我提供第一讲的讲稿，
Introducing

Greetings, welcome to particle physics
My name is Steven Pollock
I am professor of theoretical physics, nuclear and particle physics at University of Colorado, Boulder.
Those words may not amenable right now.
Nuclear and particle physics, it's a branch of physics, and by the end of the course, you are going to have a good sense of what is those words meaning and what is our study.
And that we’re going to be learning this course.

Defining particle physics

Particle physics is a branch of physics. Physics as a whole, it’s just the study of what the world is made of, how things work, how they behave, why they behave in that way. It’s trying to understand things and phenomenon, both natural and technological.
In the ordinary world, toss a ball up in the air, it follows the arch, and we can understand that path. It make sense, it follows certain laws of physics. Particle physics is a branch of the study where we really try to get down the bare bones. What is the world made of at the smaller level? What are the simplest building blocks? And most fundamental rules and ideas which govern how things work, why they behave, why they do?
Particle physics is about particles that make up the world that we’re living.
I could describe particle physics in five words. Those five words are some technical. They are defined by physicists, have a very definite meaning.
But they are also connected with everyday (????) words: Force and Energy, Matter, Space and Time.
Those are five building blocks of field of particle physics, and in fact, of much of physics itself.
Force and Energy refer to how and why things interact. Matter, what are the things, what is the world made of. Space and Time, what is the framework in which all of that stuff is going on.
As the concept of particle physics and as we go on this course, we will be dealing, especially, with the middle of the matter, and with all of the other elements
You could imagine these enormous diversity of phenomenon is overwhelmingly complicated. And you can have this sense of the world. As being filled with the whole bunch of different things, and we can study any one of them, that might be branch of science. It’s hard to imagine that you can tie them together
But we can. This is one of wonderful things that we have learned about the world. Is that everything physical from objects in this room, objects in the space, objects in the deepest, deepest galaxies, everything between all arises from very simple set of rules and very simple set of ideas and fundamental building blocks. That’s what we’re going in this course. What are those ideas? What are those building blocks? How can we make sense of them.

Dimacratus’s idea
Let’s go back 2,500 years. This is an old question people have been puzzling what the world is made of. From time(),and I guess they didn’t write it down in the days of cave people. But we do have records of Greek philosophers thinking about this, what the world is made of. We’re going to be learning in future lectures about some of those focus and ideas. But let me summarize sort of two view points that arose from 2,500 years ago. There was a Dimacratus, a Greek philosopher from about BC 400, who had the idea. He wasn’t the originator of this idea. He was following a long tradition of other philosophers who were thinking about how the world works. His idea was everything is made of little chunks. He called them atomos which means unbreakable, uncuttable. Let me give you a kind of concrete metaphor to think about Dimacratus’s idea of his real term. Let me hand you a stick of butter. Ok, this butter is an object, a physical object, and has certain characteristics. It’s yellow and it’s soft, and it melts if you heat it up. You can describe it, you can study it and understand it, and it’s a material object. Now what if I chop it in half, and throw away a half, and you’re left behind with a half of this butter. I think we will all agree there is still butter there. It’s less massive, but it’s still definitely butter. We haven’t changed the essence or character of this material. So if you want to understand butter, it doesn’t matter how much I give you. So what if I chop it in half again and again and again, so you got a pat of butter. Still butter. There is no difference between the pat of butter and the original stick except for how much I give you. And now Dimacaratus was imagining what if I keep cutting it in half and half and half again and again. Ultimately, believes Demacratus 2,500 years ago, that we will come down to a chunk, an individual, indivisible, uncuttable chunk of butter. It’s physical, it’s real, it’s a little thing that you could imagine seeing or looking at, although it turns out that it’s much smaller than anything that you could actually see with your eyes. And this is Dimacratus idea of atoms. They live in void; they move around, they interact with one another. They are real, solid, physical objects. That’s an idea, a Greek philosopher’s idea.

Aristotle’s idea
And there were other philosophers at that time who disagree completely. In fact, Aristotle, probably the most famous philosophers at that era, disagreed a lot. And most of us, we know that Dimarcratus ideas come from Aristotle’s ----.Aristotle had an idea which by modern standard, is a little more nebulous. He believed the world is made of earth, air, water and fire. But they are not physical chunks. They are more like characters, qualities. And so matter is infinitely divisible, you can keep cutting that butter into smaller and smaller bits forever. It’s butter all the way. Aristotle’s ideas survived thousands of years. He is a very brilliant man. In many respects, he had a lot of influences in field of religions, and religious philosophy, and philosophy in general. Many this scientific ideas, this one is particular, are really quite discreditive now. There is no evidence of what Aristotle had to say there is a lot of evidence for what Dimacratus believe.

