# Why do time travel paradoxes assume the future is still valid?

#### Cosmo

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I stumbled across this question at Physics Stack Exchange today. It looks like it's going to get buried, but I wanted to call attention to it here:

It's a bit hard for me to explain what I mean by "the future is still valid", but bear with me. And I hope I'm not asking something incredibly stupid

I'll refer to my version of time travel as "linear time-travel" for a lack of a better term.

Incidentally, I've read a lot about paradoxes lately, including time-travel paradoxes and about the Self-Conservation Principle. But why are almost all sources expecting the "future" to still be valid (and "loop") after time traveling?

What I mean by this is: Instead of time being a loop, why isn't it still just "a z-line"? Wouldn't that basically solve most of the paradoxes automatically (and make more sense, since those paradoxes wouldn't exist then?)

What I mean by "line" is: As soon as a person travels back in time, the whole period travelled is invalidated. That future doesn't have to happen that way anymore, and a new future is created depending on the actions of the traveling person/object. The removed "future" still existed, but now it's part of the past.

Person A1 invents a time machine and uses it to travel back in time.

Person A1 meets him-/herself (person A2) and changes the past in a way that person A2 no longer invents the time machine.

Paradox: Since person A2 didn't invent the time machine, person A1 cannot travel back in time to change the past.

Person A1 invents a time machine and uses it to travel back in time.

Person A1 meets him-/herself (Person A2) and changes the future in a way that person A2 no longer invents the time machine.

The future now unfolds differently. Person A2 no longer invents a time machine and someone else might does in the future, it doesn't matter. Person A1 is still in the past and still remembers his/her invalidated future (actually, past for person A1), living in the same time as Person A2.

I guess my question is: Why do so many paradoxes / theories seem to assume that time travel would have to 'loop', if simply having time travel that actually changes the future whitout 'loops' would remove most paradoxes and make 'more sense' (1)? Why is the theory behind it not leaning more towards 'linear' time-travel?

(1) It would make more sense to me personally, but also since many paradoxes wouldn't even exist. No grandfather paradox, no self-consistency principle, etc - don't all those paradoxes imply that time-travel wouldn't work that way anyways?

Could be something psychological.

For example, we don't talk about Standing up, walking around clockwise your desk and walking straight to the bathroom, we call it going to the bathroom.

When somebody says they are in the bathroom, you think of them arriving and the effects of such.

To complex?

All time travel paradoxes are simply "thought experiments". There is, as a matter of necessity, a lot of assuming going on with them. In the example you offer, I see person A1 "as" the paradox. As long as A1 exists, there is a paradox... he is there but, having warned A2 to avert some impending situation, he has removed the impetus for his time travel to warn A2 of it. The future, from A2's perspective, is unwritten. He experiences no paradox. If A1 ceases to exist, for whatever reason, the paradox disappears completely since, there is no one that "knows" a different future. The events that led to A1's travel did not occur for A2, who "is" A1. He will simply "pass" that period of time, without notice.

So, in that sense, A1's future IS invalid. I think each paradox would have to be considered under it's own terms.

I think...

Fantastic Cosmo! This is what actually happens in the novel I am writing, only with a lot more plot twists. Only another 50,000 words to go

I think the seventh rule of time travel is, don't make loops.

The first rule being, "Don't do stupid shit."

What you are talking about is called back force. Back force is when someone in the past has knowledge of the future and changes their decisions, changing the future.

I stumbled across this question at Physics Stack Exchange today. It looks like it's going to get buried, but I wanted to call attention to it here:http://physics.stackexchange.com/questions/200872/why-are-so-many-time-travel-paradoxes-assuming-that-the-future-is-still-valid
It's a bit hard for me to explain what I mean by "the future is still valid", but bear with me. And I hope I'm not asking something incredibly stupid

I'll refer to my version of time travel as "linear time-travel" for a lack of a better term.

Incidentally, I've read a lot about paradoxes lately, including time-travel paradoxes and about the Self-Conservation Principle. But why are almost all sources expecting the "future" to still be valid (and "loop") after time traveling?

What I mean by this is: Instead of time being a loop, why isn't it still just "a z-line"? Wouldn't that basically solve most of the paradoxes automatically (and make more sense, since those paradoxes wouldn't exist then?)

