Time.

時間

We experience time as passing with a special moment that we call now.

我們體驗到時間的流逝有一個特殊的時刻，我們稱之為 "現在"。

Now you're watching this video.

現在你正在觀看這段視頻。

Half an hour ago you were doing something else.

半小時前你還在做別的事。

Whatever you did, there's no way to change it.

無論你做了什麼，都無法改變。

And what you will do in half an hour is up to you.

至於半小時後你會做什麼，就看你自己了。

At least that's how we perceive time.

至少我們是這樣看待時間的。

But what physics tells us about time is very different from our perception.

但物理學告訴我們的時間與我們的感知截然不同。

The person who figured this out was none other than Albert Einstein.

發現這一點的人正是阿爾伯特-愛因斯坦。

I know, that guy again.

我知道，又是那傢伙。

Turns out he kind of knew it all.

原來，他對這一切都瞭如指掌。

What did Einstein teach us about the past, the present and the future?

關於過去、現在和未來，愛因斯坦教會了我們什麼？

That's what we'll talk about today.

這就是我們今天要討論的話題。

The topic we're talking about today is covered in more detail in my new book Existential Physics which will be published in August.

我的新書《存在物理學》將於八月出版，其中對我們今天討論的話題有更詳細的闡述。

You find more info about the book at existentialphysics.com We think about time as something that works the same for everyone and every object.

關於本書的更多資訊，請訪問 existentialphysics.com 我們認為時間對每個人和每個物體都是一樣的。

If one second passes for me, one second passes for you, and one second passes for the clouds above.

如果我過一秒，你過一秒，天上的雲也過一秒。

This makes time a universal parameter.

這使得時間成為一個通用參數。

This parameter labels how much time passes and also what we all mean by now.

這個參數標註了時間的流逝，也標註了我們所說的 "現在 "是什麼意思。

Hermann Minkowski was the first to notice that this may not be quite right.

赫爾曼-閔科夫斯基（Hermann Minkowski）是第一個注意到這可能不太正確的人。

He noticed that Maxwell's equations of electrodynamics make much more sense if one treats time as a dimension, not as a parameter.

他注意到，如果把時間看作一個維度，而不是一個參數，麥克斯韋電動力學方程就更有意義了。

Just like a ball doesn't change if you rotate one direction of space into another, Maxwell's equations don't change if you rotate one direction of space into time.

就像一個球從一個空間方向旋轉到另一個空間方向不會改變一樣，麥克斯韋方程從一個空間方向旋轉到時間方向也不會改變。

So, Minkowski said, we just combine space with time, to a four-dimensional space-time, and then we can rotate space into time, just like we can rotate two directions of space into each other.

是以，閔科夫斯基說，我們只需將空間與時間結合起來，形成四維時空，然後我們就可以將空間旋轉到時間中，就像我們可以將空間的兩個方向相互旋轉一樣。

And that naturally explains why Maxwell's equations have the symmetry they do have.

這也自然解釋了為什麼麥克斯韋方程具有對稱性。

It doesn't have anything to do with electric and magnetic fields, it comes from the properties of space and time themselves.

它與電場和磁場無關，而是來自空間和時間本身的特性。

I can't draw a flower, let alone four dimensions, but I can just about manage two straight lines, one for time and the other for at least one dimension of space.

我畫不出一朵花，更不用說四維空間了，但我差不多能畫兩條直線，一條代表時間，另一條代表至少一維空間。

This is called a space-time diagram.

這就是所謂的時空圖。

If you just stand still, then your motion in such a diagram is a straight vertical line.

如果你只是靜止不動，那麼在這樣的圖表中，你的運動就是一條垂直的直線。

If you move at a constant velocity, your motion is a straight line tilted at some angle.

如果你以恆定的速度運動，你的運動就是一條傾斜了一定角度的直線。

So, if you change velocity, you rotate in space-time.

是以，如果改變速度，就會在時空中旋轉。

The maximal velocity at which you can move is the speed of light, which by convention is usually drawn at a 45 degree angle.

移動的最大速度是光速，按照慣例，光速通常以 45 度角繪製。

In space, we can go forward, backward, left, right, or up and down.

在太空中，我們可以向前、向後、向左、向右或向上、向下。

In time, we can only go forward, we can't make a u-turn, and there aren't any driveways for awkward three-point turns either.

在時間上，我們只能向前走，不能掉頭，也沒有任何車道可以進行笨拙的三點轉彎。

So time is still different from space in some respects.

是以，時間與空間在某些方面還是不同的。

But now that time is also a dimension, it's clear that it's just a label for coordinates.

