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玩转量子力学

Understanding Quantum Mechanics

量子力学应该说是物理学最伟大的学说了
Quantum mechanics is supposed to be our best theory of physics.
因为它可以告诉我们
That’s because it can tell us
在我们做过的每个实验中会发生些什么
what will happen in any experiment that we’ve tried.
但科学还应该解释现象产生的原因
But science is also meant to explain why things happen.
量子力学可不会告诉我们原因
Quantum mechanics doesn’t give us why,
因为我们还完全不懂该怎么去解释它
because we still don’t understand how to interpret its meaning at all.
比如 某个物体从A出发走到B
For example, when an object starts at A and ends up at B,
量子力学说的是它走了所有可能的路径呢
does quantum mechanics say that it goes all possible ways at once
还是说在多个不同的宇宙中都存在这个物体的副本
or that there are many copies of each object in many different universes
每个副本走过的路径都不相同?
each going a different way?
又或者量子力学索性认为
Or does it say that it doesn’t even make sense
当我们不看这个物体时 讨论它并没有任何意义?
to talk about something when we’re not looking at it?
这些都是对量子力学的可能解释
These are all possible ways to interpret quantum mechanics
至少是表面解释
at least on face value.
没人懂量子力学理查德·费曼
-天才-自称天才
本视频将带你走近量子力学的基本规则
In this video we’re going to look at the basic rules of quantum mechanics
从而明白为何它的含义如此模棱两可
to understand why its meaning is so evasive.
我们会着眼于一项实验
We’re about to look at an experiment that showed
它呈现了一条很奇怪的自然规则
that nature has a very strange rule:
那就是“物体在未被测量时会表现得不同”
objects act differently when they’re not being measured.
不过 在我们介绍这一规则之前
Before we get to that though,
我们需要理解物体在什么时候算是被测量
we need to understand when objects are
在什么时候不算
and aren’t being measured.
那么来看一下这个熟悉的情境
So let’s look at this familiar situation,
你丢了一样东西
you’ve lost an object,
但你确定你把它落在了这里 或者那里
but you’re sure you left it either here or here.
但因为你现在并没有去找它在哪儿
You’re not currently measuring its position yourself,
所以目前你还无法测出它的具体位置
because you’re not looking to see where it is.
然而考虑一下 其实有很多其它东西都在测量
However, consider that there are many other things that are measuring it,
比如那些正在苹果表面反弹的光微粒
for example, think of all the light particles bouncing off it right now,
它们实际上“拍”下了你丢的东西在哪儿
they’re essentially taking a photo of where your object is.
还有那些空气粒子呢?
What about the air molecules that would have gone straight through
若你的东西不在那里 它们本该径直穿过去
where your object is if it wasn’t there
但现在却被反弹开来
but are now bouncing off that position?
它们也是在测量你丢的东西的位置
They’re also measuring your particle’s real position
这也是普遍出现的情况
and that’s how it usually is.
物体都在不断地被其它东西测量着
Objects are constantly being measured by other things.
所以“物体在未被测量时会表现得不同”的说法
So then this statement that objects act differently when they’re not being measured
并不适用于日常所见的物体
doesn’t apply for everyday objects,
这正是我们长期以来都没注意到这一规则的原因
which is why we didn’t notice this rule for such a long time.
但科学家开始把目光转向很小的东西 比如电子
But then scientists started to look at very small things like electrons.
空气粒子和光这样的东西都不会持续撞击它们
Things like air molecules and light aren’t constantly hitting them
因为它们小到足以从缝隙中穿过
because they’re small enough to sort of fit in the gaps.
那么当没人观察它们的时候
So what exactly do objects like that do
这类微粒到底在干嘛呢?
when no one is watching.
我们回到原本的情境中来
Let’s get back to the scenario where our object,
在这种情况下 我们的物体 即电子
so in this case, our electron,
一定是要么在这个位置 要么在这儿
is definitely in this spot or this spot.
但我们现在知道它没被任何东西测量
But we now know that nothing is measuring it.
我们的设想是物体在某一时刻只能存在于一个地方
We expect that objects can only be in one place at a time.
即便你并不知道它具体在哪处
