Is light a particle or a wave? - Colm Kelleher

Colm Kelleher：光，是粒子還是波？

Translator: Andrea McDonough Reviewer: Bedirhan Cinar

譯者: Elmo Wu 審譯者: Jephian Lin

You look down and see a yellow pencil lying on your desk.

當你低下頭，望著桌上的黃色鉛筆。

Your eyes, and then your brain, are collecting

你的眼睛，然後是你的腦袋，便開始接收

all sorts of information about the pencil:

各式各樣有關它的資訊：

its size,

color,

它的大小、

顏色、

shape,

distance,

形狀、

與你的距離、

and more.

及其他各種特徵。

But, how exactly does this happen?

不過，這一切到底是怎麼發生的？

The ancient Greeks were the first

古希臘人率先開始

to think more or less scientifically

以稍加科學的角度思索

about what light is and how vision works.

什麼是光，視覺究竟又如何運作。

Some Greek philosophers,

某些希臘哲學家，

including Plato and Pythagoras,

包括柏拉圖和畢達哥拉斯在內，

thought that light originated in our eyes

認為光其實源自人的眼睛

and that vision happened when little, invisible probes

而當眼睛發送 肉眼看不見的微小探測器

were sent to gather information about far-away objects.

去收集遠處物體的資訊時，便產生視覺。

It took over a thousand years

直到一千年後

before the Arab scientist, Alhazen,

阿拉伯科學家海什木才表示

figured out that the old, Greek theory of light couldn't be right.

古希臘的光學理論完全不合邏輯。

In Alhazen's picture, your eyes don't send out

海什木認為，人的眼睛根本不會發送

invisible, intelligence-gathering probes,

任何迷你情資探測器，

they simply collect the light that falls into them.

眼睛只是接收 射進來的光線。

Alhazen's theory accounts for a fact

海什木的理論 解開了希臘人

that the Greek's couldn't easily explain:

「為何有時會一片漆黑」的不解之謎。

why it gets dark sometimes.

「為何有時會一片漆黑」的不解之謎。

The idea is that very few objects actually emit their own light.

理論指出，本身會發光的物體並不常見。

The special, light-emitting objects,

會發光的物體相當特別，

like the sun

如太陽

or a lightbulb,

are known as sources of light.

或是燈泡，

我們稱之為光源。

Most of the things we see,

我們目光所及絕大部分的東西，

like that pencil on your desk,

例如桌上的鉛筆，

are simply reflecting light from a source

都只是反射光線

rather than producing their own.

本身並不會發光。

So, when you look at your pencil,

因此，當你望著你的鉛筆

the light that hits your eye actually originated at the sun

映入眼簾的 其實是鉛筆反射了

and has traveled millions of miles across empty space

穿越百萬英里

before bouncing off the pencil and into your eye,

源自太陽的光線，

which is pretty cool when you think about it.

想一想其實還蠻有意思的。

But, what exactly is the stuff that is emitted from the sun

不過，太陽散發出的究竟是什麼東西

and how do we see it?

我們又如何看得到？

Is it a particle, like atoms,

是如原子般的微小粒子？

or is it a wave, like ripples on the surface of a pond?

還是如池塘漣漪般的波？

Scientists in the modern era would spend a couple of hundred years

近代科學家耗時數百年

figuring out the answer to this question.

試圖找出真相。

Isaac Newton was one of the earliest.

艾薩克．牛頓是最早開始找尋答案的科學家之一。

Newton believed that light is made up

牛頓相信

of tiny, atom-like particles, which he called corpuscles.

光是由原子般、他稱之為光子的 微小粒子組成。

Using this assumption, he was able to explain some properties of light.

這項假設足以解釋光的某些特性。

For example, refraction,

例如折射，

which is how a beam of light appears to bend

折射是指光由空氣進入水時

as it passes from air into water.

所產生的彎曲現象。

But, in science, even geniuses sometimes get things wrong.

不過，在科學界，即使是天才也會犯錯。

In the 19th century, long after Newton died,

牛頓過世很長一段時間後，在十九世紀

scientists did a series of experiments

科學家們做了一系列實驗

that clearly showed that light can't be made up

清楚證明

of tiny, atom-like particles.

光，不可能由原子般的微小粒子組成。

For one thing, two beams of light that cross paths

仔細想想也對，兩道光束交錯時

don't interact with each other at all.

完全不會互相影響。

If light were made of tiny, solid balls,

如果光是由微小、實心的球體組成，

then you would expect that some of the particles from Beam A

我們可以預期 A 光束的微小粒子

would crash into some of the particles from Beam B.

會與 B 光束的微小粒子產生撞擊。

If that happened, the two particles involved in the collision

當兩邊的粒子發生碰撞

would bounce off in random directions.

粒子則會不規則地四處彈跳。

But, that doesn't happen.

不過事實並非如此。

The beams of light pass right through each other

兩道光束反而會直接互相穿越，

as you can check for yourself

你可以自行

with two laser pointers and some chalk dust.

用雷射筆和粉筆灰實驗看看。

For another thing, light makes interference patterns.

另外，光會有「干涉」的現象。

Interference patterns are the complicated undulations that happen

干涉指的是兩列波在空間中重疊時

when two wave patterns occupy the same space.

產生新的複雜波形的現象。

They can be seen when two objects

我們可以在

disturb the surface of a still pond,

兩個物體丟進靜止的池塘時，

and also when two point-like sources of light

或是兩道點狀光束放得很近時，

are placed near each other.

觀察到這種現象。

Only waves make interference patterns,

只有波才會互相干涉，

particles don't.

粒子不會。

And, as a bonus, understanding that light acts like a wave

更有甚者，把光理解為一種波

leads naturally to an explanation of what color is

便自然而然能夠解釋，什麼是顏色，

and why that pencil looks yellow.

又鉛筆為何看起來是黃色的。

So, it's settled then, light is a wave, right?

結論，光是一種波，對吧？

Not so fast!

先別急！

In the 20th century, scientists did experiments

二十世紀時，科學家實驗發現

that appear to show light acting like a particle.

光亦有一些粒子的特性。

For instance, when you shine light on a metal,

例如，當照在金屬上時

the light transfers its energy to the atoms in the metal

光會將能量由 一種叫「量子」的形式

in discrete packets called quanta.

分批傳遞至金屬中的原子。

But, we can't just forget about properties like interference, either.

不過，我們還是不能忽略如干涉這種波獨有的特性。

So these quanta of light aren't at all like

這些量子畢竟還是

the tiny, hard spheres Newton imagined.

與牛頓所想像的微小、實心球體有所不同。

This result, that light sometimes behaves like a particle

這讓光表現得時而像粒子，

and sometimes behaves like a wave,

時而像波，

led to a revolutionary new physics theory called

使得物理學界產生了革命性的新理論

quantum mechanics.

「量子力學」。

So, after all that, let's go back to the question,

所以，最後還是要回歸最初的問題：

"What is light?"

「光是什麼？」

Well, light isn't really like anything

光，其實獨樹一格，

we're used to dealing with in our everyday lives.

與我們日常生活接觸的事物都有所不同。

Sometimes it behaves like a particle

有時它的特性像粒子，

and other times it behaves like a wave,

有時又像波，

but it isn't exactly like either.

但與兩者 卻都不完全相同。