When I’m making small talk at parties and suchlike, revealing to others that I’m a philosopher of physics is a little bit like rolling the dice. What reaction am I going to get? The range is pretty broad, from ‘What does philosophy have to do with physics?’ to ‘Oh, that’s way above my pay grade!’ to (on happier occasions) ‘That sounds amazing, tell me more!’ to (on less happy occasions) ‘What a waste of taxpayer’s money! You should be doing engineering instead!’
Only the last of these responses is downright stupid, but otherwise the range of reactions is perfectly reasonable and understandable: philosophers of physics are, of course, not ten-a-penny, and what we’re up to is hardly obvious from the job description. So what I want to do here is sketch what the philosophy of physics really amounts to, the current state of play in the field, and how this state of play came about.
To cut to the chase: the philosophy of physics is the systematic study of our best theories of physics. This goes well beyond our current best candidates for fundamental theories of physics, such as string theory, and rather encompasses everything from Newtonian mechanics (which still constitutes the bread-and-butter of a great deal of practical physics, as well as perhaps the overwhelming majority of engineering), to Albert Einstein’s theories of relativity, to quantum mechanics, to the Standard Model of particle physics, to cosmology, and much else besides. And by ‘systematic study’ I mean something like asking questions such as these: ‘What is the conceptual and mathematical architecture of such-and-such a theory?’, ‘What would the world be like if such-and-such a theory were true?’, ‘What are the implications of such-and-such a theory for so-and-so areas of classical philosophical enquiry, like time, causality and identity?’
Given questions like these, one would be excused for thinking that the philosophy of physics is in fact continuous with physics ‘proper’ – and one would be quite right! Not only would trying to answer the above questions without a good working knowledge of physics be a forlorn hope, but also physicists themselves, especially theoretical physicists, often grapple with the structure of our best theories (for instance, there are a great many physicists working on the mathematics of general relativity, on the foundations of quantum mechanics, and so on). Ultimately, the difference between physics ‘proper’ and the philosophy of physics is really one of emphasis: philosophers are interested in understanding the structure and upshots for reality of the theories of physics that we already have, while physicists engage in the first-order practice of building new theories and then testing those theories experimentally.
The philosophy of physics isn’t some modern innovation – it’s been around for as long as physics itself
Indeed, its ultimate emphasis on bettering our understanding of the theories of physics that we already have helps to make clear that the philosophy of physics really is a discipline in the arts – despite often requiring a good understanding of physics itself. As I see it, the philosophy of physics is in fact somewhat akin to art criticism – but where the subject matter is not music or visual art or architecture, but the theories of physics constructed by some of the greatest and most creative minds of all time (from Galileo to Newton to Einstein). Philosophers of physics all share the view that physical theories – these products of the highest level of human creativity – are just as worthy of aesthetic appreciation and critical study as anything else.
So, the philosophy of physics is continuous with physics, insofar as it seeks to understand the structure of our best scientific theories of the world. It is also an artistic discipline, insofar as it engages with the critical scrutiny of certain products of the human mind – namely, physical theories. But the final thing to say here is that the field is also continuous with three other disciplines. First, with history, for in order to truly understand the content of any given theory, we must often turn to how it was developed, and the tools that physicists used to develop it. Second, with mathematics, because, often, a full understanding of the content of some given theory calls for shoring up some subtle mathematical details. And, finally, with philosophy ‘proper’, since we can’t really understand what a theory says about time, causation, identity and so on if we don’t appreciate the scrutiny to which philosophers have subjected those notions over the centuries.
All in all, this means that the philosophy of physics is an incredibly broad church – and a wonderful playground for those (like myself) who never could decide on the specific subject to which to dedicate themselves. In any case, I hope this goes some way towards giving a relatively clear sketch of what the philosophy of physics is about. What I want to do next is to stress that the philosophy of physics isn’t some modern innovation – in fact, it’s been around for as long as physics itself.
At least in the Western canon, a natural place to begin the tale would be with Aristotle, who was perhaps the greatest philosopher of all time (it’s either him or Plato!) as well as the progenitor of something like the scientific method. Aristotle engaged in a clear philosophical interrogation of the content of his theories of the world when he asked crucial questions regarding, for example, the location of the centre of the Universe. But, in the interests of constraining the narrative, let me leave Aristotle there, and dial things forward 2,000 years – to Newton.
