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光是粒子还是波?

Is light a particle or a wave?

低下头你将看到书桌上有一根黄色的铅笔。
You look down and see a yellow pencil lying on your desk.
你的眼睛,然后是你的大脑,开始收集
Your eyes, and then your brain, are collecting
关于铅笔的每一种信息
all sorts of information about the pencil:
它的大小,
its size,
颜色,
color,
形状,
shape,
长度,
distance,
以及更多。
and more.
但是,更确切地说,到底发生了什么呢?
But, how exactly does this happen?
古希腊人是第一个
The ancient Greeks were the first
近乎系统地思考
to think more or less scientifically
光是什么以及眼睛是如何工作的。
about what light is and how vision works.
一些古希腊哲学家,
Some Greek philosophers,
包括柏拉图和毕达哥拉斯,
including Plato and Pythagoras,
认为光起源于我们的眼睛
thought that light originated in our eyes
而视力则是由很多小且无形的探测物向外界探测,
and that vision happened when little, invisible probes
探测物收集远处物体的信息并送回眼睛。
were sent to gather information about far-away objects.
直到一千年前
It took over a thousand years
阿拉伯科学家阿尔哈曾
before the Arab scientist, Alhazen,
想到古希腊关于光的理论是错误的。
figured out that the old, Greek theory of light couldn’t be right.
在阿尔哈曾的想象中,你的眼睛不能发出
In Alhazen’s picture, your eyes don’t send out
无形的、可以自主聚集的探测物来达到收集信息的目的,
invisible, intelligence-gathering probes,
眼睛只是单纯地收集落入眼中的光。
they simply collect the light that falls into them.
阿尔哈曾的理论说明了一个
Alhazen’s theory accounts for a fact
古希腊人很难解释的事实:
that the Greek’s couldn’t easily explain:
为什么有时候会变暗。
why it gets dark sometimes.
有一个概念是很少的物体总是会自己发出光。
The idea is that very few objects actually emit their own light.
特殊的发光体,
The special, light-emitting objects,
比如太阳
like the sun
或者一个灯泡,
or a lightbulb,
都是光的来源。
are known as sources of light.
大多数我们所看到的东西,
Most of the things we see,
比如说你书桌上的铅笔,
like that pencil on your desk,
只是从光源反射了光,
are simply reflecting light from a source
而不是自己发光。
rather than producing their own.
所以,当你看到你的铅笔时,
So, when you look at your pencil,
落进你眼中的光其实来源于太阳
the light that hits your eye actually originated at the sun
并且在空间中穿越了数百万米的距离
and has traveled millions of miles across empty space
然后从铅笔上反射进你的眼睛,
before bouncing off the pencil and into your eye,
当你这么想的时候一切都会很酷。
which is pretty cool when you think about it.
但是,有什么东西从太阳上射出
But, what exactly is the stuff that is emitted from the sun
我们又该怎么看到它呢?
and how do we see it?
是原子一样的颗粒
Is it a particle, like atoms,
还是像池塘水面波纹一样的波呢?
or is it a wave, like ripples on the surface of a pond?
现代科学家用了一百多年
Scientists in the modern era would spend a couple of hundred years
想出了这个问题的答案。
figuring out the answer to this question.
艾萨克•牛顿是最早的一位。
Isaac Newton was one of the earliest.
牛顿坚信光的组成
Newton believed that light is made up
是微粒,类似原子的粒子。
of tiny, atom-like particles, which he called corpuscles.
在这个假定下,他发现了光的一些性质。
Using this assumption, he was able to explain some properties of light.
例如,折射。
For example, refraction,
也就是为什么一束光线会发生弯曲
which is how a beam of light appears to bend
在经过空气进入水中的时候
as it passes from air into water.
但是,在科学领域,即使天才有时也会犯错。
But, in science, even geniuses sometimes get things wrong.
十九世纪,牛顿已经逝世很久。
In the 19th century, long after Newton died,
科学家们做了一系列实验
scientists did a series of experiments
实验明显展现了光不是
that clearly showed that light can’t be made up
由微粒组成。
of tiny, atom-like particles.
通过一个实验,两束光交叉而过
For one thing, two beams of light that cross paths
却互相完全不产生影响。
don’t interact with each other at all.
如果光由固体的微粒构成,
If light were made of tiny, solid balls,
那么可以预测一些来自光束A的粒子
then you would expect that some of the particles from Beam A
将会撞到来自光束B的粒子。
would crash into some of the particles from Beam B.
如果这种事情发生,两个粒子将会发生碰撞
If that happened, the two particles involved in the collision
然后向随机方向弹开。
would bounce off in random directions.
但这并没有发生。
But, that doesn’t happen.
光束平稳地彼此交叉穿过
The beams of light pass right through each other
像你看到的一样
as you can check for yourself
两束光和一些灰尘。
with two laser pointers and some chalk dust.
令一方面,光产生了干涉。
For another thing, light makes interference patterns.
干涉是当两种波占据同一空间时发生的
Interference patterns are the complicated undulations that happen
一种复杂的波动。
when two wave patterns occupy the same space.
当两个物体落入平静的水面时
They can be seen when two objects
它们可以被观察到。
disturb the surface of a still pond,
同时两个光源
and also when two point-like sources of light
相邻放置时会发生相似的情形。
are placed near each other.
只有波才会发生干涉,
Only waves make interference patterns,
粒子是不会发生的。
particles don’t.
明白了光作为波的性质后
And, as a bonus, understanding that light acts like a wave
自然可以解释颜色
leads naturally to an explanation of what color is
还有为什么铅笔看起来是黄色的。
and why that pencil looks yellow.
所以,作为结论,光是一种波,这是对的么?
So, it’s settled then, light is a wave, right?
还没那么快!
Not so fast!
二十世纪,科学家们做实验
In the 20th century, scientists did experiments
发现光展现出了粒子的性质。
that appear to show light acting like a particle.
例如,当你将光照到金属上时,
For instance, when you shine light on a metal,
光会将能量传输给
the light transfers its energy to the atoms in the metal
金属原子。
in discrete packets called quanta.
但是,我们也不能忘记干涉性。
But, we can’t just forget about properties like interference, either.
所以光子不完全是
So these quanta of light aren’t at all like
微粒,如牛顿构想的球体。
the tiny, hard spheres Newton imagined.
结果是,光有时候有粒子的性质
This result, that light sometimes behaves like a particle
有时候有波的性质,
and sometimes behaves like a wave,
这导致了物理界的一场变革,产生了
led to a revolutionary new physics theory called
量子力学。
quantum mechanics.
所以在最后,让我们回到问题,
So, after all that, let’s go back to the question,
“光是什么?”
“What is light?”
嗯,光不完全是我们生活中接触到的
Well, light isn’t really like anything
任何一种东西。
we’re used to dealing with in our everyday lives.
有时有粒子的性质
Sometimes it behaves like a particle
其他时候有波的性质,
and other times it behaves like a wave,
但不完全符合其中任何一个。
but it isn’t exactly like either.

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