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世界观科普系列1:为什么时间不会倒流?

Why Doesn't Time Flow Backwards? (Big Picture Ep. 1/5)

物理学的基本定律——例如F=ma “重力与距离的平方成反比”
The basic laws of physics – things like F=ma, “gravity is inversely proportional
薛定谔方程,等等——并没有说任何
to the distance squared”, Schrodinger’s equation, and so on – don’t say anything
有关时间方向性的东西
about the direction of time.
它们确实将现在 过去与将来发生的事联系起来
Sure, they relate what’s going on now to what happens next, and to what happened previously,
但就时间而言没有过去与未来的差别
but there’s no distinction between forwards and backwards in time.
就微观物理学来说 过去与未来
The past and future are on an equal footing, as far as the microscopic laws of physics
是等同的
are concerned.
然而在微观世界中 确实有一条定律 时间只能向一个方向流动
In the macroscopic world, however, there is one rule that does have time going in one
这就是 热力学第二定律
direction only: the second law of thermodynamics.
这条定律表明 任何孤立的系统都倾向于熵增 也就是无序状态
That says that any isolated system will tend towards increasing entropy, or disorder.
就像冷牛奶可以与热咖啡混合在一起 变成温的牛奶咖啡 但无法
Like how cold milk and hot coffee mix together into luke-warm coffee-milk, but will never
逆转这一过程
“unmix” from each other.
一旦系统达到完全无序状态——平衡状态——就不存在
Once a system gets to its fully-disordered state — its equilibrium — there’s no more
熵增的方向了 也谈不上决定时间的流向了
direction of increasing entropy to determine the arrow of time.
我们现在能感受到的时间流逝 也就意味着我们还没达到平衡状态
So the fact that we experience the flow of time right now means that we’re not in equilibrium.
有两种可能会造成我们现在状态
There are basically two ways that could happen.
要么宇宙现在正巧处于一个特定的低熵的状态
Either the universe just happens to be, right now, in this particular, low-entropy, configuration
时间从“现在”沿着两个方向向前或向后流动 同时伴随两个方向熵增
with two directions of time flowing out forward and backward from it with increasing entropy
要么 在很久以前宇宙是从一个更低熵的状态开始的
in both directions; or at some point in the far distant past the universe started with
从这以后 无序状态(熵)就一直在增加
even lower entropy, and disorder has been increasing ever since.
[ 剧透一下:第二种说法是正确的 ]
[Spoiler alert: it’s option number two.]
这种低熵的状态就是宇宙大爆炸
That low-entropy configuration was the Big Bang.
138亿年前,宇宙曾是炽热 稠密 光滑的 并且它在急剧扩张
13.8 billion years ago, the universe was hot, dense, smooth, and rapidly expanding.
光滑而稠密的等离子颗粒在低熵状态下看上去可能是无序的
A smooth dense plasma of particles might not seem organized in low-entropy, but when the
但其实 物质密度极大的时候 粒子间的引力也是极大的
density of matter is extremely high, the gravitational force between particles is enormous.
光滑状态在面对这样的趋势的时候 就不再是一种平衡状态了 正相反
Smoothness, in the face of such tendencies, is not equilibrium, but is actually a very
它是一种低熵状态下的微妙平衡
delicately-balanced, low entropy state.
物质都有在引力作用下结块的趋势
Things want to be gravitationally clumped together into concentrated configurations
比如原恒星 原星系 甚至黑洞
like proto-stars, proto-galaxies, or even black holes.
那么高熵并处于平衡状态的宇宙看起来是什么样呢
What would a high-entropy, equilibrium universe look like?
它将是空荡荡的
It would be empty space.
而事实上 这也是我们宇宙的未来: 它正在进一步扩张 稀释
And indeed, that’s where we’re headed: the universe is expanding and diluting, and
最终所有的星体都将燃尽 黑洞也会蒸发
eventually all the stars will burn out and black holes will evaporate and we’ll be
留给我们的只有无尽的空虚
left with nothing but emptiness in every direction.
等到那时 时间的前进方向将消失 而任何类似生命或意识的东西
At that point, time’s arrow will have disappeared, and nothing like life or consciousness will
都将消亡
be possible.
而我们可以从满天的繁星和生机盎然的生物圈看出
The fact that our sky is decorated with billions of stars and galaxies, and our biosphere is
我们的起点是低熵的
teeming with life, is a reflection of our low-entropy beginnings.
我们不知宇宙为何始于这样有序的状态 但我们应该很庆幸它是这样的:
We don’t know why the universe started in such an orderly initial state, but we should
它给了我们一个非平衡状态的初始点,而这对于时间流
be glad it did: it gave us the non-equilibrium starting point that’s necessary for the
以众所周知的方式存在是至关重要的
flow of time, as we know it, to exist.
随后的其他一切——从星体和星系的形成到生命的起源
Everything that followed — from the formation of stars and galaxies to the origin of life
——就是一个熵增的故事了
— has been a story of increasing entropy.
时间的方向不是基础物理法则的一个深层特征 它的存在
Time’s arrow isn’t a deep feature of the most fundamental laws of physics; it owes
归功于我们宇宙特殊的初始状态
its existence to the specific initial conditions of our universe.
嘿 这里是Henry 感谢观看
Hey, Henry here, thanks for watching.
这是和物理学家Sean Carroll合作制作的关于时间和熵
This is the first [second, third, etc] video in a series about time and entropy made in
系列视频的第一集
collaboration with physicist Sean Carroll.
这个系列由谷歌制造与科学计划提供赞助
The series is supported with funding from Google’s Making and Science initiative,
该计划旨在令更多年轻人(以及来自各个年龄段的人)了解
which seeks to encourage more young people (and people of all ages) to learn about and
甚至爱上科学以及科学世界 这个视频是基于Sean的书
fall in love with science and the world around them, and the videos are based off of Sean’s
《纵观全局:生命的起源,意义,以及宇宙本身》 拍摄的(本书无中译本,搜书请使用英文原名)
book “The Big Picture: On the Origins of Life, Meaning, and the Universe Itself,”
你可以网上或者在任何书店中找到这本书
which you can find online or in bookstores around the world.

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