That’s a lot – enough to satisfy all human energy needs thousands of times over.

粉多的，足以滿足數千倍於全人類使用的能量。

However, the earth also radiates an equal amount of energy back into space as heat.

然而同時，地球也向太空放出等量的熱能

(An almost equal amount, anyway -- the Earth is gradually heating up, but that's another

(幾乎等量啦，地球正在暖化，此事暫且不提。)

story.)

(幾乎等量啦，地球正在暖化，此事暫且不提。)

So if energy in equals energy out, how can we possibly be using any energy to do anything?

如果輸入等於輸出，我們哪來的能量做各種事情？

What matters, it turns out, isn’t simply energy – it’s useful energy.

事實上這不只關係到能量，而是「可用的能量」。

And energy is useful when it comes packaged in a low-entropy form.

當能量在低熵狀態則容易使用。

The total amount of energy in any closed system can’t change, of course -- energy is conserved.

一個封閉系統內的能量當然不會改變─能量守恆嘛，

But that energy can be in different states of usefulness.

但能量從有用到沒用有各種層級。

There is useful energy in the water behind a dam, which will flow until the level is

水壩後頭的水釋放可用能量，直到兩邊水位等高，

the same on both sides, but will never go back “up.”

但絕對不會倒流回去。

Or in hot tea, which will melt a cold ice cube until they reach a medium temperature.

或一杯熱茶，把裡面的冰塊融化，直到兩者溫度相同，

You never see tepid tea spontaneously generate an ice cube while heating up, even though

你不可能目睹微熱的紅茶自動加溫、自動生出冰塊，

the total energy would be the same.

即使總能量仍然一致。

So, useful energy is low entropy, out-of-equilibrium energy that can be put to work, to turn a

因此有用的能量熵較低，不平衡的能量可以做功，

turbine or push a piston or melt an ice cube.

轉動渦輪以推動活塞，或融化冰塊。

Useless energy, meanwhile, can’t be put to work.

除非這能量無法做功，

It’s out there in the form of waste heat or noise or the potential energy of a ball

以廢熱、噪音或地上一顆靜止球球的位能等形式，

just sitting on the ground: we can't really do anything with it.

對我們來講都一無是處。

For example, gasoline and air carry useful, low-entropy energy.

好比說，汽油和空氣都帶有低熵的可用能量，

By burning them together, we can accelerate a car, drive for a while, then put on the

當燃燒使它們結合，我們能藉此加速汽車、開一段路、

brakes and stop.

踩煞車停止。

The total energy of the world remains constant during the trip, but useful chemical energy

整趟旅程中，世界上的能量總和不便，但有用的化學能

is converted into useless, high-entropy heat and noise.

被轉換成沒用、高熵的熱與聲音。

The energy is still there, but we can't reclaim it to power any more cars, as convenient as

能量還是在，但我們無法利用它駕車如之前那般方便。

that would be.

能量還是在，但我們無法利用它駕車如之前那般方便。

And thus it is with the earth.

地球也是同一種原理。

The sun sends us energy in a convenient, useful, low-entropy form, mainly as photons of visible

太陽給我們有用、低熵的能量， 主要是以可見光子的形式。

light.

太陽給我們有用、低熵的能量， 主要是以可見光子的形式。

This energy gets absorbed and used and degraded by various processes here on Earth, until

這股能量經由地球上的各種過程被吸收、使用、衰敗，

we radiate it away in a much higher-entropy form: lots of infrared photons.

直到我們把它們以高熵形式散發出去：大量紅外光子。

For every visible photon we receive, the Earth radiates about twenty infrared photons out

地球每接收一個可見光子，大約就會放出20個紅外光子

into space.

至太空中。

Even though there’s a balance of energy in with energy out, the entropy of the energy

即便能量輸入和輸出平衡，能量的熵仍增加了20倍。

has gone up twenty times.

即便能量輸入和輸出平衡，能量的熵仍增加了20倍。

The Sun, in fact, is only a source of useful energy because it's a hot, bright spot in

事實上，太陽本身之所以是有用的能源， 正因為它是個明亮的熱點，處在寒冷、黑暗的太空中。

a cold, dark sky.

事實上，太陽本身之所以是有用的能源， 正因為它是個明亮的熱點，處在寒冷、黑暗的太空中。

If the whole sky were the temperature of the Sun, our planet would receive a lot more energy,

若太空各處的溫度都和太陽一樣， 地球會接收到非常多的能量，

but the Earth would quickly average out to a state of high-entropy equilibrium, with

但地球很快會陷入高熵的能量平衡，

everything at the temperature of the Sun.

整個星球都變得跟太陽一樣熱。

So things like driving or living would be impossible – not because there’s insufficient

開車或生存會變得不可能─不是因為能量太少，

energy, but because you can’t do anything when all the energy has averaged out.

而是你沒辦法使用已經平衡的能量。

In equilibrium, all energy is equally useless, nothing ever changes, and time's arrow ceases

在平衡狀態下，所有能量都沒有用，事物永遠不變，

to have any meaning.

時間之箭將不再有任何意義。

Hey, Henry here, thanks for watching.

安安我是Henry，感謝收看。

This is the fourth video in a series about time and entropy made in collaboration with

本片是和物理學家Sean Carroll合作關於時間與熵

physicist Sean Carroll.

系列影片的第四集。

The series is supported with funding from Google’s Making and Science initiative,

本系列由 Google’s Making and Science initiative贊助播出，

which seeks to encourage more young people (and people of all ages) to learn about and

為了鼓勵更多年輕人(和全年齡的人們)學習

fall in love with science and the world around them, and the videos are based off of Sean’s

並愛上科學和這個世界，本片基於Sean所著

book “The Big Picture: On the Origins of Life, Meaning, and the Universe Itself,”

《The Big Picture: On the Origins of Life, Meaning, and the Universe Itself》。

which you can find online or in bookstores around the world.

你可以在線上或世界各地的書店找到。