Name: 【TED-Ed】What can Schrödinger's cat teach us about quantum mechanics? - Josh Samani
Uploaded: 2014-12-15T02:14:55.000Z
Duration: 5 min 24 s

Consider throwing a ball straight into the air.

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

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

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

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，

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?

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

is much different than the physics of the macroscopic objects

behaves according to the laws  of classical mechanics.

behave according to the laws  of quantum mechanics.

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:

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.

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

不然不會知道炸彈有沒有爆炸。

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.

In quantum physics, we could say that before our observation

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

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

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.

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

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

and it's only after  we measure its position

that we can pinpoint where it is at that moment.

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。

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

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

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,

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」，

每種結果的機率變成25%， 而不是50%。

quantum mechanics tells us it's possible to erase

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

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.

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.

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

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，

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，

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

so how do the two bombs always  conspire such that

它們的距離那麼遠 根本不能套招，

每次都約好一個爆、 一個不爆呢？

"This is just some theoretical mumbo jumbo.

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

But it turns out that quantum entanglement

has been confirmed in  real world lab experiments.

Two subatomic particles entangled in a superposition state,

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.

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。

「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

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,

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，