PHYSICS 107. Lecture 1: The Puzzle of Motion. In American universities there are three main types of physics courses for nonspecialists.

Transcription

1 PHYSICS 107 Lecture 1: The Puzzle of Motion About this course In American universities there are three main types of physics courses for nonspecialists. The first kind teaches about the physics of everyday life, featuring explanations of commonly observed phenomena. The second kind of course is the science for the citizen course. It teaches about the science behind current political issues like energy generation, global warming and other topics that one needs to understand in order to vote in an informed way. This course is the third kind. It's an attempt to give an understanding of the deepest concepts at the very forefront of modern science. This third kind of course was actually originated at the University of Wisconsin in the 1960s by Prof. Robert H. March. The intellectual climate at that time was somewhat anti-science. There was a perception that science was not relevant to our actual lives. In our more highly technological society 50 years later that may seem a bit quaint. But the excitement generated by the course not only has kept it going for 50 years at Wisconsin but the idea spread throughout the country. Ever since that time a course of this kind is taught, with some variations, at most American universities. Bob March was a good friend of mine. He died this past summer, and this course is dedicated to his memory. His book for the course, Physics for Poets, is a little outdated by now but still very much worth reading. I teach the course little bit differently from Bob, and from other people who teach the course. My conception of science is that it is just common sense, but common sense organized, analyzed, criticized, communicated, recorded, and, most importantly, tested. But it is still essentially common sense. For all four of the topics that we ll be discussing we re going to start with our common sense, and

2 we ll find that, in history, other people also started the same way. Then we're just going to see where it leads us. This wouldn't be much fun if it led only to more everyday-type consequences but, unexpectedly, that's not what happens. Instead we end up with the strangest and most surprising picture of the universe. Someone once said about quantum physics that it is not only stranger than you imagine but stranger than you can imagine. It s because of this that we are embarking on this journey, which will be a sort of journey into weirdness. The course is divided into 4 parts: Relativity, Quantum Theory, Particles, and Cosmology. Each extends common sense in a different direction. Roughly speaking, then, relativity is what we get when extend common sense about motion to very high speeds; quantum theory is what we get when we extend common sense to very short distances; particles are what we get when we chop stuff up into smaller and smaller pieces; cosmology is what we find out when we look at bigger and bigger distances and longer and longer times. Because of my approach, the first lecture or two on a given topic will not be taken from the book, but will be covered only in the course notes.

3 The puzzle of motion Surely, people have always wondered about motion. There are many natural questions that have been asked for millennia. Here are just a few examples. 1. Which things move? 2. What causes them to move? 3. Why are some things easier to move than others? We also came up with a number of other questions in class. The first people we know of who not only wondered like this, but who also gave answers in an organized written way were the ancient Greeks. They wrote things down, criticized their own ideas, organized them, and managed to pass them down to posterity. They were not the first to have theories of motion, but they were the first to have theories of motion that were coherent, cogent, and which still influence our own ideas today. We ll focus in the next lecture on Aristotle, who gave these ideas their definitive form for the ancient period. Let s focus for today on the first question: What things move? Let's ask this question but think of ourselves as if we lived in the year 400 BC around the time when Plato and Aristotle lived. This means we exclude cars, airplanes, and other man-made contrivances. Here are some answers: 1. People 2. Animals 3. Things that people or animals cause to move by throwing them, pushing them, or otherwise exerting some force on them. 4. The wind. 5. The sea. 6. Flames.

4 7. Volcanoes. 8. Earthquakes. 9. The sun. 10. The stars 11. The moon. 12. The planets We have done the first thing to go beyond common sense now: a list is the 1 st way to systematize common sense. Is there anything that all of these things have in common besides that they all move? Well, maybe not so much. The most we can do to is to classify (2 nd way!) them to some extent. Class 1. Things that are alive: examples 1 and 2. These are special because they do not need anything else to push them they originate motion and do so of their own will. Motion and life are almost the same thing: we test whether something is alive or not by seeing if it moves. Class 2. Natural phenomena: 4,5,7,8. These don t really seem to be alive, at least to us, but nevertheless there is a spontaneity and unpredictability about this class that they share with things that are alive. Class 3. Astronomical objects. This class contains examples 9,10,11,12: the sun and the moon and stars and planets. Many ancient peoples, not only the Greeks, realized that there was something special about the motion of celestial objects. It was regular and predictable. This definitely seems very different from the first two categories whose motions are very difficult to predict and certainly very irregular. Class 4. Fire. This is sort of on its own. Because of this, the special and unique nature of fire was traditionally been accorded a lot of importance by early peoples. Fire is included as one of the four elements: earth, air, fire, and water in the West, and the five elements earth, fire, water, metal, and wood in ancient China. Fire has

5 always seemed to be in a class of its own to all pre-technological people. We re not going to say much more about it in this course. Class 5. Example 3 above: things that only move when they're pushed or thrown by other things. Their motion does not come from within but is somehow imparted to them by things that are already in motion. A thrown ball is an example. The first theory of motion (not at all particular to the Greeks) is that all things that move are alive (or closely connected to things that are alive): life is the origin of all motion. We re not ignoring all the nonliving examples, but saying that their motion derives from the motion of the living things. A consequence is that although the sun may not seem to be alive, there really is a sun god and the sun is his fiery chariot. The Greeks named this god Apollo, but other peoples have similar notions and different names for the god of the sun. The moon and planets were each associated with a god or goddess and the life of the celestial being was enough to explain the motion. The same can be done for the natural motions of our second class: natural phenomena. Thus there's a god of the sea, a goddess of the earth, and gods of the winds. Indeed anything that moves need something alive to explain the motion. Plato literally thought that the earth itself was alive. After all there are earthquakes and volcanoes and so on. How else can they be explained? So now we have an early theory of motion. It is more or less what the Greeks thought prior to the 5 th or 6 th century BC, and something like it was arrived at by many other peoples in ancient times. These days we d regard this as a religious explanation of motion and call it animism. It is in many ways cogent. It takes all things that move and ascribes one sort of cause to their motion. In the fourth and fifth centuries BC, some philosophers in Greece began to regard explanations of motion purely by gods and goddesses as being unsatisfactory. After all you can t really see Apollo. The hypothesis of Apollo is what might be called an ad hoc explanation of the cause of the sun's motion. You just simply introduce one unexplained thing to explain another. This is not entirely satisfying indeed, viewed in this (rather modern) light it seems like no explanation at all. Perhaps worse is that such a theory gives no power to predict things. The

6 immortals in Greek mythology were fickle and if they are causing motion there was no way to say what is coming next. One way in which Aristotle, the subject of the next lecture, moved things forward was to give a more stringent criterion for explanation of natural phenomena. There must be GENERAL PRINCIPLES of motion if we are really interested in satisfactory explanation. If one thing causes another, the causation must be universal a like cause should always give rise to a like effect. Hopefully there will be some predictive power that comes from a more sophisticated naturalistic theory, one that does not rely on the gods.