What I mean by "line" is: As soon as a person travels back in time, the whole period travelled is invalidated. That future doesn't have to happen that way anymore, and a new future is created depending on the actions of the traveling person/object. The removed "future" still existed, but now it's part of the past.

Person A1 invents a time machine and uses it to travel back in time.

Person A1 meets him-/herself (person A2) and changes the past in a way that person A2 no longer invents the time machine.

Paradox: Since person A2 didn't invent the time machine, person A1 cannot travel back in time to change the past.

Person A1 invents a time machine and uses it to travel back in time.

Person A1 meets him-/herself (Person A2) and changes the future in a way that person A2 no longer invents the time machine.

The future now unfolds differently. Person A2 no longer invents a time machine and someone else might does in the future, it doesn't matter. Person A1 is still in the past and still remembers his/her invalidated future (actually, past for person A1), living in the same time as Person A2.

I guess my question is: Why do so many paradoxes / theories seem to assume that time travel would have to 'loop', if simply having time travel that actually changes the future whitout 'loops' would remove most paradoxes and make 'more sense' (1)? Why is the theory behind it not leaning more towards 'linear' time-travel?

(1) It would make more sense to me personally, but also since many paradoxes wouldn't even exist. No grandfather paradox, no self-consistency principle, etc - don't all those paradoxes imply that time-travel wouldn't work that way anyways?
My friend, what you are failing to realize is that any changes in a time line depending on the length of time that the incursion event take place in relation to the progress of the new time line, wopuld only be realized by the time traveler.for example traveler a invent a time machine and choose the past to travel to, by the act of traveling in time you would have to make an incursion of the timeline that already exist in the past, you cannot any eliminated any components of the timeline ( past, present, future) those components are organic. the future is unmanifested to the observers (us), but as theorized the whole of the time past present and the future would occur all at once in the 4th dimension, so that any linearization of time would only occur if you were a flat-lander, I that case time would be limited to a 1-dimensional LINEAR segment, but unfortunately your analogy just is not possible in 3-dimensional space in which time is constraint by a 3-D worldsheet composed of individual and intersecting timelines

I'm actually kind of disappointed in you guys. This is a simple question and has a simple answer.

The first thing to note is that the "future" is just another frame of time. It's not the future any more than the present or past is. Those are relative terms, not absolute. A human moving from one point to a point earlier in time is already known. In the 'past' we see his arrival. Then, once the time rolls around to the future, we see his departure. There cannot be any contradictory events if we assume a single line. Things must go according to how we saw them 'the first time'. Ultimately he can't change anything, he can't kill his grandfather. That's where Novikov's self-consistency principle comes in. It ultimately says that we can't change our own past.

The "Z-diagram" you guys mentioned is actually just evoking another timeline or alternate past. There's no restrictions, and the 'altered' future can certainly cancel out the time travel. The problem is, that this new future is NOT the same future as our traveler's departure. It doesn't override it. Both must be equally real scenarios, with our traveler switching lines.

These are simply two different models of time and time travel. Titor's narrative went with the many-worlds model, as you have a reason why your predictions aren't accurate .

"Why do so many paradoxes / theories seem to assume that time travel would have to 'loop', if simply having time travel that actually changes the future whitout 'loops' would remove most paradoxes and make 'more sense' (1)? Why is the theory behind it not leaning more towards 'linear' time-travel?"

Paradoxes rely on contradictory states. Which is much easier to arrive at if you assume both a static series of events along with a changing narrative. The grandfather paradox is when you both do and don't kill your grandfather. It's very easy to see the contradiction. What's more interesting is why the person posing the paradox assumes that changes propagate. There's no reason to assume that. If you assume the past effects the traveler's origin, that's a single line and must obey consistency. If you assume the past doesn't effect the traveler's origin, there's no paradox.

In this sense, paradoxes are when you fail at logic. Realistically speaking, a paradox will never occur. They are only the work of fiction.

As for your second question, both the models you proposed are 'linear' in nature. You're assuming the perspective of the traveler in both cases. Really, the question is whether you're dealing with one line or multiple. Multiple allows you to 'change' things without worrying about consistency.

Could be something psychological.For example, we don't talk about Standing up, walking around clockwise your desk and walking straight to the bathroom, we call it going to the bathroom.When somebody says they are in the bathroom, you think of them arriving and the effects of such.