Reviewing the two view points
But neither of those fellows was scientist in any modern sense of that word. They believe that you can understand the world just by thinking about it. Just be a philosopher, ask yourself how should the world be for to make a sense. That tends to be an unproductive way of understanding how things work. It’s much better you go to the laboratory, and ask nature, how are things? Not what we believe is the way should be. But just take data, collect information, and ask yourself: how does butter behave? And you can discover, over 2,000 years, that people began to come up concrete evidence that indeed butter is composed of what we now call molecules. And then, at later time, we discovered those molecules themselves are understood to be built up of even smaller objects that we now call atoms after Dimacratus’s original idea.

Introducing the modern view
So this puzzle about how the world works has been around for a long time, and remained unsolved for a long time. And I would argue that it has been solved today. As the course is going on, we will be really talking about what we do know, and we still don’t know. What are the remaining puzzles? And what are the deep questions that we have not yet resolved. But to a very large extend, we do know that the world is made of atoms. We have direct physical evidence. We have electron microscopes that can image individual atoms now, and even subatomic particles, the atoms themselves are made of smaller constituents, the true building block of nature are smaller than atoms. That’s a fairly recent idea, and one which we’re going to be visiting over and over again in this course. And even those subatomic particles, we have direct evidence. They leave little trails behind them in particle detectors. Kind of like the trail you see when you look at a jet airplane. Even if you can’t see the jet, you can see the direct evidence of its passing. And in this course, we’re going to learn about that data and the ideas that we have built up around the data to try to make sense of it, to try to understand it.
Let me give you an analogy, so that you can get a sense as I do, that this idea of atoms and subatomic particles is really well understood. There is another scientific idea that these Greek philosophers debated, which was our place of universe. A very different question. For instance, do we live in a flat earth? Or is it a ball? Are we the center of the universe. Or, is something else the center? Or there is no center. This is all legitimate questions for people to be asked and be curious about. And again, there were different philosophical camps 2,500 years ago. And there were few Greek philosophers who believed a contemporary version the way of the world works. But they weren’t really collecting data and making measurements, and watching the planets move the sky. And it really took 2,000 years to the zzzz and afterward for people to begin to make definite statements. And today, we all have this worldview; it’s taught to us as a little children in elementary school that the sun is the center of the solar system. And the planets go around orbits around it. And we can make sense of phenomenon like, for instance, the sun rises in East, travels across the sky, and sets in West without evoking this kind of primitive idea that we’re rest, the sun is going around once a day.
I think we can say with conviction that that’s not a scientifically valid idea. And I think pretty much everybody will agree with me. If you disagree with me, it’s worth thinking about why we know that. And even if you do agree with me, it’s worth asking yourself how do you know, why is it that you believe that picture that was given to you, handed to you by your elementary school teachers that it’s not the case that the sun is rotating around the earth once a day. There are many good reasons to believe that there are a lot of data, there are many experiments, measurements that you can make. In fact, even ones that you could make. And it’s fun to think about. And something we’re going to think about through this course. When I tell you about scientific facts, this facts are all based on evidence. And you should be always asking yourself: how do I know? Why do I believe that?
The goal of this course ,then, is to give you the framework, it’s much like the framework where I could describe the solar system, and you could have mental model. And once you have a mental model of solar system, you could understand the news reports about the satellites orbiting around the earth, or rocket ships heading out toward space to explore. You have a mental model of framework into which you can fit contemporary ideas, explorations and discoveries, and make sense of them. And what I would like to do is to give you such a mental framework to understand contemporary ideas that we have about particle physics and about fundamental building blocks. It’s a model which is not so common, although, I think lots of people do have some idea, some cool intuitive sense given to us in our elementary school, and through high school training about what the atom is. But we’ll go deeper; we’re goanna look beyond atom. And ask what they’re made of, and what, what they’re made of is made of. We’re going to follow this chain as far as we can. Particle physics goal is try to get the bottom.

