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I have a degree in theoretical physics, and also did research on general relativity. The result is cool, but it's not directly applicable to the traditional (s

I have a degree in theoretical physics, and also did research on general relativity. The result is cool, but it's not directly applicable to the traditional (sci-fi) scenario "I travel to the past and meet myself / my parents / my ancestors" The reason is simple: the authors suppose a CLOSED timelike curve, i.e. something like a circle, where you travel back in time and BECOME your younger self - which by the way only exists because you traveled back in time in the first place. A slightly different scenario would be much more interesting, but my guess is that it's much harder to analyze: a NEARLY closed timelike curve, which arrives from the past, coils around itself one or more times - like a coil, indeed - allowing causal interaction between the different spires (i.e. one can interact with its future self/selves and with its past self/selves), and finally the last spire leaves toward the future.

codethief 2y ago
> The reason is simple: the authors suppose a CLOSED timelike curve, i.e. something like a circle, where you travel back in time and BECOME your younger self Exactly. This part of the paper is not really surprising or newsworthy. If you apply periodic boundary conditions, you get periodicity, duh. In the case of CTCs, this has been known for a long time[0]. > A slightly different scenario would be much more interesting, but my guess is that it's much harder to analyze: […] Agreed. The only result I'm aware of in this context is a paper from the 90s by Echeverria, Klinkhammer, and Thorne about a thought experiment (Polchinski's Paradox) involving a billard ball entering a wormhole and colliding with its past self. Wikipedia[0] gives a good overview of the result. [0]: https://en.m.wikipedia.org/wiki/Novikov_self-consistency_principle https://en.m.wikipedia.org/wiki/Novikov_self-consistency_pri...
- the_mitsuhiko 2y ago
  This paper (among some others that are referenced in this Wikipedia article) are also cited here and referenced.
- zozbot234 2y ago
  More generally, imposing "self-consistency" on a closed cycle of interactions is just a matter of picking a fixed point. Such a fixed point will always exist if the underlying system is continuous - and continuity may always be assumed if the system be non-deterministic. (For example, a billiard ball enters a wormhole sending it to the past with probability 50%, or else it is knocked away by a billiard ball sent from the future (and does not enter the wormhole) with probability 50%. This system is self-consistent, but this is achieved by a "mixture" of two outcomes.)
  - lupire 2y ago
    Can the ball roll into wormhole, emerge in the past, hit its past self and stop, while its past self it knocked to roll into the wormhole, emerge in the past, hit its past self ...
    - zozbot234 2y ago
      Sure, this is another self-consistent solution which is discussed at length in the papers referenced above. But the neat thing about non-determinism is that it adds continuity - thus, a guaranteed existence of some self-consistent solution - even when the underlying system is discrete (as in, the ball is only allowed to either enter the wormhole on its own or be knocked off altogether - which is what creates the purported paradox).
  - codethief 2y ago
    Could you elaborate on what you mean by fixed point? Fixed point of what? And what continuity and non-determinism are you referring to exactly?
    - zozbot234 2y ago
