tomorrow, the experiences will converge. So in my model there are two dimensions of time, but there is no essential difference. When I pass the meeting point on both axes, the experiences will converge.
Thanks. So in your understanding, do the two time axes include within them an absolute time that does not depend on the events occurring in it?
As for “the same situation,” if that’s the case, it means that the whole universe changes and “jumps” back in time together with whoever travels back in time. That sounds strange in itself, and beyond that another problem comes up: what happens if I jump back to Sunday and someone else jumps back to Monday? And suppose we meet the same person — for example, I meet Haim at his workplace on Sunday, and David, who went back to Monday, meets Haim running in the park. Are there two Haims? One in T1,t and the other in T2,t? Or alternatively is it the same Haim? I don’t even know what “the same Haim” means here. It seems that what’s called for is to say that there are two parallel time axes (T1, T2), a kind of parallel universes, in each of which there is a different, parallel Haim.
And even if only I go back by myself to T2, what happens to T1? Does it cease to exist?
All this leads me to the conclusion that going back in time is a jump between universes.
And regarding the grandfather: if I won’t exist on Thursday but I did exist 50 years earlier, how is that possible, assuming my grandfather died before he brought me into the world? It seems that I exist on the t-axis independently of what happens on the T-axis, but again, does that mean I exist twice? Or that there are two versions of me?
Thanks in advance!