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Physics
Review Essays of Academic, Professional & Technical Books in the Humanities & Sciences
Philosophy of Physics
An Introduction to the Philosophy of Physics: Locality, Fields, Energy, and
Mass by Marc Lange (Blackwell) Combines physics,
history, and philosophy in a radical new approach to introducing the philosophy
of physics. An ideal guide for those who want to go beyond the equations and
discover what physics reveals about reality.
A principal aim of the philosophy of physics is to work out what the universe must (or may) be like considering the remarkable success that various theories in physics have had in predicting our observations. In other words, the philosophy of physics is concerned with interpreting physical theories -- with figuring out what they tell us about reality. As Einstein remarked, we must work from our best physical theories back to what reality is.
It might seem to you that there is nothing especially philosophical about this task -- that it is the job of physics to tell us what reality is like. Actually, this job belongs to both physics and philosophy; as we will see, there is no sharp line dividing physics from the philosophy of physics. Necessarily, there is a fair amount of physics in this book, mostly drawn from electromagnetism, relativity, and basic quantum mechanics. But the way in which these theories are treated in physics classes and textbooks is typically quite different from the way in which they are analyzed in the philosophy of physics.
I know this difference first-hand. When I was an undergraduate, I originally concentrated on physics. As I worked my way through the sequence of required courses, I found that they were primarily concerned with teaching me how various scientific theories and mathematical techniques could be used to solve various problems, such as predicting a body's path under various conditions. Though learning how to do these things was interesting, no burning desire to do so lead led me to concentrate on physics. Rather, I had wanted to know h what the universe is like in its most fundamental respects: what sorts of of things it is made of and how they work. The physics courses I took I had no time for the questions that I found myself asking as I learned more physics; there was barely enough time to review each week's set of homework problems. But more than time pressure was involved. When questions of the kind 1 thought important did arise, they were often belittled with a hostility that quite surprised me. I know now (though didn't know then) that not all physicists would have responded to my questions in this way. But I also know now (though didn't know then) that my experience was not unique; others who at some point in their educations moved from physics to philosophy underwent searing experiences very similar to mine.
This was the kind of question that I found myself asking in my physics courses. The reason I can recall this particular question so vividly is because I remember how thrilled I was to find this question raised explicitly in my textbook, and then how confused and embarrassed I was to see it dismissed in these words:
“Perhaps you still want to ask, what is an electric field? Is it something real, or is it merely a name for a factor in an equation which has to be multiplied by something else [a body's electric charge] to give the numerical value of the force [on the body we measure in an experiment?
[S]ince it works, it doesn't make any difference. That is not a frivolous answer, but a serious one.”
Even after that somewhat defensive final sentence, Purcell offers no justification for his "answer," and apparently he believes no justification necessary. Purcell's view seems to be that science is concerned only with predicting what we will observe or measure in various circumstances (such as the force on a charged body) and so long as electromagnetic theory is accurate for these purposes, it does not matter whether fields really exist over and above charged bodies. Indeed, Purcell might well say that to try to figure out whether fields are real is not only irrelevant to scientific purposes but pointless for any serious purpose. We could never have a good reason for believing in the reality or unreality of fields, so this is a merely "philosophical" issue, impossible to investigate or to debate. This attitude is not unusual in physics textbooks. Here is an excerpt from Richard Feynman's legendary Lectures on Physics, from a section entitled "What are the fields?":
You may be saying: ". . . There are electric fields at every point in space; then there are these `laws' [that do not refer to fields, instead telling us the force that one charge or current exerts directly on another, some distance away. But what is actually happening? Why can't you explain it, for instance, by whatever it is that goes between the charges?" Well, it depends on your Prejudices .... The only sensible question is what is the most convenient way to look at electrical effects. Some people prefer to represent them as the interaction at a distance of charges, and to use a complicated law. Others love the field lines.
The "only sensible question" ‑ the only question that makes any sense ‑ is whether it is "most convenient" to think about electromagnetic interactions as involving fields, not whether this way of thinking about them is true. These remarks made me feel very foolish for having thought that it not only made sense to ask, but also was important to answer the question "Are there really fields?"
I know note that it a important to answer this question and that (as we will see) good reasons can be given for answering it a certain way. Admittedly, the answer may make no difference to the way we should use the concept of an electric field to predict the path of a charged body. But the answer should make a big difference to our beliefs about what the universe is like. This is the sort of topic that the philosophy of physics investigates.
