影子有多重？：英語精讀講義

Hey, Vsauce.

嘿，Vsauce。

Michael here.

我是 Michael。

And I'm sure that

我敢肯定，

we all love to

我們都喜歡

have fun with hand

用雙手玩

shadows.

影子遊戲。

But how much does a

但一個

shadow weigh?

影子有多重呢？

It might sound like a silly question 片語搭配

這個問題聽起來或許很蠢

because it is.

因為它確實很蠢。

I mean, 表達脈絡

我是說，

a shadow cannot be put on a

你沒辦法把影子放到

scale and weighed.

磅秤上秤重。

But the material that it falls on top of

但影子落在上面的物體

can be weighed.

是可以秤重的。

And we know that light has energy.

而我們知道，光具有能量。

In fact,

事實上，

when light encounters an object,

當光照到物體上時，

it pushes that object just a little bit. 片語搭配

會給它一丁點推力。

On the surface of Earth,

在地球表面，

when sunlight is hitting it,

當陽光照射下來時，

every square inch is being pushed with a force

每平方英吋都會受到一股

of about a billionth of a pound,

約十億分之一磅的力，

which is basically nothing. 片語搭配

這基本上微不足道。

But over a large enough surface area,

但只要表面積夠大，

the results can be pretty fun.

結果就會很有趣。

On a sunny day,

在陽光普照的日子裡，

the city of Chicago weighs 300 pounds more,

芝加哥市會重上300磅，

simply because sunlight is falling on it,

純粹因為陽光照在城市上，

pushing it.

推著它。

In outer space,

在外太空，

where solar wind isn't filtered by Earth's atmosphere or magnetic field, 用詞

太陽風不會被地球的大氣層或磁場過濾，

the results are even bigger.

影響就更大了。

A spacecraft traveling from Earth to Mars

一艘從地球飛往火星的太空船

would be pushed by light 1,000 km off course. 片語搭配

會被光推離航線1,000公里。

So these things have to be factored into journeys to Mars. 片語搭配

所以，前往火星時必須把這些因素考慮進去。

We've actually already created things that can sail with light,

其實，我們早已造出能乘著光航行的東西，

giant reflective solar sails that are pushed by the Sun's light.

也就是由陽光推動的巨型反射式太陽帆。

So in a way that is calculable, 文法

所以，從可以計算的角度來看，

though difficult to measure, 文法

儘管很難測量，

an area covered in shadow technically weighs less

嚴格來說，被陰影覆蓋的區域會比較輕，

than surrounding areas being pushed by light.

這是相對於周圍受光推動的區域而言。

But enough about the Sun. 表達脈絡

不過，太陽就先談到這裡。

There are three astronomical bodies that can cast shadows on the surface of Earth

有三個天體能在地球表面投下陰影，

bright enough for us humans to see. 文法

其亮度足以讓我們人類看見這些陰影。

One is obviously the Sun,

其中一個顯然是太陽，

and the other is the Moon.

另一個是月球。

But what's the third?

那第三個是什麼？

Venus.

金星。

Pete Lawrence investigated this over at Digital Sky.

Pete Lawrence 在 Digital Sky 對此做了研究。

Now,

當時，

to make sure that the shadow he saw was caused by Venus, 片語搭配

為了確認他看到的影子是由金星造成的，

he used a tube that could be pointed at specific regions in the sky.

他使用一根可以對準天空中特定區域的管子。

Inside the tube,

他在管子裡

he put a cutout shaped like the astronomical symbol for Venus.

放入一個做成金星天文符號形狀的鏤空片。

Now,

接著，

here is light coming through the tube

這是穿過管子的光，

when pointed just adjacent to Venus, 用詞

此時管子正對準金星旁邊，

at a point in the sky relatively dark and empty to the human eye. 片語搭配

那是天空中一處人眼看來相對昏暗、空無一物的位置。

But here is what came out of the tube when pointed at Venus,

但當管子對準金星時，出來的是這個：

a Venusian shadow.

一道由金星投下的影子。

We all know that light travels quickly,

我們都知道，光傳得很快：

299,792,458 metres per second.

每秒299,792,458公尺。

C.

C。

But this light,

但這束光，

right here,

就在這裡，

in fact,

事實上，

the light coming off of your screen into your eyeballs right now,

也就是此刻從你的螢幕射進眼球的光，

is moving slightly slower than C.

移動速度略低於C。

Because C is the speed of light in a vacuum, 用詞

因為C是真空中的光速，

but all of this light is having to travel through a medium,

但這裡所有的光都必須穿過一種介質，

in this case, air. 表達脈絡

在這個例子中，就是空氣。

The speed of light in air is ever so slightly slower than C, 片語搭配

光在空氣中的速度只比C慢一點點：

298,925,574 m/s.

298,925,574公尺／秒。

This is interesting

這很有意思，

because light travels more slowly through different materials,

因為光穿過不同材料時速度會變慢，

but C remains the universal speed limit. 慣用語

但C始終是宇宙通用的速度上限。

And as long as an object doesn't go that fast, 文法

而只要物體沒有快到那種程度，

it can still outpace light in a material.

它仍然可以超過光在某種材料中的速度。

A charged particle,

一個帶電粒子，

for instance, an electron,

例如電子，

can travel through water faster than light does,

在水中的速度可以超過光，

but never faster than C.

