The fundamentals of space-time: Part 2 - Andrew Pontzen and Tom Whyntie

時空基本原理：第二部分 - Andrew Pontzen 與 Tom Whyntie

譯者: Qingqing Mao 審譯者: 瑞文Eleven 林Lim

Light: it's the fastest thing in the universe,

光：宇宙中最快的東西。

but we can still measure its speed

但我們依然有辦法測量它的速度。

if we slow down the animation,

如果我們把畫面放慢，

we can analyze light's motion using

我們可以使用時空圖來分析光的運動，

a space-time diagram,

which takes a flipbook of animation panels,

就像從側面來看一頁頁的手翻書。

and turns them on their side.

In this lesson, we'll add the single experimental fact

在這堂課裡，我們先補充一個實驗結果：

that whenever anyone measures just how fast light moves,

當任何人所測量的光速，

they get the same answer:

都會得到相同的結果。

299,792,458 meters every second,

每秒299 792 458公尺。

which means that when we draw light

這意味著

當我們把光畫在時空圖上時，

on our space-time diagram,

it's world line always has to appear at the same angle.

光的世界線必須總是呈現相同的角度。

But we saw previously that speed,

但是，我們之前看到

or equivalently world line angles,

速度即是由世界線的角度來表示

change when we look at things from

會隨著不同觀察者的視角而改變。

other people's perspective.

To explore this contradiction,

為了弄清這個矛盾

讓我們來看看如下情況：

let's see what happens if I start moving

如果我開始移動，

while I stand still and shine the laser at Tom.

而我站在這裡朝著湯姆發射雷射光。

First, we'll need to construct the space-time diagram.

首先，我們需要建構出時空圖

Yes, that means taking all of

這意味著

the different panels showing the different moments in time

取出對應每一個時刻的每一幅圖片，

and stacking them up.

然後把它們疊在一起。

From the side, we see the world line

從側面我們看到雷射光的世界線

of the laser light at its correct fixed angle,

有著固定的、正確的角度，

just as before.

就和早先的一樣。

So far, so good.

目前為止一切順利。

But that space-time diagram represents Andrew's perspective.

但是這個時空圖展示的是安德魯的視角。

What does it look like to me?

從我的視角來看是什麽樣的呢？

In the last lesson, we showed

在上一課裡

how to get Tom's perspective moving all the panels

我們展示了如何得到湯姆的視角。

只需把所有的圖片平移一點，

along a bit until his world line is completely vertical.

直到湯姆的世界線完全垂直。

But look carefully at the light world line.

但是仔細看看光的世界線。

The rearrangement of the panels

平移所有圖片的話，

means it's now tilted over too far.

會使得光的世界線傾斜過度。

I'd measure light traveling faster than Andrew would.

我所測得的光速將會比安德魯所測得的快。

But every experiment we've ever done,

但是我們所做過的實驗，

and we've tried very hard,

我們所有的努力，

says that everyone measures light to have a fixed speed.

都證明光速是恒定的。

So let's start again.

讓我們重新開始。

In the 1900s, a clever chap named Albert Einstein

在19世紀初，有個聰明的傢伙叫愛因斯坦。

worked out how to see things properly,

他想出了如何從湯姆的視角

from Tom's point of view,

來正確地看待問題，

while still getting the speed of light right.

而同時依然能得到正確的光速。

First, we need to glue together the separate panels

首先我們需要把分離的畫面都粘合起來，

into one solid block.

粘成完整的一塊，

This gives us our space-time,

於是我們有了時空。

turning space and time into

將時間和空間

one smooth, continuous material.

變成一整塊平順連續的材料。

And now, here is the trick.

現在，變戲法的時候到了。

What you do is stretch your block of space-time

你要做的是

沿著光的世界線拉伸你的整塊時空，

along the light world line,

then squash it by the same amount,

然後在垂直世界線的方向上

but at right angles to the light world line,

等量地壓縮時空。

and abracadabra!

唵嘛呢叭咪吽！

Tom's world line has gone vertical,

湯姆的世界線變得垂直了，

so this does represent the world from his point of view,

所以這代表了他眼中的世界。

but most importantly,

但是最重要的是，

the light world line has never changed its angle,

光的世界線從未改變角度。

and so light will be measured by Tom

所以湯姆所測量的光速

going at the correct speed.

也是正確的光速。

This superb trick is known as

這個傑出的戲法叫做

a Lorentz transformation.

勞侖茲轉換。

Yeah, more than a trick.

這可不是騙人的把戲，

Slice up the space-time into

重新將時空分割為一幅幅畫面

new panels and you have

你將得到物理上正確的動畫。

the physically correct animation.

I'm stationary in the car,

我靜止坐在車裡，

everything else is coming past me

所有東西都朝我移來並經過我身邊，

and the speed of light

而光速依然

works out to be that same fixed value

是那個恒定的速度，

that we know everyone measures.

和所有人測量的光速一樣。

On the other hand,

而另一方面，

something strange has happened.

有些奇怪的事情發生了，

The fence posts aren't spaced a meter apart anymore,

圍欄柱子的間隔不再是一米了。

and my mom will be worried

而我媽媽會擔心

that I look a bit thin.

我看起來變瘦了。

But that's not fair. Why don't I get to look thin?

這不公平！為什麼我看起來沒變瘦？

I thought physics was supposed to be the same

我以為物理學對每個人都是一樣的。

for everyone.

Yes, no, it is, and you do.

的確是一樣的，你也看起來瘦了。

All that stretching and squashing

所有這些，時空的拉伸和壓縮，

of space-time has just muddled together

把我們之前認為的

what we used to think of separately

獨立的時間和空間

as space and time.

都混合到了一起。

This particular squashing effect

這個特別的壓縮效應，

is known as Lorentz contraction.

被稱為勞侖茲收縮。

Okay, but I still don't look thin.

好吧，可是我依然看起來不瘦。

No, yes, you do.

不，你的確看起來瘦了。

Now that we know better about space-time,

現在我們對時空有了更多的了解，

we should redraw

我們應該重新畫出

what the scene looked like to me.

我眼中的情形是怎麽樣的。

To you, I appear Lorentz contracted.

對你來說，我表現出勞侖茲收縮。

Oh but to you, I appear Lorentz contracted.

而對你來說，我表現出了勞侖茲收縮。

Yes.

對。

Uh, well, at least it's fair.

好吧，至少這是公平的。

And speaking of fairness,

說到公平，

just as space gets muddled with time,

不僅空間被時間攪渾，

time also gets muddled with space,

時間也被空間攪渾。

in an effect known as time dilation.

有個效應叫做時間膨脹。

No, at everyday speeds,

雖然在日常的速度下，

such as Tom's car reaches,

比如湯姆的車速，

actually all the effects are much, much smaller

實際的效應非常非常地小，

than we've illustrated them.

比我們剛才展示的要小得多。

Oh, yet, careful experiments,

然而精密的實驗，

for instance watching the behavior of tiny particles

比如觀察微小的粒子

whizzing around the Large Hadron Collider

在大型強子對撞機裡的運動，

confirmed that the effects are real.

確認了這些效應是真實存在的。

And now that space-time is

既然現在時空已是被實驗驗證的事實，

an experimentally confirmed part of reality,

we can get a bit more ambitious.

我們的野心可以更大一些。

What if we were to start playing

我們是否可以開始

with the material of space-time itself?

考慮時空本身的性質？

We'll find out all about that in the next animation.

我們會在下一堂課裡找到答案。