Jackson Smith - Physics for Humanities Students


Given the portrayal of science, especially Physics, in the media, it's easy to feel slightly sceptical. Science is largely responsible (some would argue) for most of the defining characteristics of modern life, yet its presentation in the media is often woeful, with scientific achievements presented over-reverentially and inaccurately by people who have no understanding of them. From Paxo's shock whenever someone actually gets a Physics question right on University Challenge, to various science correspondents' attempts to explain what a 'five sigma' discovery is, Physics can seem bizarre and esoteric. Most arts students pride themselves on knowing at least a little about important cultural topics outside their special field, but view Physics as something that is well and truly outside their domain. However, Physics embodies one of the most important ways we gain knowledge, and an understanding of Physics is essential for anyone who wants to make statements about what knowledge is, whether objectivity is possible and how the human mind comes to understand the world around it. On this page, I will outline what Physics is, and how it links to Philosophy and the Arts.

What is Physics?

I'd like to introduce the following working definition:
Physics is the search for concepts, models and theories that can explain our past observations and predict future ones.

This definition is particularly suggestive, and should lead us quite nicely to consider a few of the interesting philosophical points about the study of Physics. I'll start, though, by explaining the terms used in this definition. The key elements of Physics; the objects that all Physicists work with; are ideas. In the above definition, I've chosen to divide ideas into three classes - concepts, models and theories.

A concept is an idea about the existence of some entity. For example, we have the concept of temperature - objects that feel hot have more of this thing we call temperature than objects that feel cold. Other common physical concepts are things like force, acceleration, pressure and rotation. Physicists often also use concepts borrowed from Mathematics, such as complex numbers. In some sense, concepts define the language we use as Physicists; the terms we use are quite strictly defined, but it's sometimes difficult to know, as a layperson, what exactly the definition being used is.

The two classes of ideas that actually explain the world around us are models and theories. These are ideas about the causal links between different concepts (e.g. force causes acceleration). I have defined two separate classes because there are, broadly speaking, two levels on which one can approach the task of explaining the world around us. Models are used to explain specific phenomena that are observed, by making analogies between the real physical system and some simplified version. For example, we can model a Hydrogen molecule, consisting of two Hydrogen atoms bonded together, as a pair of spheres joined by a spring. This simple picture can be used to explain the rotational and vibrational motion that is possible for the system. Any given model has some limited range of validity - some minimum criteria that must be fulfilled in order for the analogy to be a good one (i.e. good enough to allow us to predict the behaviour of the system). In the case of the Hydrogen molecule, our model treats the electrons in the system as a simple spring, and so cannot explain the frequencies of light absorbed by the molecule when a single electron enters an excited state. When we create a model, we are usually aware of some simplifications we have made that will limit its validity. In contrast, a theory is broader in scope than a model, and is intended to represent what's really happening in the physical system, not just an analogy. For example, in our latest theory of light, all light is made up of photons, tiny particles, and we believe these particles really exist; they're not just a convenient way of thinking about light.

As Physics progresses, our old theories are often superseded by new ones that can explain more phenomena. As such, the old theories are revealed to be models - ways of thinking about the system that are useful but in some way oversimplified. In our light example, before the 'particle theory' there existed a 'wave theory' of light. This was based on the idea that light moves through space in the same way that a wave moves on water. It is hopefully clear, then, that the boundary between models and theories is slightly blurry - in some sense, the difference is that in a model, we knowingly oversimplify, whilst in a theory we try to include every aspect of the system but often still end up oversimplifying it.

The next key part of my definition is the idea of explanation - Physics aims to explain our observations. There are two important assumptions here, which I'll hopefully expand upon in the next section. Firstly, we assume that our observations give us knowledge about the way the world is (i.e. that they are objective). Secondly, we assume that it is possible to translate this knowledge of how the world is into an explanation of how the world works - that we can get from an objective description of the world's phenomena to a set of causal relations that give a mechanism to create these phenomena. There is another branch of academia whose aims are more modest than Physics in this respect - in the field of Statistics, one is interested only in using our past experience to predict future events, not by understanding their exact origins, but through the laws of probability. Where there is no supply of past experience on which to draw, Statistics cannot operate. In contrast, the spur to new Physics is often the extrapolation of existing knowledge to new areas. If one experiment indicates that light is made of particles, then we would expect these particles to give rise to observable effects in many different situations; because we have some hypothesised model or theory of the phenomenon, new lines of enquiry are suggested to us by which to test our hypothesis.

Finally, perhaps the most important part of my definition of Physics is the idea that we need to make observations of the world around us. We do this using experiments - systematic and carefully controlled observations. Without experiments, our choice between different models and theories would be arbitrary, and ultimately up for never-ending debate. However, experiments give us a measure by which to assess different models and theories - we compare their predictions to the outcomes we measure. This is probably the most defining characteristic of Physics. In some sense, we could define Physics as the set of all topics that can be studied in this way. If a solid method existed for determining historical fact, then this also would fall into the remit of Physics, but thankfully this isn't the case and we instead live in a world of endlessly fascinating debate and sophistry.

