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There's a concept that's crucial to chemistry and physics.
在化学和物理领域里有一个非常重要的概念。
It helps explain why physical processes go one way and not the other:
这个概念可以解释为什么物理过程会这样发生，而不是另一种结果：
why ice melts, why cream spreads in coffee, why air leaks out of a punctured tire.
为什么冰会融化，为什么奶油会在咖啡中扩散，为什么空气会从穿孔的轮胎中泄露。
It's entropy, and it's notoriously difficult to wrap our heads around.
这个概念就是熵，这是一个让人很难理解的概念。
Entropy is often described as a measurement of disorder.
熵通常被描述为不规则运动的量度。
That's a convenient image, but it's unfortunately misleading.
这是一个很方便让人理解的解释，但却很容易产生误解。
For example, which is more disordered -- a cup of crushed ice or a glass of room temperature water?
比如说，以下哪种情形更加的无规则呢？是一杯碎冰块，还是一杯室温的水？
Most people would say the ice, but that actually has lower entropy.
大多数人会说冰块会更无规则，但是实际上冰块比水有更低的熵值。
So here's another way of thinking about it through probability.
这儿有另一种理解熵的方法，那就是通过概率。
This may be trickier to understand,
这个方法或许更难理解，
but take the time to internalize it and you'll have a much better understanding of entropy.
但一旦消化这个概念，你就会对熵有一个更深刻的理解。
Consider two small solids which are comprised of six atomic bonds each.
想象两个小块的固体，这两个固体都有六个化学键。
In this model, the energy in each solid is stored in the bonds.
在这个模型中，固体的能量都存在化学键中。
Those can be thought of as simple containers, which can hold indivisible units of energy known as quanta.
这些化学键可以被理解为一个简单的容器，可以用来储存不可分割的最小单位的能量，量子。
The more energy a solid has, the hotter it is.
一个固体的能量越高，温度就也越高。
It turns out that there are numerous ways that the energy can be distributed in the two solids
能量在这两个固体中分布的方式有无数种，
and still have the same total energy in each.
并且所拥有的总能量相等。
Each of these options is called a microstate.
每个分布方式都称作一种微态。
For six quanta of energy in Solid A and two in Solid B, there are 9,702 microstates.
比如说分布六个量子的能量在固体A中，两个量子的能量在固体B中，这就有9702种微态。
Of course, there are other ways our eight quanta of energy can be arranged.
当然，这八个量子在两个固体中还有其他的分布方式。
For example, all of the energy could be in Solid A and none in B, or half in A and half in B.
比如说，所有的量子可以全都分布在固体A中，而B中没有量子，还可以A，B固体各分一半量子。
If we assume that each microstate is equally likely,
如果我们假设每种微态发生的概率相等，
we can see that some of the energy configurations have a higher probability of occurring than others.
我们可以发现有些能量分布发生的概率会高于其他。
That's due to their greater number of microstates.
这是因为这样的能量分布包含更多数量的微态。
Entropy is a direct measure of each energy configuration's probability.
熵是每种能量分布状态的概率衡量。
What we see is that the energy configuration in which the energy is most spread out between the solids has the highest entropy.
我们所观察到的是，能量在固体间最分散，熵值就最高。
So in a general sense, entropy can be thought of as a measurement of this energy spread.
所以总体而言，熵可以被想成能量分散的一种衡量指标。
Low entropy means the energy is concentrated. High entropy means it's spread out.
低的熵值表明能量是集中的。高的熵值则代表能量是分散的。
To see why entropy is useful for explaining spontaneous processes, like hot objects cooling down,
为了理解为什么熵的概念可以解释自然发生的过程，比如说热的物体会冷却，
we need to look at a dynamic system where the energy moves.
我们需要理解能量流动的动态系统。
In reality, energy doesn't stay put. It continuously moves between neighboring bonds.
实际上，能量不会静止不动。而是会不停地在相邻的化学键中移动。
As the energy moves, the energy configuration can change.
随着能量的移动，能量的分布也会随之改变。
Because of the distribution of microstates,
由于微态的分布，
there's a 21% chance that the system will later be in the configuration in which the energy is maximally spread out,
能量极大程度分散的分布概率有21%，
there's a 13% chance that it will return to its starting point, and an 8% chance that A will actually gain energy.
13%的概率能量分布会回到最初的状态，固体A能量增加的概率是8%。
Again, we see that because there are more ways to have dispersed energy
别忘了，我们看到这种现象是因为分散能量的分布方式更多，
and high entropy than concentrated energy, the energy tends to spread out.
所以我们更有可能观察到高熵值，而不是能量集中的低熵值状态，能量更倾向于分散。
That's why if you put a hot object next to a cold one, the cold one will warm up and the hot one will cool down.
这就是为什么如果你把一个热的物体放在一个冷的物体旁，冷的物体会变热，而热的物体会冷却。
But even in that example, there is an 8% chance that the hot object would get hotter.
但即使是在刚刚的例子里，还是有8%的概率热的物体会变得更热。
Why doesn't this ever happen in real life? It's all about the size of the system.
那为什么这种事情从来都没有在现实生活中发生过呢？这是因为系统的尺寸。
Our hypothetical solids only had six bonds each.
我们假设的两个固体每个只有六个化学键。
Let's scale the solids up to 6,000 bonds and 8,000 units of energy,
如果我们假设每个固体有6000化学键，需要分配的总能量为8000量子，
and again start the system with three-quarters of the energy in A and one-quarter in B.
我们再次将四分之三的能量分配给A，四分之一的能量分配给B。
Now we find that chance of A spontaneously acquiring more energy is this tiny number.
现在我们可以发现，A物体能够自发获得更多能量的概率是这样一个微小的数字。
Familiar, everyday objects have many, many times more particles than this.
同理，日常物体中会包含比这多得多的小物体。
The chance of a hot object in the real world getting hotter is so absurdly small, it just never happens.
在现实世界里，一个物体会变热的概率是一个异常小的数字，小到根本不会发生。
Ice melts, cream mixes in, and tires deflate because these states have more dispersed energy than the originals.
冰块融化，奶油溶解，轮胎泄气，都是因为这些状态比原有的状态有更加分散的能量。
There's no mysterious force nudging the system towards higher entropy.
没有任何神秘的力量推着系统去往一个更高的熵值。
It's just that higher entropy is always statistically more likely.
只是因为高熵值总是在统计上更加可能发生。
That's why entropy has been called time's arrow.
这就是为什么熵又被成为时间向导。
If energy has the opportunity to spread out, it will.
如果能量有机会分散，它就会发生。