To complex?
But, the action is taking place in temporal intervals, for the most recent past to the most recent future, it is factious to annouce these acts and they hold no context when discussing time travel

I think the seventh rule of time travel is, don't make loops.The first rule being, "Don't do stupid shit."What you are talking about is called back force. Back force is when someone in the past has knowledge of the future and changes their decisions, changing the future.[/quote

Agreement regarding the dymanics of the past memory of any Back forces, but the future by very definition is un-manifested and the act of time travel is an incursion on the timeline of the time traveler and given the degree in which the individual time traveler has on future events determines the degree in which the entire time line would , be effect barring any hypotheticals scenarios. as an example if Hitler chose to be a painter with talent instead of a drug crazed homicidal dictator, if he chose time traveling as ajust a painter then any incursions on his timeline would have a minimized resultant on the whole timeline than that of a drug crazed dictator

Z diagram with John Titor case.View attachment 481
The diagram would only lend credability if John Titor actually existed to travel time, could you provide to me 1 John Titor prediction that has actually come to past?

I'm actually kind of disappointed in you guys. This is a simple question and has a simple answer.The first thing to note is that the "future" is just another frame of time. It's not the future any more than the present or past is. Those are relative terms, not absolute. A human moving from one point to a point earlier in time is already known. In the 'past' we see his arrival. Then, once the time rolls around to the future, we see his departure. There cannot be any contradictory events if we assume a single line. Things must go according to how we saw them 'the first time'. Ultimately he can't change anything, he can't kill his grandfather. That's where Novikov's self-consistency principle comes in. It ultimately says that we can't change our own past.
The "Z-diagram" you guys mentioned is actually just evoking another timeline or alternate past. There's no restrictions, and the 'altered' future can certainly cancel out the time travel. The problem is, that this new future is NOT the same future as our traveler's departure. It doesn't override it. Both must be equally real scenarios, with our traveler switching lines.

These are simply two different models of time and time travel. Titor's narrative went with the many-worlds model, as you have a reason why your predictions aren't accurate .

"Why do so many paradoxes / theories seem to assume that time travel would have to 'loop', if simply having time travel that actually changes the future whitout 'loops' would remove most paradoxes and make 'more sense' (1)? Why is the theory behind it not leaning more towards 'linear' time-travel?"

Paradoxes rely on contradictory states. Which is much easier to arrive at if you assume both a static series of events along with a changing narrative. The grandfather paradox is when you both do and don't kill your grandfather. It's very easy to see the contradiction. What's more interesting is why the person posing the paradox assumes that changes propagate. There's no reason to assume that. If you assume the past effects the traveler's origin, that's a single line and must obey consistency. If you assume the past doesn't effect the traveler's origin, there's no paradox.

In this sense, paradoxes are when you fail at logic. Realistically speaking, a paradox will never occur. They are only the work of fiction.

As for your second question, both the models you proposed are 'linear' in nature. You're assuming the perspective of the traveler in both cases. Really, the question is whether you're dealing with one line or multiple. Multiple allows you to 'change' things without worrying about consistency.
Novikov's self-consistency principle, states principle of self-consistency, which states that the only solutions to the laws of physics that can occur locally in the real Universe are those which are globally self-consistent.
The problem is that the act of time travel does not take place in the real 3-D Universe. In order for teh machine to operate the CTC takes global metric with the 4th-dimension and the in observations of cold dark matter the self-consistency principle was proven to be inconsistent by concordance cosmology

I'm not sure Titor was a real time traveler, but some of the stuff he or they talked about seemed like good points for understanding time travel. I don't think his equipment could have worked either, unless it was a straight line device. It didn't look like a straight line device, and if it was, he couldn't have skipped over to a different past.

There are a lot of different futures, but the farther you go back, the more certain the past becomes, unless it doesn't, because you find out you are in a past where world war 2 never occurred. When people attempt to change the past, they are trying to alter the past and create a "footprint" that has a more possible profitable or good out come for themselves.

I think what the Titor time line seems to indicate is that John Titior would have created several fractional time lines by moving back and forth, where both possibilities were true: that he did and did not travel in time. Every time he traveled again, he created another fractional time line.

I don't think people who have their time line changed are that aware of it. There is a condition I call "samurai disease" that makes people aware after awhile of a lot of changes and sudden breaks in their time line. Exposure to certain tachyon weapons may also cause something similar to this to occur which is like having extremely accurate deja vu and premonitions.

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