In my opinion, the greatest scientist of all time is undoubtedly Sir Isaac Newton (1643-1727). One can make straightforward points to back this up: his achievements laid much of the groundwork for modern science, and for the paradigm of leveraging mathematics in order to solve problems in physics (which led to the Enlightenment idea of the ‘clockwork universe’). Moreover, Newton invented substantial new areas of mathematics, including calculus. However, it’s really at a more fine-grained level that one sees Newton’s genius: his magnum opus, the Philosophiæ Naturalis Principia Mathematica (1687), or The Mathematical Principles of Natural Philosophy, not only reads much like a piece of modern mathematics, but also – and this is much less known – contains insights that substantially presage many of the steps Einstein himself made on his path towards general relativity around two and a half centuries later. The Principia also contains philosophical reflections on the nature of space, time, and motion (as well as some damning interrogations of earlier thinkers, in particular René Descartes) that are as crisp and clear as the very best modern philosophical works.
Great physicists have been, on several important occasions, also great philosophical thinkers
Let me focus on the latter of these. With Newton’s theory of universal gravitation on the table, ‘natural philosophers’ of the day (the precursors to modern scientists) were able to interrogate long-held philosophical presuppositions regarding the nature of space and time with newfound rigour and precision. This is best exhibited in the Leibniz-Clarke Correspondence, a series of letters between Gottfried Wilhelm Leibniz (1646-1716) – himself a polymath philosopher, scientist and mathematician (he independently invented the calculus alongside Newton) – and Samuel Clarke (1675-1729), who was a prominent supporter of Newton at the time. In their letters (five on each side, before the correspondence was cut short by Leibniz’s death), Leibniz and Clarke explored various philosophical questions arising out of Newton’s new mechanics, for example: what is the difference between merely apparent (‘relative’) motion and true (‘absolute’) motion? How can we possibly determine ‘global’ properties of the Universe, such as its overall velocity? And how to understand identity in physics – could there, for example, be two physical bodies alike in all respects?
None of this work – which is obviously philosophical in nature, and which in fact counts as exemplary philosophy of physics – would have been possible without the framework afforded by Newton’s new physics. So this corroborates my point that developments in the philosophy of physics are often deeply intertwined with developments in physics, and that great physicists have been, at least on several important occasions, also great philosophical thinkers.
But this is just one data point. What happened after Newton? The standard narrative has it that philosophy and physics then went their separate ways: like a good midwife, ‘natural philosophy’ might have been of invaluable assistance in the birth of modern science, but after the birth she had no role to play. In the wake of the work of Immanuel Kant (1724-1804) and especially his successors including G W F Hegel (1770-1831), philosophy arguably became less concerned with the details of first-order science and its implications, and more committed to the investigation of loftier matters such as the limitations to the possibility of our knowledge, the possibility of access to a ‘transcendent’ reality, and so forth. Meanwhile, physics earned its stripes as an ‘exact science’, with the development over the course of the 18th and 19th centuries of the ‘analytical mechanics’ of Joseph-Louis Lagrange, William Rowan Hamilton, Adrien-Marie Legendre and others, which built upon Newton’s work and rendered it substantially more mathematically sophisticated.
There’s some truth in this narrative of amicable divorce, but there’s also a sense in which it’s not entirely fair, because – albeit perhaps less obviously than in Newton’s time – philosophical reflection by physicists and upon physics by philosophers persisted into the subsequent centuries. Here’s one example. In the 19th century, a mathematical revolution was precipitated by János Bolyai (1802-60) and Nikolai Lobachevsky (1792-1856), who showed that there could be consistent non-Euclidean geometries (that is, alternatives to Euclid’s geometry as presented in his Elements) in which, say, the sum of interior angles of a triangle is either more or less than 180 degrees. This work was brought to maturity by Bernhard Riemann (1826-66) in his habilitation thesis, but it was not long until this led to philosophical questions of the greatest depth: given the possibility of these geometries, need it in fact be the case that, as a matter of strictest necessity (as Kant had it), the structure of space must be Euclidean, or are there in fact other possibilities for the geometry of space? This question, which became known as the ‘Problem of Space’, came to occupy some of the greatest minds of the time, including the physicist Hermann von Helmholtz (1821-94), the mathematician Sophus Lie (1842-99) and the polymath Hermann Weyl (1885-1955).
