影子有多重？：英语精读讲义

Hey, Vsauce.

嘿，Vsauce。

Michael here.

我是 Michael。

And I'm sure that

我敢肯定，

we all love to

我们都喜欢

have fun with hand

用双手玩

shadows.

影子游戏。

But how much does a

但一个

shadow weigh?

影子有多重呢？

It might sound like a silly question 短語搭配

这个问题听起来或许很傻

because it is.

因为它确实很傻。

I mean, 表達脈絡

我是说，

a shadow cannot be put on a

你没法把影子放到

scale and weighed.

秤上称重。

But the material that it falls on top of

但影子所落在的物体

can be weighed.

是可以称重的。

And we know that light has energy.

而我们知道，光具有能量。

In fact,

事实上，

when light encounters an object,

当光照到物体上时，

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

会给它一丁点推力。

On the surface of Earth,

在地球表面，

when sunlight is hitting it,

当阳光照射下来时，

every square inch is being pushed with a force

每平方英寸都会受到一股

of about a billionth of a pound,

约十亿分之一磅的力，

which is basically nothing. 短語搭配

这基本可以忽略不计。

But over a large enough surface area,

但如果覆盖的表面积足够大，

the results can be pretty fun.

结果就会很有意思。

On a sunny day,

在阳光明媚的一天，

the city of Chicago weighs 300 pounds more,

芝加哥市会重上300磅，

simply because sunlight is falling on it,

仅仅因为阳光照在城市上，

pushing it.

推着它。

In outer space,

在外太空，

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

太阳风不会被地球大气层或磁场过滤，

the results are even bigger.

其影响就更大了。

A spacecraft traveling from Earth to Mars

一艘从地球飞往火星的航天器

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

会被光推得偏离航线1000千米。

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

因此，火星之旅必须把这些因素考虑进去。

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

事实上，我们已经造出了能借光航行的东西，

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

也就是由阳光推动的巨型反射式太阳帆。

So in a way that is calculable, 語法

所以，从可以计算的角度来看，

though difficult to measure, 語法

尽管很难测量，

an area covered in shadow technically weighs less

严格来说，被阴影覆盖的区域会更轻，

than surrounding areas being pushed by light.

这是相对于周围受光推动的区域而言。

But enough about the Sun. 表達脈絡

不过，太阳就先说到这里。

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

有三个天体能在地球表面投下阴影，

bright enough for us humans to see. 語法

其亮度足以让我们人类看见这些阴影。

One is obviously the Sun,

其中一个显然是太阳，

and the other is the Moon.

另一个是月球。

But what's the third?

那么第三个是什么？

Venus.

金星。

Pete Lawrence investigated this over at Digital Sky.

Pete Lawrence 在 Digital Sky 对此进行了研究。

Now,

当时，

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

为了确认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.

他使用了一根可以对准天空中特定区域的管子。

Inside the tube,

他在管子里

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

放入了一块做成金星天文符号形状的镂空片。

Now,

现在，

here is light coming through the tube

这是穿过管子的光，

when pointed just adjacent to Venus, 用詞

此时管子正对准金星旁边，

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

那里在人眼看来是天空中一处相对昏暗而空旷的位置。

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

而管子对准金星时，出来的是这个：

a Venusian shadow.

一道由金星投下的影子。

We all know that light travels quickly,

我们都知道，光传播得很快：

299,792,458 metres per second.

每秒299,792,458米。

C.

C。

But this light,

但这束光，

right here,

就在这里，

in fact,

事实上，

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

也就是此刻从你的屏幕射入眼球的光，

is moving slightly slower than C.

移动速度略低于C。

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

因为C是真空中的光速，

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

但这里所有的光都必须穿过一种介质，

in this case, air. 表達脈絡

在这个例子中，就是空气。

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

光在空气中的速度只比C慢一点点：

298,925,574 m/s.

298,925,574米/秒。

This is interesting

这很有意思，

because light travels more slowly through different materials,

因为光穿过不同材料时速度会变慢，

but C remains the universal speed limit. 習語

但C始终是宇宙通用的速度上限。

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

而只要一个物体没有快到那个程度，

it can still outpace light in a material.

它仍然可以超过光在某种材料中的速度。

A charged particle,

一个带电粒子，

for instance, an electron,

例如电子，

can travel through water faster than light does,

在水中的速度可以超过光，

but never faster than C.

但绝不会超过C。

When this happens,

当这种情况发生时，

we get something analogous to a sonic boom. 用詞

就会出现类似音爆的现象。

We get a photonic boom. 習語

我们会得到“光子爆”。

In a sonic boom,

在音爆中，

the sound information propagating off of the object

从物体向外传播的声音信息

is in the form of compression waves. 短語搭配

以压缩波的形式存在。

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

当物体越来越接近声速，

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

也就是这些波离开它的速度时，

each new wave has less time

每一道新波所拥有的时间都会越来越少，

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

来不及躲开下一道波，

until eventually the waves collapse all into each other

直到最后，所有波都叠在一起，

and the density and pressure is enormous,

密度和压力变得极其巨大，

causing a sonic boom.

