Philosophical thinking brings with it a heathy dose of skepticism about many things that are often taken for granted. This is generally a good thing and can lead to greater clarity. It can also lead to ideas that look downright bizarre to the general population: the idea that you might be a brain-in-a-vat; the idea that the universe could have come into being five seconds ago; the idea that you are really a spacetime worm stretching from your birth to your death!

In this book we apply the principles of philosophy to theories of physics – it is important to emphasize that we are dealing with theories of physics in philosophy of physics: these are the entities that link us to the world and are that which we must interpret. Again, this involves clarity through skepticism, but also leads to views that strike many as odd: the idea that it takes a mind to ‘collapse a quantum state;’ the idea that there are many worlds (one for each choice in quantum mechanical experiment); the idea that the future can causally influence the present. The good news is that these latter examples are grounded in our best physical theories: odd or not, they seem to offer possible interpretations of these theories (i.e. they are ‘ways the world could be given the truth of the theory’). Modern physics can even throw some light on what might be considered ‘purely philosophical’ issues, such as ‘what is the relationship between an object and its properties?’, ‘can two objects share exactly the same properties?’, ‘is the future open or fixed?’, ‘does time flow?’, and ‘are you really a four-dimensional spacetime worm?’

There are several books on philosophy of physics that do a very good job at introducing the reader to the basic issues, with varying levels of difficulty. The present book aims to provide a snapshot of the central topics, methods, and problems of modern philosophy of physics in a very elementary manner. The audience for this book is the absolute beginner, albeit one with a modicum of mathematical ability (or an ability to at least not glaze over at the mere sight of some mathematical formulae). But it nonetheless aims to be a complete course, in that it covers all the main areas (classical and quantum, relativistic and non-relativistic, statistical and non-statistical) and provides supplementary readings across a range of skill levels, so indicating the work that needs to be accomplished to reach research-level philosophy of physics.

The book is written for both early stage philosophers and physicists:

• Physicists who want more than rules and algorithms for churning out numbers to compare with experiment, and want to know ‘what it all means.’
• Philosophers who want more than watered-down, quasi-journalistic physics, and who are not satisfied with philosophical discussions of the nature of the physical world that do not engage with physics.

I’m not going to tell you to shut up and calculate; but I’m not going to tell you not to calculate either! A little bit of computational skill is vital in good philosophy of physics, though one could probably be a good physicist without having a philosophical bone in one’s body. I would argue, however, that one could not be a great physicist without a good head for philosophical thinking. Despite the necessity of having a good grasp of the mathematical details of physical theories in order to be a proficient philosopher of physics, this book makes do with the bare minimum: it is a stepping stone to the many books of a rather more mathematically involved nature. Where matters get a little technical (or where there are interesting diversions, historical or otherwise), I have relegated these to endnotes – these often contain suggestions for interesting, usually more advanced readings.

The book begins, in Chapter 1, with some general considerations about philosophy of physics itself, as a discipline. Central to this is the idea that philosophy of physics concerns itself primarily with interpreting the representations of physical systems that can be found in physics (usually the best available physics, couched in mathematical representations). We also consider the question of why these mathematical representations seem to be so good at gripping onto the world.

Chapter 2 introduces some basic concepts from physics: the states, observables, and dynamics that form the bricks and mortar of the world-pictures (or ‘ontologies’) according to our theories. This machinery is then used to introduce symmetries in physics in Chapter 3, in which we also begin to see how philosophical issues emerge from symmetry – symmetry will play a central role in the chapters that follow.

Chapters 4 to 7 apply all of this foregoing discussion to specific examples, starting with spacetime theories in Chapters 4 and 5 (including the theories of relativity), then statistical physics (in Chapter 6), and quantum mechanics (Chapter 7). Chapter 8 provides ‘tasters’ of seven more cutting edge issues in philosophy of physics, which might provide more scope for future research projects for budding philosophers of physics.

Each chapter includes a handful of further readings, organized according to difficulty: ‘fun,’ ‘serious,’ and ‘connoisseur’ level. This book should be supplemented with at least the fun readings (or suitable extracts) in order to provide a rounded picture. Together with these, and perhaps several of the readings from the endnotes, this would provide material enough for a semester-long course in introductory philosophy of physics.

I should perhaps end with a brief note on the endnotes in this book: there are lots of them! P. G. Wodehouse mercilessly slammed footnotes in his autobiography Over Seventy: An Autobiography with Digressions (Herbert Jenkins, 1957):

When I read a book I am like someone strolling across a level lawn thinking how jolly it all is, and when I am suddenly confronted with a ⁽¹⁾ or a ⁽²⁾ it is as though I had stepped on the teeth of a rake and had the handle spring up and hit me on the bridge of the nose. I stop dead and my eyes flicker and swivel. I tell myself that this time I will not be fooled into looking at the beastly thing, but I always am, and it nearly always maddens me by beginning with the word ‘see.’

However, endnotes, while ameliorating some of Wodehouse’s complaints, also face his wrath:

Slightly, but not much, better than the footnotes which jerk your eye to the bottom of the page are those which are lumped together somewhere in the back of the book. These allow of continuous reading, or at any rate are supposed to, but it is only a man of iron will who, coming on a ⁽⁶⁾ or a ⁽⁷⁾, can keep from dropping everything and bounding off after it like a basset hound after a basset.

Some explanation is therefore in order for filling a book with almost ninety endnotes. These notes serve two functions:

1. to cope with the inevitable difference in skill sets and diverse backgrounds of the likely readers of this book: some of you will be at ease with philosophical concepts, but perhaps confused by certain aspects of mathematics and physics; and some of you will have the opposite problem. Endnotes provide some opportunity for filling out such concepts.
2. to enable readers’ specific special interests to be attended to (mostly via suggestions for further readings) in an unobtrusive way (Wodehouse’s remarks notwithstanding): some of you would be keen to know where they can learn more about conventionalist principles in science, for example, while others find that incredibly boring and will want to focus on the more mind-boggling, metaphysical issues, or some more technical issues.

A reader with no inclination to bounce around the book can rest assured that (again, Wodehouse’s remarks notwithstanding) the entire book can be read without ever turning to the back, or perhaps browsing through them separately at their leisure.

Sydney, October, 2015    DPR