Hi. It's Mr. Andersen and this AP Physics essentials video 13. It is on wave-particle
duality. And before we talk about that we should identify what a particle is and what
a wave is. They are both ways that we can transfer energy from one point to another.
And so they both would allow me to knock this can over. Let me use a cannon ball first.
And so that is a particle. I am transferring energy through a particle from point A to
point B. The cannon ball is going from one point to another and I have knocked the can
off. Now how else can I knock it off? Using a wave, well you see that there is a chain
here. I could add energy to it at one side. That would create a wave that would knock
the can off. Now you can see the chain is not moving. That the wave continues to move
back and forth. It is transferring energy through the medium. And so objects can either
be particles, like a baseball or they can be waves, like sounds waves. You are listening
to sound waves right now coming from the speaker on your computer or the speaker in your headphones.
How did it get there? Well I vibrated the air. My microphone picked up on that. And
then your speakers vibrate in the air. But it is not like sound is moving from me to
you. It is just vibrating the air. And so what scientists have found is that there is
this duality. Particles act like waves and waves act like particles. And in this video
we are going to talk about how particles have wave properties. And this was first seen in
the double slit experiment, which is kind of counter-intuitive. But it will show you
what is going on in the world of the very small. The world of quantum mechanics. And
so let's set this double slit experiment up. What we have are two slits on this side. And
what we are going to do is shoot particles at it. So you could imagine that we are shooting
baseballs or bullets or marbles or even spray paint that are going through those two slits.
And so what do you think will happen? Well those particles are going to move through
those slits and they are going to hit some kind of a screen. Now what do you think it
is going to look like on that screen if I turn it so you can see it? Well you probably
guessed this. In other words the particles are not all moving in a straight line. But
they are moving through the slit. They are hitting this screen and then we are seeing
that if we turn it sideways. And so what do you think happens if we were to close up one
of these? Well the particles are not going to make their way through. If we close this
one what is going to happen? Particles are not going to make it through there as well.
And so if we remove both then we are back to where we started. So you should not have
learned anything new. This is how the world works. Now let's say we are using waves now.
And so imagine instead of shooting spray paint through these slits we now have got a wave
tank where we are generating waves on the left side. Those waves are moving through
the two slits. And then we have detectors on the screen that are showing us what is
the amplitude of those waves. How much energy is being transferred through? So if we kind
of simulate that it is going to look like this. And so what we are seeing is interference.
You can see that as those waves move through we are increasing the waves in some places,
decreasing the waves in some places. And what you see is an interference pattern that would
look like this. Because those are moving as waves they can interfere with each other.
And so you do not have two discrete units. That is how waves look in a double split experiment.
And so then scientists said well let's look at something very, very small. Let's look
at an electron, which we tend to think of as being a particle. And so let's shoot particles
through this double slit experiment and see what we see on the other side. So you might
image they are going to move through. If we turn it towards you might think it would look
just like those marbles or that spray paint did. But what you see is an interference pattern.
So we see these things that we think of as particles and they are starting to act as
waves. They are interfering with one another. And so scientists thought well maybe they
are all going every which way. And so they are interfering with each. What if we just
shoot them one at a time. So let's just shoot a few electrons at a time. And what they find
is as they do that over minutes and minutes and then hours and then hours, what is happening
is this interference pattern starts to emerge. And so this is crazy to scientists. So what
is going on. Are they somehow interfering with other electrons in the future or in the
past? Are they somehow dividing in half and then interfering with each other, those two
parts of that electron? And so scientists want to test that. So what they do is they
observe it. In other words they put flashes of light that are going to be released when
they incident with an electron and they see what happens these to see which one of these
holes is the electron going through. And as they start to observe it something even weirder
happens. It goes back to acting like a particle again. And so again, what is this showing
us? That these particles that we think of as discrete little units are actually behaving
like waves. And it opens up this world of quantum mechanics. And so you could do this
if you wanted to. This works with any kind of a light source, like a laser light source
where it is moving in one coherent plane. And so what you could do is just shine a laser
through these two slits. You could do this is a physics classroom. And what you will
get is this diffraction pattern on the other side. It is something that we can observe
but it is being created at the quantum level. And what do I mean by that? Well we live in
the world classical mechanics. We live in a world where everything travels much less
than the speed of light. And things are much larger than the size of an atom. And so we
live in this world of classical mechanics. That is mostly what you are going to learn
in Physics I and II. But know this, that as things get really really fast we have to adjust
and use relativistic mechanics. And as things get really, really small we have to use quantum
mechanics. And so the double slit experiment does not work in our classical mechanics world.
But it does work in the world of the very very small. And so did you learn this, how
classical particles like electrons and photons can have wave-like properties? I hope so and
I hope that was helpful.