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

但如果你能回訪我們所在宇宙的更早的時間點，便會產生一個著名的悖論。

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

如果你在你祖父小時候就把他殺了的話會怎麼樣？

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

如此一來你的父親或母親就不會出生，所以你也不會出生，

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

所以你也不可能回到過去殺死你的祖父。

Paradox.

悖論出現了。

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,

祖父悖論最簡單的解決方法是，當你回到過去時，你實際上不是回到你自己的歷史，而是回到一個副本，

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

而你的所有作為都只會影響到該宇宙其他的未來，而不是你自己的過去。

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

但這種解釋很無聊，因為它只是避開了悖論而已。

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... past – no 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.

我們就試著按照自相矛盾的時間線，一路通往它自相矛盾的結論吧。

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

你回到了過去，殺死了你的祖父，所以你沒有出生，因此也沒有回到過去，

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

因此你的祖父沒有被殺，所以你能被生下來，讓你能回到過去然後殺了祖父，然後一直循環下去...

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

我的說法讓這看起來像是線性接連發生的事件，但其實是兩個平行發生的故事。

Is that even possible?

這有可能嗎？

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.

這樣的現象叫做量子疊加，而這個現象也是造成雙縫實驗、許多原子與分子、太陽地核中的聚變反應等產生的奇怪狀況背後的原因。

So if the universe were to exist in a superposition of two states – your grandfather is alive and your grandfather is dead – then the natural result is a superposition of two states:

因此如果宇宙確實是以疊加模式呈現出兩種不同狀態，也就是你的祖父還活著和你的祖父已經死去的狀態，最後自然就產生了兩種疊加的狀態：

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

你成功出生並回到過去殺死了你的祖父，以及你沒有出生的兩個狀態。

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

而這又產生了兩種疊加狀態：你的祖父死去，和你的祖父活著的兩個狀態。

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

有藉於此，至少從邏輯的觀點來看，這個反覆的時間軸完全是前後一致的，並沒有產生任何的矛盾。

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

而我們也可以藉由將問題視為一個馬可夫鏈穩定態解法來處理掉矛盾－但我就不在這裡細談了。

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

當然這些對於祖父矛盾的解法並不代表封閉的時間迴圈是真的可能發生的狀況。

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.

事實上，關於此種時間迴圈現象的討論能應用在部分對複雜性理論的研究上，並能推敲出既然時間會迴圈，那便代表回到過去是不可能的。

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

但其實我想說的重點是，有時我們以為一個情境創造出了一個矛盾，實際卻並沒有，

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.

而真正唯一一個矛盾其實是，我們的思想能扭曲到去妄想會有進行時光旅行來殺死祖父的孫子，卻沒能靈活到想到時間是可以被扭曲的。

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.

對我來說科學就是充滿好奇心、勇於提問並嘗試去找出答案。

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.

所以我想要謝謝他們贊助這部影片，並希望這部影片能解答其他人在未來可能會有的疑問。

Just love being outside on a beautiful day!

能在這樣風和日麗的日子裡出門走走真是太棒了！