但現在，時間也是一個維度，很顯然，它只是座標的一個標籤。

There's nothing universal about it.

這沒有什麼普遍性。

There are many ways to put labels on a two-dimensional space, because you can choose your axes as you want.

在二維空間上貼標籤有很多種方法，因為你可以隨意選擇軸線。

The same is the case now in space-time.

現在的時空也是如此。

Once you have made time into a dimension, the labels on it don't mean much.

一旦你把時間變成了一個維度，上面的標籤就沒有什麼意義了。

So what, then, is the time that we talk about?

那麼，我們所說的時間是什麼呢？

What does it even mean that time is a dimension?

時間是一個維度，這意味著什麼？

Do other dimensions exist?

還有其他維度嗎？

Supernatural ones?

超自然的？

That could explain the strange sounds you've been hearing at night?

這可以解釋你晚上聽到的奇怪聲音嗎？

No, that's a separate problem I'm afraid I can't help you with.

不，這是另一個問題，恐怕我幫不了你。

It was Albert Einstein who understood what this means.

正是阿爾伯特-愛因斯坦理解了這一點。

If we also want to understand it, we need four assumptions.

如果我們還想了解它，就需要四個假設。

The speed of light in a vacuum is finite, it's always the same, nothing can go faster than the speed of light, and all observers' viewpoints are equally valid.

真空中的光速是有限的，它始終保持不變，沒有什麼能比光速更快，所有觀察者的觀點都同樣有效。

This formed the basis of Einstein's theory of special relativity.

這構成了愛因斯坦狹義相對論的基礎。

Oh, and also, the observers don't have to exist.

哦，還有，觀察者不一定存在。

I mean, this is theoretical physics, so we're talking about theoretical observers, basically.

我的意思是，這是理論物理學，所以我們談論的基本上是理論觀測者。

So if there could be an observer with a certain viewpoint, then that viewpoint is equally valid as yours.

是以，如果有一個觀察者持有某種觀點，那麼他的觀點與你的觀點同樣有效。

Who or what is an observer?

誰或什麼是觀察員？

Is an ant an observer?

螞蟻是觀察者嗎？

A tree?

一棵樹？

How about a dolphin?

海豚怎麼樣？

What do you need to observe to deserve being called an observer, and what do you have to observe with?

你需要觀察什麼才能被稱為觀察者？

Believe it or not, there's actually quite some discussion about this the scientific literature.

信不信由你，科學文獻中其實對此有不少討論。

We'll sidestep this interesting discussion and use the word observer the same way that Einstein did, which is a coordinate system.

我們暫且不討論這個有趣的問題，而是像愛因斯坦那樣使用 "觀察者 "這個詞，也就是座標系。

You see, it's a coordinate system that a theoretical observer might use, dolphin or otherwise.

你看，這是一個理論觀察者可能使用的座標系，不管是海豚還是其他。

Yeah, maybe not exactly what the FBI thinks an observer is, but then if it was good enough for Einstein, it'll be good enough for us.

是的，也許和聯邦調查局認為的觀察者不完全一樣，但如果對愛因斯坦來說足夠好，對我們來說也足夠好。

So Einstein's assumption basically means any coordinate system should be as good as any other for describing physical reality.

是以，愛因斯坦的假設基本上意味著，在描述物理現實時，任何座標系都應該和其他座標系一樣好。

These four assumptions sound rather innocent at first, but they have profound consequences.

這四個假設初聽起來相當天真，但卻有著深遠的影響。

Let's start with the first and third.

讓我們從第一項和第三項開始。

The speed of light is finite, and nothing goes faster than light.

光速是有限的，沒有什麼能比光速更快。

You're probably watching this video on a screen, a phone or laptop.

您可能正在通過螢幕、手機或筆記本電腦觀看這段視頻。

Is the screen there now?

螢幕現在在嗎？

Unless you're from the future watching this video as a hologram in your space house, I'm going to assume the answer is yes.

除非你來自未來，在你的太空屋中全息觀看這段視頻，否則我認為答案是肯定的。

But a physicist might point out that actually, you don't know.

但物理學家可能會指出，實際上，你並不知道。

Because the light that's emitted from the screen now hasn't reached you yet.

因為現在螢幕發出的光還沒照到你。

Also, if you are from the future watching this as a hologram, make sure to look at me from the right.

另外，如果你來自未來，正在觀看全息影像，請務必從右邊看我。

It's my good side.

這是我好的一面。

Maybe you hold the phone in your hand, but nerve signals are ridiculously slow compared to light.