Approach to study particle physics
And this leads me to an idea. It’s kind of philosophical idea and approach to understand the world, which I call reductionism, which everybody calls reductionism. It’s the idea that when you look at something complicated, there are many way to try to understand it. One of ways is try to ask what is it made of? What are the kind of more universal building blocks? That’s a reductionistic approach, because we reduce the complicated thing to simple thing. Now this is not the only way to approach science. If you’re looking at humam being interacting with one another and you say what’s going on? How do I understand the conflicts and discussions, human psychology, sociology? You could in principle think about that by investigating human beings, and looking at various aspects of their interactions. Or, you could be a reductionist, you say, let me study human brain, let me try to understand human behavior in terms of chemical and physical makeup of brain. It’s certainly not the only way to understand human behavior, but I would argue it’s a useful way. We can come up with drugs, if somebody is dysfunctional, in some cases, it really does boil down to chemical
In other cases, it requires psychotherapy. There are lots of possible approaches that you could imagine. The reductionism one, being only one of the many, but a very powerful one. If you ask a doctor, how does the human body work? Especially a sort of western doctor. I think their approach is, to a large extend, reductionistic. They say: well, the human body is collection of organs. And those organs are fairly universal. People have hearts, dogs have hearts, and worms have hearts. And so we can understand the complicated system in terms of fundamental building blocks, and how they fit together. You get heart, lungs, bones and skin. We need to understand not only what those parts are, but how they hold together. Now, it’s not the only approach to medicine, it may ultimately just be one of many useful approaches. But I think we can argue that it’s a definitely useful, valuable, and powerful scientific approach to do that, to think reductively. And as a physicist, we tend to think reductively to deeper and deeper level. I mean: what’s a heart? What is a heart? How does it work? Well, to me, the way the heart works, primarily, is made of a bunch of cells of certain type. And they have certain structure and interact with one another in certain well-defined ways. So again, I understand this complicated thing by reducing it into components and how they fit together. And we can keep asking that question. Ok. What is a cell? How does it work? Why does the cell behave in the way they does? And the answer is, at least one answer is: well, the cell is made of various protein molecules which fit together in certain configuration. Great. So what’s a protein molecule? Well, it’s a bunch of atoms, Hydrogen, Carbon, Nitrogen and oxygen, and whatever it is makes of a protein molecule fitting together in a certain way, obeying certain rules. So we keep understanding complicated things in terms of simpler ones which are more universal. By the time when we get down to these atoms that make up molecules, easy atoms are constituents of everything in the world. So you understand how they work in a protein, you also understand how they work in a frame of a car. So there is an appeal and value to follow this reductionistic approach, even if it is not the only way. Particle physicists just ask: how far can we go. We’ve got these atoms. What are they? Now we have a mental model. They are made of nuclears and electrons which orbit around it. It’s a kind of classical pictures which we’ll be talking about in future lecture in this course. And then we can ask, ok, what’s the nuclear made of? And we understand it in terms of smaller particles still, protons and neutrons which themselves are made of something called quarks, a bizarre word which we’re going to be learning a lot about. So anything that’s unfamiliar at the beginning of this course hang on, we’re going to be defining what’s a quark, what’s a electron? What’s a lepton? All of the buzz words of this field, I’m going to make sense, because it’s really just simple story. And in order to make sense of them, it’s nice to have some mental framework model. And that model can be quite simplistic. It’s perfectly legitimate to have a kind of na?ve view to start with, that give you the essence of the story. I have to apology for reductionism, because there are lots of discussion in scientific community about how useful and how important the reductionism is. For example, supposing I want to teach you music. I said I want you to understand motza. There are various approaches, one of approaches might be to sit down to the piano, listen to the motza music, and figure out what’s the individual motsza I am hearing, I might write them down on a piece of paper with some lines. Using a scientific notation to encode which notes the music is made of, how long they last, and how loud the soft they are. Of course that’s the way musicians get music. A pianist sits down and reads that notation, so can play moatza. So basically, as a reductionist, I said mostza are just zzzz notes on the piano. I have reduced music to the simple fundamental building blocks. There’re idiat keys, and then various properties they have. How long they last. That’s a reuctionist approach, of course, it’s not completely satisfied. It’s not just the names， the notes and the order. There are something deeper about music. That’s compelling and beautiful. And so there is more to understanding music than just knowing the notes. But clearly, knowing the notes is useful and important and valuable. So this is where we headed, we’re going to be learning about building blocks and how they fit together.

