Paradox-free time travel is theoretically possible • Hacker News Board

Re: Paradox-free time travel is theoretically possible

#211

Post by JoeAltmaier » Mon, Jan 25, 2021, 1:14 PM UTC

tsimionescu wrote: post #210

JoeAltmaier wrote: post #209

Earlier quoted context omitted.

Again recapitulating what a paradox is. If time travel works at all (the premise) then I can look up a historical event, and send a hand grenade back through time to that event. What then? The response "you obviously can't do that" isn't answering anything. What happens when I do? Deliberately, to defy the argument that it's impossible. What then? That's the whole issue.

Please answer this question: what happens if, today, you decide to not have gone to the store yesterday? What stops you from deciding this? The way this particular idea of time travel works is like this: 1. A time traveler throws a grenade, killing Joan of Arc. 2. History happens. 3. The time traveler who killed Joan of Arc is born. 4. The time traveler goes back in time and kills Joan of Arc. 5. We hear of this even…

Answer: I have a time machine. If that means anything, it means I can go back to yesterday and decide not to go to the store. Otherwise what is it? Some kind of television into the past?

Re: Paradox-free time travel is theoretically possible

#212

Post by tsimionescu » Mon, Jan 25, 2021, 4:03 PM UTC

JoeAltmaier wrote: post #211

tsimionescu wrote: post #210

Earlier quoted context omitted.

Please answer this question: what happens if, today, you decide to not have gone to the store yesterday? What stops you from deciding this? The way this particular idea of time travel works is like this: 1. A time traveler throws a grenade, killing Joan of Arc. 2. History happens. 3. The time traveler who killed Joan of Arc is born. 4. The time traveler goes back in time and kills Joan of Arc. 5. We hear of this even…

Answer: I have a time machine. If that means anything, it means I can go back to yesterday and decide not to go to the store. Otherwise what is it? Some kind of television into the past?

It is a loop between the present and the past. You keep talking though about deciding to do things, and in this paradigm, there is no such thing. I am not 'deciding' to write this post, it is simply a consequence of the evolution of the universe and my life so far. I may feel like I am free to write it or not, but that is only an illusion.

Just as much, if you have a time machine, and go to the past, you will do exactly what you had already done in the past. It is not something you have a choice on, it is simply a consequence of the universe so far. The same would be true if you went to the future - all your future actions were decided the moment you were born, the moment the Earth formed, the moment the universe was formed. There is no choice.

Re: Paradox-free time travel is theoretically possible

#213

Post by JoeAltmaier » Mon, Jan 25, 2021, 5:51 PM UTC

tsimionescu wrote: post #212

JoeAltmaier wrote: post #211

Earlier quoted context omitted.

Answer: I have a time machine. If that means anything, it means I can go back to yesterday and decide not to go to the store. Otherwise what is it? Some kind of television into the past?

It is a loop between the present and the past. You keep talking though about deciding to do things, and in this paradigm, there is no such thing. I am not 'deciding' to write this post, it is simply a consequence of the evolution of the universe and my life so far. I may feel like I am free to write it or not, but that is only an illusion. Just as much, if you have a time machine, and go to the past, you will do exac…

Ah. Then I decide to disbelieve in the deterministic model of the universe proposed. And I cannot be faulted for that, because its a consequence of the evolution of the universe. So we're done here.

Re: Paradox-free time travel is theoretically possible

#214

Post by tsimionescu » Tue, Jan 26, 2021, 8:29 AM UTC

JoeAltmaier wrote: post #213

tsimionescu wrote: post #212

Earlier quoted context omitted.

It is a loop between the present and the past. You keep talking though about deciding to do things, and in this paradigm, there is no such thing. I am not 'deciding' to write this post, it is simply a consequence of the evolution of the universe and my life so far. I may feel like I am free to write it or not, but that is only an illusion. Just as much, if you have a time machine, and go to the past, you will do exac…

Ah. Then I decide to disbelieve in the deterministic model of the universe proposed. And I cannot be faulted for that, because its a consequence of the evolution of the universe. So we're done here.

Agreed! I said from the start - I'm not claiming that this is how reality works. I'm just pointing out that it is self-consistent and that it does solve the paradox.

I myself can't entirely believe this notion of a fully deterministic universe. Especially one that doesn't even involve randomness (though randomness by itself wouldn't bring in any possibility of choice).

Re: Paradox-free time travel is theoretically possible

#215

Post by simiones » Tue, Jan 26, 2021, 4:30 PM UTC

schwartzworld wrote: post #95

littlestymaar wrote: post #68

The time-travel paradox arises only if you presuppose the existence of free will: if we don't, and just consider human beings as physical objects acting deterministically, then there can be no paradox: the time traveler will simply do what was already had been done in his past.

Ah yes, can you change the past or was there a time traveler version of you running around failing to change things the whole time? See season 5 of Lost for endless discussion on this point. You also can avoid paradoxes if it turns out that time travel just puts you on an alternate timeline. There's a different paradox with this version of time travel though. Suppose I'm a kid and future-Me comes back and tells me a…

It may be that, just as it's impossible for you to be killed by yourself from the future in this type of universe, it's also impossible to create information out of nothing, so the specific scenario you described would just be impossible.

This could be a kind of conservation law - imagine that instead of a story you had a golden coin, that you receive from your future self, keep, and give back to your past self after you travel, you've added some mass to the universe, which would be impossible because of conservation of energy.