One consequence of living in the modern world is that there is almost no-one to whom physics is totally unfamiliar. Therefore, the question of what physics ``is'' is already not an isolated. It is inevitably mixed in with what physics is perceived to be, and with how it affects us. Since those considerations influence how we think about the subject, and even the meanings we assume for the words, they need to be addressed as well. They also tell us a lot about our current relation to the subject, and about what could be changed.
Going to school, reading the news, listening to radio or television, it is hard to remain unaware that Physics exists, and that somehow it is something people care about. Few subjects have a more hallowed reputation, in both the good and the bad sense. Even if one has no clear idea what physics ``is'', it is apparent that somehow it is responsible for many things we value, and that we have come to depend upon. The most obvious of these are technologies that we use every day, radios, cars, electric tools, lights and appliances, even the buildings in which we live and work. The technologies are so numerous that it would be quicker to try to list any daily activities that have not come to involve some technology based on physics. That list might have no members at all. At the same time, we are also aware that, for good or ill, we have chosen to accept responsibilities that physics has made possible. For instance, it is easier to die for a simple mistake driving a car than it is walking, and the poisons, pollution and wars we can make today have a seriousness that was never possible before. All this occurs simply because physics has made us more powerful. Overall, we have chosen to embrace this power because it offers us too much good to refuse. If nothing else, the life and quality of life that have been created by our power to overcome poverty and disease far outweigh everything destroyed in all modern wars combined. This may be a useful perspective to keep in mind.
We are also exposed to a ``Wow!'' aspect of physics, especially its more modern topics. The amazing claims of relativity and quantum mechanics that delighted and confounded the popular culture of the early 1900's have found so many applications in computers, laser technologies, medicine and nuclear energy that they have almost become passé, because no-one today seriously doubts that the results are correct. However, we have a whole new set of claims that seem too wild to be true, which have not yet become common, about black holes, cosmology and high energy particles. News from the ``forefront'' of Physics contains statements about the earliest moments of the universe, made as matter-of-factly as if someone had just been there and returned with a report. As one comes to appreciate just how many things have been understood within the domain of Physics, its scope alone becomes breathtaking.
Because physics has produced so many useful things, and because the statements of Physics have indeed been correct very many times, it has earned a reputation not only of great value, but of great reliability. Physics does properly deserve to be called one of mankind's crowning intellectual achievements. The bad side of this ``hallowed reputation'' is that it encourages us to be passive observers, either trusting or simply aquiescent, of the things we are told. In doing so, it is rapidly separating us into two cultures. We use the technologies and depend on them, but for the most part we do not understand them. We hear news from the forefronts, but there is a distressing tendency to receive it like doctrine, issuing from some priesthood. Should we trust it? Is it ridiculous? It is as if this news comes from some ``other'' culture, removed from the practical reality of everyday life. One of the main motivations for this book is that there should not be ``two cultures'', divided in their relation to physics. Not only is it undesirable, but for this subject in particular, it was never necessary, as we shall see.
Much like its news, the history of physics is decorated with colorful characters. Everyone has heard of Galilleo, Newton and Einstein. Most people alive since the 1940s have heard of Hans Bethe, Robert Oppenheimer and even Richard Feynman, and many have heard of Enrico Fermi, or Pierre and Marie Curie. The list of popular ``physics heros'' (and ``physics villians'') goes far beyond this.
The history of physics, particularly the last 300 years, seems astride with giants of intellect, more densely than in all the centuries preceding. Unfortunately, while we generally know that remarkable people have contributed to physics, what we fail to appreciate is how physics has created remarkable people. This is important because it affects how we expect ourselves to relate to physics, to its established successes and its current speculations. If we understood this one point, it would be our greatest protection against making physics into a priesthood.
