Through light, we could experience distant stars and look back at the beginning of existence itself.

有了光線，我們才能看到遙遠的星星，探尋存在的起源。

But, what is light?

但，什麼是光？

In a nutshell, light, is the smallest quantity of energy that can be transported.

簡單來說，組成光線的光子就是能量傳輸的最小單位。

A photon, an elementary particle without a real size that can't be split, only created or destroyed.

光子是基本粒子，沒有真正的大小，也不能分裂，只能產生和消亡。

Light also has a wave-particle duality, being kind of a particle and a wave at the same time (although this is a lie).

光子也有「波粒二象性」，既是粒子又是波的一種（雖然這是謊言）。

Also when we say light, we actually mean visible light which is a tiny part of the electromagnetic spectrum: Energy in form of electromagnetic radiation.

另外，當我們說到「光」的時候，一般都是指「可見光」，它只佔有整個電磁頻譜的極小部分：電磁輻射形式的能量。

Electromagnetic radiation consist of an enormous range of wavelengths and frequencies.

電磁輻射包括很大範圍的波長和頻率。

Gamma rays have the smallest wave lengths because they are the highest energy photons.

伽馬射線的波長最短，因為其光子的能量最高。

But most gamma rays are just under ten picometers, which is still way smaller than a hydrogen atom.

但多數伽馬射線只有不到十皮米的波長，比氫原子小得多。

For reference, a hydrogen atom compared to a cent is about as big as a cent compared to the Moon.

作為參考，氫原子之於一分硬幣的大小，相當於硬幣之於月亮。

Visible light is in the middle of the spectrum in a range of about 400 nanometers to 700 nanometers: about the size of a bacteria.

可見光位於頻譜的中間，波長介於 400 至 700 納米，大概是細菌那麼大。

On the other end of the spectrum, radio waves can be up to 100 kilometers in diameter.

在頻譜的另一端，射頻電波的波長可以高達 100 公里。

While the biggest wave lengths we know exist can span from 10,000 kilometers to a baffling 100,000 kilometers, way larger than Earth.

而已知最大波長可能超過十萬公里，比地球大得多。

From a physics stand point, all these different waves are the same.

從物理的角度看，這些不同的電磁波都是一樣的。

They all have the wave-particle duality and travel at 'c', the speed of light, just at different frequencies.

它們都具有「波粒二象性」，也以光速「c」前進，僅僅是頻率不同。

So what makes visible light special, then?

所以可見光有什麼特別之處嗎？

Well... Absolutly nothing.

呃⋯⋯ 沒有。

We just happen to have evolved eyes, that are good at registering exactly this part of the electromagnetic spectrum.

我們只是剛好演化出眼睛，其擅於準確地接收這個頻率的電磁波。

This is not a complete coincidence though.

但這也不完全是巧合。

Visible light is the only set of electromagnetic radiation that propagates when in water, which happens to be where most eyes first evolved, millions of years ago.

可見光是僅有的在水中容易傳播的電磁波，而最早的眼睛，在數百萬年前，就是在水中進化出現的。

That was a smart move, because light not only interacts with matter, it's also altered by it and can be used to gather information about the world around us, with almost no delay.

眼睛是一項偉大的發明，因為光不僅會與物質發生相互作用，還會被它改變，並且可以幾乎無延遲地用於收集有關我們周圍世界的信息。

Which is arguably really helpful for survival.

可以說是對於動物的生存很有幫助。

Okay, where does light come from?

那麼，光是從哪裡來的呢？

A vast range of electromagnetic waves are created when atoms or molecules drop from a higher state of energy to a lower one.

當原子或分子由高能態轉變為低能態的時候就會發出各種頻率的電磁波。

They lose energy and emit it in the form of radiation.

它們失去能量並以輻射的形式發出。

At the microscopic level, visible light is created when an electron within an atom that is in an excited state drops to a lower energy state and loses this excess energy.

從微觀角度看，當原子中的一個電子從激發態跌至低能態並散發掉其多餘能量時，就會發出可見光。

The same way, incoming light can elevate an electron to a higher state of energy by being absorbed by it.

同樣，入射光可以被電子吸收，從而將電子提升到更高的能量狀態。

Macroscopically, the moving charge of the electron creates an oscillating magnetic field, which creates an oscillating electric field perpendicular to it.

從宏觀角度看，電子的移動電荷會產生一個震盪的磁場，該磁場又會產生一個垂直的電場。

These two fields move themselves through space, transferring energy from one place to another, carrying information about its place of creation with it.

兩種磁場在空間中前進，傳輸能量，也傳送關於它的誕生地的訊息。

Why of all the things in the universe is light the fastest thing there is?

為什麼光是宇宙萬物中速度最快的呢？

Let's change the question: What is the fastest way to travel through space in the universe?

從另一個角度問：在宇宙中穿越太空最快的方式是什麼？

It's c, exactly two hundred ninety-nine million, seven hundred ninety-two thousand, four hundred and fifty-eight metres per second in a vacuum, one billion kilometres per hour.

就是「c」，真空中 299,792,458 米每秒，每小時十億公里。

Electromagnetic radiation just happens to move this fast.

電磁輻射正好就是這麼快。

Any particle that has no mass travels at c, without acceleration or any in between.

其實，任何零質量的粒子都只會以光速前進，沒有加速過程。

The light that has been released from a candle does not speed up until it reaches light speed, at the very instant of its creation, its speed is c.

就是說，當光從蠟燭中出來的時候，不會從零加速到光速，產生的瞬間光就已經是光速前進。

So why is c, the speed of light, finite then?

那麼，為什麼光速「c」是有限的呢？

Well, nobody knows.

呃⋯ 沒人知道。

Our universe is just built this way.

宇宙就是這樣。

We don't have the smart answer here.

我們也不知道如何回答。

So light is part of a spectrum, an elementary particle that also behaves like a wave, propelled by two perpendicular fields, travelling at the speed limit of the universe.

所以光是一種頻譜的一部份，是一種像波一樣的基本粒子，由兩種垂直的場推進，以宇宙最高速度前進。

Okay, that's nice and all, but what about the crazy stuff about travelling at the speed of light, and time, the twin paradox, quantum stuff, and things like that?

好了，就這樣。不過其他更加古怪的東西，像以光速前進對時間的影響、孿生悖論、量子的東西，還有諸如此類的東西呢？

We have to save them for another video.

我們會留到下一支影片再說。

For now, let's just be happy that we've evolved eyes that pick up waves of information permeating the universe.

現在，該為我們已經進化出能吸收遍及整個宇宙的信息波的眼睛而感到高興。

Making us see things and put our existence into perspective.

讓我們看得到並透視事物。