Feynman hints at one respect in which the reality of fields would make a difference. Consider a magnet and a compass some small distance away. The magnet is not in contact with the compass needle or with anything that is also touching the compass needle. Yet the magnet somehow manages to cause the needle to turn. This is weird: we tend to think that a cause must be in contact with its effect. How can the magnet affect the needle without something passing between them or their being in direct contact themselves? We will look closely at this question throughout the course of this book. For the moment, I shall say only that if fields are real, then the direct cause of the needle's motion is local to the needle: the magnetic field where the needle is. There is then no gap in space (or time) between the cause and its effect. But for the field to cause the needle to move, the field mast be real, not merely a device we use to simplify our calculations.
The question of whether causes must be local to their effects is the main subject around which this book is organized. It is simultaneously a question of metaphysics ‑ concerning the nature of cause and effect ‑ and a question of physics. To pursue this question, we will have to grapple with many others: Are fields real? Is energy a real stuff that flows around or merely a calculational device? What about electric charge? Do material objects ever really come into contact? According to the theory of relativity, is energy actually matter, or is matter nothing but energy, or what? Does the "spooky action at a distance" in quantum mechanics undermine the idea that a cause must be local to its effect? Some of the issues with which we will have to wrestle involve general questions: about cause and effect, the way in which scientific theories are confirmed by evidence, the sense in which a theory can "unify" various phenomena, and the relation between a scientific theory and its philosophical interpretation. Other issues we will investigate are more closely associated with particular parts of physics.
I have tried to write the most introductory text there could be in the philosophy of physics that does not make undue sacrifices in either the philosophy or the physics. Accordingly, I have tried to presuppose a minimum of detailed prior experience with philosophy or physics. The issues on which I will concentrate might naturally strike any attentive reader of an introductory physics textbook. I have tried to motivate these issues in that spirit, sometimes using excerpts from physics textbooks to propel the discussion. Many of these issues not only were important in the development of classical physics, but also persist today in connection with the most up‑to‑date physical theories. This
book does not pursue them nearly that far; in many respects, it merely scratches the surface. Nevertheless, it will leave you in a position to think rigorously about some of these issues for yourself. Accordingly, at the close of each chapter, I have included some questions for you to ponder. After all, it is much more fun to participate in these efforts than just to be a spectator.
It is also generally more fun to read books that stick their necks out once in a while by taking a stand than books that just give everlengthening collections of arguments, objections, and replies on all sides of various questions. So from time to time, you will find me defending some controversial philosophical view. You should take my conclusions to be provisional; by the time you read these words, I may even have changed my mind, for the arguments I will offer are not utterly conclusive. Perhaps that is for the best: I want you to read with an eye toward improving on what I say.
For that matter, probably none of the physical theories discussed in this book is completely true, though many of them may in some sense approximate the truth. For instance, although much of this book concerns the classical theory of electromagnetic fields, this theory cannot be married easily to charged quantum particles. And, of course, the coming years will undoubtedly bring new physical theories revealing the limitations of even our best current theories. So although the theories I shall examine are accurate enough for certain kinds of practical applications, their interpretation can give us only limited insight into reality. What, then, is the point of wrestling with their interpretation?
The answer is twofold. To begin with, many of the questions encountered in interpreting today's cutting‑edge theories arose earlier in connection with the simpler theories investigated here. That is partly because classical electromagnetic field theory is the prototype for many current theories. But even if this were not the case, the theories I examine would still illustrate the interesting things that can happen when we take a theory from physics, a theory that seems straightforward enough when we are using it merely to predict our observations, and try to use it to describe reality.
Despite the continuing importance of the issues I shall discuss, several of them seem to have slid beneath the radar of current philosophy of physics, which is devoted largely to relativity, quantum mechanics, and beyond. This is understandable but unfortunate. An introduction that focuses primarily on the hottest topics in current physics not only makes little contact with the physics that most of us know most about, but also gives the impression that metaphysically speaking, everything was straightforward until relativity and quantum mechanics came along. This is a highly inaccurate picture. Philosophical puzzles arise From such familiar features of classical physics as collisions, potential energy, fields, and energy flow ‑ puzzles with which nineteenth‑century physicists were deeply engaged, as we shall see.
Furthermore, concentration on relativity and quantum mechanics may give the impression that philosophical work contributes nothing positive to science, but merely comes along after the real work has all been done, trying to cause trouble for some theory that scientists them find pretty satisfactory. After all, quantum mechanics was not developed in order to address philosophical problems with classical physics, and though philosophical grounds for discontent with quantum mechanics were pointed out shortly after its initial development, these concerns have as yet led to no amendment to the theory. On the other hand, Einstein arrived at relativity precisely by thinking about what he perceived to be problems in the interpretation of classical electromagnetic theory. But these issues are neglected by texts that start with relativity and quantum mechanics, failing to look at classical electromagnetism; they emphasize only the experimental difficulties for classical physics that relativity addresses.
