 Hi, I'm Zor. Welcome to Inizor Education. I would like to... Basically, it's like the end of the Physics 14's course. It's a topic which I believe is very important. And it's about units. We are measuring all the physical characteristics of certain things, processes or objects in certain units. And I just would like to talk about a few aspects of measuring and units. Okay, now, I'll probably touch most of the units. I mean, not all of them, obviously. But I believe I will just talk about most of the units, explain where they're coming from and what's their base. Okay, so this lecture is part of the course called Physics 14's. I think it's the last topic, which will contain a certain number of lectures for different units. And it's all presented on the website Unizor.com. The same website has the course called Math 14's, which is kind of prerequisite. I mean, you probably... You don't have to learn everything about Math from this particular website. Maybe you know it. That's okay. But in any case, whatever the knowledge is in the course Math 14's is required basically for studying physics, especially calculus or vector algebra. Now, the website is completely free. There are no ads. What actually makes this website kind of stand alone, it has both lectures, which are recording videos basically, and textual part. So each lecture has the textual notes, which basically is like a textbook. You can read it and that would be like learning from the book or you can watch the lecture and that would be like sitting somewhere in a classroom. And probably the best place to do both. So you might start from reading and then watching or watching and then reading, whatever. But they kind of... It's about the same thing. Every lecture and every note for this lecture are about the same thing, but maybe slightly different aspect. It's always useful. In addition, the website contains many problems, which needs to be solved. Some of them are solved in the corresponding lecture. Now there are exams, which you can take as many times as you want until you will get the perfect score. So that's all about the website and now let's go back to our topic. So units in physics. Well, people were measuring different things in a different way since the time in memorial, obviously. Now, everything is fine and certain measures were established like, for example, a foot for the legs in English-speaking countries like Brits, United States, Canada. At the same time, other countries developed other measures and when it was necessary to do something together, they had problems. So let me just exemplify it a couple of times. The legend says, and I'm not sure whether it's right or wrong, the legend says that Columbus, when he basically discovered America, he was thinking that he was somewhere in Asia and that's because, and again, I'm not sure that's true, but that's what legend says, it's because he was kind of mixing Roman miles and nautical miles, which are different, maybe. A little bit more recent event was, I think in 1999, when NASA has lost a Mars orbiter, apparently because certain things were measured in, like, pounds per square inch instead of kilograms per square meters or something like this. And this, well, that's a pressure, basically. And this incompatibility actually caused some particular part designed for certain pressure, but people who designed it were thinking that this is the pressure, the number which was there was expressed in, like, kilograms per square meter. Instead it was actually calculated in pounds per square inch and that was a disaster, basically. So obviously there is a need for something which all the people accept as the system of units and all of them must actually use the same thing. Well, for historical reasons it's kind of difficult. Nevertheless, the RISA system, which is kind of universally accepted, officially accepted, let's put it this way, by practically all countries, and it's called CSI. I think it stands like Systema International or something like that. I think it's in French because it was probably originated somewhere in Europe. In any case, by 1960, actually, this system became official de facto standard. Not that we don't deviate from this system. I mean, wherever you go in the United States you will see, like, speed on the road is measured in miles per hour, not kilometers per hour. Although in Europe it's mostly kilometers. Well, sometimes somewhere in England maybe they still have miles. But mostly Europeans are switched to, they call it metric system. Well, metric system is just part of the sea. Metric system is about measuring the lengths, actually, in meters. That's why it's metric. But there are other things, like kilograms instead of pounds, etc. So there is absolutely no need to talk about necessity to develop the system and the system was developed. Now, what does it mean it was developed? It means that people have agreed that there is a unit of length, which is called meter, which is the unit of time, which is second, which is the unit of mass, which is kilogram, etc. All right, they have agreed. But now, how do we know that my kilogram is the same as your kilogram? Well, apparently at that time they have established certain physical standards, physical standard of a meter, which was kind of a rod made of some precious metal or alloy. The same thing with the kilogram, there was just piece of metal, which considered to be one kilogram, etc. Speaking about seconds, for example, the time, well, people were basically having, all the people have watched that. So somewhere there was the watch, I don't know where, maybe in Paris, maybe in Greenwich, New London, or whatever. So they were considered to be the precise watch, and all other watches have to agree with this. Obviously, these physical objects could not be the real standard for lengths, for mass, or for time, because everything is changing in our world, including, even if it's made of precious metal, it still kind of temperature might deviate a little bit. Well, you can try to maintain proper temperature, but now that your temperature is also measured in something, how do you know that measurement is correct? So we are trying to use our artificial objects to measure something which is supposed to be objective, and at that time it was a very important decision made. Instead of having our objects, real physical objects, like the rod, which has the lengths of, we consider to be one meter, to be standards, we really have to build our standards based on something which exists in nature and seemingly constant. So let's say we have determined that the speed of light in vacuum is such and such. All right, now such and such means what? Let's say kilometers or meters per second. All right, that's fine, but now the question is, how do I measure it next time? Well, next time I will measure it again in meter, where is the meter? The meter is in Paris or somewhere else. Why don't we do the process in reverse? Why don't we say, okay, if the speed of light is such and such, whatever it is. We have calculated today, for instance. It's like 2, 9, 9, blah, blah, blah, blah, blah, blah. I don't remember exactly, meters per second. All right, if this is what we have determined today, now to be completely independent of the future, we are considering the speed of light is this, the definition and the meter is such a length that the speed of light would be exactly this number. So we are kind of postulating the constant itself and from the constant we have the meter. So the meter is 1 over this number of the length which the light in vacuum covers in one second. Now that actually is much more objective definition of the meter. It's 1 over certain number of certain lengths which is a constant, which is the length the light covers in one second. Okay, now there is a problem what is the second and it looks like the second should be defined before and that's actually the