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The basic laws of physics – things like F=ma, "gravity is inversely proportional to the distance squared",
物理学的基本定律，比如F=ma，重力大小和距离的平方成反比，
Schrodinger's equation, and so on – don't say anything about the direction of time.
薛定谔方程等等—都没有谈及时间的方向。
Sure, they relate what's going on now to what happens next, and to what happened previously,
当然，它们将现在发生的事情与接下来发生的事情，以及之前发生的事情联系起来，
but there's no distinction between forwards and backwards in time.
但是在时间上没有向前和向后的区别。
The past and future are on an equal footing, as far as the microscopic laws of physics are concerned.
就物理学的微观定律而言，过去和未来是平等的。
In the macroscopic world, however, there is one rule that does have time going in one direction only: the second law of thermodynamics.
但在宏观世界中，有一条规则是时间只向一个方向运动：热力学第二定律。
That says that any isolated system will tend towards increasing entropy, or disorder.
该定律认为，任何一个孤立系统的熵将趋向于增加或无序。
Like how cold milk and hot coffee mix together into luke-warm coffee-milk, but will never "unmix".
比如冰牛奶和热咖啡混合成为微温咖啡牛奶，但永远不会“不混合”。
Once a system gets to its fully-disordered state -- its equilibrium -- there's no more direction of increasing entropy to determine the arrow of time.
一旦一个系统达到完全无序的状态—平衡态—再也没有更多增加熵的方向来决定时间的箭头了。
So the fact that we experience the flow of time right now means that we're not in equilibrium.
所以我们现在经历的时间流意味着我们不是处于平衡状态。
There are basically two ways that could happen.
基本上有两种可能的方式。
Either the universe just happens to be, right now, in this particular, low-entropy,
要么现在宇宙恰好是处于这个特殊的、低熵的状态，
configuration with two directions of time flowing out forward and backward from it with increasing entropy in both directions;
时间无论往两个方向中的哪一个流动，它的熵都会增加；
or at some point in the far distant past the universe started with even lower entropy, and disorder has been increasing ever since.
要么在遥远的过去，宇宙的初始熵更小，从那时起，无序一直在增加。
Spoiler alert: it's option number two.
剧透警告：是第二种选择。
That low-entropy configuration was the Big Bang.
低熵状态就是大爆炸。
13.8 billion years ago, the universe was hot, dense, smooth, and rapidly expanding.
138亿年前，宇宙是热的、稠密的、平滑的且快速扩张。
A smooth dense plasma of particles might not seem organized and low-entropy,
一个光滑稠密的粒子等离子体似乎不是有组织且低嫡的，
but when the density of matter is extremely high, the gravitational force between particles is enormous.
但当物质密度极高时，粒子间的引力就会非常大。
Smoothness, in the face of such tendencies, is not equilibrium, but is actually a very delicately-balanced, low entropy state.
在这样的情况下，平滑度是不均衡的，但确实是处在一个很微妙平衡且低熵状态。
Things want to be gravitationally clumped together into concentrated configurations like proto-stars, proto-galaxies, or even black holes.
万物都想通过引力聚集在一起，如原始恒星、原始星系甚至黑洞。
What would a high-entropy, equilibrium universe look like? It would be empty space.
一个高熵平衡的宇宙会是什么样子？它将是一个空无的空间。
And indeed, that's where we're headed:
事实上，这正是我们现在的走向：
the universe is expanding and diluting, and eventually all the stars will burn out and black holes will evaporate
宇宙正在膨胀和稀释，最终所有的恒星都会燃烧殆尽，黑洞也会蒸发
and we'll be left with nothing but emptiness in every direction.
我们在每一个方向上除了虚无将一无所有。
At that point, time's arrow will have disappeared, and nothing like life or consciousness will be possible.
那时，时间的箭头将消失，生命和意识都是不可能的。
The fact that our sky is decorated with billions of stars and galaxies,
我们的天空装饰着数十亿颗恒星和星系，
and our biosphere is teeming with life, is a reflection of our low-entropy beginnings.
我们的生物圈充满了生命，这个事实是我们低熵起点的反射。
We don't know why the universe started in such an orderly initial state,
我们不知道宇宙为什么会以如此有序的初始状态开始，
but we should be glad it did: it gave us the non-equilibrium starting point that's necessary for the flow of time, as we know it, to exist.
但我们应该为此高兴：它给了我我们一个非平衡的起点，这个起点是时间流存在的必要因素。
Everything that followed -- from the formation of stars and galaxies to the origin of life -- has been a story of increasing entropy.
接下来的一切—从恒星和星系的形成到生命的起源—都是一个熵增加的故事。
Time's arrow isn't a deep feature of the most fundamental laws of physics;
时间箭头不是最基本物理定律的深层特征；
it owes its existence to the specific initial conditions of our universe.
它的存在要归功于宇宙的特定初始条件。
