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You may know that it takes light a zippy eight minutes to reach us from the surface of the Sun,
或许你知道太阳光从太阳表面到达地球需要8分钟，
so how long do you think it takes light to travel from the Sun's core to its surface?
但是你认为太阳光要多久才能从太阳中心到达太阳表面呢？
A few seconds or a minute at most?
几秒钟还是顶多一分钟？
Well, oddly enough, the answer is many thousands of years. Here's why.
扶好下巴。答案是需要好几千年。下面来解释一下。
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,
光子从太阳中心流出，
they interact with matter and lose energy, becoming longer wavelength forms of light.
与物质相互作用损失能量，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.
不过这一过程十分复杂。
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质子并且改变方向，
acting like a bullet ricocheting off of every charged particle it strikes.
就像子弹一样，每次撞击改变一次方向。
The question of how far this photon gets from the center of the Sun after each collision is known as the random walk problem.
经过无数次撞击的光子，到达表面到底要经过多久，这是一个随机漫步问题。
The answer is given by this formula: distance equals step size times the square root of the number of steps.
这个公式会告诉我们答案：距离等于步长乘以步数的平方根。
So if you were taking a random walk from your front door with a one meter stride each second,
如果你从你家前门出发，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 just to travel one kilometer.
会花费你11天时间、走100万米的路程，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 to reach you?
所以光子要用多久才能从太阳中心到达地球呢？
We know the mass of the Sun and can use that to calculate the number of protons within it.
我们知道太阳的质量，并且可以以此来计算质子的数量。
Let's assume for a second that all the Sun's protons are evenly spread out,
我们暂时假设太阳所有的质子都均匀摊开，
making the average distance between them about 1.0 x 10^-10 meters.
使得它们之间的平均距离约为10的负10次方公尺，
To random walk the 690,000 kilometers from the core to the solar surface would then require 3.9 x 10^37 steps,
从核心到太阳表面随机漫步69万公里，将需要3.9乘10的37次方步。
giving a total travel time of 400 billion years. Hmm, that can't be right.
需要四千亿年的总时间。嗯，这不太对。
The Sun is only 4.6 billion years old, so what went wrong?
太阳只有46亿岁，那哪里出错了？
Two things: The Sun isn't actually of uniform density and photons will miss quite a few protons between every collision.
有两件事：实际上太阳的密度并不均匀，光子每次碰撞会错过一些质子。
In actuality, a photon's energy, which changes over the course of its journey,
实际上，一个光子的能量会随着它行进的过程而改变，
determines how likely it is to interact with a proton.
因而决定它如何与质子相互作用。
On the density question, our models show that the Sun has a hot core, where the fusion reactions occur.
至于密度的问题，我们的模型显示太阳有一个热核，是核融合反应发生的地方。
Surrounding that is the radiative zone, followed by the convective zone, which extends all the way to the surface.
它的周围是辐射区，其次是对流区，就这样一直延伸到表面。
The material in the core is much denser than lead,
核心的密度比铅还要大得多，
while the hot plasma near the surface is a million times less dense with a continuum of densities in between.
而靠近表面的热电浆密度则小了一百万倍。两者之间则是连续密度。
And here's the photon-energy relationship.
这里是光子与能量的关系。
For a photon that carries a small amount of energy, a proton is effectively huge,
相对于一个携带少量能量的光子，质子其实非常大，
and it's much more likely to cause the photon to ricochet.
它更容易使光子被弹飞。
And for a high-energy photon, the opposite is true. Protons are effectively tiny.
而对于高能光子，情况正好相反。质子相对来说非常微小。
Photons start off at very high energies compared to when they're finally radiated from the Sun's surface.
与光子在太阳表面被辐射出去时的能量相较，质子出发时的能量非常高。
Now when we use a computer and a sophisticated solar interior model
现在我们使用计算机和一个复杂的太阳内部模型，
to calculate the random walk equation with these changing quantities, it spits out the following number: 170,000 years.
用这些变量计算随机漫步方程，它算出来的是以下数字：17万年。
Future discoveries about the Sun may refine this number further,
未来有关太阳的发现可以进一步细化这个数字，
but for now, to the best of our understanding,
但是现在，尽我们的理解，
the light that's hitting your eyes today spent 170,000 years pinballing its way towards the Sun's surface,
今天进入我们眼睛的光，花了17万年，以弹珠台游戏的方式到达太阳表面。
plus eight miniscule minutes in space.
加上在宇宙穿梭的微小8分钟。
In other words, that photon began its journey two ice ages ago, around the same time when humans first started wearing clothes.
换句话说，光子在两个冰河时期前就开始了它的旅程，同一时间人类才刚开始穿衣服。