Even if you don’t know where that place is,
但你知道它一定在其中的某一个地方
you know it’s in one of the places
只不过不知道到底是哪一个
you just don’t know which
这往往是对的
and this is usually true.
但在这个例子中
But in the case where nothing at all
我们的研究对象电子没有被任何物体测量
is measuring our object like our electron,
下面的实验会证明我们的设想并不正确
the following experiment will show us that this isn’t true.
电子在未被测量时
The electron can’t be at one place at a time
在某一时刻并不会只存在于一个地方
while it’s not being measured.
这个实验叫“双缝实验”
This experiment is called the double-slit experiment.
下面我们来看看这个实验怎么做
And here’s how it goes.
我们每次只会从这里发射一枚电子
We’re going to fire electrons one at a time from here.
自制电子枪!
DIY electron gun!
*请勿在家尝试
*Please don’t try this at home.
当它们抵达后面的这堵墙时
When they get to this back wall,
电子撞击的位置会发亮
the place where the electron hits lights up.
要怎么做出一堵碰到电子就会亮的墙呢?
How do we make a wall that lights up on contact with electrons?
这个物理过程分为两步
The physics is a two-step process.
第一步 找到个聪明的实验专家
First, find one of those clever experimentalist people.
第二步 请他们用实验物理“魔法”帮帮你
And two, ask them to use their experiemental physics magic to help you.
简单吧?
Simple.
实验原理(来自理论物理学家)
.
重点在于我们知道粒子从哪儿出发
The point is we know where the particle starts
通过某种方式 我们能测出它最终到了哪儿
and somehow, we measure where it ends.
但在中间过程 我们没有测量它
But in the middle, we don’t measure it
也希望没有别的东西在测量它
and hopefully nothing else is measuring it either.
然后在这之间放一个屏障 上面开两扇小门
Then put in a barrier with two tiny doors in it.
显然 朝这个方向发射的电子
Obviously there’s a good chance
很有可能没法从门里穿过去
to the electron fired towards this barrier doesn’t make it.
我们把没通过的电子全部忽略
But we’re going to ignore any that don’t get through.
那些通过了的电子
The electrons that do get through
必然是从门里过去的
must gone through via the doors.
按照之前的期望 电子是从其中一个门过去的
We’d expect that it went through one of these doors,
只是我们不知道是哪一个门
it’s just that we don’t know which one.
但我们会发现 当电子没被任何东西测量时
But we’ll see that, while the electron isn’t being measured by anything,
情况并不是我们期望的那样
this isn’t the case.
它并不是从单独的一个门里过去的
It doesn’t go through a single door.
为了证明这一点 我们先预测一下
To show this we’re going to predict
如果真像如我们所想 电子会在哪儿着陆
where the electrons would have landed if they acted as we expected
然后我们会发现这与实际的实验结果不同
and see that it’s different from the actual result of this experiment.
要做预测的话 我们得先知道
To get our prediction, we need to know
如果电子确定只从单个门穿过 它会如何运动
what an electron that definitely went through only one door would do.
所以在实际做实验时 我们先关上一扇门
So let’s close one door and actually do this experiment
你可以看到 电子从剩下这扇门中穿过
As you can see,electrons going through this door
最终堆积在这扇门后
end up in a pile behind the door.
当然如果我们开另一扇门来做这个实验
And of course if we had done it with the other door open instead,
电子同样会聚集在那扇门的后面
then similarly electrons would end up behind that door in a bunch.
所以如果我们知道电子从其中一扇门穿过
So if we know an electron goes through one of the doors
那我们就能知道它大概会在哪儿着陆
then we know approximately where it will land.
现在假设两扇门都打开
Now say both doors are open
我们预测每个电子都只从一扇门中穿过
and we predict each electron has to go through one door.
我们会确保电子的发射速度足够慢
We’re going to make sure that the electrons are fired slowly enough
以免不同的电子同时穿过屏障
so that different electrons can’t go through together
而互相干扰
and then interact with each other.
现在我们预测一部分电子会从这扇门穿过
Now we predict that some of our electrons will go through this door.
但根据单缝实验的结论 我们可以知道
But we know electrons going through this door end up here,
从这扇门穿过的电子最终会出现在这儿
