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Dylan16807 6y ago

> Try dealing with something of consequence, like killing your former self (as in: the person in that point of time-space which carries the mutable signifier of “yourself” which is younger than yourself) or giving yourself a kilogram of gold from the future, which I think would violate the law of preservation of energy or whatever. You can make up impossible scenarios without time travel too. Like someone that shoots themselves (lethally, thorough and immediate) and then a month later they go out to lunch and eat a ham sandwich. The ability to write out a scenario doesn't mean the scenario is possible. The theory is that "I go back in time and kill myself" is just as impossible as "I kill myself then a month later eat a ham sandwich". And for largely the same reason: both scenarios violate causality, so the universe can't be shaped that way.

deleted 6y ago
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yrimaxi 6y ago
Shooting yourself and then later going out to lunch is simply impossible; it is not a paradox. A paradox is when two contradictory things are true at the same time. The grandfather problem is a paradox if time travel is possible. If it isn’t possible then it is simply an impossibility. > And for largely the same reason: both scenarios violate causality, so the universe can't be shaped that way. So you agree that time travel is impossible. Great... we agree. However I was discussing this with someone who apparently thinks that it is concievable... I wasn’t merely making up impossible scenarios; I was demonstrating how their theory gives rise to paradoxical scenarios.
- Dylan16807 6y ago
  > A paradox is when two contradictory things are true at the same time. > The grandfather problem is a paradox if time travel is possible. If it isn’t possible then it is simply an impossibility. It's only a paradox if you're making certain assumptions about how time travel works. You need to admit that these are assumptions. Consider the airplane on a treadmill paradox. There is a hidden assumption that it's possible to accelerate the treadmill in isolation to match the wheel speed, but the logic fails because the speed of the treadmill is entwined with the speed of the wheels on the plane. > So you agree that time travel is impossible. Great... we agree. No, that's not what I said. > I was demonstrating how their theory gives rise to paradoxical scenarios. It doesn't. You're assuming you can choose your actions independently from the time travel. The theory says that every choice is entwined with the fabric of the universe, so that by the time you see that an instance of time travel exists, the critical non-paradox-causing choices are already set in stone.
- vidarh 6y ago
  > A paradox is when two contradictory things are true at the same time. No. A paradox is when two contradictory things appears to be true at the same time. It simply signifies that our model is flawed or incomplete. The grandfather problem is a paradox if our model of time travel is one which is impossible. That is the point. It is simply a way of demonstrating why there needs to be constraints that makes our more naive notions of time travel impossible. As such it constrains the set of possible valid models to those in which the grandfather paradox can't be expressed within the constraints of the model. > I wasn’t merely making up impossible scenarios; I was demonstrating how their theory gives rise to paradoxical scenarios. You've done no such thing. The paradoxical scenario you imagine can't be expressed in the model I proposed, and so it solves the paradox.
- tsimionescu 6y ago
  The key here is that, if time travel is possible, you would experience the events like this : 1. You start your day normally 2. You meet yourself from the future and discuss some niceties. 3. Many years later, you enter a time machine, and you arrive back at time 2, where you have that conversation with yourself that you remember. The past has already happened, so it can't change. So, if you haven't met a time traveler so far, that is proof that no one will ever time travel to the moment and place where you just were, even if time travel becomes possible in the future. Now, you may ask, 'what happens if at step 3 I decide to shoot myself, or not have that conversation?'. Here the answer is simple : that question makes no sense. You have already done our not fine these things, there is no power of decision that could influence this. It's like asking "what if I now decide that yesterday I didn't go to the store". The fact that the event at time 2 happened is fixed both in the past and the future. It was in fact fixed at the time of the universe's creation. Now, you may not accept determinism, or you may believe in free will, and there are good reasons for both (quantum indeterminism, subjective experience, etc.). In either case, I don't think you can believe in time travel, since all of the paradoxes reappear. But if you believe the past and the future are already set in stone, there is no paradox if the two cross each other in certain ways. But it doesn't make sense about what you 'will decide to do', because this is as fixed as what you ate yesterday.
  - JoeAltmaier 6y ago
    I think that argument is just recapitulating what a paradox is. It doesn't address "what if you try to do something that didn't happen", which is the root question.
    - tsimionescu 6y ago
      No, the argument is saying that the question you are asking makes no sense. You can't "try" to do anything differently, because these things have already happened. Again, it's just like asking "what if I try to not go to the shop yesterday, what would happen to the bread I'm eating?". Can you decide today to not do something yesterday? No. In the same way, a time traveler can't "choose" to modify the past - it's something that's already happened. Of course, this does imply that the future is also set in stone, so in fact you can't choose to go the shop or not tomorrow any more than you can choose to have done so yesterday. This is not a paradox, it's just counter to our subjective experience.