但絕不會超過C。

When this happens,

當這種情況發生時，

we get something analogous to a sonic boom. 用詞

就會出現類似音爆的現象。

We get a photonic boom. 慣用語

我們會得到「光子爆」。

In a sonic boom,

在音爆中，

the sound information propagating off of the object

從物體向外傳播的聲音資訊

is in the form of compression waves. 片語搭配

是以壓縮波的形式存在。

And as the object gets closer and closer to the speed of sound, 文法

當物體愈來愈接近音速，

the speed that those waves are moving away from it at,

也就是這些波離開它的速度時，

each new wave has less time

每一道新波擁有的時間都會愈來愈少，

to get out of the way of the next, 片語搭配

來不及避開下一道波，

until eventually the waves collapse all into each other

直到最後，所有波都疊在一起，

and the density and pressure is enormous,

密度和壓力變得極其巨大，

causing a sonic boom.

因而引發音爆。

Normally,

一般來說，

when a charged particle moves through a material

當帶電粒子穿過一種

whose molecules can be polarized,

分子可以被極化的材料時，

the molecules give off photons. 片語動詞

這些分子會釋放光子。

But each photon has room to fly away,

但每個光子都有空間飛散開來，

and the waves all destructively interfere with each other,

而且所有波都會彼此相消干涉，

so no radiation is given off.

所以不會放出輻射。

But the faster the particle goes, 文法

但粒子的速度愈快，

the less room the photons have away from each other,

光子彼此分開的空間就愈小，

and their waves begin to constructively interfere,

它們的波便開始發生相長干涉，

giving off a photonic boom,

從而產生「光子爆」，

Cherenkov radiation. 用詞

也就是契忍可夫輻射。

Astronauts,

太空人，

especially those who have gone all the way to the Moon,

尤其是那些一路去過月球的太空人，

have reported seeing flashes of light.

都曾表示看見過閃光。

Many people attribute this to high-speed particles moving through the liquid inside their eye 片語搭配

許多人認為，這是高速粒子穿過眼內液體所造成的，

faster than light normally would,

它們的速度超過了光通常在其中傳播的速度，

causing photonic booms right inside their body.

因而直接在人體內引發「光子爆」。

Speaking of the speed of light, 表達脈絡

說到光速，

here's a great question a few of you have sent

這是你們當中幾個人傳來的一個很棒的問題。

me. It involves a possible way of going faster than C.

它涉及一種可能超過C的方法。

Here's how it goes.

問題是這樣的。

Let's say I want to push a button that is a light-year away from me, 講解

假設我想按下一個離我一光年遠的按鈕，

which means that it would take light,

這表示即使是光，

the fastest possible thing in the universe,

這個宇宙中速度最快的東西，

a year just to get from me to the button.

也要花一年才能從我這裡抵達按鈕。

Okay, well,

好吧，那麼，

what if I built a board one light-year long 文法

如果我造了一塊一光年長的板子，

all the way from me to the button, 片語搭配

讓它從我這裡一路延伸到按鈕，

and then I pushed one end of the board?

然後推動板子的一端呢？

Would the other end immediately push the button?

另一端會立刻按下按鈕嗎？

And if so,

如果會，

did I just break the speed of light?

那我是不是剛剛突破了光速？

Did I just send information faster than light?

我是不是剛剛以超光速傳遞了資訊？

Well,

其實，

we're not talking about the speed of light anymore,

我們現在談的已經不是光速了，

are we? 文法

對吧？

We are talking about the speed of push.

我們談的是「推動」的速度。

When you push a rigid object,

當你推動一個剛性物體時，

what you are really doing is putting a series of compression waves 講解

你真正做的是讓一連串壓縮波

through the object,

穿過這個物體，

which move at the speed of sound in the object's material.

它們會以該物體材料中的音速傳播。

The information about,

關於這件事的訊息——

"Woah, we've been pushed,

「哇，我們被推了，

you should move,"

你該動起來了。」

is sent via that compression wave,

會由那道壓縮波傳遞，

and it only travels at the speed of sound.

而且只能以音速前進。

So when pushing a normal day-to-day size type object,

所以，當你推動日常生活中一般大小的物體時，

it feels almost instantaneous.

感覺幾乎是瞬間發生的。

But when pushing a light-year-long board,

但如果推的是一塊一光年長的板子，

it would take a lot longer.

那就要久得多。

A cool way to see this in action 文法

想親眼看見這個過程，有個很酷的方法：

is to look at an object in which compression waves travel more slowly.

觀察一個壓縮波傳播得比較慢的物體。

For instance,

例如，

what Veritasium has done,

看看 Veritasium 做過的實驗，

blowing minds by showing Slinkys being dropped. 慣用語

他們展示 Slinky 彈簧圈自由落下的過程，讓所有人大開眼界。

The information telling the Slinky that,

告訴 Slinky 彈簧圈的訊息——

"Hey, uh, we're moving,"

「嘿，呃，我們開始動了」——

travels through the Slinky slow enough that a slow-mo camera can see it happen. 文法

在 Slinky 中傳得夠慢，慢動作攝影機可以拍下整個過程。

If you haven't watched all of Veritasium's Slinky videos,

如果你還沒看過 Veritasium 所有的 Slinky 影片，

you've missed 片語動詞

那你可就錯過

out. In fact,

精彩內容了。事實上，

you should watch all of their stuff.

你應該把他們的所有作品都看一遍。

It is superb.

真的非常棒。

But to wrap things up, 片語動詞

不過，在結束之前，

here's the point.

重點是：

The speed of push is not instant,

推動的速度並不是瞬時的，

and it's certainly not the speed of light.

當然也不是光速。

But light can push you.

但光確實能推動你。

In fact,

事實上，

technically,

嚴格來說，

you weigh more when the lights are on 文法

開燈時你會比較重，

than you do when the lights are off.

關燈時則比較輕。

I've put links in the description

我在說明欄放了一些連結，

to all kinds of cool things you should definitely check out

裡面有各種很酷的內容，絕對值得你去看看，

for fun

既有樂趣，

and for science.

也有科學。

And as always,

一如往常，

thanks for watching.

謝謝收看。