because that’s what happened in the single door experiment.
同样地 从另一扇门穿过的电子最后会出现在这儿
And similarly others will go through the other door and end up here.
所以若每个电子只从一扇门穿过
So if each electron goes through just one door,
那么我们就必然会看到两堆电子
then we must get two piles.
然而这个预测完全错了
And yet this prediction is completely wrong.
我们只能承认
We have to conclude that the electrons
每个电子不只从一扇门中穿过
didn’t go through just one door each.
但我知道你们中有些人在想
But I know what some of you are thinking.
那电子一定是同时从两扇门中穿过了
Then they must have gone through both.
换句话说 电子其实是被砍成了两部分
In other words the electron quite literally gets cut into two.
一部分走这边 另一部分走另一边
Part of its mass goes one way and the other part the other way.
然后这两部分撞到了一起
Then these two parts of the object bump into each other
再反弹开来 砸到墙上形成了这个奇怪的图案
causing them to ricochet wildly and the weird pattern
或者根据其它版本的解释
Or in some versions of this explanation
电子一分为二
the particle splits into two
平铺成波扩散出去 互相干涉
and then flattens out into two waves that interfere with each other.
不幸的是 这些解释仍然存在很多问题
Unfortunately there are many problems with these sorts of explanations.
首先若它裂成两半或者扩散出去
Firstly if it splits into two bits or if it spreads out,
当你在墙上测量时
then when you measure it at the wall
你看到的应该是两个点或者很大一片亮痕
you’d expect to get two dots or a spread out smear.
但这都与实际情况不符
That’s not what happens though.
一个电子通过后只会留下干净利落的一点
One electron through gives you one crisp dot.
但有人想用复杂的波理论来解释这一现象
But some people believe a more complicated version of this wave story to get around this.
就是说 电子在你没有看它的时候是波
That goes it’s a wave when you’re not looking
但一旦你开始测量它 它就会变回粒子
but when you measure it, it becomes a particle again.
这就表示 一旦电子撞到后墙
That would mean that as soon as the electron hits the back wall
它就得立即把自己那一大滩零碎
it has to immediately pick up all of its mass
从它散布的所有地方全部捡回来
from everywhere it’s spread out to
然后收拢还原成一个粒子
and gather into a particle again.
但挪动这一大滩东西是需要能量的
But moving mass around takes energy.
电子能从哪儿获得能量呢?
Where’s the electron supposed to get that from?
有一种感觉是波的确在这个实验中出现了
There is a sense that waves are involved in this experiment,
但远比你以为的更不易察觉
but it’s much much more subtle than you may have been led to believe.
无论如何 重要的是
Anyway, the point is
双缝实验告诉了我们一些重要的事情
the double-slit experiment showed us some important things.
当一个物体没有被别的东西观察时
Where nothing watches what an object does,
它并不只做一种选择
it doesn’t just do one of the options.
我们没法直接看到这个过程
We can’t see that directly,
但我们可以从实验结果推断出来
but we can infer that from the experiment.
而且它也不是被物理切分成几部分
And the object doesn’t physically split up
然后每部分各做各的选择
and have parts of itself to do each of these options.
那么在我们看不见的地方它到底在干嘛?
So, what exactly is the object doing behind our backs and
我们又该如何预测这个实验的结果呢?
how are we supposed to predict the results of this experiment?
它们太疯狂了
They’re crazy,
顺便一提 并不只双缝实验的结果出乎意料
And it isn’t just the double-slit experiment that had unexpected results, by the way.
很多其它实验也表明
Lots of other experiments showed that
在我们不做观察时 粒子做了某些很可疑的事
particles are doing something really fishy when we’re not looking.
这就是为什么科学家提出了量子力学
And that’s why scientists invented quantum mechanics.
这个数学理论能很好地解释某个实验现象
It’s some nice maths that you can tell an experiment too
它也会告诉你实验可能的结果是什么
and it tells you what you can expect.
目前为止量子力学都是正确无误的
And so far it’s always been exactly right,
这可是个了不起的成就
which is a grand achievement.
但我们还是想知道粒子到底在做什么
But we still want to know what’s the particle actually doing.
然而量子力学并没有给我们答案