Physics Contextualised

Physics shares many of its foundations with Philosophy. Indeed, Physics only emerged as a separate subject within the last few centuries of its millennia-long evolution. That said, modern Physics is characterised by a sturdy pragmatism that belies its philosophical roots. This is largely due to fact that in Physics we have experiments to test our hypotheses. As such, the precise method by which we generate hypotheses is not quite the focal point of our work. This experimental side of Physics can give the impression of a vast gulf between itself and Philosophy (and the Arts). However, Physics can be contextualised alongside these other fields of study, which are seen to share the same basic perspective as Physics, with the crucial difference that less quantifiable evidence is available to test hypotheses.

The key philosophical traditions from which modern Physics inherits are the same ones which justify any attempt to gain objective knowledge about the causal relations of the world from human experience of it. Induction, empiricism and reductionism are three tenets whose influence on Physics is clearly manifest, though there are other ideas that have passed from Philosophy into Physics, and sometimes back again. Notably, Einstein's principle of Relativity was inspired partly by the work of Philosopher of Physics, Ernst Mach. Following Einstein's rise to fame, 'relativity' entered the zeitgeist, and was highly influential (and often misinterpreted) in the development of ideas of cultural relativism and the vague notion that reality is ultimately subjective.

In a pragmatic sense, the success of Physics justifies its philosophical bases - a viewpoint based on these traditions has given us sufficient understanding of the objects of our everyday experience to allow us to manipulate them as we desire; creating engines, refrigerators, computers, space shuttles, the internet and all the other technologies that define our era. Anyone arguing against these philosophical bases must be at pains to explain the apparent effectiveness of Physics.

Creativity, also, has its place in Physics. Thanks to our ability to perfom experiments, we have a method to distinguish good hypotheses from bad. This gives us the creative freedom to generate any number of hypotheses, safe in the knowledge that only those that match our experience of reality will propagate. This creativity exists in the Arts, but, in that arena, the measure of a given hypothesis is usually the convincingness of its proposal; its appeal to the intuition of the reader. As such, one is constantly led to question one's own perspective to safeguard against cultural and other biases. In Physics, our constant recourse to experiments allows us to cut through this mire of confusion.

Doing Physics

Simplification and comprehensibility:
What do we mean when we say we 'understand' a physical system? As I argued above, we understand a system when we possess a set of causal relations that allow us to successfully predict the behaviour of the system, i.e. we have both a qualitative description of the system, and the ability to make quantitative predictions about it. Physics progresses because we continue to take our understanding of existing systems and apply it to new ones. Whilst this process is essential for the advancement of the frontier of knowledge, one can only really trust the predictions of a theory/model within the range of situations in which it has been tested - any use of the theory outside this range is in some sense an experiment. However, if we are willing to concede this, then what predictions can we trust? If we perform an experiment on one particular system, e.g. a pendulum, then how can we be confident that the theory we develop will apply to any other pendulum? In some sense, we must define the essential features of a physical system that must be present for our theory to apply - what makes a pendulum a pendulum? The only real answer I can give to this is that one must use physical intuition - the experience gathered by studying many different systems, which may apply more or less tangentially in any specific case. In our pendulum example, it is intuitively clear that our pendulum will behave very differently if it is under water. It will behave slightly differently if it is at a different altitude, though this is perhaps only apparent from experience. You may conclude that all pendulums are different, and there is no point having a general theory to treat them all. However, it is only by considering such general systems that we are able to comprehend the world at all. The point is that the complex systems of our everyday experience have behaviour that can be deconsstructed into a combination of many simple behaviours, each analogous with the behaviour of some simple system, e.g. a pendulum. It is because of this deconstruction that we can apply the intuition we gain from studying very simple and unrealistic systems to the study of much more complex and realistic ones.

The human process of extrapolation:
This process of extrapolation, from simple systems to much more complex ones, is a key feature of the way we do Physics. A remarkable property of the human mind is its ability to see the similarities in very different systems. For example, an atom is a very different system to a pendulum, but in some ways it can be useful to think of the two systems as equivalent - intuition gained by doing so has contributed greatly to our understanding of the atom. Of course, we should not be too surprised at the mind's ability to draw analogies between very different systems - this is after all the basis of metaphor.

Quantifying knowledge - errors and statistics:
In the above discussion, I have made much of the fact that Physicists use experiments to test their models and theories. In practise, this is done by using the hypothesised model/theory to predict the value of a quantity that can be measured, then doing an experiment to measure that quantity. How close does the measured value have to be to the predicted one in order for it to count as evidence in support of the hypothesis? If our theory predicts, for example, that the angle between two particles should be 15 degrees, but it is measured to be 14 degrees, then is this a success or failure of the theory? Statistics is invaluable in quantifying the significance of such a discrepancy - using the techniques of statistics, we can combine the various imperfections in our measurement to find just how likely it is that our measured value matches up to the predicted value, seen through the haze of the imperfect process of measurement. Using statistics, we can assign an 'error' to our measured value - the amount by which we may realistically have mis-measured the value. For our earlier example, an error of 1 degree or more would mean our measurement does match our hypothesis. In many areas, the progress of Physics depends on our developing ever more sophisticated and accurate measuring techniques to reduce the errors on our measurements, and thus test our theories more stringently.

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Copyright - Jackson Smith 2013