The thing to appreciate is that no change took place in the species ``man'' 300 years ago. People have wanted to understand nature and build useful tools for as long as the species has existed. The special change that happened 300 years ago was that man learned something new. Whatever it was, it offered ordinary people, the same kind as had always existed, the possibility to make the extra-ordinary discoveries that we still admire and use, even centuries later. Certainly, many of the contributors to physics were remarkable people. The point is that there were probably neither more nor fewer such people than there had been for millennia. The difference in what they have achieved arose from the difference in the tools they had to use, and all of we current people should expect to benefit from using the same tools in the same way.
This is our first suggestion that, in spite of the fact that Physics clearly encompasses a huge amount of knowledge, and has an increadible range of applications, it must somehow be manageable by real, normal people. We will see shortly, in nuts-and-bolts detail, how this is accomplished. The fact that it is possible at all is our first reason that physics should be something everyone understands: simply that it can be.
Thus, our cultural ambiance, without telling us what physics is, informs us that something exists which is important and trustworthy. At the same time, though, it encourages us to make the knowledge, and the people who create and use it, remote from daily life. Our first task, then, has been to regain perspective, and to realize that the very success of physics in shaping the modern world should be telling us that it is special, precisely because it is something that we can use.
But what is Physics?
It is remarkable how difficult it is to find a simple answer to this question. Generally, there are three audiences with respect to physics, and three different kinds of frustration that come from asking it. More or less, they spring from the same source.
Most of us are neither students nor practicing physicists, and rely on news or the ``layman's'' press, to review what is known or tell us about the present state of the field. That writing probably suffers worst from the ``two-culture'' prejudice, with the laymen being the ``outside'' culture. This means that Physics is not presented as something to be understood, questioned and used by the reader, now or ever. At best, it may be something interesting for browsing, and at worst, simply something to be accepted.
Not very different is the experience of students who do not expect to become physicists by profession. In some sense, because as a society we know physics is ``important'', we feel that it should be taught, but again there is too much of a tendency for ``facts'' to be taught as ``culture'' (a somewhat ironic play on words), and not as things the student is seriously expected to question or use. Text is often more methodical than the news, and less sensationalistic, but it is seldom offered, intellectually, inter pares.
Oddest of all, though, is the experience of students learning physics, who intend themselves to become practicing physicists. One would expect these people, if anyone, to be told right from the start, what physics is and what about it matters. Instead, most experience something like a condensed version of ``the news'', and spend many years just trying to juggle and remember facts, with no clear idea how they all fit together.
In all three cases, the problem is that the reader is expected to take a passive relation to the subject, and that the subtle but extremely important methods that give order to the knowledge are not made clear. In the course of a lifetime, a single person can occupy many of these categories. That is why we have mentioned all three perspectives. Even a working physicist or engineer, if he has not used Quantum Mechanics, can feel like a layman in this area.
Nominally, this book is a text on modern physics for introductory students. It follows a familiar format of segmented text and problems. As we have said, it also purports to start from a logical beginning. These should not be taken as limitations, though. The same attention to order and arrangement that is intended to make the topic intelligible to non-physics students should make the individual details clearer to physics majors as well. They are, after all, drawn from the practices of the profession. Similarly, the problems and subdivision of the text should not be ``inappropriate'' for non-student laymen. Physics is a tool. Problems are examples of that tool in action. The hierarchy into which the ideas naturally fall recommends a similar segmentation of the description.
Above all, the material in this book is not intended to be ``cocktail party physics''. It is real, get-to-an-answer and understand-how-you-got-there physics. We will prove answers with pictures, because that reduces the number of new words to be learned. Pictures have always been one of the mind's most powerful reasoning tools, because people are visual animals. We will also use equations, when they are the easiest tools to use. Here as well, though, we will start from the beginning, and define what we mean as we go. None of the material or methods is particularly ``student material'' or ``physics student material''; it is general ``thinking-person material''.