But this quantum mechanics doesn’t tell us.
毕竟它只是一种数学理论
It’s just a bunch of maths after all.
其实有些物理学家认为
In fact some physicists say that
讨论一个未被测量的物体是没有意义的
it doesn’t even make sense to talk about an object that isn’t being measured,
因为它相当于不存在
that it doesn’t exist.
这对我来说太疯狂了
This seems crazy to me
幸亏许多其他人也这么觉得
and thankfully to many others, too.
我愿相信即便我没看着它 月亮依然在那儿
.
最近更多物理学家又开始解决这个问题了
Recently, more physicists have started to tackle this question again
神奇的是 他们甚至开始用更多的数学手段
and they’ve even started to tease meaning out of this mathematics,
来研究这个数学理论的意义
amazingly, by using more mathematics
现在到了激动人心的提问时间了
It’s a really exciting time to be asking questions about this theory.
这是我制作这一系列量子力学视频的原因
That’s why I’m making a series of videos on quantum mechanics.
第一部视频会教你有关量子的定律法则
The first set of videos will teach you the rules of quantum,
内容比较偏数学 但真的不难
which are mathematical, but they’re really not hard
你不用提前学任何数学或物理知识就能听懂
and you don’t need any background in maths or physics already to understand them.
之后我们会涉及很多有趣的话题
After that we’ll cover fun topics
包括最终的量子计算
including quantum computing eventually,
这是我博士正在攻读的方向
which is what I’m doing my PhD on.
下一个视频上传之后
When the next video is up
点击右上角你就可以看到了
you can click here to see it.
如果想知道何时更新 请您订阅本视频
Feel free to subscribe if you want to know when that happens.
同时你也可以试着做做家庭作业
Meanwhile you can try these homework problems
来检测一下你听懂了多少
to see how much you understood from this video.
如果你愿意 把答案写在评论里
Write your answers in the comments if you’d like.
我会看的
I’ll read them
这很有助于让我知道哪部分最难
and it helps me a lot to understand which bits were the trickiest.
第一题 做双缝实验时我们说过
First question. We talked about doing the double-slit experiment
要用很小的物体 比如电子
with small things, like electrons,
但理论上有没有告诉我们
but is there any reason in theory
为什么不能用更大的东西来做这个实验?
that we couldn’t have done it with bigger things
要想实验成功 你该怎么做呢?
and what would you need to do to make it work?
第二题 虽然在实验的中间过程中
Second question. We didn’t measure
我们没有测量电子经过的路径
where the electron went in the middle of the experiment
但这是可以做到的
but it is possible.
那么如果我们要通过测量每扇门
And so if we were to measure each door
来观察电子从哪一扇门穿过
to see which one the electron went through,
我们会发现什么呢?
what would we find
后墙上的图案又会变成什么样呢?
and what would happen to the pattern on the back wall?
最后 双缝实验还有另一种解释
Finally, there is another way to interpret the double-slit experiment,
我觉得很酷炫
which I think is really cool.
叫“导航波理论”
It’s called pilot-wave theory.
我也做了一个相关视频
And I’ve made a video about it.
“导航波理论”认为粒子仅从一扇门中穿过
In that theory the particle goes through just one door.
尽管我们认为这是不可能的
Even though our argument said that that’s not possible.
为我们的观点找到论据
Find the floor in our argument.
同样 对于“导航波理论”中粒子又是为何有奇怪的表现
Also comment on why particles still act strange
欢迎留言讨论
in the pilot-wave theory.
我希望你们对其中几个问题能有点儿头绪
I hope you have a goal at some of these questions
有任何不懂的都可以在下面留言提问
and ask anything that you didn’t understand below.
我们在“玩转量子力学”系列的下个视频中再见
I’ll see you in the next video of the Understanding Quantum Mechanics series.

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视频概述

本视频借助经典的双缝实验,对量子力学的一条基本规则做了简单介绍。课后题目:1. 可不可以用更大的物体做双缝实验,要想成功应该怎么做?2. 双缝实验中,如果观测电子如何穿过屏障上的门缝,会发生什么,后墙上的图案又会变成什么样?3. “导航波理论”认为电子只穿过了其中的一个门缝,而我们讲到的理论不这么认为,请找出论据。

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视频来源

https://www.youtube.com/watch?v=8Dso6Fv1FUw

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