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  • As far as we know, time only moves in one directionforward.


  • But if you could loop back to visit the universe at an earlier point in time, a famous paradox arises.


  • What if you killed your grandfather when he was a child?


  • Then your father or mother wouldn't have been born, so you wouldn't have been born,


  • so you wouldn't have been able to go back in time to kill your grandfather in the first place.


  • Paradox.


  • The simplest resolution to the grandfather paradox is that when you go back in time, you're actually not going back into your own history but to a copy,


  • and everything you do there influences the new alternate future of that universe, not your own past.


  • But that's boring, because it just avoids the paradox.


  • If what you do when you go back in time actually influences your own past, and the effects of your time travel do loop back to the present... future... pastno problem.

    如果你真的回到以前去改變你的過去,而你所進行的時間旅行真的會回過頭來影響到現在... 未來... 過去的話,那也沒問題。

  • Let's just follow the paradoxical timeline through beyond its paradoxical conclusion.


  • You go back in time, kill your grandfather, thus you aren't born so you can't go back in time,


  • thus your grandfather isn't killed, thus you are born, so you go back in time and kill your grandfather, and so on...


  • I'm showing this as a looping linear series of events, but really it's two entangled histories happening in parallel.


  • Is that even possible?


  • Well, I don't know about the time travel part, but subatomic particles regularly do multiple different things in parallel.

    這個嘛,時間旅行可不可能實現我不知道, 但次原子粒子確實是會在同一個平行時空做出多種不同行為的。

  • It's called quantum superposition, and is responsible for the weirdness of the double slit experiment, many properties of atoms and molecules, fusion in the sun's core, and so on.


  • So if the universe were to exist in a superposition of two statesyour grandfather is alive and your grandfather is deadthen the natural result is a superposition of two states:


  • you're born and able to go back in time to kill your grandfather, and you're not born.


  • And the natural result of these is a superposition of two states: your grandfather is dead, and your grandfather is alive.


  • And so, at least from a logical perspective, this looping timeline is entirely consistent and there's no paradox.


  • And a similar paradox-free solution can be obtained by viewing the problem as a steady-state solution to a Markov chainbut I won't go into that here.


  • Now of course nothing about these solutions to the grandfather paradox suggests that closed time loops are actually possible.


  • In fact, some of the implications this kind of time loop have in the study of complexity theory seem to suggest that time loops, and thus time travel into the past, must be impossible.


  • But the main point is sometimes we think a situation creates a paradox when it doesn't,


  • and really the only paradox is how our thinking can be twisted enough to dream up time-traveling murderous grandsons, but not twisted enough to think about twisting time.


  • I want to say a big thank you to Google for supporting this video.

    我想向 Google 贊助這部影片表達大大的感謝。

  • Science for me is all about curiosity, asking questions and trying to find answers.


  • Google has always been one of the first places that I go when I have a question and want an answer— I'm sure it's the same for you!

    而 Google 一直是當我有疑問並想要尋求解答時第一個前往的地方-我相信各位也是如此吧!

  • So I'd like to thank them for supporting this video, which hopefully will be an answer to other people's future questions.


  • Just love being outside on a beautiful day!


As far as we know, time only moves in one directionforward.


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祖父悖論的解決方案 (Solution to the Grandfather Paradox)

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    Summer 發佈於 2021 年 10 月 04 日