To answer the question, then, Physics is the discipline through which people try to understand how the natural world behaves. Of course, that is too simple an answer. The whole point of this book is to provide a better one. Since ancient times, people have wanted to ``understand'' nature, and have regarded one or another piece of ``wisdom'' as a step in that direction. Physics, as the thing that has changed the shape of the world in 300 years, is something more than that. It involves a particular way of observing, and a particular use of language to describe what is observed. These are special because they have taught people how to be precise and unambiguous in describing what they see. With the ability to describe precisely then comes the ability to predict. The day-to-day business of physics is predicting what some part of nature will do in a given circumstance, and checking to see if each prediction comes true. Physical ``knowledge'' is just a large set of rules for making predictions about how nature will behave. The special methods of physics are designed to make the most possible use of those rules that work best, and to spot and weed out the ones that have ever failed. In some sense, that is the whole answer. Our work in the rest of the book will be to show through experience what the words in it mean.
In its significance to us as people, though, physics is many other things as well. It was not obvious, throughout most of human history, that there were any rules for predicting the behavior of nature, which might never fail. If they did exist, no-one expected that they could be found by people. The behavior of nature was simply one aspect of fate. It might be feared, but it would never have occured to anyone that it could be predicted with an absolute consistency, much less controlled. The success of physical law, starting with the work of Newton and continuing like wildfire since, has first made possible the attitude that the world is a knowable place. In human culture, that is a revolution.
Physics has not only changed how we relate to nature; it has also changed how we relate to each other. People can learn new things today at a rate that wasn't even imaginable a century ago. Much of that ability springs from the methods of physics, and the rest from the synergy of what has already been learned. As noted, it has also made us more powerful actors. The newfound ability to know so many special things, and the power that comes from acting on them, has created our level of specialization, and its resulting interdependence. The very interdependence and power that have made our lives better, though, have also created a dependence on physics itself, which never existed before. People raised in today's modern cultures simply do not know enough of the right things to survive, half-alone and in parallel, as their ancestors did. We depend on our technology and our inter-relations to make us useful for much of anything at all. With our newfound potency, we also depend on physics to find solutions to the ever-larger problems we are able to create.
This is just one reason that the relation of physics to our Culture is so important, and that a passive relation to physics is very dangerous. If the culture is to give us the tools that we need to remain productive and stable, we need to understand what we are protecting. By the nature of the subject, this is not well served by a passive relation to the knowledge or the practice, or one that presumes a division of interdependent people into two skew cultures.
Even modern technology, though, and all of what we have become with it, express only a small part of what physics is and what it gives us. Modern Physics extends man's understanding of nature out almost to the edge of ``what it is possible to see'' in the universe, and back to some very small fraction of a second after its ``beginning''. The combination of knowledge about the very large and the very small, which is necessary to accomplish this, goes by the name cosmology. Cosmology, as a matter-of-fact and predictive description of the beginning and entirety of the universe, was not even conceivable as a subject before the current century. All this has been worked out while we were sitting here on one very small planet. Yet most of it can be understood from experiences that are accessible to us all.
Modern physics is not only revolutionary because of how far it takes us, though. It is also remarkable for the level of coherence it has achieved among everything that people understand to date. Building this coherence is a process generally known as ``unification''. It is perhaps easiest to illustrate with an example.
The collection of chemicals that exist in nature is overwhelmingly complex. The set that can exist, including the ones that generally don't, goes far beyond that. Such complexity and diversity is a very hard thing for the human mind to handle. Knowing something about one chemical, does one then know anything about any others? Can anything be proved or predicted when such complex behavior is possible? It can be very hard to say.
The crack in the dyke, that made chemistry a knowable field, was Mendeleev's contribution to the so-called ``atomic theory of matter''. The ancient Greeks had insisted that, at some level, all of matter was made of pieces which were small and indivisible. They had no way to show that this was true, though, or to do anything useful with such a claim. Mendeleev's achievement was to give meaning to ``atoms''. He found a way to classify every chemical he could identify as a composition of a few ``elements''. The chemicals could differ tremendously, but the elements that made them were always the same few. More importantly, Mendeleev could assign definite properties to each of the elements, and show how the possible chemical species were constrained by a few rules for combining them. The most important step of all, though, was his recognition that every bit of the same kind of element was the same as every other bit. These bits were the ``atoms'' the Greeks had sought, something that was repeatable because it couldn't be broken down into a continuum of other things.
