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The earth receives about 90 petajoules of energy from the sun every second.
地球每秒从太阳接收大约90千焦耳的能量。
That's a lot – enough to satisfy all human energy needs thousands of times over.
这已经足够满足人类能源需求了，这个量也是人类所需的数千倍之多。
However, the earth also radiates an equal amount of energy back into space as heat.
然而，地球也以热量的形式向太空辐射等量的能量。
(An almost equal amount, anyway -- the Earth is gradually heating up, but that's another story.)
（一个几乎相等的量，地球正在逐渐升温，但那是另一回事了。）
So if energy in equals energy out, how can we possibly be using any energy to do anything?
如果能量输入等于能量输出，那么我们怎么才能利用能量呢？
What matters, it turns out, isn't simply energy – it's useful energy.
事实证明，重要的不仅仅是能量，而是有用的能量。
And energy is useful when it comes packaged in a low-entropy form.
当能量以低熵形式出现时，它是有用的。
The total amount of energy in any closed system can't change,
任何封闭系统的总能量是不会改变的，
of course -- energy is conserved. But that energy can be in different states of usefulness.
当然，能量是守恒的。但是这种能量可以处于不同的有用状态。
There is useful energy in the water behind a dam,
水坝后面水含有用能量，
which will flow until the level is the same on both sides, but will never go back "up."
它会一直流动，直到两边的水位相同，但不会回到“逆流而上”。
Or in hot tea, which will melt a cold ice cube until they reach a medium temperature.
或者放在热茶里，热茶会融化冰块，直到它们达到中等温度。
You never see tepid tea spontaneously generate an ice cube while heating up,
你永远不会看到温热的茶在加热时会自发产生冰块，
even though the total energy would be the same. So, useful energy is low entropy,
即使总能量是一样的。因此，有用的能量是低熵的，
out-of-equilibrium energy that can be put to work
不平衡的能量，可被用来
to turn a turbine or push a piston or melt an ice cube.
转动涡轮或推动活塞或融化冰块。
Useless energy, meanwhile, can't be put to work.
同时，无用的能量不能被利用。
It's out there in the form of waste heat or noise or
它以废热、噪音
the potential energy of a ball just sitting on the ground: we can't really do anything with it.
或球的势能的形式存在于那里，我们真的不能利用它来做任何事情。
For example, gasoline and air carry useful, low-entropy energy.
例如，汽油和空气携带有用的低熵能量。
By burning them together, we can accelerate a car,
通过燃烧它们，我们可以使汽车加速，
drive for a while, then put on the brakes and stop.
开一段时间，然后踩刹车，然后停下来。
The total energy of the world remains constant during the trip,
在这段行驶过程中，总能量保持不变，
but useful chemical energy is converted into useless, high-entropy heat and noise.
但是有用的化学能量被转换成无用的、高熵热和噪音。
The energy is still there,
能量还在那里，
but we can't reclaim it to power any more cars, as convenient as that would be.
但我们不能再利用它来驱动更多的汽车，无法像之前那样方便利用。
And thus it is with the earth. The sun sends us energy in a convenient,
因此地球也是如此。太阳给我们传送便捷的，
useful, low-entropy form, mainly as photons of visible light.
有用的和低熵的能量，主要是以可见光的光子形式。
This energy gets absorbed and used and degraded by various processes here on Earth,
这种能量被地球上的各种过程吸收、利用和降解，
until we radiate it away in a much higher-entropy form: lots of infrared photons.
直到我们以一种更高熵的形式辐射出去：大量的红外光子。
For every visible photon we receive,
对于我们接收到的每一个可见光子，
the Earth radiates about twenty infrared photons out into space.
地球辐射出大约二十个红外光子进入太空。
Even though there's a balance of energy in with energy out,
即使能量输入与能量输出之间存在平衡，
the entropy of the energy has gone up twenty times.
能量的熵也上升了20倍。
The Sun, in fact, is only a source of useful energy
事实上，太阳只是一种有用能量的来源，
because it's a hot, bright spot in a cold, dark sky.
因为它是寒冷黑暗太空中的发光发热的点。
If the whole sky were the temperature of the Sun,
如果整个天空的温度与太阳一样，
our planet would receive a lot more energy,
我们的地球将接收更多的能量，
but the Earth would quickly average out to a state of high-entropy equilibrium,
但是地球会迅速平均化达到一个高熵的平衡状态，
with everything at the temperature of the Sun.
所有的东西都达到太阳的温度。
So things like driving or living would be impossible –
这样就不可能开车或生存
not because there's insufficient energy,
不是因为能量不足，
but because you can't do anything when all the energy has averaged out.
而是因为当所有的能量都达到平衡时，你什么也做不了。
In equilibrium, all energy is equally useless, nothing ever changes,
在平衡中，所有的能量都是一样无用的，没有任何变化，
and time's arrow ceases to have any meaning.
时间轴静止，时间也不再有任何意义。
Hey, Henry here, thanks for watching.
嘿，我是亨利，谢谢你的收看。
This is the fourth video in a series about time
这是与物理学家肖恩·卡罗尔一起制作的
and entropy made in collaboration with physicist Sean Carroll.
关于时间和熵的系列视频中的第四期。
The series is supported with funding from Google's Making and Science initiative,
该系列得到了谷歌“制造与科学”计划的资助。
which seeks to encourage more young people (and people of all ages)
该计划旨在鼓励更多的年轻人（以及各个年龄段的人）
to learn about and fall in love with science and the world around them,
了解科学，爱上科学和他们周围的世界。
and the videos are based off of Sean's book "The Big Picture:
本期视频是基于肖恩的书“《大图景》：
On the Origins of Life, Meaning, and the Universe Itself,"
论生命的起源、意义和宇宙本身”。
which you can find online or in bookstores around the world.
你可以在网上或世界各地的书店找到。
