What can Schrödinger's cat teach us about quantum mechanics? - Josh Samani

「薛丁格的貓」與量子力學 - 賈許‧薩馬尼

譯者: Aaron Shoo 審譯者: Adrienne Lin

Consider throwing a ball straight into the air.

把一顆球直直往上拋，

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，

你想必在日常生活中 已經看過無數次了。

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

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?

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

我們還能用日常所見來回答嗎？

不行吧，怎麼說？

Definietly not. Why?

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

因為物理系統 在這麼微小的世界裡，

is much different than the physics of the macroscopic objects

和在構成我們日常生活經驗的

you see around you all the time.

宏觀世界是非常不同的。

The everyday world you know and love

你所知而深愛的周遭，

behaves according to the laws of classical mechanics.

主要是「古典力學」的範疇；

But systems on the scale of atoms

而在原子的世界裡，

behave according to the laws of quantum mechanics.

則要靠「量子力學」來解釋。

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

量子的世界是非常詭異的。

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

有一個有名的例子 可以形容量子世界有多詭異：

Schrödinger's cat.

「薛丁格的貓」。

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

一名不太喜歡貓的物理學家， 把貓放進一個箱子裡，

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

裡面還有一顆炸彈， 有50%的機率在封箱後爆炸。

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,

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

所以我們也不知道貓是死是活。

In quantum physics, we could say that before our observation

在量子力學中，

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，

而是兩種可能的混合，

with a 50% chance for each.

各佔50%的機率。

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

這樣的假設也在 量子力學的世界裡，

like an electron orbiting in a hydrogen atom.

例如繞著氫原子轉的電子。

The electron isn't really orbiting at all.

電子其實沒有特定軌道，

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

比較像是繞著原子核亂飛。

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

所以有機會出現在不同地方，

and it's only after we measure its position

除非我們實際測量電子所在，

that we can pinpoint where it is at that moment.

才能確定某個時點的電子在哪。

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

就像我們不知道貓的死活，

until we opened the box.

除非我們打開箱子。

This brings us to the strange and beautiful phenomenon

這讓我們進入 一個怪異又美麗的現象

of quantum entanglement.

--「量子糾纏」。

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

假設現在不只一隻貓在箱子裡， 而是有兩隻貓在不同箱子。

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

如果用這兩隻貓重複 「薛丁格的貓」實驗，

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

可能會有四種不同結果。

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

可能兩隻貓都活著， 或是兩隻貓都死了，

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

又或是某一隻活著 但另一隻死了。

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

所以又再次進入「疊加狀態」，

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

每種結果的機率變成25%， 而不是50%。

But here's the cool thing:

但有趣的來了：

quantum mechanics tells us it's possible to erase

量子力學跟我們說，

兩隻貓同時活下來 或死去是不可能的。

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

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

也就是說只剩兩種可能，

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

只會是某隻貓活著， 然後另一隻死掉。

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

這種現象的專有名詞， 是兩隻貓的狀態「糾纏」。

But there's something truly mindblowing about quantum entanglement.

但「量子糾纏」有一個 很神奇的部分：

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

如果你準備了互相糾纏的 兩隻貓在箱子裡，

then move the boxes to opposite ends of the universe,

把箱子丟到宇宙的兩端，

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

實驗的結果還是會一樣。

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

一定是其中一隻貓活著， 另一隻會死掉，

even though which particular cat lives or dies is completely undetermined

只是在我們看到結果前，

不知道是哪隻活、哪隻死。

before we measure the outcome.

How is this possible?

這怎麼可能呢？

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

為什麼兩隻相隔如此遙遠的貓，

can be entangled in this way?

還會這樣糾纏呢？

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

它們的距離那麼遠 根本不能套招，

so how do the two bombs always conspire such that

所以炸彈到底怎麼能

one blows up and the other doesn't?

每次都約好一個爆、 一個不爆呢？

You might be thinking,

你可能會說，

"This is just some theoretical mumbo jumbo.

「這只是理論上來說啦，

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

真實世界又不會照著演！」

But it turns out that quantum entanglement

但實際上「量子糾纏」

has been confirmed in real world lab experiments.

已經被實驗證明了。

Two subatomic particles entangled in a superposition state,

兩個疊加狀態下的「次原子粒子」，

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

如果其中一顆朝特定方向轉， 另一顆就會朝反方向轉。

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

結果一定會這樣， 儘管兩顆粒子之間

for information to pass from one particle to the other

不可能溝通或傳遞訊息，

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

代表它們的轉向 完全由「量子糾纏」來決定。

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

「量子糾纏」因此 毫不意外地變成

of quantum information science,

「量子資訊學」的核心，

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

研究如何將量子世界的特殊性，

in our macroscopic world,

應用到現實生活的新興領域。

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

比如說「量子密碼學」研究讓間諜 更隱密地互相傳送訊息，

or quantum computing, for cracking secret codes.

或是用「量子計算」破解密碼。

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

我們的日常生活開始往 量子物理的領域發展。

Quantum teleportation may even progress so far,

「量子遙傳」再繼續發展下去，

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

也許有一天你的貓 可以去到更安全的星系，

where there are no physicists and no boxes.

一個沒有物理學家 和箱子的地方。