It is a question so profound that few people would even think to ask it yet its effects are all around.

這是個非常深奧的問題，很少有人會想到這個問題，但它的影響力仍舊存在。

Human beings live in a perpetual present inexorably sealed off from the past, but moving relentlessly into the future.

人類活在永恆的當下，冷酷地與過去隔絕，但無情地邁向未來。

For most people time seems to be something that is just out there a thing ticking away in the background fixed immutable.

對大多數人來說，時間似乎就是在那裡，在背景滴答作響，固定不變的東西。

Time seems to go in one direction in one direction only, but physicists see it much differently.

時間的行進似乎只有一個方向，只有向前行進，但物理學家可不這麼想。

John Archibald wheeler said time is what stops everything from happening at once.

John‧Archibald‧wheeler 說過，時間是阻止一切發生的原因。

I wish I could explain what it would be like.

我希望我可以解釋那會是甚麼樣子。

For there not to be time.

如果沒有時間的話。

But I can't, we experience time.

但我沒辦法，我們已經體驗時間的存在。

We are psychological creatures that have time and that's kind of one of our defining characteristics, so if time weren't around

我們是有時間概念的心理生物，那是我們生物特徵的定義之一，所以如果沒有時間

we don't really have even a good way to

我們真的沒有一個好的方法

describe what happens because happens, right?

來描述發生甚麼事，因為就是發生了，對吧？

That's a verb right there that has to do with time.

那是個必須有時間概念才能用的動詞。

So I think once you start wondering about this you get much further into the realms of philosophy than physics.

所以我想一旦你開始對這個感到好奇，你會進一步深入哲學領域，而不是物理領域。

I actually don't think it's that mysterious.

我其實不覺得它有這麼神秘。

We live in a world that's full of stuff and this world happens over and over again with the stuff in slightly different positions.

我們生存的世界充斥各種事物，各種事物在略有不同的地點不斷發生。

Time is just the label on all those different moments that constitute the history of the universe.

時間只是為了構成這個宇宙的歷史，而在不同時刻貼上的標籤

It's very much like the page numbers in a book.

就像書裡的頁碼一樣。

One of the great minds who changed the way science thinks about time was Albert Einstein.

改變科學界思考時間概念的偉大思想家之一就是 Albert‧Einstein。

In 1905 he published his special theory of relativity

西元 1905 年他公開了他的狹義相對論

which described the motion of objects moving near the speed of light.

狹義相對論描述了物體接近光速的運動狀態。

In it he demonstrated that time passes differently in different places depending on how those places are moving with respect to one another.

在其中，他證明時間在不同位置流逝的程度不同，取決於這些位置相對的運動方式。

So first Einstein studied the effect of Fast motion,

所以，首先，Einstein 研究高速移動的影響

and he found that space and time got mixed up if you moved more quickly and that's his special theory of relativity.

他發現如果你移動的越快速，空間與時間就會越混亂，那就是他的狹義相對論。

The theory of relativity is so called because

會稱作相對論的原因是

even the difference between time and space is relative to what observer is doing the measuring.

即使時間與空間的差異是相對於觀測者的觀測方式。

In particular to how fast you're moving.

特別是看你運動的速度有多快。

This is something that we never notice in our everyday lives, but once you start moving close to the speed of light relativity becomes crucial.

這是我們日常生活中不會注意到的事，但一旦你開始以接近光速的速度運動時，相對性變得至關重要。

Einstein showed that the faster one travels the slower time goes for the traveler at the speeds at which humans move this is imperceptible.

Einstein 表示，當旅行速度越快，對處於那個速度之下的旅行者來說時間過得越慢，這是人類運動速度下無法察覺的。

But for someone traveling on a spaceship at speeds close to that of light

但對接近光速的太空船上的旅行者來說時間會變慢

time would slow down compared with its passage for people on earth.

