 Hi, I'm Zor. Welcome to a new Zor education. Today we will continue talking about units of measurement in physics. And obviously, the most important system of units is called C, system international knowledge, something like this. In the previous lecture, we have explained how a unit of time, a second, can be defined using certain fundamental constants, which basically occur in nature and seemingly constant, not changing with time. That was resonant oscillations of atom of cesium, cesium 133, cesium or cesium. Now, that was the time. Now, today we will talk about measurement of the lengths in the unit and how it was derived. Now, this lecture is part of the course called Physics for Teens. It's presented on a website, Unizor.com. Website is totally free. There are no advertisements. Everything is basically completely at your disposal. You can use it as much as you want. There is a prerequisite course on the same website called Math for Teens. Math is absolutely necessary for studying physics, especially calculus. So, what else? Yeah, there are notes for each lecture on this website. So, if you watch this lecture from, let's say, YouTube, you don't have this benefit. But if you do it from the website, you will have basically the textual part, like a textbook, which is at the same time you can basically read it. And for the same topic, whatever is in the lecture, there is the same thing in the text. Maybe slightly different. Maybe with some more details. Plus, the website contains many tasks, problems, exams, which you can take as many times as you want to get the perfect score. And there are other functionalities of the website. We are not talking about this right now. So, back to unit of lengths. Lengths. Now, this lecture and all other lectures dedicated to units will be very short. So, you don't need a lot of patience. Just 10-15 minutes concentration. So, how did people measure lengths? Well, the most important unit of measurement of lengths in the world, which is part of the system of units called C, is meter. Well, in the United States, we are using less the meter and more something like a foot or a mile. Everywhere else in the world, meter is really de facto standard. So, I'll talk about how meter actually came to be and what is exactly the definition of the meter. Well, one of the first definitions was something like one, 10 million, 10 millions part of the distance between the North Pole and equator. Well, it was decent definition, but obviously it's not really precise, as we understand. Now, then, somewhere in 19th century, they have decided to make a meter. So, they made a rod made of the alloy of platinum and aridium, which they were thinking is having the lengths exactly one meter. It was made of this alloy and it has a very peculiar section, something like this. That's the section of the alloy and it's about a meter and it's exactly a meter, as they understood the meter at the time in 19th century. They put it in some laboratory in France. I think the city is called Severs near the Paris. And so, there was some kind of an organization which was supposed to take care of basically this thing, to store it in constant condition, temperature, humidity and whatever else. Well, about 100 years later, people realized that, well, no matter how precise we have made this particular meter made of platinum and aridium, it's still changing with time and it's not good for definition of what actual meter is, considering the precision which we really have to make right now. Okay, so, they have decided and that's the general direction of all the definitions of all the units in C. They decided to hook it with some kind of a natural physical constant and the choice was the speed of light in vacuum. Now, we have already defined a second as a unit of time. So, now we can define if the speed is defined, the speed is basically distance per unit of time and the time is already defined one second as in the previous lecture we really talk about. It's a certain number of oscillations of atom of caesium 133. Well, actually, I have this number here, something like 9 billion 192 million 631 770 oscillations. So, considering the oscillations is something like a natural constant under specific oscillations of atom when it goes from one state to another. So, that was considered to be a constant to, well, the precision which probably we cannot exceed at least right now. So, the second was defined. Now, the speed of light was measured many times and approximately it was 299,792,458 meters per second at the time. Well, obviously, again, it was not exactly this because meter was, you know, not exactly a precise kind of measurement at the time, but that was one of the latest. So, what physicists decided to define meter as such a distance which if used to measure the speed of light this number will be exact. So, in one second light would cover exactly this number of meters and that's basically definition of meter which means meter is the distance which light covers in this many seconds, 58. So, one over this number, piece of a second, fraction of a second is the distance which is equal to one meter by definition. So, the speed of light is this exactly. So, that actually made a meter to be, well, basically based on some physical natural constant which we think is constant, speed of vacuum, speed of light in the vacuum. So, now it becomes kind of objective. You see, before the definition of the meter was this particular road, metal road which they have made. That's our making, it's subjective definition of meter. We, the people, decided how it is. And again, it was not really precise because it's changing with the time no matter how well you preserve it. Now, this definition is not changing with the time, hopefully. At least as far as we understand, tomorrow or a year after the speed of light will be exactly the same as today. So, that's why connecting the lengths with some objective natural physical constant is definitely preferable way. Exactly the same as we defined the second in the previous lecture. Again, it's not something which we have built like a clock or we have observed that the moon is 28 days or something like this. No, this is all changing things but something which is not changing according to our understanding of nature right now. The oscillations of atom of season when it goes from one particular state to another. That seems to be objective and it seems to be constant. Same thing with the speed of light. So, again, the whole tendency of changing the units of measurement and that was actually quite recently. I mean, things changed in 2018, 2016. I mean, it's quite, within the last 10 years, even less. So, these definitions are moving towards more objective, more physical constant-related values which supposedly not change, not changing with time. Okay, so we have defined meter. Now, obviously, we have kind of derived units from meter fractions like decimeter, which is one tenth centimeter, which is one hundredths of a meter. Millimeter, which is one thousandth of a meter. Then what we have after millimeter, we have micrometer, which is one millionths. Then nanometer, one millionths and picrometer is 10 to the minus 12. So, one over 10 to the 12th degree of a meter. As well as the multiple units, decimeter, hectometer and kilometer. So, decimeter is 10 meters, hectometer is 100 meters and kilometer is 1000 meters. Although, personally, I never used deco and end-hecto. So, kilometer is more kind of accepted measurements. Now, in the United States of America, meters are used mostly for technical and physical kind of research and other things. But at the same time, quite popular are feet and miles and inches as fractions of the food. Now, it's still kind of tradition and Americans try to preserve this particular tradition, although most other countries actually switch to metric system with meters. Not always, by the way. For instance, in Great Britain, you will probably have both metric system in some places and old English system in some other places. Although the tendency is towards metric system, it seems simpler and generally acceptable. So, well, that's it for measuring the lengths. It's a short lecture, as I was telling before, and I will have a few more lectures for different measurements of different physical quantities. So, thanks very much and goodbye.