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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.

Example with the paradox:

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.

"normal" time-travel

Example without the paradox:

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.

"linear" time-travel

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?

Community
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Katai
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  • I have sometimes asked the same questions, but I wasn't thinking about physics at the time, I was thinking about Sci-Fi stories. This does not sound like a physics question. (P.S., Try reading "Thrice Upon a Time" by James P. Hogan. It may be more to your liking than many other time-travel stories.) – Solomon Slow Aug 17 '15 at 16:17
  • @jameslarge well, I wasn't actually sure. But I'm mainly relating to a paradox that appears to be physics related (self-conservation principle) so I assumed that sci-fi wouldn't be the right place for this. Especially since I'm curious about "why" time travel is depicted as "loop", even if it would create so many strange paradoxes! So I figured, since the paradox seems to be physics related, this is the right place! – Katai Aug 20 '15 at 14:54

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The Novikov consistency principle is just an expression of how it is assumed that the laws of physics still apply : even in situations where spacetime can be twisted into having closed timelike curves, you still have to solve differential equations stemming from some initial conditions. And ideally, we want those to be continuous and smooth. Also, we cannot change the future because this all happens in the same spacetime (the same manifold, mathematically), hence you cannot have a function with two values at the same point.

Methods where spacetime would allow different futures and different pasts have been proposed, mostly by David Deutsch and Mark Sharlow, usually with some combination of non-Hausdorff manifolds (branching spacetime) and the Many World Interpretation. There are some topological results and some results regarding causality in such spacetimes, as well as theories linking the concept to interpretations of quantum mechanics, but as far as I know it is a rather minority theory and outside of topological results, there is very little physics done on the topic. It seems to be mostly idle speculation.

Slereah
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  • I am aware of the Deustch model and the postselection models for time travel in quantum mechanical context. But branching spacetime seems kinda new to me. I should give this a read. Is it somehow related to parellel universe or are they separate cocenpts? – Secret Aug 17 '15 at 10:27
  • "Parallel universes" isn't really a concept in physics, although several things are similar to the notion. Branching spacetimes are somewhat similar to the idea, yes, in the descriptions they usually get. – Slereah Aug 17 '15 at 10:30
  • Yes I see - I'll give those topics a read. Of course it's speculation, I was generally just wondering why all speculation takes such a "complicated" direction, that (at least to me) seems too full of paradoxes to even work at all, instead of an (again, to me) much more simple and better working approach. – Katai Aug 20 '15 at 14:59
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Sometimes you get a time machine when you were not trying to get a time machine, such as with a wormhole or a FTL transport. The FTL only makes a time machine if space is finite or if you use multiple transports so since the time travel is accidental and involves how multiple parts connect together it isn't obvious how to break it into a before and an after (which is the actual point).

A wormhole doesn't start out as a time machine it becomes one as the two mouths move differently or have matter get near them differently. So at what point should things start behaving differently? Also a wormhole is just following Einstein's field equation (albeit with exotic matter) at what point are you saying we should stop using GR (Einstein's field equation)? And what should we use instead?

As for FTL, again people just use SR (e.g. have particles that are already moving FTL) or GR (e.g. use Einstein's field equation with exotic matter). So if you propose to do something else: when, why, what, and how?

Timaeus
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