vidarh 6y ago

> Try dealing with something of consequence, like killing your former self (as in: the person in that point of time-space which carries the mutable signifier of “yourself” which is younger than yourself) or giving yourself a kilogram of gold from the future, which I think would violate the law of preservation of energy or whatever. It is entirely irrelevant what it is. The point is that if the system is immutable it is impossible to represent a combination of state transitions that alters an existing state, as such the set of possible space-time combinations would in that case only include those where the time travel does not introduce a paradox that depends on change. The same way the timeline does not include any states where I fold an elephant, because it's not possible to do so. You have not presented any paradoxes that can work with an immutable space-time other than perhaps if said space-time introduces divergent timelines at any point of mutation, in which case the paradox can also easily be resolved. > And yet if time travel backwards is possible you simply cannot deny that one would experience (!) that time travel as steps 1–3. Temporal experience. Hah! You can experience it that way, but your "do something" step 2 would be deterministic and could not possibly include steps that introduced a paradox. > There’s absolutely nothing hard about the concept of time being immutable; You're still thinking in terms of a mutable space-time. I did not talk about time being immutable, but the entirety of space-time. Your "do something" step does nothing that changes anything in an immutable space-time, because everything is fixed from the start. Nothing moves. No action is taken. Everything is unalterable. Time, and cause and effect, in that scenario is simply an illusion of consciousness at every point of a fixed 4d (or more) canvas that never changes in any way. > The true puzzle is time travel which introduce paradoxes, which your original comment says absolutely nothing about. My original comment gave immutability as an example of a possible solution. Time travel with paradoxes is not possible. That is the entire point of a paradox: The problem as stated reveals a problem with the presumed model. The solution is to identify and remove flaws in the model. Removing mutability is one such solution.

yrimaxi 6y ago
> It is entirely irrelevant what it is. The point is that if the system is immutable it is impossible to represent a combination of state transitions that alters an existing state, Alters? Aha! But you cannot alter that which is immutable! Seems that you still don’t get the concept. See how silly it is to language police people on such subjets? > would in that case [only include those] where the time travel does not introduce a paradox that depends on change. Highlighted the problematic part with brackets. This is just hand-waving. “Only those”? Your theory would have to be able to explain concrete examples like the grandfather problem, or else it doesn’t rise to the level of being a theory that can explain the paradox. > The same way the timeline does not include any states where I fold an elephant, because it's not possible to do so. Irrelevant unless there is an actual elephant folding paradox that I’m missing. Impossible things can be explained; paradoxes cannot since they reveal a contradiction in the model. > You have not presented any paradoxes that can work with an immutable space-time other than perhaps if said space-time introduces divergent timelines at any point of mutation, in which case the paradox can also easily be resolved. This is the first time (?) you’ve mentioned divergent timelines or hinted at non-just-straight-line-time. And yes, that would probably be perfectly adequate, and is the best answer you’ve given thus far. > You can experience it that way, but your "do something" step 2 would be deterministic and could not possibly include steps that introduced a paradox. “Determinism” is either irrelevant or non-sufficient. “Could not possibly” is pure hand-waving as there is nothing which makes sure that you are able to do something benign (like greet your former self) but not do something like kill yourself. > You're still thinking in terms of a mutable space-time. I did not talk about time being immutable, Wow, I have to hand it to you. I did mention “space-time” once (although only once) and I utterly failed to be rigorous and spell out the whole package deal, namely space-time. What an utter embarrassment. Not that it changes my point in any way. > My original comment gave immutability as an example of a possible solution. Time travel with paradoxes is not possible. You can’t resolve the Barber Paradox by simply saying that the Universe wouldn’t allow the barber to shave himself. That’s effectively what you’re doing here, only using vaguely hard science terms with a seasoning of “you still don’t get it”.

EliRivers 6y ago

You've made a hell of a mess of all this. The premise presented is simple. The past is fixed, the universe is consistent, if you travel back in time that always happened. You're saying there's a puzzle about something that the premise at hand simply says is impossible. You might as well imagine being able to create energy out of nothing and then say that's a puzzle in a universe in which we have conservation of energy. Just because you can imagine something impossible doesn't make it a paradox. And I’ll reiterate: I accept all of these premises No you don't. You keep saying "but what if you go back and change the past?"

I don’t think this really gets at the paradox. What if you shot yourself?

I don’t think this really gets at the paradox. What if you shot yourself?