Sunlight is way older than you think - Sten Odenwald

翻译人员: Xinlong Zhang 校对人员: Kai Ma

You may know that it takes light a zippy eight minutes

或许你知道太阳光从太阳表面

to reach us from the surface of the Sun,

到达地球需要8分钟

so how long do you think it takes light

但是你认为太阳光要多久

to travel from the Sun's core to its surface?

从太阳中心到达太阳表面呢

几秒钟 顶多一分钟？

A few seconds or a minute at most?

Well, oddly enough, the answer is many thousands of years.

扶好下巴 答案是需要好几千年

Here's why.

这里是解释

Photons are produced by the nuclear reactions deep in the core of our Sun.

光子在太阳中心深处通过核反应产生

As the photons flow out of the core, they interact with matter and lose energy,

光子从太阳中心流出

与物质相互作用损失能量

becoming longer wavelength forms of light.

形成波长更长的光波

They start out as gamma rays in the core,

光波在核心以伽马射线形式产生

but end up as x-rays, ultraviolet or visible light as they near the surface.

但是在表面却衰减为X射线紫外线或者可见光

However, that journey is neither simple nor direct.

然而过程十分复杂

Upon being born, each photon travels at a speed of 300,000 kilometers per second

光子刚刚产生时候速度是每秒30万千米

until it collides with a proton and is diverted in another direction,

直到它撞击到另一个光子并且改变方向

acting like a bullet ricocheting off of every charged particle it strikes.

好比子弹一样

每次撞击改变一次方向

经过无数次撞击

The question of how far this photon gets from the center of the Sun

光子到达表面到底要经过多久

after each collision

is known as the random walk problem.

就是一个随机漫步问题

The answer is given by this formula:

这个公式会告诉我们答案

distance equals step size times the square root of the number of steps.

距离等于步长乘以步数的平方根 （一步指撞击间隔）

So if you were taking a random walk from your front door

如果你从你家前门出发

with a one meter stride each second,

每秒走一米并且每次随机换一次方向

it would take you a million steps and eleven days

会花费你11天时间 经过100万米的路程

just to travel one kilometer.

才能离开1千米远的距离

So then how long does it take for a photon generated in the center of the sun

所以光子要用多久

才能从太阳中心到达地球

to reach you?

We know the mass of the Sun

我们知道太阳的质量

and can use that to calculate the number of protons within it.

可以通过质量计算光子的数量

Let's assume for a second that all the Sun's protons are evenly spread out,

不妨假设这一秒钟内光子均匀扩散

making the average distance between them about 1.0 x 10^-10 meters.

假设它们每次传播0.1纳米的距离

To random walk the 690,000 kilometers from the core to the solar surface

从太阳核心到表面是69万千米的距离

would then require 3.9 x 10^37 steps,

用随机漫步的话3.9X10^37步

giving a total travel time of 400 billion years.

大概需要4千亿年的时间

Hmm, that can't be right.

桥豆麻袋 这不科学啊

The Sun is only 4.6 billion years old, so what went wrong?

太阳公公才460万岁好嘛 所以一定是哪里出问题了

Two things:

这里有两点

The Sun isn't actually of uniform density

第一 太阳并不是均匀的

and photons will miss quite a few protons between every collision.

第二 每次撞击之间光子会失去一些质子

In actuality, a photon's energy,

实际上 在这个过程中

which changes over the course of its journey,

光子的能量是在不断变化的

determines how likely it is to interact with a proton.

并且能量决定着与质子反应的可能性

On the density question,

在密度这个问题上

our models show that the Sun has a hot core,

我们假设的是

where the fusion reactions occur.

溶解反应在太阳的核心中发生

Surrounding that is the radiative zone,

围绕核心的是辐射传播区域

followed by the convective zone, which extends all the way to the surface.

再其外是通向表面的对流区域

The material in the core is much denser than lead,

核心的物质比金属铅还要密集

while the hot plasma near the surface is a million times less dense

表面附近的连续密度等离子体

with a continuum of densities in between.

相比之下稀疏了100万倍

And here's the photon-energy relationship.

这是光子-能量的关系

For a photon that carries a small amount of energy,

对于携带少量能量的光子而言

a proton is effectively huge,

质子的影响是巨大的

and it's much more likely to cause the photon to ricochet.

很可能引起光子的反弹

And for a high-energy photon, the opposite is true.

对于携带大量能量的光子而言

Protons are effectively tiny.

质子的影响就很微小

Photons start off at very high energies

光子出发时相对于到达表面时候而言

compared to when they're finally radiated from the Sun's surface.

携带大量的能量

Now when we use a computer and a sophisticated solar interior model

现在我们用电脑运算一下先进的太阳内部模型

to calculate the random walk equation with these changing quantities,

计算出随机路程中数量的变化

it spits out the following number: 170,000 years.

答案是17万年

Future discoveries about the Sun may refine this number further,

未来的发现也许会精确这个数字

but for now, to the best of our understanding,

但是现在我们最好的理解是

the light that's hitting your eyes today

今天我们看到的阳光

spent 170,000 years pinballing its way towards the Sun's surface,

经历了17万年跌跌撞撞的路途

plus eight miniscule minutes in space.

别忘了还要再加上8分钟的太空之旅

In other words, that photon began its journey two ice ages ago,

也就是说 在两个冰河世纪之前 人类刚开始穿衣服的时候

around the same time when humans first started wearing clothes.

今天的阳光就已经启程了