They were able to leverage their philosophical maturity and insight to make significant steps in physics
Here’s a second example of the persisting relationship between physics and philosophy. During the 19th century, physicists – most notably Ernst Mach (1838-1916) – worried a great deal about various assumptions in Newton’s mechanics, particularly the assumption that space and time are inert background structures in which physical events unfold. In opposition to this, Mach laid down a principle (dubbed by Einstein ‘Mach’s principle’) according to which there can be no spacetime without matter, and sought to reformulate Newtonian physics around such a principle.
These two threads found their confluence in the second golden age of interaction between physics and philosophy, at the turn of the 20th century – a golden age led by luminaries such as Henri Poincaré (1854-1912), Weyl and, of course, Einstein himself (1879-1955). This was an age in which – just as in the time of Newton and Leibniz, and Aristotle well before them – the greatest scientists and mathematicians were once again the greatest philosophers, and as such were able to leverage their philosophical maturity and insight in order to make significant steps in physics itself.
Perhaps the most telling example of this is Einstein’s development of the general theory of relativity (which remains our best theory of space, time and gravitation) over an approximately 10-year period from 1905 to 1915. This made essential use of both the insights gained from the Problem of Space and the possibility of non-Euclidean geometries, and of Mach’s principle, at least as heuristics for the development of the theory. Indeed, Einstein acknowledged explicitly throughout his life the influence that his philosophical training had on his physics, as well as (reciprocally) the influence that his physical developments had on his philosophy of science.
What this narrative makes clear is that the extent to which there is non-trivial interaction between physics and philosophy can fluctuate over time, but the philosophy of physics – the critical interrogation of our best theories of physics – has been around for a long time, whether as practised by ‘pure’ philosophers, or by the physicists themselves (or by ‘natural philosophers’, when there was no clean distinction between the two disciplines).
But if this is right, then why does the philosophy of physics remain a rather niche discipline in modern times? Well, as I’ve stressed, the interactions between physics and philosophy can wax and wane. In fact, after what I’ve described as the second golden age of interactions at the turn of the 20th century, and especially after the debates between Einstein and Niels Bohr (1885-1962) on the new quantum theory in the late 1920s, in which the latter intimated cryptically that reality at the fundamental level is ineffable, and that we should give up on trying to secure a complete picture of the world at the fundamental level, things took a rather different turn. At some point later in the 20th century, many physicists developed an attitude of outright hostility towards philosophy. This came to a head – and somewhat unfortunately made it into the public consciousness – in the writings of highly respected physicists such as Steven Weinberg – in whose book Dreams of a Final Theory (1992) was a chapter provocatively titled ‘Against Philosophy’ – and Stephen Hawking, who, even more provocatively, declared in 2010 that ‘philosophy is dead.’
What were the sources of this unfortunate shift in attitude? For better or worse, Bohr’s philosophy was not only obscure (what does it mean, after all, for reality to be ‘ineffable’?), it also suggested that it’s simply misguided for physicists to have pretensions to ‘understanding reality’. Bohr was one of the most influential figures of his day, so it’s not too surprising that these instrumentalist ideas, according to which physics is a mere ‘instrument’ for making experimental predictions, and in turn according to which we should no longer have such grandiose ambitions as ‘understanding the fundamental nature of reality’, took root in physics at large.
It’s not uncommon to hear physics lecturers across the globe making pejorative comments on philosophy
The second source of the shift was that, after Einstein’s work on general relativity – which, as we have seen, was shot through with philosophical thinking – the theory entered the doldrums for several decades. Very few physicists actually worked on it because it was regarded both as too difficult to learn and too difficult to test, until the arrival of the work on singularities and black holes by Roger Penrose, Robert Geroch, Hawking and others in the 1960s. At the same time, however, particle physics was thriving, with rapid developments in quantum field theory in the 1930s and ’40s – work that would eventually lead to the development of the Standard Model of particle physics. That work was both very collaborative and very pragmatic: physicists were content to develop tools without necessarily understanding the mathematics they were manipulating, let alone the conceptual foundations of the work. It’s not difficult to see that this hands-on, workmanlike attitude would be in tension with philosophical reflection on the craft.
