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

从薛定谔的猫里学到量子力学 - Josh Samani

翻译人员: Nical Lee 校对人员: Bighead Ge

Consider throwing a ball straight into the air.

竖直地向上扔一个球

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

你能预测当球离开你的手以后的运动吗？

Sure, that's easy.

当然，这很简单。

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

在到达某个最高点之前，这个球将一直上升。

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

之后它将下落，又一次回到你的手上。

Of course, that's what happens,

当然，事情就是这样的。

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

你之所以知道，是因为你已经无数次目击过这样的事情发生。

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

你一直在整个生命的日常现象里观察到物理

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

但是，假设我们去探索一个关于原子物理的问题

like what does the motion of an electron

就像是，一个电子绕氢原子核的运动是什么样

around the nucleus of a hydrogen atom look like?

就像是，一个电子绕氢原子核的运动是什么样

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

我们能依据日常的物理经验去回答吗？

Definietly not. Why?

肯定不行。为什么？

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

因为，微观系统运行的物理定律

is much different than the physics of the macroscopic objects

与宏观物理的定律有很大不同

you see around you all the time.

那些你见到或是围绕在你身边的物理

The everyday world you know and love

每天，你所见到和所深爱的

behaves according to the laws of classical mechanics.

被经典力学定律所掌控着。

But systems on the scale of atoms

但是，原子规模的系统

behave according to the laws of quantum mechanics.

被量子力学定律所掌控。

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

这种量子世界被证明是个十分奇怪的地方。

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

一个有名的思想实验给出了一个说明量子世界奇怪的例子，

Schrödinger's cat.

薛定谔的猫

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

一个实际上并不特别喜欢猫的物理学家，将一只猫放进盒子中，

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

并一同放了一颗炸弹，在盖子盖上后，有50%的机率爆炸。

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

除非我们我们重新打开盖子，我们没有办法得知

whether the bomb exploded or not,

炸弹有没有爆炸

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

因此，也无法知道猫是活着还是死了。

In quantum physics, we could say that before our observation

在量子物理中，我们可以说在观测之前，

the cat was in a superposition state.

那只猫处于叠加态。

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

既不是活着也不是死亡，而是两种可能性的混合。

with a 50% chance for each.

每种可能都有50%的可能性。

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

在量子规模中，同样的事情发生在物理系统上，

like an electron orbiting in a hydrogen atom.

就像一个电子绕氢原子核运行

The electron isn't really orbiting at all.

电子不是真正的绕轨道运行。

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

就像是空间中的任何地方，都在一瞬间

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

存在某处比其他地方有更大的可能性，

and it's only after we measure its position

并且只有在我们测量过它的位置之后，

that we can pinpoint where it is at that moment.

我们才可以精确的知道那个时刻它在哪。

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

很多现象，像是我们不知道这只猫是死是活

until we opened the box.

直到打开盒子

This brings us to the strange and beautiful phenomenon

这些把我们带入奇怪而又美丽的量子缠绕现象

of quantum entanglement.

。

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

假设，现在我们有两只猫，在两个不同的盒子里

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

如果我们对这一对猫重复“薛定谔的猫”的实验

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

这个实验的结果有四种可能

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

两只猫都活着或是都死了

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

亦或是，一只猫活着一只死了，或者反一下

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

这个两只猫的系统又存在于叠加态，

每种结果都有25%的可能性，而不是50％。

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

But here's the cool thing:

但是，这里有很有棒的事

quantum mechanics tells us it's possible to erase

量子力学告诉我们有可能去清除

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

叠加态中的结果，即两只猫都活着或死去。

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

换言之，可以是两只猫的系统，

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

这个的结果将总是一只猫活着，另一只猫死去。

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

这个科学术语就是猫的纠缠态。

But there's something truly mindblowing about quantum entanglement.

但是，关于量子纠缠有些真正吸引人的事情。

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

如果你在这种纠缠态中，准备两只在盒子中猫的系统

then move the boxes to opposite ends of the universe,

然后把盒子移到宇宙的另一端

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

实验的结果将总是相同。

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

一只猫总是活着，而另一只总是以死亡结束。

even though which particular cat lives or dies is completely undetermined

尽管在观测结果前，

具体哪只猫死亡或活着是完全不确定的，

before we measure the outcome.

How is this possible?

这怎么可能？

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

宇宙两端的猫的状态

can be entangled in this way?

怎能这样被纠缠？

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

它们相隔太远以至于不能即使沟通。

so how do the two bombs always conspire such that

那么，两个炸弹怎么能总是策划好

one blows up and the other doesn't?

一个爆炸而另一个无恙？

You might be thinking,

你一定在想，

"This is just some theoretical mumbo jumbo.

这只是一些理论上的胡言乱语

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

这种事情不可能在真正的世界上发生。

But it turns out that quantum entanglement

但是，这证明了，量子纠缠

has been confirmed in real world lab experiments.

（量子纠缠）已经在真实世界的实验室被证实了。

Two subatomic particles entangled in a superposition state,

两个亚原子的颗粒在叠加态纠缠，

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

在这种状态下，如果一个颗粒以一个方向旋转，另一个颗粒，必定以反方向旋转

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

事实就是这样，甚至，当没有办法

for information to pass from one particle to the other

将信息从一个颗粒传给另一个

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

去暗示，怎么服从缠绕的法则旋转。

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

不足为奇的是，

of quantum information science,

纠缠是量子信息科学的核心，

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

这是一个正在发展的领域，研究在我们的宏观世界

in our macroscopic world,

怎样应用奇怪的量子世界的定律

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

就像量子密码，间谍可以互相发送安全信息，

or quantum computing, for cracking secret codes.

或者量子计算，去破解密码。

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

日常的物理可能开始看上去有点像奇怪的量子世界。

Quantum teleportation may even progress so far,

量子远距离传送，甚至可能发展的十分迅速

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

某天，你的猫可能逃脱去了另一个星系，

where there are no physicists and no boxes.

那里既没有物理学家，也没有盒子。