Can you predict the motion of the ball after it leaves your hand?

你知道球會往哪邊跑嗎？

Sure, that's easy.

當然，太簡單了！

The ball will move upward until it gets to some highest point,

球向上到達一定高度後，

then it will come back down and land in your hand again.

會垂直落下， 回到你的手中。

Of course, that's what happens,

沒錯吧， 就這麼簡單，

and you know this because you have witnessed events like this countless times.

你想必在日常生活中 已經看過無數次了。

You've been observing the physics of everyday phenomena your entire life.

你從出生到現在， 每天都在觀察周遭的物理現象。

But suppose we explore a question about the physics of atoms,

但當我們想到原子物理的問題時，

like what does the motion of an electron

比如氫原子的電子怎麼運動？

around the nucleus of a hydrogen atom look like?

我們還能用日常所見來回答嗎？

Could we answer that question based on our experience with everyday physics?

不行吧，怎麼說？

Definitely not. Why?

因為物理系統 在這麼微小的世界裡，

Because the physics that governs the behavior of systems at such small scales

和在構成我們日常生活經驗的

is much different than the physics of the macroscopic objects

宏觀世界是非常不同的。

you see around you all the time.

你所知而深愛的周遭，

The everyday world you know and love

主要是「古典力學」的範疇；

behaves according to the laws of classical mechanics.

而在原子的世界裡，

But systems on the scale of atoms

則要靠「量子力學」來解釋。

behave according to the laws of quantum mechanics.

量子的世界是非常詭異的。

This quantum world turns out to be a very strange place.

有一個有名的例子 可以形容量子世界有多詭異：

An illustration of quantum strangeness is given by a famous thought experiment:

「薛丁格的貓」。

Schrödinger's cat.

一名不太喜歡貓的物理學家， 把貓放進一個箱子裡，

A physicist, who doesn't particularly like cats, puts a cat in a box,

裡面還有一顆炸彈， 有50%的機率在封箱後爆炸。

along with a bomb that has a 50% chance of blowing up after the lid is closed.

除非我們打開箱子，

Until we reopen the lid, there is no way of knowing

不然不會知道炸彈有沒有爆炸。

whether the bomb exploded or not,

所以我們也不知道貓是死是活。

and thus, no way of knowing if the cat is alive or dead.

在量子力學中，

In quantum physics, we could say that before our observation

我們會說在開箱以前，

the cat was in a superposition state.

貓咪處於「疊加狀態」；

It was neither alive nor dead but rather in a mixture of both possibilities,

牠既不是生也不是死，

with a 50% chance for each.

而是兩種可能的混合，

The same sort of thing happens to physical systems at quantum scales,

各佔50%的機率。

like an electron orbiting in a hydrogen atom.

這樣的假設也在 量子力學的世界裡，

The electron isn't really orbiting at all.

例如繞著氫原子轉的電子。

It's sort of everywhere in space, all at once,

電子其實沒有特定軌道，

with more of a probability of being at some places than others,

比較像是繞著原子核亂飛。

and it's only after we measure its position

所以有機會出現在不同地方，

that we can pinpoint where it is at that moment.

除非我們實際測量電子所在，

A lot like how we didn't know whether the cat was alive or dead

才能確定某個時點的電子在哪。

until we opened the box.

就像我們不知道貓的死活，

This brings us to the strange and beautiful phenomenon

除非我們打開箱子。

of quantum entanglement.

這讓我們進入 一個怪異又美麗的現象

Suppose that instead of one cat in a box, we have two cats in two different boxes.

--「量子糾纏」。

If we repeat the Schrödinger's cat experiment with this pair of cats,

假設現在不只一隻貓在箱子裡， 而是有兩隻貓在不同箱子。

the outcome of the experiment can be one of four possibilities.

如果用這兩隻貓重複 「薛丁格的貓」實驗，

Either both cats will be alive, or both will be dead,

可能會有四種不同結果。

or one will be alive and the other dead, or vice versa.

可能兩隻貓都活著， 或是兩隻貓都死了，

The system of both cats is again in a superposition state,

又或是某一隻活著 但另一隻死了。

with each outcome having a 25% chance rather than 50%.

所以又再次進入「疊加狀態」，

But here's the cool thing:

每種結果的機率變成25%， 而不是50%。

quantum mechanics tells us it's possible to erase

但有趣的來了：

the both cats alive and both cats dead outcomes from the superposition state.

量子力學跟我們說，

In other words, there can be a two cat system,

兩隻貓同時活下來 或死去是不可能的。

such that the outcome will always be one cat alive and the other cat dead.

也就是說只剩兩種可能，

The technical term for this is that the states of the cats are entangled.

只會是某隻貓活著， 然後另一隻死掉。

But there's something truly mindblowing about quantum entanglement.

這種現象的專有名詞， 是兩隻貓的狀態「糾纏」。

If you prepare the system of two cats in boxes in this entangled state,

但「量子糾纏」有一個 很神奇的部分：

then move the boxes to opposite ends of the universe,

如果你準備了互相糾纏的 兩隻貓在箱子裡，

the outcome of the experiment will still always be the same.

把箱子丟到宇宙的兩端，

One cat will always come out alive, and the other cat will always end up dead,

實驗的結果還是會一樣。

even though which particular cat lives or dies is completely undetermined

一定是其中一隻貓活著， 另一隻會死掉，

before we measure the outcome.

只是在我們看到結果前，

How is this possible?

不知道是哪隻活、哪隻死。

How is it that the states of cats on opposite sides of the universe

這怎麼可能呢？

can be entangled in this way?

為什麼兩隻相隔如此遙遠的貓，

They're too far away to communicate with each other in time,

還會這樣糾纏呢？

so how do the two bombs always conspire such that

它們的距離那麼遠 根本不能套招，

one blows up and the other doesn't?

所以炸彈到底怎麼能

You might be thinking,

每次都約好一個爆、 一個不爆呢？

"This is just some theoretical mumbo jumbo.

你可能會說，

This sort of thing can't happen in the real world."

「這只是理論上來說啦，

But it turns out that quantum entanglement

真實世界又不會照著演！」

has been confirmed in real world lab experiments.

但實際上「量子糾纏」

Two subatomic particles entangled in a superposition state,

已經被實驗證明了。

where if one spins one way then the other must spin the other way,

兩個疊加狀態下的「次原子粒子」，

will do just that, even when there's no way

如果其中一顆朝特定方向轉， 另一顆就會朝反方向轉。

for information to pass from one particle to the other

結果一定會這樣， 儘管兩顆粒子之間

indicating which way to spin to obey the rules of entanglement.

不可能溝通或傳遞訊息，

It's not surprising then that entanglement is at the core

代表它們的轉向 完全由「量子糾纏」來決定。

of quantum information science,

「量子糾纏」因此 毫不意外地變成

a growing field studying how to use the laws of the strange quantum world

「量子資訊學」的核心，

in our macroscopic world,

研究如何將量子世界的特殊性，

like in quantum cryptography, so spies can send secure messages to each other,

應用到現實生活的新興領域。

or quantum computing, for cracking secret codes.

比如說「量子密碼學」研究讓間諜 更隱密地互相傳送訊息，

Everyday physics may start to look a bit more like the strange quantum world.

或是用「量子計算」破解密碼。

Quantum teleportation may even progress so far,

我們的日常生活開始往 量子物理的領域發展。

that one day your cat will escape to a safer galaxy,

「量子遙傳」再繼續發展下去，

where there are no physicists and no boxes.

也許有一天你的貓 可以去到更安全的星系，