相對於地球上的人們看到的太空船航行時間。

One of Einstein's great insights was to understand

Einstein 其中一個偉大的見解是

that different people moving at different speeds through the universe.

瞭解不同的人以不同的速度在這個宇宙運動。

See.

注意。

-Different events as being simultaneous. -There was another important aspect of Einstein's theory which he didn't even realize when he published it.

-不同的事件好像是同時發生的。 -Einstein 的理論還有另一個重要面向是他在公開時沒有意識到的。

The time was woven into the very fabric of space itself.

時間被編織入空間本身的結構中。

It was actually not Einstein but another German mathematician named Hermann Minkowski

其實不是 Einstein 提出的，是另一個德國數學家，名叫 Hermann‧Minkowski。

who pointed out that Einstein's theory could be thought of as a theory of a single

他指出 Einstein 的理論可以視為

Four-dimensional space-time. And Einstein when he first heard this idea said well, that's just a mathematician talking.

一個單獨四維時空的理論。而當 Einstein 第一次聽到這個想法時說：嗯，那只是一個數學家的說法。

That's not really something that is very useful to physicists

對物理學家來說那不是個很有幫助的東西。

But once he tried to fit gravity into his theory of relativity. He realized the space-time concept was crucial.

但一旦他試著把重力加入他的相對論中。他發現時空概念相當重要。

Einstein used this insight to help develop his general theory of relativity which incorporated gravity.

Einstein 利用這個見解幫他發展了包含重力的廣義相對論。

He published it in 1915 ten years after his special theory of relativity which was focused solely on motion.

在僅專注於運動的狹義相對論發表後 10 年，也就是西元 1915 年他公開了廣義相對論。

Einstein tried very hard he worked for ten years after inventing special relativity to fit gravity into the framework.

發明狹義相對論後，Einstein 努力了 10 年才將重力加入這個框架。

And eventually he realized he needed a new framework, so he invented what we call the general theory of relativity.

最終他明白他需要一個新的框架，所以他發明了我們口中的廣義相對論。

Where Einstein says that this space-time stuff this four-dimensional world in which we live has a life of its own.

Einstein 說，時空這個東西，這個我們生活的四維空間，有它自己的生命。

Space-time itself can move, it has dynamics, it has curvature

時空它本身可以移動、它有動態、它有曲率

and it's that curvature that geometry of space-time that you and I experienced as the force of gravity.

而就是那樣你和我經歷的時空曲率、時空幾何為重力的力量。

And then he found the time was also changed by your proximity to a very massive object.

然後他發現在你接近一個質量很大的物體時，時間也會改變。

So like a black hole is the best example.

所以，像黑洞就是個絕佳的例子。

But also just how close we are to the earth or to the Sun or anything else that has a gravitational field

但我們多靠近地球或多靠近太陽，或任何有重力力場的物體

changes the passage of time.

都會改變時間的流逝。

With the general theory of relativity he demonstrated that massive objects warped the fabric of space-time.

在廣義相對論中，他證明了大質量物體會扭曲時空結構。

It is this curvature that causes time to slow down near them.

就是這個曲率導致時間在接近它們的時候變慢。

According general relativity,

根據廣義相對論

time slows down near massive bodies.

時間接近大質量物體會減慢。

It slows down any place that gravity pulls you toward it.

任何重力將你拉近的地方都會減慢。

Time slows down in proportion to the gravitational pull of a nearby object so the effect would be strong near a black hole.

時間減慢的程度與重力拉近的物體成比例，所以接近黑洞的影響力很大。

But milder near the earth, but even here it can be detected.

但接近地球的影響沒那麼大，但即使在這裡也可以被發現。

Einstein's theories had to be taken into account when the GPS system was set up, otherwise it would have been inaccurate.