从而引发音爆。

Normally,

通常，

when a charged particle moves through a material

当带电粒子穿过一种

whose molecules can be polarized,

其分子可以被极化的材料时，

the molecules give off photons. 短語動詞

这些分子会释放光子。

But each photon has room to fly away,

但每个光子都有空间飞散开来，

and the waves all destructively interfere with each other,

而且所有波都会彼此相消干涉，

so no radiation is given off.

所以不会放出辐射。

But the faster the particle goes, 語法

但粒子的速度越快，

the less room the photons have away from each other,

光子彼此分开的空间就越小，

and their waves begin to constructively interfere,

它们的波便开始发生相长干涉，

giving off a photonic boom,

从而产生“光子爆”，

Cherenkov radiation. 用詞

也就是切伦科夫辐射。

Astronauts,

宇航员，

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

尤其是那些一路去过月球的宇航员，

have reported seeing flashes of light.

都曾报告看到过闪光。

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

许多人认为，这是高速粒子穿过眼内液体所致，

faster than light normally would,

它们的速度超过了光通常在其中传播的速度，

causing photonic booms right inside their body.

从而直接在人体内引发“光子爆”。

Speaking of the speed of light, 表達脈絡

说到光速，

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

这是你们当中几个人发来的一个很棒的问题。

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

它涉及一种可能超过C的方法。

Here's how it goes.

问题是这样的。

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

假设我想按下一个离我一光年远的按钮，

which means that it would take light,

这意味着即使是光，

the fastest possible thing in the universe,

这个宇宙中速度最快的东西，

a year just to get from me to the button.

也要花一年才能从我这里抵达按钮。

Okay, well,

好吧，那么，

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

如果我造了一块一光年长的板子，

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

让它从我这里一直延伸到按钮，

and then I pushed one end of the board?

然后推动板子的一端呢？

Would the other end immediately push the button?

另一端会立刻按下按钮吗？

And if so,

如果会，

did I just break the speed of light?

那我是不是刚刚突破了光速？

Did I just send information faster than light?

我是不是刚刚以超光速传递了信息？

Well,

其实，

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

我们现在讨论的已经不是光速了，

are we? 語法

对吧？

We are talking about the speed of push.

我们讨论的是“推动”的速度。

When you push a rigid object,

当你推动一个刚性物体时，

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

你真正做的是让一连串压缩波

through the object,

穿过这个物体，

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

它们以该物体材料中的声速传播。

The information about,

关于这件事的信息——

"Woah, we've been pushed,

“哇，我们被推了，

you should move,"

你该动起来了。”

is sent via that compression wave,

会由那道压缩波传递，

and it only travels at the speed of sound.

而它只能以声速行进。

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

所以，当你推动日常生活中普通大小的物体时，

it feels almost instantaneous.

感觉几乎是瞬间发生的。

But when pushing a light-year-long board,

但如果推动的是一块一光年长的板子，

it would take a lot longer.

那就要久得多。

A cool way to see this in action 語法

想直观看到这一点，有个很酷的方法：

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

观察一个压缩波传播得更慢的物体。

For instance,

比如，

what Veritasium has done,

看看 Veritasium 做过的实验，

blowing minds by showing Slinkys being dropped. 習語

他们展示 Slinky 弹簧自由落下的过程，让所有人大开眼界。

The information telling the Slinky that,

告诉 Slinky 弹簧的信息——

"Hey, uh, we're moving,"

“嘿，呃，我们开始动了”——

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

在 Slinky 中传得很慢，慢动作摄影机足以拍下这个过程。

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

如果你还没看过 Veritasium 的所有 Slinky 视频，

you've missed 短語動詞

那你可就错过

out. In fact,

精彩内容了。事实上，

you should watch all of their stuff.

你应该把他们的所有作品都看一遍。

It is superb.

真的非常棒。

But to wrap things up, 短語動詞

不过，在结束之前，

here's the point.

重点是：

The speed of push is not instant,

推动的速度并不是瞬时的，

and it's certainly not the speed of light.

当然也不是光速。

But light can push you.

但光确实能推动你。

In fact,

事实上，

technically,

严格来说，

you weigh more when the lights are on 語法

开灯时你会更重，

than you do when the lights are off.

关灯时则更轻。

I've put links in the description

我在简介里放了一些链接，

to all kinds of cool things you should definitely check out

里面有各种很酷的内容，绝对值得你去看看，

for fun

既有乐趣，

and for science.

也有科学。

And as always,

和往常一样，

thanks for watching.

感谢观看。