也許你把手機握在手裡，但神經信號的傳輸速度比光線慢得離譜。

If you couldn't see your hand and someone snatched your phone, it'd take several microseconds for information that the phone is gone to even arrive in your brain.

如果你看不見自己的手，而有人搶走了你的手機，那麼手機不見了的資訊甚至需要幾微秒才能傳入你的大腦。

So how do you know your phone is there now?

那麼，你怎麼知道你的手機現在在那裡？

One way to answer this question is to say, well, you don't know, and really you don't know that anything exists now other than your own thoughts.

回答這個問題的一種方法就是說，好吧，你不知道，而且除了你自己的想法之外，你真的不知道現在還有什麼東西存在。

I think, therefore I am, as Descartes summed it up.

我思故我在，笛卡爾如是總結。

This isn't wrong, I'll come back to this later, but it's not how normal people use the word now.

這並沒有錯，我稍後會再談這個問題，但這不是現在正常人使用這個詞的方式。

We talk about things that happen now all the time, and we never worry about how long it takes for light to travel.

我們經常談論現在發生的事情，卻從不擔心光的傳播需要多長時間。

Why can't we just agree on some now and get on with it?

為什麼我們不能現在就達成一些共識，然後繼續呢？

I mean, think back of that space-time diagram.

我的意思是，回想一下那個時空圖。

Clearly, this flat line is now, so let's just agree on this and move on.

很明顯，這條平線就是現在，所以讓我們就此達成一致，繼續前進吧。

Okay, but if this is to be physics rather than just a diagram, you have to come up with an operational procedure to determine what we mean by now.

好吧，但如果這是物理學而不僅僅是圖表，你就必須想出一個操作程序來確定我們現在的意思。

You have to find a way to measure it.

你必須找到衡量它的方法。

Einstein did just that in what he called Gedankenexperiment, a thought experiment.

愛因斯坦就是在他所謂的 "Gedankenexperiment"（思想實驗）中做到了這一點。

He said, suppose you place a mirror to your right and one to your left.

他說，假設你在右邊和左邊各放一面鏡子。

You and the mirrors are at a fixed distance to each other, so in the space-time diagram it looks like this.

你和鏡子之間的距離是固定的，所以在時空圖中看起來是這樣的。

You send one photon left and one right and make sure that both photons leave you at the same time.

你向左和向右各發送一個光子，並確保兩個光子同時離開你。

Then you wait to see whether the photons come back at the same time.

然後，你就等著看光子是否同時回來。

If they don't, you adjust your position until they do.

如果他們不這樣做，你就調整你的位置，直到他們這樣做。

Now remember Einstein's second assumption.

現在請記住愛因斯坦的第二個假設。

The speed of light is always the same.

光速始終不變。

This means, if you can send photons to both mirrors and they come back at the same time, then you must be exactly in the middle between the mirrors.

這意味著，如果你能同時向兩面鏡子發送光子，而它們又同時返回，那麼你一定正好位於兩面鏡子的中間。

The final step is then to say that at exactly half the time it takes for the photons to return, you know they must be bouncing off the mirror.

最後一步就是說，在光子返回所需的一半時間內，你知道它們一定是從鏡子上反彈回來的。

You could say, now, at the right moment, even though the light from there hasn't reached you yet.

你可以說，現在，在正確的時刻，儘管那裡的光線還沒有照到你。

It looks like you've found a way to construct now.

看來你已經找到了施工方法。

But here's the problem.

但問題就在這裡。

Suppose you have a friend who flies by at some constant velocity, maybe in a spaceship.

假設你有一個朋友，他以某種恆定的速度飛過，也許是在飛船裡。

Her name is Alice, she's much cooler than you, and you have no idea why she's agreed to be friends with you.

她叫愛麗絲，比你酷多了，你不知道她為什麼同意和你做朋友。

But here she is, speeding by in her spaceship left to right.

但她就在這裡，乘坐飛船從左到右飛馳而過。

As we saw earlier, in your space-time diagram, Alice moves on a tilted straight line.

正如我們之前看到的，在你的時空圖中，愛麗絲在一條傾斜的直線上移動。

She does the exact same thing as you, places mirrors to both sides, sends photons and waits for them to come back, and then says, when half the time has passed, that's the moment the photons hit the mirrors.

她做了和你完全一樣的事情，在兩側放置鏡子，發送光子並等待它們回來，然後說，當時間過去一半時，就是光子擊中鏡子的時刻。

Except that this clearly isn't right from your point of view.