Reasons for studying particle physics
But let me in the first lecture before going into any detail which comes later in this course; ask a kind of big question: why is it that we care? I can’t, I can’t just give you a hard zzzz answer. It’s perfectly legitimate. People will ask you， why are you taking this course? Why would you spend time and energy and money to try to understand particle physics? There are many reasons that I can think of, many of personal reasons. As the course goes on, I am going to be asking this question at various levels over and over again. Why do we care about this stuff? It‘s interesting at many levels and many ways. Let me throw out a couple of ideas to you today. And it’s something you should be thinking about in this course as we go along. When people say to me： Why do you study particle physics when I am sitting in the airplane, the person next to me learns I am a physicist. After their initial guard reacton, they want to know what we get out of this. What’s the reason for studying particle physics?

All sciences are tied together as the first reason
I think their thinking about technology, that’s what scientists are famous for, what has come out of this particle physics research that affects everyday lives. And I can list something. Ah… particle physics is a bit of uncertain field. We’re trying to look at microscopic objects. And many times these objects are so disconnective from everyday ordinary world. It’s little bit hard to point you finger at some specific application. But there have been at many. There are a lot of medical technologies that make use directly of what we have learned about particle physics, from radiation therapies, proton beam therapies to imaging human being brains with antimatter scans. All of these stuffs are really in a direct sense to use particle physics. If you allow me to generalize particle physics to modern physics which include building ideas, quantum theory and relativity. All of a sudden, I can point just about all of technology from television sets, microwave ovens to cell phones. All of that stuff relies upon and was developed from ideas of quantum theory. If you really focus on particle, particle physics. Well, we’ve developed magneticology from the accelerator that we use to study particles.

The particles themselves don’t always enter into our life. And so maybe that I have to kind of backoff. Say, you know, maybe this is not the reason why we study particle physics. Let me ask you this: why do we go to the moon in the late 60’s and early 70’s. when people ask NASA: what was the point of going to the moon? There are many different answers that they can give. And every now again, you hear them say: well, goss, the space program helped us develop tent and zzzz. That’s a nice idea, I am grateful when I am camping, we have tent. It’s really not the reason, to me at all, for why we went to the moon. As you can imagine, there are a lot of deeper, more profound reasons. Now you can think about spin off in another sense. The space program teaches us about astronomy. It teaches us about materials, it teaches us about geology. In a similar way, particle physics can spin off. What we’ve learned when we’re studying particle physics is mathematical description of how particle work together, interact and form a complex structures. If you generalize that idea, you realize that you can apply it to something other than the fundamental building blocks. A biologist might take the same mathematical structures, and apply it to cells, and we can understand how organism build that cells functions in terms of a theory which is really the same theory, a same mathematical framework as particle physics have developed. And this has happened over and over again. In history all of sciences are tied together. There is no branch of science that is completely disconnective from particle physics. Nor is particle physics completely disconnective from geology, or astronomy, or chemistry. It all ties together. So we can talk about applications in this scientific interacting sense. And sometimes it’s even fairly applied. We have discovered particle’s nature in particle physics. Exotic particles with crazy names like muon and neutrino. And we have begun using those, for example, as astronomical tools, we can use them to look at distant objects in a way we’ve never been able to before. We’re going to be learning about that in this course. Those mysterious particles all do make sense. There is a simple picture that you can have in your mind what a neutrino is, what a MUON is. And then, you will be able to understand at some fairly deeper level. What is a technology, why does it work? So this is a spin off still in some abstract sense.


Another reason for studying
But I think we can, we can motivate studying particle physics at level that’s not really even about applications. It’s not about applying to technology; it’s not about applying to other fields. It’s our human curiosity about the world. We are puzzle solvers. I think it’s genetic. If you go back a million years and there are some cave people. You know I could imagine one cave person watches a rock trembling down the cliff, and saying, en… rocks follow a parabolic arch, next time when rocks are bouncing toward me, I know which way to step, so doesn’t rock me in the head. We want to know what the world is made of. At least I do. And since you watch this course, probably you do too. What are those building blocks? Can we understand them? It’s the same kind of appeal of crossword puzzle. Except, this one is real, it’s not artificial. The world has provided us with this puzzle. And we want to know: how do we put it together? How do we solve the mystery of nature?

