Regarding the logic of time travel.

[Falkon303]

Ok, so I am very interested in time travel, but I wonder about the logic/mechanics of it.

Let's pretend in your past, you went to the park, ate a hotdog, and went home, and you have a great memory of it because it was very windy that day.

Now let's say later in your life that the CIA employs you as a lab rat for a time travel experiment, and you teleport back to the moment you had the hotdog, and have a chat with yourself. What I imagine would happen is that if you return to the future, it would be completely different, and there may in fact be a new version of yourself as well as your current self because you had created a new future version of yourself, and your time travel self wasn't affected because when you time travel, the past-future relativity is broken.

Anyways, I believe that's how it works, but I'd love to hear what you all have to say on the subject. Also, if you could, please explain in simple terms as any physics or convoluted mathematical explanations aren't really what I am asking about. The science and math of time travel is one thing, but the basic logic and mechanics is another.

I guess I should have posted this in the Time Travel forum. Well, hopefully a time traveller will answer it.

 

[Ayasano]

The view I personally take is that when you turn on the time machine it will simply break before you can go back, or some other event will stop you from meeting yourself. You could only go back and meet yourself if you remember meeting your future self in the first place.

Now, the reason it breaks can be one of two things:

1. The chronology protection conjecture is correct and it's simply impossible to change the past.
2. Changing the past would erase the existing future like in your example and replace it with a new one. Time travellers from that future would like very much not to be erased and sabotage your machine.

#1 is simpler but I like the idea that history is full of time travellers making sure other time travellers don't screw with history.

However, this doesn't mean you can't use time travel for fun and profit. You can't change a known event. That is to say, if you have no recollection of meeting yourself on that day, you can't go back and meet yourself, but you can go back and talk to someone else. Just don't try and reveal time travel to the world because unless you remember that happening, the people from further Up will just show up and shut you down as soon as you try it.

Note that paradox is avoided altogether regardless of which reason is correct. #1 is obvious, but in #2, how would the people stopping you know that you were going to erase them if you hadn't done it yet?

The answer is that you have to think in four dimensions. They know because all of history is already written, including all of the people going back and forth trying to change events and being stopped. It's just that nobody knows about all of history.

Let's say you have 3 people, A, B and C. A wants to go back and meet himself, thereby changing history and erasing C, born far in the future, from existence. A's time machine is blown up by C before he can turn it on, which B witnesses, but B knows that there's no way C could have known about the machine without being told. So B goes and tells C about the machine, at which point C goes back in time and blows it up, thus fulfilling the stable time loop.

Does your head hurt yet? And that's just a simple example of a stable time loop.

 

[Falkon303]

"You could only go back and meet yourself if you remember meeting your future self in the first place." - So this would mean that any time travel to any previous dates prior to 2014 has already happened/occurred?

I was watching a National Geographic show on time travel, and it was saying that if we change past events it screws up the present future, which is more along the lines of what I believe.

Just trying to get a clear picture. Thanks for the detailed reply.

 

[Ayasano]

The short answer is yes. You just don't know about it because when you have all of time and space at your disposal, staying hidden becomes trivial. And who knows, maybe you will have gone back to a date prior to 2014? You just made sure not to bump into yourself because you don't remember it.



It's hard for me to explain, but in terms of the basic mechanics of time travel, that's what I believe too. However, that fails to take into account the survival instincts of humans. I know I, personally, would rather not be erased from history.

Look at it this way. If one person can invent time travel, then it stands to reason so can others. Our universe is going to be around for many billions or even trillions of years, so barring some universe-wide extinction-level event before we or any other species invent time travel, it also stands to reason that over the lifetime of the universe many people will invent time machines. If you consider it likely that a species as a whole may adopt time travel, the number of time travellers moving around goes up by several orders of magnitude.

We also know that due to the Butterfly Effect, small changes to an initial state of a complex system can result in entirely different end states. The classic "stepping on a bug in the age of the dinosaurs".

The result of these two assumptions is that there are some trillions of time travellers who want history to stay the same. So while the basic mechanics of time travel might allow us to change history, in practice you won't be able to because someone will show up and take care of you. I, personally, find that kind of comforting.

(And don't worry about changing something by accident. You can only change a known event. If you don't know the outcome in advance, how do you know which one to pick to make it different?)

 

[Falkon303]

Yes, but even existing in a time that you hadn't previously would change everything. If I go back in time, and I am in a crowd, even if I don't talk to anyone, I have changed the future because I have made other people observe me, which leads to a change in thought, or even more, having to walk around me, which changes the timing of their events, which branches out/etc. I am not saying that it means anything drastic would occur, but it definitely would change the future.

It's possible that there are regulators that control time travel, but if there's not you would literally be changing the future of reality realtime. I wonder if people would stay the same people, or if they'd be entirely different beings, or if we'd go into a chaotic state of different futures until the change stopped.

 

[Ayasano]

But that's just it, in that scenario you wouldn't change anything. You were always in that crowd, long before the thought of time travel even entered into your mind.

 

[Ayasano]

I'm going to borrow some terminology from the Continuum RPG here because it makes explaining things easier.

Up = forward in time, away from the Big Bang.
Down = backward in time, towards the Big Bang.
Age = your subjective past, the time that you've experienced
Yet = your subjective future, that which you have yet to experience.

Let's say you're standing in a room. This is where the loop begins. There is another room next door to yours, and the door is closed. You travel Down 30 seconds and across into the room next door. You then wait 31 seconds and go through the door into the original room. From the perspective of someone else standing in that room, they would see you dissapear and then step through the door 1 second later.

While you were standing in the main room, there was a future you standing in the room next door, waiting for you to leave. When you leave, they step through and the loop ends.

 

[Falkon303]

So what if I travel back in time and convince myself to get a tattoo that I don't currently have?

 

[Ayasano]

Right before you leave someone appears, punches you in the face, and then politely asks you to not mess with the space time continuum.

 

[Falkon303]

Right before you leave someone appears, punches you in the face, and then politely asks you to not mess with the space time continuum.

lol

Seriously though. I am curious if you'd change, or if the minute you teleported your actual future was destroyed and you'd only have a future relative to the impact of your time travel.