只不過，從你的角度來看，這顯然是不對的。

Because the mirrors to her right are in the direction of her flight.

因為她右邊的鏡子正對著她飛行的方向。

So the light takes longer to get there than it does to the mirrors on the left, which move towards the light.

是以，光線到達那裡所需的時間比到達左邊鏡子所需的時間要長，而左邊的鏡子是朝著光線移動的。

You would say that the photon which goes left clearly hits the mirror first, because the mirror's coming at it.

你會說，向左的光子顯然首先擊中了鏡子，因為鏡子正對著它。

From your perspective, she just doesn't notice, because when the photons go back to Alice, the exact opposite happens.

從你的角度來看，她只是沒有注意到，因為當光子回到愛麗絲那裡時，發生的情況正好相反。

The photon coming from left takes longer to get back, so the net effect cancels out.

從左邊來的光子需要更長的時間才能返回，是以淨效應抵消了。

What Alice says happens now, is clearly not what you think happens now.

愛麗絲所說的現在發生的事情，顯然不是你認為的現在發生的事情。

For Alice on the other hand, you are the one moving relative to her.

另一方面，對於愛麗絲來說，你是相對於她移動的那個人。

And she thinks that her notion of now is right and yours is wrong.

她認為她的 "現在 "概念是對的，而你的是錯的。

So who's right?

那麼誰是對的？

Probably Alice, you might say, because she's much cooler than you, she owns a spaceship after all.

你可能會說，也許是愛麗絲，因為她比你酷多了，畢竟她擁有一艘飛船。

Maybe, but let's ask Einstein.

也許吧，但讓我們問問愛因斯坦。

Here is where Einstein's fourth assumption comes in.

這就是愛因斯坦的第四個假設。

The viewpoints of all observers are equally valid.

所有觀察者的觀點都同樣有效。

So, you're both right.

所以，你們都是對的。

Or, to put it differently, the notion of now depends on the observer.

或者換一種說法，"現在 "的概念取決於觀察者。

It is observer-dependent, as physicists say.

正如物理學家所說，它取決於觀察者。

Your now is not the same as my now.

你的現在和我的現在不一樣。

If you like technical terms, this is also called the relativity of simultaneity.

如果你喜歡專業術語，這也叫做同時性相對論。

These mismatches in what different observers think happens now are extremely tiny in everyday life.

在日常生活中，不同觀察者對現在發生的事情的看法不一致是極其微小的。

They only become noticeable when relative velocities are close to the speed of light, so we don't normally notice them.

只有當相對速度接近光速時，它們才會變得明顯，是以我們通常不會注意到它們。

If you and I talk about who knocked at the door right now, we won't misunderstand each other.

如果你和我談論現在是誰敲門，我們就不會誤解對方。

If we'd zipped around with nearly the speed of light however, referring to now would get very confusing.

然而，如果我們以接近光速的速度飛馳，那麼提到現在就會非常混亂。

This is pretty mind-bending already, but wait, it gets wilder.

這已經很讓人匪夷所思了，但等等，還有更瘋狂的。

Let us have a look at the space-time diagrams again.

讓我們再來看看時空圖。

Now let us take any two events that are not causally connected.

現在，讓我們任選兩個沒有因果關係的事件。

This just means that if you wanted to send a signal from one to the other, the signal would have to go faster than light, so signaling from one to the other isn't possible.

這就意味著，如果你想從一個空間向另一個空間發送信號，信號的傳播速度必須超過光速，是以從一個空間向另一個空間發送信號是不可能的。

Diagrammatically, this means if connect the two events, the line has an angle less than 45 degrees to the horizontal.

用圖解法表示，這意味著如果將兩個事件連接起來，直線與水平線的夾角小於 45 度。

The previous construction with the mirrors shows that for any two such events, there is always some observer for whom those two events happen at the same time.

前面的鏡像結構表明，對於任何兩個這樣的事件，總有某個觀察者認為這兩個事件是同時發生的。

You just have to imagine the mirrors fly through the events, and the observer flies through directly in the middle.

你只需想象鏡子在事件中飛過，而觀察者直接從中間飛過。

And then you adjust the velocity until the photons hit both events at the same time.

然後調整速度，直到光子同時擊中兩個事件。

Okay, so any two causally disconnected events happen simultaneously for some observer.

好吧，那麼對於某個觀察者來說，任何兩個因果關係斷開的事件都是同時發生的。

Now take any two events that are causally connected, like eating too much cheese for dinner and then feeling terrible the morning after.

現在我們來看看有因果關係的兩個事件，比如晚餐吃了太多奶酪，第二天早上感覺很糟糕。

Find some event that isn't causally connected to either.

