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
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,
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:
这个a b和c只是一些数字 下个视频我会讲
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
That means the wavefunction
for that particle is a superposition of going through door 1
and door 2.
因此 从下一个视频起 我们开始看到
As we ’ ll start to see from the next video onwards,
this wavefunction idea is incredibly
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.
因此 如果我们说 粒子同时进行着所有可能行为 这种话就很草率
That’s why it’s very sloppy to say thingslike: the particle is doing all possible things
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
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
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
I had so much fun reading them,
and I was so proud you put in the time and thought to
do that- thank you.