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- [Voiceover] So let's think a little bit about waves. The classical notion of what a wave is, is it's a disturbance traveling through a medium. Disturbance traveling through a medium. So what do we mean by disturbance and what do we mean by a medium? We could come up with some fairly simple examples. Let's say that I have a little puddle here. So that's my little puddle. And let's say I were to drop a stone in that puddle. So this is my stone. It gets dropped in the puddle. We've all seen what happens. Right when it goes into the water, that pushes that water down, pushes the water around it out, then that water comes back up. Then you have this traveling disturbance. You have this traveling disturbance. These kind of rings that start to radiate outward from where I actually dropped the pebble. You can actually see them move outwards. You can see the crests of those waves move outwards. We're talking about a disturbance. Well this is that disturbance. The pond was completely flat, but then we agitate it. We pushed it down. It pushed the water outwards. Then that pushed the water next to it up and down, up and down. It traveled outwards from the point of that initial disturbance. We have this disturbance traveling through the medium. What's the medium here? The medium here is the water. The initial disturbance was the rock disturbing that water. But then that water disturbed the water around it, which disturbed the water around it. That disturbance kept traveling through the medium. We can give other examples of this. If I take a really long string and hold it right there. That's my hand. The string is, let's just say, attached to something. Let's say it's a really long string and right now there's some slack in it. It's attached to a wall. Let's say that I were to start moving my hand up and down. So I move my hand up and down. What's going to happen? If I just did it once, what I'm gonna have is this lump of string move from the left to the right. So that's what it's gonna look like at first. Then a few seconds later, it's gonna look like this. It's gonna look something like this. You're gonna have this disturbance, which is this wave, this lump that I just generated. It is going to move to the right. What's the medium? The medium in this case is the string. It is the string. What just happened there? When I jerk the string up, I'm disturbing those string molecules right next to my hand. They're pulling on the string molecules next to them pulling on the sting molecules next to them. Then you have this traveling lump. Once again, these are both examples of waves. Disturbances traveling through a medium. Now I can give other through a medium. I can give other examples. Sound waves. What causes sound waves? You have a bunch of air particles all bouncing around at actually surprisingly fast speeds. But if you disturb them, if you cause something to, say, compress a bunch of air particles right here. These air particles get ultra-compressed. Some type of clap happens, so these get ultra-compressed. Then they're gonna bounce on the ones next to them. Once again, you have this disturbance traveling through the medium. In this case, the medium is the air. The medium is the air. So, now that we've seen some classical notions of waves, let's think about something a little bit more mysterious. That is the notion of light. Light definitely has wave like properties. Wave light can interfere with each other. We can do a whole other videos on light. If I were to take a barrier like that with two small slits in it. If I were to shine some light, one way to think about it is these slits are the only place where the disturbance gets through and then it would cause the light to propagate from each of them like that. Maybe I'm just drawing the crests of the waves, but also come out from here. You can see where the two crests meet. They're going to constructively enhance each other. They're going to constructively interfere. If you were to put some type of a detector right over here, you would see the bright points of the light and you would see the dark points of the light. So light definitely behaves like a wave. I'm just showing you even one example of light behaving like a wave. But if it is a wave, that means it needs to be or if we use our classical logic, you would say, well that means it's a disturbance traveling through a medium. But what is that medium? We have light coming from our sun. We call it sunlight. We have light coming, I keep wanting to make it a - actually our sun is white, we tend to draw it as a yellowish color, but that's just because of its - what happens is it goes through the atmosphere. So you have the sun, you have the earth. This is not drawn to scale. Somehow that light is able to reach us over 93 million miles. What's it going through? Is it a disturbance traveling through a medium? Well, for a long time, people said, well it must be. It has these wave-like properties. It seems to be traveling with a velocity, the speed of light. So there must be medium that it is traveling through. People theorize what this medium is. They called it the luminiferous ether. Let me write that down cause it's a fun word. It's a good name for a band. Luminiferous. Lumine, no I'm not spelling it right. Luminiferous ether. It was this idea that maybe, even in the vacuum of space, you have this substance. This luminiferous ether that the light is traveling through. That it's somehow a disturbance in that luminiferous ether. Now, what we're gonna do in future videos, is we're gonna test that. We're gonna see if that's actually the case because what's interesting about if there is a luminiferous ether, we're not gonna be stationary relative to that luminiferous ether. In fact, we're orbiting around the sun. The sun is orbiting around the center of the galaxy. There's no way that we're gonna be stationary relative to that luminiferous ether. If we're not stationary relative to that luminiferous ether, we should be able to detect how light behaves. If it's going in the direction of ether or if it's going against the direction of the relative movement of the ether relative to us. So anyway, I'll leave you there. This mystery of science. Waves, disturbance through a medium. We saw that with the water, with the air, with the string. But what about light? Is there a luminiferous ether? Put a question mark there.