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Topic review

### Expand view Topic review: Past-self advice

Guest wrote:
a1s wrote: Stable time loop (a.k.a "immutable time-line"): You can travel to the past (and "affect" it) but you can't actually change it, since your present is already the result of you going back in time. This vaguely contradicts causality.
Closed timelike curves may be capable of existing in our universe, or not, but I see nothing inherently contradictory with them. The whole point is that they are stable, so they contain no possibility of a contradiction.
I didn't say they were exactly contradictory, I don't know enough about general relativity to say one or the other. That being said, from a layman's point of view going back in time (to affect it in any way. Again note how affect⊃change) seems to violate the basic rule of causality which is "stuff happens after the stuff that made it happen happened"
a1s wrote:Causality says that effect must happen after cause (and in fact at least d/c seconds after, where d is distance between where the cause and effect happen, and c is the speed of light) that means that time travel violates it, in any cases that matter.
(If finding out your grandfather was a monster in 2010, leads you to travelling from 2020 to 2000 in order to kill him, that would mean an event in 2010 caused an effect in 2000. That's a contradiction, some would say, so time travel doesn't exist, QED.)
Note that in the above example you killed your grandfather, after he was an abusive asshole (perhaps this was a navigation mistake on your part, or perhaps this is one of those weird Novikovian effects)

Edit: Novikov claimed that time travel (in a stable timeline) would be inherently surrounded by improbability, because the things that make the most sense from a personal-narrative point of view (such as killing grandpa before he caused harm, which was the whole point of the trip) are statistically eliminated as impossible due to "causing" a paradox)

a1s wrote: Stable time loop (a.k.a "immutable time-line"): You can travel to the past (and "affect" it) but you can't actually change it, since your present is already the result of you going back in time. This vaguely contradicts causality.
Closed timelike curves may be capable of existing in our universe, or not, but I see nothing inherently contradictory with them. The whole point is that they are stable, so they contain no possibility of a contradiction.

Jigglypuff wrote:
a1s wrote:(If finding out your grandfather was a monster in 2010, leads you to travelling from 2020 to 2000 in order to kill him, that would mean an event in 2010 caused an effect in 2000. That's a contradiction, some would say, so time travel doesn't exist, QED.)
But if you went and then told yourself "By the way, I know he's already dead, but in 2020 you have to travel to 2000 and kill your grandfather", then you've created a new cause for the same effect.
Well, yes. However it's not your responsibility to fix causality (have you been playing C°ntinuum by any chance?) rather it's supposed to be physically impossible to violate.
The next argument however could be that the root cause of the event wasn't in 2010 when you found out, but in 1990s when he was abusive. Which, obvi, is before 2000.
hussam wrote:
a1s wrote:While physics-wise the first model is more likely (though itself a distant second to the "time travel is impossible" theory,) both appear equally often in fiction.
Another thing I thought of is: A tree fell on me in 2010 and broke my leg thus ruining my basketball career. I invent a time machine in 2020, go back in time, cut down the tree a day in advance. This means my leg never broke and I never had to invent a time machine.
This could be the multithreaded time notion mentioned in the original post.
You have stumbled upon what is known as the grandfather paradox. (Good job, no irony intended, on coming up with it all by yourself)

a1s wrote:(If finding out your grandfather was a monster in 2010, leads you to travelling from 2020 to 2000 in order to kill him, that would mean an event in 2010 caused an effect in 2000. That's a contradiction, some would say, so time travel doesn't exist, QED.)
But if you went and then told yourself "By the way, I know he's already dead, but in 2020 you have to travel to 2000 and kill your grandfather", then you've created a new cause for the same effect.

a1s wrote:While physics-wise the first model is more likely (though itself a distant second to the "time travel is impossible" theory,) both appear equally often in fiction.
Another thing I thought of is: A tree fell on me in 2010 and broke my leg thus ruining my basketball career. I invent a time machine in 2020, go back in time, cut down the tree a day in advance. This means my leg never broke and I never had to invent a time machine.
This could be the multithreaded time notion mentioned in the original post.

Jigglypuff wrote:Causality isn't a problem if you told yourself to tell younger you when they reach your age.
Causality says that effect must happen after cause (and in fact at least d/c seconds after, where d is distance between where the cause and effect happen, and c is the speed of light) that means that time travel violates it, in any cases that matter.
(If finding out your grandfather was a monster in 2010, leads you to travelling from 2020 to 2000 in order to kill him, that would mean an event in 2010 caused an effect in 2000. That's a contradiction, some would say, so time travel doesn't exist, QED.)

Of course it's possible that causality is only mostly true, unless you're studying a very unusual case. (the same way that Newtonian mechanics are mostly true, unless you want to deal with nano-second-precise measurements, circumluminal speeds, or stellar-scale distances)

Causality isn't a problem if you told yourself to tell younger you when they reach your age.
As for my past advice, I wish I had some sort of sports almanac to quote to my younger self...

hussam wrote:Alternate universes are parallel ones.
This might actually be irrelevant, since by the time you learn to travel between universes, you're moving at super-relativistic speeds and can time travel (probably. That or die horribly when physics give out.)

However that's not what I mean. There are generally 2 interpretations of time travel:

Stable time loop (a.k.a "immutable time-line"): You can travel to the past (and "affect" it) but you can't actually change it, since your present is already the result of you going back in time. This vaguely contradicts causality.
Multiple universes: You can't go to your own past, however you can travel to (or create) a universe identical to yours, but for the fact that you time traveled there. This resolves causality but raises a lot of practical questions (as you pointed out)

While physics-wise the first model is more likely (though itself a distant second to the "time travel is impossible" theory,) both appear equally often in fiction.