[MUSIC] Now I'm really excited to introduce to the part of quantum mechanics. Quantum mechanics conceptually is as exciting as special relativity that it's completely changed our fundamental understanding of how things work. And technically it's much more exciting than relativity at least for the moment being because it impacts much greater to modern technology. For example the CPU in our cell phone and computer is made from the principle of quantum mechanics indirectly, not directly but from the principle and more modern technology. For example, quantum information, quantum computing are starting to directly use the consequence of quantum mechanics to further change the world. So this is why quantum mechanics is so exciting. Historically, quantum mechanics follows a very different path compared to the development of relativity. Why it is the case because relativity comes with a minimal set of principles which we are not familiar with, essentially just one which is the speed of light for different observers are the same in the vacuum, okay? For sure there are two different other principles but they are more familiar to us. This is the only thing which is unfamiliar to us which is on the one hand, easy to be postulated by one person, Einstein and another thing is from that everything else is a logical, just following the logic. A decent physicist is able to build up the whole building of special relativity by him/herself. However, quantum mechanics is different. It follows from a number of different principles which are very counterintuitive, which if we are not seeing the experiments we would never have imagined. For example, the world is linear postulated by quantum mechanics that one system can be in superposition of another system. There are just the same system but different states exist together in the sense of superposition and eventually you can measure the consequence of that and even our real world possibly the whole real world is a superposition of different worlds, which we are not sure at this moment. But it's possible and we are one existence, but there might be other existence at the same type, simply we do not know. And the second is probability that there is a big debate between Bohr and Einstein, if the God play dice. If our world is determined from the very beginning, there are probabilities to evolve into different states and so far it looks like from every evidence. That Bohr is right that our world is built on probabilities. We have to rely on the probabilities to get our outcome of experiments and unitarity. That the probability evolves with time. With a reasonable way that the probability is not smaller than one exceeding one, but exactly one. And uncertainty that our classical understanding of the world is, a particle. If I want to predict a particle, I have to know its initial position and momentum. And now in quantum mechanics you are unable to get the full information precisely including the particle's momentum and position. You will have an uncertainty in accessing that information. And they are the fundamental postulates of quantum mechanics. More technically, you can put them into four or five postulates as we will encounter later. This is quantum mechanics, a system which looks much more complicated compared to relativity and it is not a one person effort to build up this quantum mechanics. It is collective effort, by many scientists at the beginning of the 20th century, having said so many difficulties of learning quantum mechanics don't get scared. Execellent students can learn quantum mechanics excellently and I think so value. And here are some suggestions of learning quantum mechanics, assuming if you have some experience of learning special relativity and here I would like to tell you what's the difference in here. For sure, the physical content is completely different and I'd like to share with you What is the mindset? What the way of thinking, way of learning, what is the difference? The one thing is I hope you can pay more attention to experiments and applications. Look closer to how these experiments are different in the quantum behavior compared to what we would have expected in the classical behavior. Look closer to these experiments use these experiments to try to build up a better way of thinking a better physical picture. And number two is more closely between physics and mathematics in special relativity. The way of doing things is we first have a thought experiment and logically from the fundamental postulates of relativity, how this system should evolve is more logic than mathematics, but in quantum mechanics on the one hand is more abstract that sometimes our logic doesn't work in that way. And on the other hand we have more mathematical guidance to tell us how the world should evolve. So we'd like to rely more on the mathematics instead of our logical thinking to understand our physics. For sure for any theory you have to rely on both mathematics and the logical thinking. But here the proportion is different in special relativity and quantum mechanics. And having that said, don't be scared and try to build up your physical picture of what's happening in quantum mechanics. And now here is the physical picture: Alice in her quantum wonderland. Suppose that Alice, drink a bottle of "drink me", and become smaller, smaller and smaller. What will she find if she is as large as an atom or even smaller than an atom? What she will find is, first of all, when she knows where she is, she doesn't know how fast she's going. And if she knows how fast she's going, she doesn't know where she is. And if she is walking in brightness, the light is brighter, shining to her, she knows better where she is and when the light is shining to her, she doesn't feel the light as continuous waves, but she's feeling that the lights are like bullets, biu-biu-biu, on her. And this is very strange experience. When she would like to come into a classroom for learning, she would choose not to come through the entrance but from a wall she directly cross a wall into the classroom. And if she prefers to get into the classroom by entrances, she can get into both of the entrances. If there are two entrances of the classroom, she can get into both of the entrances at the same time and then she has preferred positions to end up in the classroom compared to other positions. And if she has made a lot of friends in this microscopic world, for example, a lot of electrons then she finds a great difficulty to identify which electron is which electron. she would like to name pne electron is named the Charlie, another electron is named Bob. But then very quickly she is unable to distinguish who is who and this is the wonderland of quantum mechanics. In this set of lecturers, I will introduce to you all of them and much more.