Someone on Quora asked, “If time travel were possible, is it recommended to meet the “past self” (or the “future self”) or can this cause some kind of a paradox?” Here’s my answer, which gets into the possibility of time travel in general.
Any amount of interaction with the past on the same timeline would would eventually, per the butterfly effect if nothing else, either slightly or substantially change the state you were in when you traveled back into the past, which would change your effect on the past, which would slightly or substantially change the state you were in when you went back into the past, etc. etc., causing a paradox.
Nature wouldn’t have an arbitrary threshold by which it says, “ok, if you change your past self more than THIS much, I’ll cause a paradox.” There would always be a paradox. And paradoxes don’t exist in nature, of course, only in thought experiments.
If you really could change the past, which changes your state when you go into the past, which changes the past, which changes your state when you go into the past, which changes the past, etc., the timeline itself would change an infinite number of times, and at what rate? Infinity times per second?
And time as we know it is (whether in actuality or just in our models—either way) progression along a timeline.. so to change the timeline itself would imply a second dimension to (otherwise linear) time that we know nothing about and have never detected. And if there’s a second dimension to time, it should be there under all circumstances, not just when somebody decides to travel back in time on their own timeline..
(Not that I’m saying the existence of a second dimension to time would make time travel into the past on your own timeline possible.. it would still be the case that, because the change would be in an endless loop, the timeline would have to change an infinite number of times (in other words, it would never stop changing, as infinity is by definition never reached), before the conclusion of what happened as a result of you going back into the past is resolved.)
So this leaves two possible options for traveling back in time:
- When you travel back in time, you actually travel to an alternate timeline, i.e. a parallel universe, whose present corresponds to your past.
(It couldn’t be that its present corresponds to your present but you actually travel backward to its past, because then if there’s any interaction at all between that universe and ours, including but not limited to that of multiple time travelers traveling between them, then a causal paradox would arise as described above.
It can’t be that the two universes have no temporal relationship to each other either, because then either (a) you’d never be able to get to it, or (b) you could pick any point of time to travel to within the other timeline that you want and likewise a traveler from there to here could pick any point on this timeline they want, so again, causal paradoxes would arise.)
- Any effect you have while traveling backward in time on the same timeline actually fulfills the past rather than changes it. This possibility seems unlikely to me, since it would put obvious restrictions on what you’re able to do when you’re in the past, such as killing your past self. And how would those restrictions be put into place? What would stop you from doing such a thing? Serendipity? Who would orchestrate such serendipity?
While either of those two modes of time travel may be possible, in neither case does it matter at all how you interact with yourself: in the first case because it’s not on the same timeline so it won’t affect the same you that went back into the past, and in the second case because any way you can possibly affect yourself will actually fulfill the past rather than change it.
So my conclusion is that either (a) time travel is categorically impossible, or (b) you can travel in time, but there’s no possibility of causing a paradox so it doesn’t matter what you do.
One thing I didn’t go into in my Quora post is the possibility that the progression of time is not a strict mechanical phenomenon that can be understood, or in-principle simulated, from the bottom up, but rather a kind of abstract, dynamic weave or tapestry that’s self-healing and can efficiently absorb small paradoxes, but can be significantly disturbed by more stark paradoxes causing causal rifts of some kind.
To give a sense of this time-weave’s dynamicism / organicalness, one thing I’ve read about it is that it can be locally, temporarily sped up to force an issue and predict the future but without 100% accuracy / probability of accuracy.
It seems that this view of time’s progression would be especially compatible with a brain- or mind-like model of the universe. I make a rational argument for idealism (as in the metaphysical philosophy) here.
In addition, I’ve had many experiences that seemed to indicate a mental or psychological characteristic to reality. For example, I was once working on a short Python program that had a bug. I identified the bug and fixed it, but the program still wouldn’t work right. So I only added some print statements strictly for the purpose of debugging / proving to myself that the program should work correctly, and then it worked. Then I took out the debugging statements so that I had the same program again that I had before I added the debug statements that wasn’t working, and it still worked. I believe software engineers weren’t kidding (except inasmuch as they refuse to believe their own experiences because they violate the modern mechanistic story we tell ourselves about the universe) when they coined the term “Schrodinbug” (defined here).