 Hi, I'm Zor. Welcome to a new Zor education. Today we will continue talking about zero relativity and in particular, we'll talk about time. You see, there is a concept of relative and absolute. So let's just talk about these two terms first. What relative means? Relative means relative to something else. Now, what is definitely relative and everybody knows it's relative? Well, position is relative. For instance, I'm positioning at whatever three meters from this room and three meters from this wall, right? From this wall. Now, that's relative to the walls, but I'm positioned, let's say 25 meters above the ground because I'm in the building. So again, it's all quantitative variation of my position are supposed to be evaluated relative to something else. I mean everybody understands it. Now, the movement itself, yes, I am moving with my building where I am around the earth axis and with the earth I'm moving around sun. Now, this marker is moving if I'm doing such a movement, but it's moving relative to the walls of the room. It's not moving relative to my hand. So the movement itself is also relative. Now, but there is one thing which we kind of intuitively accept as absolute, which is time. Now, what does it mean absolute time? Well, if you have two clocks, which are completely synchronous, very good Swiss-made clocks, whatever. And one of them is in possession of the one person, another is in possession of another person, and that other person goes somewhere else, returns back, and the clocks are supposed to be showing exactly the same time, if they're good clocks, obviously. In this particular sense, the time was always considered to be absolute. So no matter how the clocks are moving or stanging or not moving, whatever, they are supposed to show exactly the same time. Okay. That's understanding, and based on this understanding, Newton basically had created the whole mechanics, and it was very fine, completely corresponding to experiment, etc. So from this grew a relativity principle. Principle of relativity, and I think we were talking about this before, basically states that if you have two reference frames, two systems of coordinates, if you wish, and one is moving with the same speed, with the same direction, relative to another, and they are inertial systems, which means there are no external forces acting on them. There should be no experiment which is conducted inside each of the system that show that this system is moving and this one is standing or vice versa. So all the laws are supposed to be the same. So if you are in the, let's say, in the train, inside some kind of a cabin, no matter what experiments you are conducting. Now, what if the train is moving with some constant speed relative to another person who is stanging somewhere on the ground and looks at this train? So no experiment should be conducted, no experiment is possible to conduct. Inside of this train, that show that the train is really moving. Unless he sees it in the window, etc. So it's a closed car or whatever. So that was a principle of uniformity, if you wish, of all the inertial systems. And that worked fine. And in this principle of relativity, worked fine. Everybody understood it, everybody agreed with it. Now, there was a problem. The problem was that number one Maxwell equation showed that the speed of propagation of electromagnetic oscillations, including light, is constant depending only on the media. Let's say it's in the vacuum. So it's always the same in the vacuum. So no matter whether the source of light is moving or standing or moving with the same with this speed or moving with that speed, different speed, different direction, no matter where it is, the light is always moving with the same speed. Now that was actually that led into invention of such an artificial object as either. So the light apparently was thinking about is propagating inside the ether, and ether is oscillating, basically. Now, but that was not really an adequate theory. It contradicted many different things. And then not only theoretical results, also experiments results showed that the light really propagates with exactly the same speed. And this contradicts the principle of relativity in Newtonian mechanics. Why? Okay, it's a very simple experiment. So let's consider this particular train is moving with a very high speed. 150 meters per microsecond. I use microseconds, which is one millionth of a second, to be more or less close to the speed of light. Now, speed of light C is 300 millions meter per second, so it's 300 meters per microsecond. Okay? Now, let's say you have the source of light right in the middle of this car or train or whatever, and the distance between this and the back wall is 90 meters and to the front is 90 meters. So 90 meters is the distance which light is supposed to cover when it goes to both directions. Now, principle of relativity tells that it should be exactly the same time to reach the front wall as the back wall, regardless of the speed of the car itself or train. Because otherwise, it would be an experiment which would kind of determine my absolute movement and there is no such thing as absolute movement, movement is relative. All right, so let's just calculate