this post was submitted on 26 Mar 2024
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Assuming time travel exists: is it possible to alter the past?
If an event occurs, and you decide to travel back in time to change/prevent that event: It has no longer occurred in the way that caused you to want to change it; thus you never travel back to change it, and it does occur...
The Grandfather Paradox, I'm partial to that one as well.
I was playing with this recently. Suppose you are playing rock, paper, scissors with yourself from a few minutes into the future. Your future self “remembers” what you will play and so as long as you play normally, future self always wins. But change the rules a bit and play where future you goes first.
In a normal game, you should always win because you clearly see how future you played, but future you played to counter what future you remembers present you playing…
E.g. future you remembers playing paper, and so plays scissors. You see scissors and go go play rock, but that should be impossible because future you doesn’t remember playing rock.
The weird thing to me is not that the second scenario (where future you goes first fails) but that playing normally (both going at the same time) works. I think the paradox emerges when future knowledge is introduced to the past. In the normal game, future you does not expose future knowledge until the exact moment you play and cause that knowledge to exist in your present, but in the altered game, the introduction of future knowledge creates a feedback loop.
Of course the game isn’t needed. Simply seeing future you conveys the fact that you exist in the future. Should you, for example (and please don’t do this) see near future you then stab your arm with scissors, you will miss or be stopped because future you does not have a wounded arm.
I wonder what happens if future you’s arm is out of sight. would you be able to stab your arm then only for future you to then reveal a wounded arm?
I think that just shows that time travel doesn't exist.
Perhaps. Unless you consider multiverse theory: The idea that the act of traveling to the past splits the timeline into two realities. One containing the original (to your perspective) timeline with the event(s) that caused you to travel back, and a second where you've arrived in the past to alter those events and the results there of.
Not sure I believe it, but it's a theory none the less.
Or maybe it's only possible to travel forward in time. Closer to our current understanding of the universe.
The multiverse conception of time travel is nice and fine for fiction, but in reality, if you think about what such a time machine would have to do, it would have to create an entire new universe, with all precise details exactly in place, and it would have to have a lower entropy than your origin reality. Good luck building such a machine
I like the idea of splitting timelines if reverse time travel is possible, but it does have some consequences. The biggest one being that it means you can't actually travel back in time. Time travel may even be relatively simple but as it has no effect on the primary timeline you will never be able to change the past as it appears there; travelling back in time simply creates an alternate reality. As far as the primary timeline inhabitants are concerned, you have either died or vanished (or maybe nothing appeared to happen at all) but you have not travelled in time. It also means it's impossible to return to your original timeline as further reverse time travel will only create new alternate timelines, the closest you can get is a timeline that closely resembles your home one.
Another fun approach is that infinitely many alternate timelines already exist (think Many Worlds), travelling back in time simply means you spontaneously form in another world through quantum fluctuations or something equally hand-wavey. The thing I find interesting about this one is that it doesn't necessarily involve time travel at all. You form with the memories of having travelled in time, sure, but you have just spontaneously formed through quantum fluctuations so it's reasonable to assume your memories have too; it may have just been a randomly formed memory that didn't actually happen. Since it's just random fluctuations there'd also be infinitely many universes where you spontaneously pop into existence with no time travel memory, so I suppose in a way this never was time travel. The original timeline would be unaffected by this kind of travel as you can only move to universes where you have already spawned in.
The way I see it the only way to actually change the past in your current timeline arguably involves destroying the universe. You'd have a single timeline and each instance of reverse time travel cuts off your timeline's future and links back to a previous point from which time can continue. You can visualise this timeline as a piece of string, time travel is a loop in that string. If you travel back in time by a year, everything you did in that past year is within that loop off to the side of the primary timeline; the loop starts and ends at the same point. Time travel would essentially delete your future and plonk you back onto the primary timeline. No need to worry about the grandfather paradox; you were born in a loop off to the side of real time so killing your grandfather doesn't change that loop. It works around the bootstrap paradox for similar reasons; the information was created in some loop somewhere, even if it appears to have created itself on the prime line. It's a nice thought experiment but the problem here is that if you travel back in time but fail to change the conditions which caused the time travel you may have just ended the universe in an infinite time loop.