The problem with these texts is not merely their historical inaccuracy or their inability to account for Einstein's own remarks, such as this: What led me directly to the Special Theory of Relativity was the conviction that the electromotive force induced in a body in motion in a magnetic field was nothing else but an electric field. Rather, the most harmful consequence of this approach is that it makes the questions investigated by philosophy of physics appear "merely philosophical" in a pejorative sense: marginal, detached from the concerns that actually drive innovation in physics. In fact, contrary to the remarks I quoted earlier from Purcell and Feynman, "philosophical" concerns have been (and continue to be) integral to progress in physics, as we will repeatedly see. I hope, then, that this book encourages students of physics and philosophy to continue their fruitful tradition of thinking hard about what reality has got to be like in order for our physical theories to succeed as well as they do in predicting our observations.
Quantum Non-Locality and Relativity: Metaphysical Intimations of Modern
Physics
by Tim Maudlin (Blackwell) Modern physics was born from two
great revolutions: relativity and quantum theory. Relativity imposed a locality
constraint on physical theories: since nothing can go faster than light, very
distant events cannot influence one another. Only in the last few decades has it
become clear that quantum theory violates this constraint. The work of J. S.
Bell has demonstrated that no local theory can return the predictions of quantum
theory. Thus it would seem that the central pillars of modern physics are
contradictory.
Quantum Non-Locality and Relativity examines the
nature and possible resolution of this conflict. Beginning with accurate but
non-technical presentations of Bell's work and of Special Relativity, there
follows a close examination of different interpretations of relativity and of
the sort of locality each demands. The story continues with a brief discussion
of the General Theory of Relativity. This second edition also includes a new
author's preface and an additional appendix.
Quantum Non-Locality and Relativity introduces philosophers to the
relevant physics and demonstrates how philosophical analysis can help to resolve
some of the problems. All of the physics is presented from first principles, and
as much as possible is presented pictorially.
Feynman Lectures on Gravitation by Richard B. Feynman, edited by
Fernando Mornigo and William Wagner (Westview) Based on the
in-class lectures of Richard Feynman, this book covers a wide range of topics in
physics and provides a window to the thoughts of a brilliant Nobel laureate. It
shows how a classical field theory like General Relativity can be derived from a
quantum field theory. It also points out the extreme difficulty of accomplishing
this in the case of gravity and ending up with a consistent, anomaly free
theory. Deep, complex and difficult going but well worth the effort to see the
elegance of the connection between General Relativity and QFT
Readers of this book will benefit from familiarity with both quantum field
theory and relativity as well as a certain amount of mathematical
sophistication. Don't be fooled by the similarity of title to other "Feynman
Lectures on..." because this book is based on an upper level graduate physics
course and assumes the background of a typical PhD student in physics.
Feynman Lectures on Gravitation are based on notes prepared during a
course on gravitational physics that Richard Feynman taught at Caltech during
the 1962-63 academic year. For several years prior to these lectures, Feynman
thought long and hard about the fundamental problems in gravitational physics,
yet he published very little. These lectures represent a useful record of his
viewpoints and some of his insights into gravity and its application to
cosmology, superstars, wormholes, and gravitational waves at that particular
time. The lectures also contain a number of fascinating digressions and asides
on the foundations of physics and other issues.
Characteristically, Feynman took and untraditional non-geometric approach to gravitation and general relativity based on the underlying quantum aspects of gravity. Hence, these lectures contain a unique pedagogical account of the development of Einstein's general relativity as the inevitable result of the demand for a self-consistent theory of a massless spin-2 field (the graviton) coupled to the energy-momentum tensor of matter. This approach also demonstrates the intimate and fundamental connection between gauge invariance and the Principle of Equivalence.
Advanced Solid State Physics by Philip Phillips (Westview)
Phil Phillips received his bachelor's degree in chemistry and mathematics from
Walla Walla College in 1979 and his Ph.D. in physical chemistry from the
University of Washington in 1982. After a Miller Fellowship at Berkeley, he
joined the faculty in the Department of Chemistry at Massachusetts Institute of
Technology (19841993). Phillips came to the University of Illinois depart of
Physics in 1993 where is now a Full Professor of Physics. Philip Phillips is a
theoretical condensed matter physicist who has an international reputation for
his work on transport in disordered and strongly correlated low-dimensional
systems. His research lies predominantly in quantum, phase transitions with a
special emphasis on insulator-superconductor transitions, Mottness, and
competing order in strongly correlated electron systems.
Advanced Solid State Physics provides an accurate
exploration and solid foundation for students and researchers of this
fast-growing field. Provides ample background that underpins the principles of
solid state physics, and moves quickly to an overview of current research in
this fast-moving field.
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