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• 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.

Hi. It's Mr. Andersen and this AP Physics essentials video 13. It is on wave-particle

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# 波動粒子の二元性 - その1 (Wave-Particle Duality - Part 1)

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羅紹桀 に公開 2021 年 01 月 14 日