1
Interpreting Physical Theories

In this chapter we get to grips with what philosophy of physics is all about and what kinds of questions it deals with. We also introduce some basic general concepts and terminology from philosophy: ontology, epistemology, etc. And also introduce such essential philosophy of science concepts as 'theory,' 'model' and so on - the concepts on which our later discussions will be based (the tools of the trade, so to speak). A key point that will be emphasized here is that there is often a difficulty in understanding how some (empirically) successful theory (formulated in 'the language of mathematics') can map on to physical reality - there is an additional question discussed of how mathematics can perform its feat of allowing the formulation of precise, successful physical laws. There is often, for example, a multiplicity of possible 'ways the world could be' according to the mathematical structure, while still preserving the theory's empirical success. We indicate that symmetries often lie at the root of (the most interesting of) these situations - a fact that will form the basis of much of this book.

1.1 Does the World Need Philosophers of Physics?

Does the world really have any need for philosophers of physics and the odd trade they ply? If we were to put them all on a spacecraft, in the style of the Hitchhiker's Guide to the Galaxy, say, on the Golgafrincham Ark Fleet's 'Ship B,' occupied by telephone sanitizers, public relations executives, advertising account executives, and other such 'worthy' tradespeople, would the human race be worse off or all the better for it?

Of course "need" can mean many things, each depending on the purpose or use. If we believe that there is a need to think especially deeply about physical theories, about why they work so well, and what they can tell us about the nature of reality: then there is a need for philosophers of physics. If we mean is there a practical use for philosophy of physics, then it is less clear that they have anything to contribute to the world, though that is not a clear-cut matter. But the main problem we face in answering this question is knowing where to draw the line between physics and philosophy of physics. If we were to end up throwing David Bohm, Niels Bohr, Albert Einstein, Werner Heisenberg, Erwin Schrödinger, and Hermann Weyl onto Ship B then you would agree, I hope, that the world would be all the worse for it. Yet each of these physicists was a 'philosopher-physicist': they even wrote books on the philosophy of physics. In days gone past they would have been called 'natural philosophers,' like Newton. I think natural philosophy is still a very useful term to use to describe one who studies the natural sciences philosophically, and if I had my way it would be back in operation to describe philosophers of physics and philosopher-physicists. I can think of several living (as of 2015, and long may they continue) physicists that fit the traditional mold of natural philosopher, among them Julian Barbour, Rudolph Haag, Roger Penrose, Carlo Rovelli, Lee Smolin, Max Tegmark, Gerard't Hooft, and Dieter Zeh. What characterizes them is that they think deeply about the foundations of their subject, and especially about the nature of space, time, and matter: our primary subject matter (or rather, the subject matter of our subject matter: theories of space, time, and matter).

And yet the philosophy of physics is often frowned upon by physicists. When physicists lapse into discussing philosophy it is seen to be just that: a lapse. Richard Feynman is famously reported to have said that "Philosophy of science is about as useful to scientists as ornithology is to birds" - there is a fairly sizeable catalogue of Feynman's anti-philosophy quotes to draw from and I think much of today's anti-philosophical spirit has a lot to do with Feynman worship (though there are many worse people to worship.). Again, as above, this might well be true, of course, depending on how one interprets 'useful.' 'Use' is just like 'need' again. When it comes to computing values of physical quantities to be compared with experiment, admittedly philosophers might not be of much use. They won't be too upset that they aren't of use in this sense: it's not what they live for. But if one is facing some problem in the foundations of physics, then the more generalist approach of a philosopher (or a philosophical approach) might be of some use after all - likewise, ornithology might well be useful to birds, in terms of conservation of a species for example!

Need and usefulness need to be more carefully circumscribed. One can easily make a case for a need for a philosophical approach to physics, and for the usefulness of such an approach. Given this, one can make a case for having a specific discipline, a dedicated community of scholars, devoted to such an approach, putting aside as secondary those core aspects of physics itself. Hence, becoming a philosopher of physics rather than a physicist involves a trade-off: you put the computing-intensive aspects aside in favor of the critical, interpretive aspects. Some people can do both, but they are the exception.

A philosophical approach to physics will direct attention to aspects of physics that are usually deemed sacred by the average practicing physicist. This can lead to advances by opening up new lines of enquiry, suggesting hypotheses that would be unthinkable at the everyday level of physics. In this book we will meet several examples in which this has occurred, mostly based on the work of philosopher-physicists. One often finds that thought experiments (or gedanken experiments) lie at the root of the really major advances in physics, the revolutions. Such thought experiments usually probe some foundational assumption, concerning space, time, locality, causality, determinism, matter, force, and so on. This kind of approach amounts to philosophical thinking (in the sense of conceptual analysis).

In this sense, then, philosophy is a fundamental part of the development of physics. One can't really do physics without making certain assumptions, however minimal, about how the theories one is using map onto reality (even if it is only mapping onto observable reality, as some believe [anti-realists, such as instrumentalists or constructive empiricists, who give unobservables the 'silent treatment'], though even this is not so benign a claim as they think).1 One needs to take a stance on what the components of the theories refer to, what they are about (this is the meaning of the philosophical term 'ontology': what there is).

This is one of the primary functions of philosophers of physics: interpretation of physical theories (on which, see §1.3 below). Mapping between theory and world. It is almost never a trivial matter; especially since physical theories often make use of all sorts of idealizations, approximations, and indirect methods of representing their 'target' systems. A key assumption underlying philosophy of physics, then, is that the job of physics is to say something about the structure of reality, about what the universe is really like: what objects there are, what properties they have, how they behave, how they relate to one another, and so on. Much of this book will be devoted to specific examples of this type and will highlight the ways in which interpretive controversies emerge. The interpretive controversies point to the existence of 'epistemological' considerations: what we can know. If there are multiple possible interpretations of some theory, then it seems we are limited in what a theory can tell us about the world. We will meet this in a very stark way in §5.1, in which an argument is presented that claims to show that our choice of world-geometry is largely conventional (that is, there is no fact of the matter that can decide, so we use other considerations to choose). We consider what interpretation amounts to in §1.3. First, let's take a look at a historical feature of philosophy of physics: the issue of its relative recency.

1.2 Why No Ancient Philosophy of Physics?

Philosophy of physics is a fairly modern discipline, emerging from various specializations that occurred at the close of the nineteenth century. Why didn't philosophy of physics exist earlier? Why wasn't there such a thing in the days of Democritus and Parmenides? We are happy to still discuss Newtonian mechanics as philosophers of physics, but it seems that stretching further back doesn't quite work in the same way (even with Plato and Aristotle): why is this? One important factor is that the pre-Socratics, and those working before the Medieval period, relied on naked eye observations. There were no amplifications of vision as there have been since Galileo modified the looking glass (until then a simple gypsy toy) into a telescope for scientific usage. There was, in fact, no real experimental method. Some kinds of experiments could no doubt be said to have occurred, but this was not viewed as the royal road to worldly knowledge as it is today. This meant that theoretical structures were far more heavily based on what was delivered through the senses. However, as we will see in a...