      A fixed point involving the dynamics of "complex interpersonal interactions" (to quote the above-linked Wikipedia article) that are typically involved in these purported time-travel paradoxes. Continuity of the underlying physics is enough to ensure that such a fixed point will definitely exist, and allowing for non-determinism is just a convenient way of recovering a sort of continuity even if the underlying physics is assumed to not be continuous. (These concerns are somewhat comparable to those that involve issues of so-called "metastability" in electronic circuits and indeed other physical systems which are designed to only have a limited number of "discrete" states.)
moralestapia 2y ago
>which by the way only exists because you traveled back in time in the first place No, you keep going forward all the time, but on a dimension closed on itself. That's the whole point.
- Sharlin 2y ago
  If time is closed on itself, then by definition there can be no change from one "round" to another, you have to return to the exact world state you started in. Otherwise it wouldn't be closed. Just like a coil is not a closed shape even if its projection (a circle) is.
deleted 2y ago
[deleted]
PaulHoule 2y ago
Most 'time loops' in science fiction might better be described as time knots. I think of https://en.wikipedia.org/wiki/Predestination_(film) https://en.wikipedia.org/wiki/Predestination_(film) which is much more complicated than the usual time travel scenario; presumably the protagonist leaves but doesn't really enter since the protagonist is their own mother and father (the matter that makes them up does enter since they eat and breathe the way everybody else does; thinking the story through I'd think if I was going to have such a miraculous and singular existence I'd rather be a fantastic creature of some kind [dragon?] as opposed to a relatively boring intersex person capable of both reproductive roles) Also https://en.wikipedia.org/wiki/The_End_of_Eternity https://en.wikipedia.org/wiki/The_End_of_Eternity which tames the complexity of time travel by presupposing 'eternity' has a second time dimension, making large-scale engineering of history practical. 'Eternity' itself owes it's existence to a time loop which is ultimately broken by the protagonist.
zmgsabst 2y ago
Your younger self doesn’t have to be a future state of your present self, you just have to induce it, eg, being your own father or grandfather. Your younger self doesn’t have to be in your future if you allow some overlap (father) or short gap (grandfather) on the circle.
vyrotek 2y ago
This reminds me of comic strips where someone interacts with an adjacent frame.
pdonis 2y ago
> a NEARLY closed timelike curve, which arrives from the past, coils around itself one or more times - like a coil, indeed - allowing causal interaction between the different spires (i.e. one can interact with its future self/selves and with its past self/selves), and finally the last spire leaves toward the future. The classic sci-fi story describing this is Heinlein's By His Bootstraps. Note, though, that even in this version, the causal interactions are fixed: the same person experiences the events multiple times from different viewpoints, but the events have to be the same each time. They can't change. In Heinlein's story, the main character tries to do something different at one of these interactions and finds that he can't.
- j1elo 2y ago
  also a good example is the movie Triangle
  - lupire 2y ago
    And the TV show Dark
cgh 2y ago
Isn't the cool part of this the assertion that the arrow of time flips at points of minimum and maximum entropy? In other words, it's two parallel timelines, not a continuous loop of entropic time. The article dedicates itself to proving this assertion with a bunch of math of which I understood maybe 10%. I am not a physicist, etc so if I sound daft then that's why.
harrison_clarke 2y ago
futurama solved that: fry travelled back and became his grandfather's younger self, rather than his own younger self

