[What is time?]

[什麼是時間？]

The earliest time measurements were observations of cycles of the natural world, using patterns of changes from day to night and season to season to build calendars.

最早出現的時間測量是觀察自然世界的循環，用白天到黑夜、一季到下一季的改變模式，來建立日曆。

More precise time-keeping, like sundials and mechanical clocks, eventually came along to put time in more convenient boxes.

更精確的時間記錄，比如日晷以及機械時鐘，最終都出現了，並把時間放在更方便的盒子中。

But what exactly is it that we're measuring?

但，我們在測量的到底是什麼？

Is time something that physically exists, or is it just in our heads?

時間有實體的存在嗎？還是它只是我們腦中的想像？

At first the answer seems obvious—of course time exists, it constantly unfolds all around us, and it's hard to imagine the universe without it.

一開始，答案似乎很明顯——時間當然存在，它持續在我們身邊展開，我們也很難想像沒有時間的宇宙。

But our understanding of time started getting complicated thanks to Einstein.

但，因為愛因斯坦，我們對於時間的了解開始變複雜。

His theory of relativity tells us that time passes for everyone, but doesn't always pass at the same rate for people in different situations, like those travelling close to the speed of light or orbiting a supermassive black hole.

他的相對論告訴我們，對每個人，時間都會過去，但過去的速度就會因情況而異了，比如在接近光速旅行的情況，或是在超大質量的黑洞旁邊運行的情況。

Einstein resolved the malleability of time by combining it with space to define space-time, which can bend, but behaves in consistent, predictable ways.

愛因斯坦解決了時間的可塑性，將時間與空間結合， 來定義「空間—時間」，它可以被彎曲，但行為仍然是一致、可預測的。

Einstein's theory seemed to confirm that time is woven into the very fabric of the universe.

愛因斯坦的理論似乎確認了時間是被織入到宇宙中的。

But there's a big question it didn't fully resolve:

但還有一個大問題， 是這個理論沒有完全解決的：

Why is it we can move through space in any direction, but through time in only one?

為什麼在空間中我們能朝任何方向移動，但在時間中的方向只有一個？

No matter what we do, the past is always, stubbornly, behind us.

不論我們怎麼做，過去始終都很固執地留在我們身後。

This is called the arrow of time.

這就是所謂的「時間箭頭」。

When a drop of food coloring is dropped into a glass of water, we instinctively know that the coloring will drift out from the drop, eventually filling the glass.

當一滴食用色素被滴入一杯水中，我們直覺上就知道，該色素會擴散出來，最終充滿整個杯子。

Imagine watching the opposite happen.

想像一下相反的狀況。

Here, we'd recognize time as unfolding backwards.

在這裡，我們認為時間是反向呈現的。

We live in a universe where the food coloring spreads out in the water, not a universe where it collects together.

我們所住的宇宙中，食用色素散佈在水中各處，而不是集合起來成一滴的。

In physics, this is described by the second law of thermodynamics, which says that systems will gain disorder, or entropy, over time.

在物理上，可以用熱力學第二定律來描述它，該定律說的是，系統會隨著時間得到混亂或是無序。

Systems in our universe move from order to disorder, and it is that property of the universe that defines the direction of time's arrow.

在我們宇宙中的系統， 會從有秩序變成混亂，正是宇宙的這項特性定義了時間箭頭的方向。

So if time is such a fundamental property, it should be in our most fundamental equations describing the universe, right?

所以，如果時間是這種根本的特性，它就應該要出現在我們用來描述宇宙的最根本的方程式當中，對吧？

We currently have two sets of equations that govern physics.

目前，我們有兩組支配物理的方程式。

General relativity describes the behavior of very large things, while quantum physics explains the very small.

廣義相對論描述宏觀現象，而量子物理解釋微觀現象。

One of the biggest goals in theoretical physics over the last half century has been reconciling the two into one fundamental "theory of everything."

在過去半個世紀， 理論物理最大的目標之一，就是將兩者調解整合成為單一的「萬物之理」。

There have been many attempts—none yet proven—and they treat time in different ways.

許多人嘗試過，卻沒一個被證明——它們都以不同的方式來看待時間。

Oddly enough, one contender called the Wheeler-DeWitt equation, doesn't include time at all.

很怪的是，其中一個稱為「惠勒—德威特方程式」的角逐者，它完全沒有納入時間。

Like all current theories of everything, that equation is speculative.

和目前所有的萬物之理一樣，該方程式純屬理論推測。

But as a thought experiment, if it or a similarly time-starved equation turned out to be true, would that mean that time doesn't exist, at the most fundamental level?

但就思想實驗來說，如果它或同樣缺乏時間的方程式最後證明是真的，那是否意味著時間不存在於最根本的層級上？

Could time just be some sort of illusion generated by the limitations of the way we perceive the universe?

時間是否可能只是一種幻覺，因我們受限於對宇宙的感知方式而產生出來的幻覺？

We don't yet know, but maybe that's the wrong way of thinking about it.

我們還不知道答案，但也許那是錯誤的時間思考方式。

Instead of asking if time exists as a fundamental property, maybe it could exist as an emergent one.

不問時間是否真的是一種根本特性的存在，而是問它是否是一種突顯特性的存在。

Emergent properties are things that don't exist in individual pieces of a system, but do exist for the system as a whole.

突顯的特性不存在於系統中的個別部分，而是存在於整個系統當中。

Each individual water molecule doesn't have a tide, but the whole ocean does.

個別的水分子沒有潮汐，但海洋卻有。

A movie creates change through time by using a series of still images that appear to have a fluid, continuous change between them.

電影透過時間創造出改變，使用一系列靜態影像，各影像間有著流暢、連續的改變。

Flipping through the images fast enough, our brains perceive the passage of time from the sequence of still images.

只要非常快速翻動這些影像，我們的大腦就認為靜態影像序列是有的時間經過的。

No individual frame of the movie changes or contains the passage of time, but it's a property that comes out of how the pieces are strung together.

電影中沒有任何一個畫格會改變，單一畫格也不包含時間經過，但當各部分被串接在一起時，就會產生出時間這種特性。

The movement is real, yet also an illusion.

動作是真實的，同時也是種幻覺。

Could the physics of time somehow be a similar illusion?

時間的物理學有沒有可能在某種層面上也是類似的幻覺？

Physicists are still exploring these and other questions, so we're far from a complete explanation.

物理學家仍然在探究這些及其他問題，我們離找到完整的解釋還很遙遠。

At least for the moment.

至少目前是如此。

If time does exist, you'll probably want a brief history of it.

如果時間真的存在，你或許會想知道它大略的歷史。

Explore more of the fundamental questions of our existence by downloading a free audio version of Stephen Hawking's modern classic at audible.com/teded.

至 audible.com/teded 下載免費的霍金博士現代物理經典有聲書，來探索更多關於我們存在的根本問題。

Every trial created encourages Audible to continue supporting Ted-Ed's nonprofit mission.

每一個試閱都是鼓勵 Audible 來繼續支持 Ted-Ed 所推動的非營利任務。