Hello everyone. I'm really excited to be with you in the journey of relativity and cosmology. First of all, we all know that we exist in space and time. But have you thought about what do we mean by the words in here. What do we mean by existence? What do we mean by space? What do we mean by time? Actually, nowadays, in modern physics, we do not fully understand what do we mean by existence, space, and time, but I promise you that we have understood much more compared to hundreds of years ago. Back to hundreds of years ago, back to Newton. What do we mean by space and time? Space and time were absolute in the concept of Newton. They are something that cannot be changed. They existed there absolutely. However since 100 years ago everything has changed. Since 100 years ago, we understood special relativity. In special relativity, space and time appearing to look two different things were put together. When we are moving with a constant velocity, it appears to us that space and time can project into each other, just like when we are rotating a object in our three space dimensions. This is special relativity, that space and time become together, one object that can transform into each other. Further, we have general relativity that space and time are not only one object unified, but also it is dynamical. Dynamical means that space and time, can be affected by matter. Matter can curve spacetime and gravity is interpreted as how matter moves in extremal, for example, shortest distance in this curved space and time. This will give us a theoretical ground to further study what is our universe and how to understand the matter in the universe, the evolution of the universe, and that is cosmology. Further along this journey, we hopefully in the future, understand better about space and time. For example, think about if space and time are fundamental constructions in nature, or they are emergent in a sense that they are made of more fundamental building blocks. Are they? we don't know. That corresponds to quantum gravity, a emerging field in the past decades which still people are actively studying. This is about the whole journey of space and time, starting from 100 years ago to the frontier of our research. In our course, we will cover these parts, starting from special relativity to general relativity and eventually to cosmology. In the first part of the course, we will start from understanding special relativity. What shall we learn? After we understand special relativity, we will understand a few things. First let's, imagine a scenario. Imagine that Alice is moving in a spaceship, and the speed of the spaceship is very close to the speed of light, and Bob is standing on the ground. In this scenario, what happens to Alice and Bob watching each other? For example, what will Bob observe about Alice? After learning special relativity, you will understand that actually, Bob will see that the time of Alice will slow down and everything that Alice is doing, for example, the heart rate, the watch, et cetera. Every consistent clock of Alice will slow down according to Bob. This is not understandable in Newton's concept of time because if time were absolute, then Alice and Bob must have the same time so nobody will slow down. But I will show you that in special relativity Alice indeed slow down as watched by Bob and Bob will slow down as watched by Alice and how that happens, how that is consistent. What about space? Similarly, Bob will find Alice's space. For example, Alice's ruler that contracted and Alice similarly, see Bob's ruler get contracted. This again is not understandable in Newton's absolute space and time because absolute space means Alice and Bob has the same notation of space which is not the case here. Another amazing thing is what if Alice, is going out for a trip and returns back and the whole trip takes one year? In this case, what Bob considers, Bob will consider that the whole trip takes 10 years. How would that be possible if space and time were absolute? Is not possible if Newton's absolute space time, but you will see it is the case in special relativity. Also, imagine that if Alice is already pretty far away and Alice and Bob wrote letter to each other at about the same time. In this case, there will be an argument in special relativity that Alice considers she wrote the letter first and Bob considers he wrote the letter first. How would that be possible? In Newton's mechanics, there must be one person who wrote the letter first absolutely, but we will see that in special relativity what actually happens when the motion is pretty first close to speed of light, both of them can be correct in the sense that they do not have to agree in the notion of the same time. Finally, acceleration. If Alice want to accelerate her spaceship, according to Bob, can Alice's spaceship exceed the speed of light due to acceleration or not? Bob will see that no matter how hard Alice is trying to accelerate her spaceship, how fast the fuel in the spaceship is burning. The spaceship only accelerate a little bit, is not going to exceed the speed of light no matter how hard Alice tries. Again, this is very different from Newton's mechanics and gave us new views about what is energy, momentum, acceleration, et cetera. This is what we will learn in special relativity.