codethief 2y ago

> The reason is simple: the authors suppose a CLOSED timelike curve, i.e. something like a circle, where you travel back in time and BECOME your younger self Exactly. This part of the paper is not really surprising or newsworthy. If you apply periodic boundary conditions, you get periodicity, duh. In the case of CTCs, this has been known for a long time[0]. > A slightly different scenario would be much more interesting, but my guess is that it's much harder to analyze: […] Agreed. The only result I'm aware of in this context is a paper from the 90s by Echeverria, Klinkhammer, and Thorne about a thought experiment (Polchinski's Paradox) involving a billard ball entering a wormhole and colliding with its past self. Wikipedia[0] gives a good overview of the result. [0]: https://en.m.wikipedia.org/wiki/Novikov_self-consistency_principle https://en.m.wikipedia.org/wiki/Novikov_self-consistency_pri...

the_mitsuhiko 2y ago
This paper (among some others that are referenced in this Wikipedia article) are also cited here and referenced.
zozbot234 2y ago
More generally, imposing "self-consistency" on a closed cycle of interactions is just a matter of picking a fixed point. Such a fixed point will always exist if the underlying system is continuous - and continuity may always be assumed if the system be non-deterministic. (For example, a billiard ball enters a wormhole sending it to the past with probability 50%, or else it is knocked away by a billiard ball sent from the future (and does not enter the wormhole) with probability 50%. This system is self-consistent, but this is achieved by a "mixture" of two outcomes.)
- lupire 2y ago
  Can the ball roll into wormhole, emerge in the past, hit its past self and stop, while its past self it knocked to roll into the wormhole, emerge in the past, hit its past self ...
  - zozbot234 2y ago
    Sure, this is another self-consistent solution which is discussed at length in the papers referenced above. But the neat thing about non-determinism is that it adds continuity - thus, a guaranteed existence of some self-consistent solution - even when the underlying system is discrete (as in, the ball is only allowed to either enter the wormhole on its own or be knocked off altogether - which is what creates the purported paradox).
- codethief 2y ago
  Could you elaborate on what you mean by fixed point? Fixed point of what? And what continuity and non-determinism are you referring to exactly?
  - zozbot234 2y ago
    A fixed point involving the dynamics of "complex interpersonal interactions" (to quote the above-linked Wikipedia article) that are typically involved in these purported time-travel paradoxes. Continuity of the underlying physics is enough to ensure that such a fixed point will definitely exist, and allowing for non-determinism is just a convenient way of recovering a sort of continuity even if the underlying physics is assumed to not be continuous. (These concerns are somewhat comparable to those that involve issues of so-called "metastability" in electronic circuits and indeed other physical systems which are designed to only have a limited number of "discrete" states.)

moralestapia 2y ago

>which by the way only exists because you traveled back in time in the first place No, you keep going forward all the time, but on a dimension closed on itself. That's the whole point.

Sharlin 2y ago
If time is closed on itself, then by definition there can be no change from one "round" to another, you have to return to the exact world state you started in. Otherwise it wouldn't be closed. Just like a coil is not a closed shape even if its projection (a circle) is.

deleted 2y ago

[deleted]

PaulHoule 2y ago

Most 'time loops' in science fiction might better be described as time knots. I think of https://en.wikipedia.org/wiki/Predestination_(film) https://en.wikipedia.org/wiki/Predestination_(film) which is much more complicated than the usual time travel scenario; presumably the protagonist leaves but doesn't really enter since the protagonist is their own mother and father (the matter that makes them up does enter since they eat and breathe the way everybody else does; thinking the story through I'd think if I was going to have such a miraculous and singular existence I'd rather be a fantastic creature of some kind [dragon?] as opposed to a relatively boring intersex person capable of both reproductive roles) Also https://en.wikipedia.org/wiki/The_End_of_Eternity https://en.wikipedia.org/wiki/The_End_of_Eternity which tames the complexity of time travel by presupposing 'eternity' has a second time dimension, making large-scale engineering of history practical. 'Eternity' itself owes it's existence to a time loop which is ultimately broken by the protagonist.

zmgsabst 2y ago

Your younger self doesn’t have to be a future state of your present self, you just have to induce it, eg, being your own father or grandfather. Your younger self doesn’t have to be in your future if you allow some overlap (father) or short gap (grandfather) on the circle.

vyrotek 2y ago

This reminds me of comic strips where someone interacts with an adjacent frame.

pdonis 2y ago

> a NEARLY closed timelike curve, which arrives from the past, coils around itself one or more times - like a coil, indeed - allowing causal interaction between the different spires (i.e. one can interact with its future self/selves and with its past self/selves), and finally the last spire leaves toward the future. The classic sci-fi story describing this is Heinlein's By His Bootstraps. Note, though, that even in this version, the causal interactions are fixed: the same person experiences the events multiple times from different viewpoints, but the events have to be the same each time. They can't change. In Heinlein's story, the main character tries to do something different at one of these interactions and finds that he can't.

j1elo 2y ago
also a good example is the movie Triangle
- lupire 2y ago
  And the TV show Dark

cgh 2y ago

Isn't the cool part of this the assertion that the arrow of time flips at points of minimum and maximum entropy? In other words, it's two parallel timelines, not a continuous loop of entropic time. The article dedicates itself to proving this assertion with a bunch of math of which I understood maybe 10%. I am not a physicist, etc so if I sound daft then that's why.

harrison_clarke 2y ago

futurama solved that: fry travelled back and became his grandfather's younger self, rather than his own younger self