設置全球衛星定位系統時必須考慮 Einstein 的理論，否則就不精準了。

These ideas about space and time being relative sound very

這些空間與時間相關的想法聽起來非常

wild and far out and removed for everyday experience. But to physicist, they are very very important. There's something we observe every day

瘋狂又天馬行空而且遠超日常經驗。但對科學家來說，它們非常非常重要。有些東西我們每天都會觀察到。

One of the interesting things about Einstein's equations is that none of them suggest time goes in only one direction

Einstein 的等式中都有一個很有趣的點是，沒有一個說明時間是單向的。

They work equally well if time goes forwards or backwards.

無論時間往前或往後都能讓等式成立。

This is in contrast to everyday experience

這與日常經驗形成對比。

One of the conundrums is that

其中一個難題是

the laws of physics for the most part do not distinguish between time going forward and time going backwards and yet the actual

物理學定律大部分不會區分出時間是向前或後退

world that we experience certainly is different forward to backwards.

但我們經歷的現實世界中向前或向後肯定是不同的。

One scientist who puzzled over the directionality of time was Arthur Eddington a 20th century

一位對時間的方向性感到困惑的科學家是 20 世紀的天文學家 Arthur‧Eddington

astronomer who defined the concept of the arrow of time based on observations made by the 19th century physicist Ludwig Boltzmann.

他根據 19 世紀物理學家 Ludwig‧Boltzmann 的觀測結果定義了時間之箭。

In space there's no preferred direction. There's no difference in the laws of physics if you look left or look right.

在空間中沒有優先方向。不論你向左或向右，在物理學定律中是沒有差別的。

But there seems to be an enormous difference between the past and the future.

但在過去與未來中，似乎有非常大的差異。

That's what we call the arrow of time, and it's a little bit mysterious.

那就是所謂的時間之箭，但這有點深奧。

Why is the past so different from the future? We think it's not a difference in the deep down fundamental laws of physics.

為什麼過去與未來如此不同？我們認為在深層的物理學基本定律中這是沒有差別的。

It's actually just a difference because of the universe in which we live.

事實上是因為我們生存的這個宇宙讓它有所差異，

The arrow of time is just time as we experience one second following one other second following another second.

時間之箭指的就是時間，就是我們經歷了這一秒後，接著是下一秒，再緊接著另一秒。

It's just the ... psychological experience of time and the fact that

這只是對時間的心理感受，事實上

things change as we experience them, but it seemed to happen in the world forward. So buildings get built,

事物改變的同時我們也正經歷著那些改變，但似乎只發生在向前邁進的世界中。所以建築物建成了

but they decay slowly, and it's that decay, there's at least partially responsible for the arrow of time.

但建築物腐朽的速度很慢，至少腐朽的部分可以作為時間之箭成立的原由。

It's the increase of entropy. So in thermodynamics we know that things get more random over time.

就是熵的增加。所以在熱力學中我們可以知道隨著時間推移，事物會變得越不確定。

And that appears to be very fundamental to our experience of time and what's happening in the universe.

那似乎對我們對時間的感受以及宇宙發生的事相當重要。

The arrow of time is based on the second law of thermodynamics, which says the disorder known as entropy increases with time.

時間之箭是基於熱力學第二定律而來，指稱為「熵」的混亂現象會隨著時間增加。

For example a building left untouched will slowly decay into its surroundings. It will disintegrate into a more chaotic state.

例如一棟閒置建物會慢慢腐朽融入周遭環境。它會慢慢崩解進入更混亂的狀態。

But it is highly unlikely that the building will become more orderly over time.

但隨著時間流逝，它不太可能變得更加有秩序。

This is because there are many more ways for a system to be disorderly than orderly.

這是因為讓一個系統變得更無秩序的途徑比更有秩序的途徑多。

There can be many ways for something to break for instance,

舉例來說，有很多方法可以讓某個東西毀壞

but only one way for it to be put back together again.

但只有一個方法可以讓它再次回復原狀。

A system will be less disordered in the past and more disordered in the future. This is the arrow of time.