it. If it's 90 meters with the speed of light 300 meters per second, you would have that the T to the front is equal to 90 divided by 300, which is 0.3 microsecond, and time to the back would be the same. Okay. Now, let's talk about this person with stanging on the ground. From his perspective, while the light goes to the back wall, the back wall goes towards the light, and while the light goes to the front wall, the front wall goes further during this time. So let's just determine the time for light to reach both walls from his perspective. Well, let's say T back is equal to what? Well, T back multiplied by speed of light. That's the cover, that's the distance covered by light, which is the initial 90 meters minus the distance the back wall moves towards the light. So that would be 90 meters minus during the same time. With speed V, the back wall moves towards the light, from which TV is equal to 90 divided by C plus V, which is 450, which is 0.2 microsecond. Now, T front, so that's the time for light to reach the front wall. It's supposed to cover 90 meters. Plus, the distance the front wall will move away from the light, which is, and so that would be 90 divided by C minus V. C minus V is 150, so 90 divided by 950 to 0.6 microsecond. Well, that's the difference. So what does it mean? Something which is simultaneous for the person inside is not simultaneous for outside observer. That's a contradiction. It means time is not absolute. So simultaneousness is not really absolute principle, which basically we didn't think so when Newton invented his mechanics. He was thinking that the clocks everywhere are exactly the same and showing exactly the same time. That's what time means, absolute time means. Now we see it's difference. Okay, how about another simple example? What if I will introduce a delay of one second, 0.1 microsecond to this sensor. So sensor receives the light, but then it waits 0.1 microseconds before indicating something. So that would add so that would be plus 0.1, which is 0.4. So now the signal received by the front wall is less than the signal received by the back wall. So this event is, this event is earlier than this event. Right? Okay. T, Tf is less than Tb. Now what happens in this case? From this perspective, this is increased by 0.1. So 0.3, but Tf now is still greater than Tb. This is 0.6 and this is 0.3. So you see there are two different things. In this case the front wall receives earlier, the signal earlier than the back wall. From this perspective the front wall receives later than the back wall. So if one event precedes another for one particular guy, it follows for another. So that's what I mean saying that the constancy of the speed of light contradicts the principle of relativity. So we have to really change the whole concept of time to basically put them together. Because you see we cannot really doubt that the speed of light is constant. Many experiments and Maxwell equations show that this is really always constant. Nothing we can do about it. That's the fact. And principle of relativity is something which physicists believed all the time. This is also the fact. And if they contradict to each other, we need another way of basically approaching the concept of time. Time must be also changing in the system which is moving to basically bring the bring together, bring into correspondence the constant speed of light and principle of relativity. If we want to retain the principle of relativity, which is kind of our understanding of the nature, the constancy of the speed of light, which is a fact. It's not just our understanding. It's a result of experiments. You see the principle of relativity is something we invented. The constancy of the speed of light is a fact which we have confirmed by experiments. So if we want to bring them together, we need something else, not just Newtonian mechanics. And that's what Einstein did in the beginning of 20th century. It shows that time is not absolute and there is a dependency somehow between the time as it's perceived by the person inside and the time perceived as the person outside. So not only speed is relative, the time is also relative. And that was the purpose of the whole explanation. And that's why we can say that answer to this question is the time absolute. We should answer it negatively. And then the subsequent work of Einstein and some other people really brought this theory of relativity in basically in coordination with principle of relativity. Principle of relativity was confirmed and the theory of relativity was also confirmed experimentally. We just need more precise experiments, more precision in our clocks and our speed, etc., etc. to really detect that, you see, this is a very high speed. Now, while this speed was really very small, the difference between the timing was very, very small. I specifically used a very high speed, half of the speed of light for the movement of this train to show that the result difference. Okay, so I suggest you to read the notes for this lecture there on unizord.com. You choose the course relativity for all. This is the menu option Einstein's view. And that's where you will find the lecture about absolute time. So thanks very much and good luck.