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#### 《理解量子力学》#2 波函数

The Wave Function | Quantum Mechanics 2

Last time we met quantum mechanics,

and got aquatinted to the two rules of the theory.

If you haven ’ t seen that introduction video yet,

do go watch that first.

In the next few videos I ’ ll

teach you how to use those rules properly and then we can

start applying it in awesome ways.

Last video we talked about the double slit experiment,

and I ’ ll just remind you of

the moral of that story.

We learnt that, for whatever reason,

objects seem to act differently when they ’ re not

being watching them compared to when theyare.

Say a particle has several possible statesit could be in.

Normally we ’ d think,

regardless of whether its being watched, it must act normal- doing
——处于那些可能性中的一种
just one of those things,

but this experiment showed us that that can ’ t be true.

What we don ’ t know though is…

what exactly are they doing behind our backs?

Quantum mechanics deals with this by basicallyside stepping the issue.

It totally gives up

on talking about what the object is actually doing all together.

Instead it only talks about something called the wavefunction associated with that particle.

What is this wavefunction? Umm,

well.

Let me tell you what it does first.

To do that,

we need the superposition principle- which I ’ ll explain more precisely this time.

Going back to this situation, this rule tells us how the wavefunction looks.

It says the wavefunction is in each of thepossible states.

We write that like this,

the wavefunction equals the particle ’ s state in option 1

plus in option 2 etc:

That a, b, and c are just some numbers- and they ’ re the subject

of the next video but we’ll ignore themfor now.

The important part is:This wavefunction, that is somehow associated

with the particle,

contains each of the possible things states the particle could be in.

For example, if we looked at the double slit experiment,

we don ’ t know which door the

particle took.

That means the wavefunction

for that particle is a superposition of going through door 1

and door 2.

As we ’ ll start to see from the next video onwards,

this wavefunction idea is incredibly
——我们可以用它预测
useful- we can use it to predict the outcome

of any experiment done so far, and it ’ s

the reason we get all the quantum phenomena

that you may have heard of, like entanglement,

the Heisenberg uncertainty principle etc.

It’s very useful… but hopefully it’sa little unsatisfying.

What does this wavefunction tell us about what the particle is doing?

There are plenty possible interpretations.

Maybe it means that while we aren ’ t looking,

the particle really does split up and do all

the possible things in its wavefunction.

Or perhaps the particle really only does one

of those things- but unlike in classical physics,

it still cares about the other options init’s wavefunction.

Or maybe, the particle doesn ’ t really exist

at all until it ’ s measured.

We don’t know.

That’s why it’s very sloppy to say thingslike: the particle is doing all possible things
——事实上我们没有证据粒子在做什么
at once- actually we don ’ t have a clue what the particle is doing,

in quantum mechanics

we only know about the wavefunction.

Another example of this sloppiness is when people say the particle is a wave.

What they really mean is that the wavefunction looks wavelike in some circumstances. Again,

we don ’ t really understand what that says

about the particle, because we don ’ t

really know how the wavefunction is relatedto the particle.

The crazy thing about quantum mechanics is,

we don ’ t need to understand it to be able

to use it.

In fact, some people think it doesn

’ t make sense to talk about the meaning of quantum

mechanics at all, because anything said would just be speculation with no evidence.

But actually we ’ ll see that by very carefully studying quantum mechanics,

people have derived

at least some concrete things about the meaning- even

though we ’ re still a long way from
——如果我们能理解的话
understanding quantum mechanics- if it’seven possible at all.

And with that, I ’ ll leave it for today,

but it ’ s time for your homework!

For the first task, I’d like you to justspeculate.

For example, make up some interpretations

for the wavefunction or tell me what you think

are nice or not nice interpretations for thewavefunction.

What does nice mean to you?

The second question is,

do you think we should even worry about the interpretation of scientific

theories?

Isn ’ t it

enough that the theory makes the right predictions- even if we don ’ t understand

why it works?

Can questions about the meaning of a theory be considered scientific questions at all?

Feel free to bring up examples etc to back up your answer.

Even if you only have partial answers or you want to ask a question,

feel free to leave

it as a comment- I love reading them.

Before I go, can I just say,

I was blown away by how seriously you guys answered the questions

last time.

I had so much fun reading them,

and I was so proud you put in the time and thought to

do that- thank you.