The third reason to study
I think there is another level which for me, some kind of, I don’t know, personal satisfaction. So when I look at a rainbow, it’s a beautiful phenomenon, and some people would even say it’s magical, and I understand it, used to that word, and I feel that way. When I look at a rainbow, I feel that it’s magical, and I love it. I will point it out to my friend, look, there is a rainbow that we all enjoy it. But I think I get a deeper satisfaction from my understanding of rainbow as a physical phenomenon. I know what’s going on. There is little water droplet in the rain. And the sunshine is reflecting internally. And I understand enough optics to know why the colors appear, in the directions that they appear, at the angle it appears. And that doesn’t take away the magic it adds to me.To the pleasure and experience of physical phenomenon. Because even though it is magical mysterious, it also make sense. The world is really fundamentally simple. At some level, from a rainbow to music, to hard and soft objects, anything you could think of all boils down to fundamental particles, interact with fundamental laws. It’s not about the facts themselves; It’s about putting together those facts into a framework.

Conclusion
Particle physics is, in many ways, regarded as fundamental, it’s also for that reason, the most abstract in any sciences. If you’re a reductionist, you’re trying to understand complex phenomenon. A biologist is trying to understand human body, is doing something very useful and very fascinating. The human body and all its complexity is a great science. To me it’s a little overwhelming. A simple, physicists go for the simple, underlying truth. And particle physicists really try to get down to the bottom as far as we can. Now I don’t want to argue that we’ve done, I don’t want to argue that today we know what the bottom level is. And began, this lecture saying we do have a deep understanding of the world. You will learn in this course about that deep understanding. It goes by the title the standard model of particle physics. And I am going to try to convince you it is a deep true statement in scientific sense of the word truth. Not it is the end of the line. Maybe we will understand these deep truths in terms of yet deeper ones. That’s certainly possible. And it’s one of the open questions.

At the end of these classes how can we go beyond the present statement of understanding? So our goals here are, without getting into heavily mathematic, in fact, without getting into mathematic at all, that you can still create for yourself and understand it a conceptual model, the standard model of particle physics. So you can understand when people are telling you in the news about some new discovery, when you relearning something that you’ve learned in your youth. It all fit together. The picture is really quite straight forward. In this course we’re going to begin with a sort of historical format. I believe there are many ways to approach a broad field of human understanding. I think it’s nice to look back at the historical evolution of these ideas. Some of these ideas are pretty wild; some of them are pretty abstract. And when that happens, we’re going to just pause, and look at the people who discover it, and experiments that they did, which convince them of this fascinated nature. We’re going to look at the progressing human thought which has reached a truly spectacular level. This is very accurate; this is quantum physics which allows you to make calculations, and predictions, to design new apparatus. So we’re going to be following this historical path, but not entirely. There is a mystery that appears in 1930’s, which we’ve solved today. I’m certainly not going to leave you handling. I want the story to begin to form cohering zzz right from the start. So we’ll be progressing along feeding all the ideas into standard model of particle physics. As I said, you don’t have to be a physicist. In fact, I don’t expect that anybody know any particle physics, nuclear physics. You don’t really even have to know what an atom is except for some vague memories from your youth.


Particle physics is big field of knowledge. In this first lecture, I’ve just tried painting very broad strokes. What is physics? What is particle physics？ And very important, I’ve been talking about motivation why are we interested in this stuff. Now given the complexity of this topic. I think it’s really worth for a while to continue at this introductory level for one more lecture. We’ll start to get mete of material in lecture 3. but next time, I want to continue, first of all I want to be specific about what is particle physics, let’s define it. And I want to also talk a little bit more specifically about how big are this things we’re talking about, and how small is the atom, how small is the subatomic particle. It’s nice to set real scales, so we can think about this in an intuitive sense. I want to talk about the structure of the atom. In the little bit more detail. I’d like to begin to introduce some of the buzz words that we’ll be using throughout this course next time so that we have a framework, some place to hang all these ideas, which we’re going to be discussing. And at that point, at the end of the next lecture, we’re going to be equipped to create an outline for the course. I really haven’t done that yet. I want to talk about where we head at, what’s the structure of the course, how it was organized. And at that point, we will really be able to begin our detailed-study of standard model of particle physics.