To some extent, this attitude persists today. It’s not uncommon to hear physics lecturers in universities across the globe making pejorative comments on philosophy (often from a position of some ignorance); the rather more reflective physicist N David Mermin coined the saying ‘Shut up and calculate!’ in 1989 to encapsulate the instrumentalist attitude many physicists seem to have towards the foundations of their subject matter. Despite all this, I think there’s a wealth of evidence that the interaction between physics and philosophy, and the discipline of philosophy of physics itself, is very much on the up – perhaps even entering a third golden age.
One practical reason is that the workmanlike attitude of those physicists working on quantum field theory in the mid-20th century is no longer obviously sufficient for doing good physics. The Standard Model of particle physics is now complete, and (with the discovery of the Higgs boson at the Large Hadron Collider in CERN) extensively empirically confirmed. That said, while plenty of extensions to the Standard Model (most notably, those that invoke supersymmetry) have been developed, none have been tested experimentally, and the prospects for empirical confirmation of any particular such model don’t look great, because it is simply too expensive to build the particle colliders needed to probe the energy scales where one would expect experimental signatures of such models. As a result of issues such as these, many physicists have come to newfound appreciation for the value of philosophical reflection on different areas of physics.
Let me take some examples to back up this point. Over the past few decades, fields such as quantum information theory and quantum computing have become significant within physics, and these fields have had to grapple with concepts that are adjacent to philosophers’ concerns. For instance, in order to build a viable quantum computer, it’s widely recognised that the phenomenon of ‘decoherence’ (roughly: the suppression of quantum mechanical effects at macroscopic levels) must be controlled. Decoherence, however, turns out to play a key role in essentially all known interpretations of quantum mechanics – that is, the projects to understand what quantum mechanics says about the nature of the world; the projects that find their natural home in the philosophy of physics. As such, it’s unsurprising that physicists working in these areas of quantum theory have been much more open to dialogue with philosophers on quantum mechanics than their predecessors might have been.
Dialogue with practising physicists is the best it’s been in decades
As a second example, physicists now much better appreciate that, when exploring the space of alternative theories of gravitation to Einstein’s general relativity, it is helpful to taxonomise these with reference to whether they satisfy his ‘equivalence principle’ and, since it is philosophers who best understand such principles, physicists have in recent years entered renewed dialogue with them on such matters. And, as a final example, cosmologists have recently begun to worry about whether modern cosmology will be ‘permanently underdetermined’ by evidence, and therefore we will remain forever in the dark as to the large-scale structure of the cosmos. This invites newfound dialogue between physicists and philosophers in order to explore these possibilities and assess the prospects for overcoming such issues.
In the present day, the philosophy of physics is once again in a good place. There are a great many of us distributed across the globe (almost 300, according to this list) and the community as a collective has a great deal of expertise in all four fields of physics, philosophy, mathematics and history. Dialogue with practising physicists is the best it’s been in decades. (Even the job description of ‘natural philosopher’ is resurging: witness the recently opened Radboud Centre for Natural Philosophy in Nijmegen in the Netherlands, and the fact that Sean Carroll at Johns Hopkins University has the newly created title of Homewood Professor of Natural Philosophy.) Given that careful attention to foundational and philosophical issues was an integral component of many of the previous leaps forward in physics, this can only be a good thing.
Even if one doesn’t have pretensions to developing a quantum theory of gravity, when one really attends deeply to the structure of our best theories of physics, one finds in all corners subtle, fascinating and superlatively beautiful issues to explore. To take just one example (and to circle back to my suggestion that the philosophy of physics be understood as a form of art criticism), the mathematics of ‘gauge theory’ – which can be used to make rigorous and conceptually clear sense of the foundations of the Standard Model of particle physics and quantum field theory – is as beautiful and elegant and sumptuous as any Botticelli or Caravaggio or Monet. As such, the payoffs and insights we can gain from it – both aesthetically and metaphysically – are at least as great as any from the fine arts. It’s in the philosophy of physics that we have the liberty to study and criticise such edifices and their conceptual upshots, without necessarily feeling the pressure to develop new theories or build new experiments – crucial as those are, of course.