一個系統會在過去會比較有秩序，而在未來會更加混亂。這就是時間之箭。

So how can the arrow of time be reconciled with Einstein's equations if time can go forwards and backwards according to relativity?

所以如果根據相對論的內容，時間可以向前或向後，時間之箭怎麼會跟 Einstein 的等式一致呢？

Does that mean it's possible to go backwards in time?

是不是意味著回到過去是有可能的？

We actually don't know whether it's possible to travel backwards in time. We do know it's possible to travel forwards.

其實我們不知道回到過去是不是有可能。我們知道去到未來是有可能的。

Yesterday I travel forwards 24 hours, and here I am right now.

我昨天花了 24 小時的時間旅行到未來，所以我現在在這裡。

You can't help but travel forward in time. Our best guess is that you just can't travel backward in time

你沒辦法不跟著時間向前進。我們最理想的猜測是你沒辦法在時間概念中回到過去。

The theory of relativity does allow time travel to the future. If I go down near a black hole

相對論確實承認時間會向前行進。如果我落下接近一個黑洞

and stay there for one hour you may go forward by seven years by the time.

並待在那裡 1 小時，你可能隨著時間經過了 7 年。

I come back, so you've moved forward in time seven years while I move forward in time just one hour.

我回來後，你已經經歷了 7 年時光，而同時我只經歷 1 小時時間。

Einstein's theories do allow for the formation of wormholes in space, these are shortcuts that link otherwise distant places in the space-time continuum.

Einstein 的理論確實承認太空中蟲洞的形成，這些是時空連續體中連結其他遙遠距離的捷徑。

So a wormhole is

所以蟲洞是

a hole in space and time

在時空中的一個洞

that comes out somewhere else.

出口在某個地方。

And it does it much faster than the speed of light potentially so it would be a way of

而且它的速度可能遠高於光速，所以它可能是個違反

violating what we think of otherwise as a fundamental speed limit in Einstein's theories.

我們認為的 Einstein 理論的基本速度限制的方法。

And it does it in a way that doesn't obviously violate Einstein's theories,

它是以沒有明顯違反 Einstein 理論的方法運作

but we're not sure that there's any actual way you can build a wormhole.

但我們不確定是不是有任何方法可以建立蟲洞。

They might be shut off from the rest of space and time and so could never actually use it to do anything.

它們可能會從整個時空中關閉，所以實際上可能沒辦法用它來做任何事。

Although wormholes are theoretically possible, they're a highly implausible proposition.

雖然蟲洞在理論上可行，它還是個非常難以置信的主張。

That's because the equations suggest enormous masses and energies would be required to create and manipulate one.

因為等式說明，建造與操作一個蟲洞需要非常龐大的質量與能量。

There are two problems of the wormholes one is if you don't have one how you create it.

現在對於蟲洞有 2 個問題，首先，如果你連一個都沒見過，你怎麼創造一個蟲洞出來。

And the other is if you do have one. How do you hold it open,

另一個是，如果你真的創造了一個出來。你要怎麼讓它維持開啟

so it doesn't collapse and prevent you from going through? And in both cases,

讓它不會瓦解並阻止你穿越？而在這 2 個事實中

what we understand about these questions

我們怎麼去理解這些問題

points toward a

都指向一個結論

conclusion that wormholes are probably forbidden by the laws of nature.

蟲洞可能是被自然法則禁止的。

What remains then is a mystery.

剩下的就是一個謎。

Theory fails to forbid traveling backwards in time, but practice suggests it might just as well be forbidden.

理論沒辦法阻止我們回到過去，但實務說明這可能就是被禁止的。

For now it would appear the arrow of time cannot be reversed.

現在看來，時間之箭無法被逆轉。

No one knows why time passes, but it seems that no matter how people look at it.

沒人知道為什麼時間會流逝，但人類怎麼看似乎也無所謂。

It goes in one direction, in one direction only.

時間是單向道，只有前進這個方向。