# Understanding Quantum Mechanics

Quantum mechanics is supposed to be our best theory of physics
That's because it can, tell us what will happen in any experiment that we've tried
But science is also meant to explain, why things happen
Quantum mechanics doesn't give us why because, we still don't understand how, to interpret its meaning at all
For example when an object starts at. A and ends up at b does quantum mechanics say that it goes all possible ways at once
Or that there are many copies of each object in many different universes each going a different, way
Or, does it say that it doesn't even make sense to talk about something when we're not looking at it
These are all possible ways to interpret quantum mechanics at least on face value
In this video we're going to look at the basic rules of quantum mechanics to understand why its meaning is so evasive
We're, about to look at an experiment that showed that nature has a very strange rule
Objects act differently when they're not being measured
Before we get to that though?
We need, to understand when objects are and aren't being measured so let's look at
This, familiar situation, you've lost an object but, you're sure you left it either here or here
You're, not currently, measuring its position yourself because you're, not looking to see, where it is
However consider that there are many other things that are measuring it for example think of all the light particles bouncing
off it right now they're essentially taking a photo of where your object is
What about the air molecules that would have gone straight through
Where your object is if it wasn't there but are now bouncing off that position they're
also, measuring your particles real position, and that's how it usually is
Objects are constantly being measured, by other things so then this statement that objects act differently when they're not being measured
Doesn't apply for everyday objects which is why, we didn't notice this rule for such a long time
But then scientists started to look at very small things like
electrons
Things like air molecules and light aren't constantly hitting them because they're small enough to sort of fit in the gaps so what exactly
Do objects. Like that do when no one is watching
let's get back to the scenario, where our object so in this case our electron is
Definitely in this spot or this spot but, we now know. That nothing is measuring it?
We expect that objects can, only be in one place at a time
Even if you, don't know, where that place is you know it's in one of the places you just don't know
Which and this is usually true?
But in the case, where nothing at all is measuring our object like our electron the following experiment will show
Us that this isn't true the electron can't be at one place at. A time, while it's not being measured
This experiment is called the double slit experiment. And here's how. It goes we're. Going to fire electrons one at a time from here
When they get to this back, wall the place where the electron hits lights up how
Do, we make a wall that lights up on contact with, electrons the physics is a two-step process
First find one of those clever experimentalist people and to
The point is we know, where the particle starts and somehow, we measure where it ends?
But in the middle, we don't measure it and hopefully nothing else is measuring it either
then put in a barrier with, two tiny doors in it
Obviously there's a good, chance that an electron fired towards this barrier doesn't make it but we're going to ignore any that don't get through
The electrons that do get through must have gone through via the doors
We'd expect that it went through one of these, doors it's just that, we don't know, which one?
But we'll see that, while the electron isn't being measured. By anything this isn't the case it doesn't go through a single door
To show, this we're going to predict
Where the electrons would have landed if they acted as we expected and see that it's different from the actual result of this experiment
To get our prediction, we need to know, what an electron that definitely went through only one door would do
So let's close one door and actually do this experiment
As you can see electrons going
through The store end up in a pile behind the door and of course if we had done it with the other door open instead then
Similarly electrons would end up behind that door in a bunch
So if we know an electron goes through one of the doors then, we know approximately, where it will land
Now, say both doors are open, and we predict, each electron has to go through one door
We're going
To, make sure that the electrons are fired slowly enough so that different electrons can't go through together and then interact with, each other
Now, we predict that some of our electrons will go through, this, door but we know electrons going, through this?
Door end up here because that's what happened in the single door experiment and
Similarly others will go through the other door and end up here
So if each electron goes through just one door then
We must get two piles and yet this prediction is completely wrong?
We have to conclude that the electrons didn't go through just one door each
But i know, what some of you are thinking?
Then they must have gone through both in other words the electron quite literally gets cut
Into two part of its, mass goes one way and the other part the other way
Then these two parts of the object. Bump into, each other causing them to ricochet wildly and the weird pattern
Or in some versions of this explanation the particle splits into two and then flattens out into two waves that interfere with
each other
Unfortunately there are many problems with, these sorts of explanations
Firstly if it splits into two bits or if it spreads out
Then when you measure it at the wall you'd expect to get two dots or a spread out smear
That's not what happens though?
one electron through, gives you one crisp dot
But, some people believe a more complicated version of this wave story to get around
This, that goes it's a wave when you're not looking but when you measure it it becomes a particle again
That would mean that as soon as the electron hits the back, wall it has to immediately pick up all of its
Mass from everywhere it's spread out to and gather into a particle again
But moving, mass around takes energy
Where's the electron supposed to get that from
There is a sense that waves are involved in this experiment but it's much much more subtle than you, may have been led to believe
Anyway, the point is the double slit experiment showed us some important things, where nothing
Watches what an object, does it doesn't just do one of the options, we can't see that directly but
we can infer that from the experiment and
The object doesn't physically split up and have parts of itself to do each of these options
So, what exactly, is the object doing behind our backs and how are, we supposed to predict the results of this experiment they're?
Crazy, and it isn't just the double-slit experiment that had unexpected results
by the way loads of other experiments showed that particles are doing something really fishy, when we're not looking and
That's why, scientists invented quantum mechanics it's
some nice maths that you can tell an experiment to and it tells you, what you can expect and
So far it's always been exactly right which is a grand achievement
But we still want to know, what's the particle actually doing
But this quantum mechanics doesn't tell us it's just a bunch of maths after all
In fact some physicists say that it doesn't even make sense to talk, about an object that isn't being
Measured that it doesn't exist
This, seems crazy, to me and thankfully to many others too
Recently, more physicists have started to tackle this question, again, and they've even started to tease meaning out of this mathematics
That's why i'm making a series of videos on quantum mechanics
The first set of videos will teach you the rules of quantum, which are mathematical but they're really not hard and you don't need
Any background in maps or physics already to understand them
After that we'll cover fun topics including quantum computing eventually which, is what i'm doing my phd on?
When the next video is up you can click here to see it feel free to subscribe if you, want to know, when that happens
Meanwhile you can try these homework problems to see how
Much you understood from this video
Write your answers in the comments if you'd like i read them and it helps me a lot to understand which bits were the trickiest
First question, we talked, about doing the double slit experiment with, small things, like, electrons but
Is there any reason in theory that we couldn't
Have done it with bigger things, and what would you need to do to make it work
Second question, we didn't measure, where the electron
Went in the middle of the experiment but
It is possible and so if we were to measure each door to see, which one the electron went through what
would we find and what would happen to the pattern on the back wall
Finally there is another way to interpret the double slit experiment, which i think is really cool it's called pilot-wave theory