找一些與兩者都沒有因果關係的事件。

Let's say this event is a supernova going off in a distant galaxy.

假設這個事件是一顆超新星在遙遠的星系中爆炸。

There are then always observers for whom the supernova and your cheese dinner are simultaneous, and there are observers for whom the supernova and your morning after are simultaneous.

是以，總有一些觀察者認為超新星和你的奶酪晚餐是同時發生的，也有一些觀察者認為超新星和你的早晨是同時發生的。

Let's then put all those together.

然後，讓我們把所有這些放在一起。

If you're comfortable with saying that something, anything, exists now which isn't here, then, according to Einstein's fourth assumption, this must be the case for all observers.

如果你願意說現在存在著某種不在這裡的東西，任何東西，那麼，根據愛因斯坦的第四個假設，所有觀察者都必須如此。

But if all the events that you think happen now exist, and all other events exist, then all events exist now.

但是，如果你認為現在發生的所有事件都存在，而其他所有事件都存在，那麼所有事件現在都存在。

Another way to put this is that all times exist in the same way.

另一種說法是，所有時間都以同樣的方式存在。

This is called the block universe.

這就是所謂的 "塊宇宙"。

It's just there.

它就在那裡。

It doesn't come into being.

它不會出現。

It doesn't change.

它不會改變。

It just sits there.

它就停在那裡。

If you find that somewhat hard to accept, there is another possibility to consistently combine a notion of existence with Einstein's special relativity.

如果你覺得這有點難以接受，那麼還有另一種可能，即把存在的概念與愛因斯坦的狹義相對論連貫起來。

All that I just said came from assuming that you're willing to say something exists now even though you can't see or experience it in any way.

我剛才所說的一切都源於一個假設，那就是即使你無法以任何方式看到或體驗到某樣東西，你也願意說它現在就存在。

If you're willing to say that only things exist which are now and here, then you don't get a block universe.

如果你願意說只有現在和這裡的事物才存在，那麼你就不會得到一個塊狀宇宙。

But maybe that's even more difficult to accept.

但也許這更讓人難以接受。

Another option is to simply invent a notion of existence and define it to be a particular slice in spacetime for each moment in time.

另一種選擇是簡單地發明一種存在的概念，並將其定義為每一時刻的特定時空切片。

This is called a slicing, but unfortunately it has nothing to do with Pisa.

這就是所謂的切片，但不幸的是，它與比薩毫無關係。

If it had any observable consequences, that would contradict the fourth assumption that Einstein made.

如果它有任何可觀測的結果，那就與愛因斯坦的第四個假設相矛盾了。

So it's in conflict with special relativity, and since this theory is experimentally extremely well confirmed, this would almost certainly mean the idea is in conflict with observation.

是以，它與狹義相對論相沖突，而由於這一理論在實驗上得到了極好的證實，這幾乎肯定意味著這一觀點與觀測相沖突。

But if you just want to define a now that doesn't have observable consequences, you can do that, though I'm not sure why you would want to.

但如果你只是想定義一個沒有可觀察後果的現在，你也可以這麼做，雖然我不知道你為什麼要這麼做。

Quantum mechanics doesn't change anything about the block universe because it's still compatible with special relativity.

量子力學並沒有改變塊狀宇宙的任何東西，因為它仍然符合狹義相對論。

The measurement update of the wave function, which I talked about in this earlier video, happens faster than the speed of light.

波函數的測量更新（我在之前的視頻中提到過）比光速更快。

If it could be observed, you could use it to define a notion of simultaneity.

如果可以觀測到，就可以用它來定義同時性的概念。

But it can't be observed, so there's no contradiction.

但這是無法觀察到的，所以並不矛盾。

Some people have argued that since quantum mechanics is indeterministic, the future can't already exist in the block universe, and that therefore there must also be a special moment of now that divides the past from the future.

有人認為，既然量子力學是不確定的，那麼未來就不可能已經存在於塊狀宇宙中，是以，也必須有一個特殊的現在時刻來劃分過去和未來。

And maybe that is so.

也許就是這樣。

But even if that was the case, the previous argument still applies to the past.

但即便如此，前面的論點仍然適用於過去。

So, yeah, it's true, for all we currently know, the past exists the same way as the present.

所以，沒錯，就我們目前所知，過去和現在的存在方式是一樣的。

So, you thought, this is a video about special relativity, but then I've been talking about quantum mechanics again.

所以，你會想，這是一個關於狹義相對論的視頻，但後來我又一直在講量子力學。