It rapidly became clear that the great complexity of chemistry was a complexity of the ways atoms could be combined. All of that complexity, though, could no longer obscure the following increadibly important observation: all the pieces of any given chemical substance must be exactly the same. Many chemicals might be possible, but there was no freedom for a bit of any given chemical to be different from any other bit, because they were all made from the same tiny, identical atoms, according to the same few, simple rules. Moreover, no matter what was done with a chemical, the only building blocks that could be extracted from it were the ones put in to start. Thus, chemistry might be complicated, but it was constrained, and it was predictable. The age of alchemy was brought to an end by this one observation. Our entire process of controlling, inventing and finally engineering chemicals has grown as we have learned to apply the few simple rules Mendeleev discovered (and some refinements of them).
This reduction of many special cases to a description involving only a few rules is an example of the process of unification. One of the hallmarks of modern physics is that it currently provides a theory of fundamental processes (basic building blocks, like Mendeleev's elements) that describes every experience with nature that we have ever been able to measure. We don't know all the rules for complexity, or how to use them. At the same time, it is almost unbelievable that, with the ability to measure things that happen inside stars, and events like those that occured just after the beginning of the universe, we have never seen a part of nature that requires more than the building blocks we currently understand. When such simplification is possible, we should be making use of it.
Finally, in the course of discovering how nature behaves, we have learned many things about how it is possible for people to think, that are revolutions of a different sort. These are results of the practice of physics, ``behind the scenes'', yet the other laws could not have been found, and indeed cannot even be expressed, without them.
One thing we have learned in the course of doing physics is a new meaning for the notion of confidence. This refers to much more than just the ``hallowed reputation'' we gather from the news. It is true that physical law has achieved a whole new level of trustworthiness never before known. The special thing, though, is where that confidence comes from, and the ramifications for how it can be used. We will look at this in the next section.
Another thing we have learned is a new paradigm, both for the use of language and for the very notion of abstraction. At its deepest, it gives us a whole new philosophy of what it means for something to be ``right'' or ``wrong''. Practically, it not only makes the new notions of confidence in physics possible, but also leads to a way to organize the knowledge that makes it very easy to use, even in the hands of a single person. The philosophy matters precisely because of its implications for everyday life.
The ``new understanding of abstraction'' takes the largest and most specialized body of knowledge people have ever had, and makes it navigable. In the same step, it overcomes what could be called the ``library problem'', which is becoming increasingly significant in more and more aspects of modern life. As ever more specialized and interdependent people, we generate more information, and we also want to use more information that other people have created. Metaphorically, our problem is that our libraries become so crowded that we cannot get past each other to find what we want to know. Even when problems of ``lookup'' (in a literal sense) are solved, we still have the problem of having to read it all, if we depend on sources outside ourselves for our information or our confidence. The organization of physical knowledge is chosen precisely to get the most possible use from the fewest possible ``facts'', which would have to be remembered or looked up. The way physics enables us to solve very many problems, while intellectually still ``traveling light'', is itself a paradigm for how people can relate, and how they can moderate their dependences. It is also the best way to avoid library problems of various sorts that has yet been found.
Finally, physics has attained one thing, within a very small subculture, that has never been possible before, except rarely among isolated individuals. It has defined a new relation of ideas to the social authority of groups. This is a subtle point, which will be clearer at the end of the next section. It hinges on the way physics provides a notion of what is trustworthy, that is simply independent of group concensus. It does not subvert it, nor does it require it. In other words, the value of an idea is not determined by how it is endorsed within a group. The increadible, yet subtle, importance of this separation is that within physics, ideas can be examined with the harshest of scrutiny, by people who are in the closest of co-operation. They can also be checked, across cultures and across time, in a way that requires no common reference except nature itself. This relation, of people to ideas, is at bottom responsible for both the tremendous growth of knowledge within physics, and for its unparalleled integrity.