The Theory of Going back in time to alter something

[Inquisitive Stone]

Say, if you went back in time and changed something. Like for example, you went back to the day that you decided indefinetly that you were going to be an engineer, but then you go back and make your past self change his/her mind. Wouldn't you disapear as you made your past self decide not to be an engineer, then you wouldn't be there because that choice would lead to many different choices.

 

Also, if you were to show yourself to your past self, what would happen??

[Spyman]

energetix, do you think you are a duplicate of your past self or the past self that has aged ?

[Inquisitive Stone]

Nah...

 

I dont think I'm either of those.

I just thought up of it because its one of those paradoxes that just pop into my head.

[mooeypoo]

That sounds like the "Grandfather" paradox; If you go back in time and kill your own grandfather before he had time to produce your parent, you wouldn't have existed to go back in time and kill him, which means you do exist, which would mean you killed him, which means you don't --- and so on.

 

A paradox.

 

Since time travel isn't really possible, it's impossible to tell what would really happen. The paradox can be easily solved if you imagine time as a "tree" of possibilities. Meaning, when you went back in time and changed your own mind, you created a brand new reality in which you won't go back in time. The "alternate" reality can still exist.

 

That's all philosophical.

 

But it sure as heck makes for good movies...

[ydoaPs]

it's impossible to tell what would really happen.

No, it's not. He'd fail at whatever he tried to change. Any time travel to the past would be a causal factor into the present. It's not like things happen more than once.

 

These "paradoxes" aren't paradoxes at all; they're errors in thinking caused by following the traveler instead of the timeline.

[Sisyphus]

Since time travel isn't real*, none of the ways it's portrayed are "right." However, I greatly prefer the "whatever happens, happens" version, as it's less ridiculous and allows for more interesting (IMO) plots.

 

*Yes, I know we subjectively experience "moving forward in time." I mean you can't take a piece of the universe (like a person) in 2009 and mash it into the universe in 1909.

[Moontanman]

If your future self went back in time wouldn't you be much bigger than your past self due to the expansion of the universe? Hey, it's time travel! Seriously, if you did go back and change the past wouldn't you be stuck in the past unable to return to a future that no longer existed?

[Edtharan]

No, it's not. He'd fail at whatever he tried to change. Any time travel to the past would be a causal factor into the present. It's not like things happen more than once.

 

These "paradoxes" aren't paradoxes at all; they're errors in thinking caused by following the traveler instead of the timeline.

I agree, in part. If you look at any Quantum system (and in this you can think of the universe as a quantum system), then it is expressable as a wave function. One of the features of a wave is that it can have interference (like in the two slit experiment).

 

A Time Traveller would act as interference on a Universal scale.

 

If you think about Sum over Histories, the History can take any path as long as two of the points are fixed: The point where the Time Traveller arrives in the Past and the point where the Time Traveller leaves the future.

 

So long as these two points remain fixed for the Universe, any intermediate history should be possible. Any history that causes a violation of these points, would suffer from negative interference and end up not being possible.

 

Interestingly, this also includes any historical path that loops multiple times through the time loop. This makes the number of histories effectively infinite, but even so, there are also an infinite number that are excluded.

 

When working out the path that a particle takes, they use this sum over histories in the analysis. This even works for multiple particles. In these Antimatter works just like normal matter going backward through time (it doesn't mean that it does, it just works like it does). So this type of analysis might be used to analyse Time Travellers.

 

Instead of Antimatter acting like a time traveller, you have a time traveller, and the analysis works with multiple particles too. So using a sum over histories, and by applying interference to work out which histories are impossible, you can work out the effects that Time Travel might have.

[mooeypoo]

Since time travel isn't real*, none of the ways it's portrayed are "right." However, I greatly prefer the "whatever happens, happens" version, as it's less ridiculous and allows for more interesting (IMO) plots.

 

*Yes, I know we subjectively experience "moving forward in time." I mean you can't take a piece of the universe (like a person) in 2009 and mash it into the universe in 1909.

I much prefer the "Back to the Future" approach where time isn't linear.. I have absolutely no physical reason to think that, but it just seems more reasonable to me that time isn't necessarily linear and fixed.

 

But this is all philosophical... time travel to the past isn't real. yet?

[Mr Skeptic]

I would resolve your paradox the same way as I would resolve the grandfather paradox. Send a letter that has instructions to do what needs to be done, and also that the letter must be sent to the past (to the same place/time you sent it). In doing this you cause an event that will later cause itself and your intended effect, removing yourself from the causal chain and resolving the paradox.

[padren]

Paradoxes are mental traps that exist due to the incomplete manner of how we model the universe in our minds. The universe itself doesn't require our models to operate, and thus are free of them.

 

Take Maxwell's Demon - it caused some interest but ultimately it only is perplexing if you don't count all the variables in play, and that's why paradoxes are so difficult... we can't always figure out what we are missing.

 

If time travel is somehow possible, my bet is that you can only go back/forward to different versions of reality and that both versions always have and will exist.

 

 

I think the most telling element in these discussions is the nature of time over time:

 

Take looking at a huge stone at 12:00pm exactly Oct 2 2009 that has been tilted to the left for 1000 years. You go back in time a few minutes, and tilt it to the right, and hide in the bushes at exactly 12:00pm Oct 2 2009 again but this time it's tilted to the right. Now, you have the normal paradox issue here but we also have to ask "If at that exact moment the rock started out leaning left and ended up leaning right (still in that exact moment) how much time did it spend in that one instant facing the left, vs facing the right?"

 

If you view points in space as locations that can either contain something or not, we expect that to change over time naturally since matter moves around so much. You can describe a 3D object as a series of x,y,z points in the same way a picture can be described as a series of x,y points but the 3D object that is described has to be static. There is no room for more information. To describe a 3D object that changes over time, you need x,y,z,t points. That can describe a rippling bucket of water for instance, where x,y,z points could only describe it's form frozen at one given moment. But now you are talking about going back and changing time. For time to change over time we need another dimension. If one exists that allows for it, my layman's guess would be 'probability' but I honestly have no idea. If I had to bet though, I'd bet you couldn't change time over time and still talk about the whole mess in terms of space/time (ie, x,y,z,t) because you'll always be sort one dimension simply from the perspective of describing the data of the event.

 

From the x,y,z,t perspective, it will always be a baffling paradox.