second part of today's lecture. So the first part I have just concluded with the idea that our measurement units must be somehow hooked and derived actually from some objective reality. So we have to find some objective reality for time and say that the second is defined by that constant. The question is what's the constant? Okay, so now we are going to the second part of this lecture. Time unit. Now, first we have to find some objective physical constant which exists without our measurement without anything. Well, for historical and some other reasons what has been actually taken as a standard, as a constant from which we can derive the time unit. Time unit is a second. That was decided, the second should be. So we have to measure this constant as we know it right now and that would be some kind of a constant. Then we should really postulate, we should really say that this is the definition of the time of the second and then that would be the definition and that would be a standard from which the second actually can be always checked gates. So what is this physical characteristics? As I was saying for historical and other reasons what they have chosen, they have chosen the metal called cesium. I think it has 55 protons and 133 total number of particles in the nucleus, protons and neutrons. Now, so what about this metal? Well, first of all it's a metal. Well, actually what's very important and kind of very interesting about well, metal usually is hard, right, but you can melt it. The temperature of melting is 28.4 degree of celsius, which is about 84 degree Fahrenheit, which is just slightly more than the room temperature. So just a little bit warmer than a normal room temperature and this metal is melting. This is beside the point, it's just an interesting quality of this metal. It occurs naturally. Now, what about this particular properties which help us to define the time? If you irradiate this atom of cesium with, let's say, microwave, you will excite its electrons and even the nucleus can actually change its spin. But whatever it is, it gets into some state of excitement. As we know, many other atoms actually will do, if you will bombard them with photons of electromagnetic field, for example. Now, after they are excited, at certain moment they go back to normal, to the ground level of their energy. So first they consume energy and they can consume in different chunks, whatever it is, but then they are going back to the normal state and they emit certain energy. Well, when they emit certain energy it's related, if you remember from the discussion about atom structure, the electrons are going to a next level. So if you will take a look at whatever electromagnetic field around this is, you will see that it emits photons. It emits electromagnetic oscillation of certain frequency and that frequency is the property of the element. So different elements, when they go from excited mode to normal, emit certain electromagnetic oscillations of certain frequency which is specific for this element. So they have decided that whatever this particular frequency emitted, frequency of electromagnetic oscillations emitted by this particular atom, when it goes from excited state to normal, that is measured and it was, I don't remember if I have it, it was certain number, I don't remember exactly the number, whatever the number is. So this number is a frequency, it hurts. So electromagnetic oscillations emitted by this atom when it goes from excited state to a normal state has certain frequency. Okay, fine. Now, what does it mean it has certain frequency? It's certain oscillations per second, that's what hurts is. It's one over second actually, it's one over second. Sometimes it's S, sometimes it's SAC, it doesn't really matter. I think standard is one over S, but in many cases people are using SAC for abbreviation. So if this is the number of oscillations which this particular atom makes in one second, as we know about second right now. So what physicists did, they did basically reverse operation. They have said, okay, one second is the amount of time this thing does this particular number of oscillations. So they started from the objective reality, the oscillations of this thing. It has certain frequency which is basically a characteristic of this metal. And they're saying, okay, since we have this frequency, we have these oscillations. Okay, one second is such time when number of these waves is exactly this number. So they have defined this constant as being constant and derived the second from it. So this is extremely important, it's a philosophical actually difference. We do not define subjectively based on some clock or something. What is the second? We are defining this based on some objective reality, oscillations of these things and wavelengths and is basically, everything is constant, whatever the constant is. So whenever we have amount of time during which this number, this number which they have decided as being the constant, the world constant, number of oscillations happened, then this particular time period is called a second. So now we can always reproduce a second by watching this particular atom changing from excited state to a normal state, watching the oscillations of electromagnetic field emitted by this particular transformation and just count, okay, as soon as we count this particular number, that means the time during which number of oscillations is such is a second. And that's what's very important. Now, basically that's all I wanted to say as a kind of preambular to system of units, international system of units, the standard. And again, what's most important is the way how we have decided to define our measurement units. We have decided to define it based on certain objective reality, like for example, the time unit, the second, is defined as the time needed for oscillations of this particular kind to be this particular number. So number is a standard. Okay, now obviously with any particular number, we can divide it into tens, hundreds, thousands because we're talking about international system, so the number 10 is always very important. So as far as the second is concerned, we have defined millisecond, which is one thousandth, microsecond, which is one millionth, nanosecond, which is one billionth, nanosecond, one trillionth, et cetera. Now, there are also some, I would say, non-standard but still common units of time, like minute and hour, minute is 60 seconds and hour is 3,600 seconds. These are also derived units. So it's easier to talk about derived units if the main unit, which is in the case of time, is a second as soon as it's defined and is defined objectively, not based on some clock or rotation of the earth around the sun, for example. Everything is changing. There are not too many really objective constants in the world and one of them is the one which is oscillations, for example, of this particular atom. Probably they could have done some somehow differently, but that's how they have decided and that's how it is the standard. And there are atomic clocks based exactly on this principle which are reproducing these transformations of atoms of cesium and they're very, very precise. Now, I think the latest precision is so we have made a clock which makes a difference in one second in, like, more than a million years, something like this. So that's how precise this atomic clock is. All right, that's it for today. I will continue talking about other items on this, either units of the measurements, but again, this lecture is very important because I wanted to talk about what's the main philosophical approach to define the units of measurements. It's the objective constants which exist somewhere in the world which do not really change. So we are postulating these constants and then we derived our units of measurements postulating the constant to be derived the unit. Thank you very much and good luck.