Realism In Physics

Following the recent discussion about the existence of electrons, I can’t help but feel like paraphrasing Jules (played by Samuel L. Jackson) in Pulp Fiction, “say [electron] again, say [elec-tron] again, I dare ya, I double-dare ya motherf***er, say [electron] one more god damn time!” But, I just wanted to throw in my 2 cents on the question around realism in physical theories, based on my discussions with physicists. I am, as always, open to correction but I feel that identifying certain nuances in the different positions might help to progress some discussions beyond the archetypal “philosophy is useless to physicists” or it’s counterpart “physicists don’t know they’re doing philosophy”. Perhaps identifying the subtle nuances (maybe even sleights of hand) that people employ might help us to identify different questions to ask, which can help progress both philosophical discussions and those in physics. Maybe not, sure what do I know?!


Philosophers and Physicists 

That philosopher’s and physicists often see things differently, is an understatement – not least when it comes to the value of philosophical inquiry. It should come as no surprise then that, sometimes, philosophers and physicists understand certain ideas differently. One such example would be with respect to the notion of realism in physcis – or perhaps what is sometimes referred to as “anti-realism”.

In philosophy, broadly speaking, realism is the perspective that posits the existence of an objective reality independent of human perception or thought. Realism asserts that the external world exists regardless of whether it is being observed or conceptualized, challenging notions that reality is solely a construct of subjective experience or consciousness. While it can also take this meaning in the field of physics, there is a possible nuance in it’s application to the mathematical formalism of physical theories. Nowhere is this more evident than in the field of  quantum mechanics.


The Realism of Mathematical Objects

The world of quantum mechanics has always challenged our intuitions about the nature of reality, including the idea that certain properties of the world exist regardless of whether they are being observed. In the often-heated debates among physicists and philosophers (about the existence of…..what?), it’s crucial to understand that when some physicists speak of anti-realism, they aren’t necessarily rejecting realism altogether. Instead, their focus is on the realism of mathematical objects within physical theories. “Anti-realism” in this context is simply a denial of the idea that all mathematical objects in physical theories e.g. the wave function, are real ontological entities or “beables”, to use the word coined by John Bell. This approach to quantum mechanics (or indeed physics in general) is often called instrumentalism (when not accompanied by further philosophical considerations).

The instrumentalist interpretation of quantum mechanics is a pragmatic philosophical stance that refrains from making ontological claims about the underlying reality of quantum entities. Instead of positing the existence of unobservable entities like particles in definite states, instrumentalism treats the mathematical formalism of quantum mechanics as a predictive tool. According to this view, the primary function of quantum theory is to provide a calculational framework that accurately predicts the outcomes of experiments without necessarily insisting on a concrete, objective reality for the entities involved. Instrumentalists argue that while the mathematical machinery of quantum mechanics is remarkably successful in making predictions, attempting to assign definite properties or states to quantum particles may be an unnecessary metaphysical pursuit. Thus, instrumentalism allows scientists to focus on the practical utility of the theory without committing to a specific ontological interpretation of the quantum world. It’s worth baring in mind that it is even possible for a physicist to be an anti-realist with regard to the mathematical formalism of physical theories, while remaining a realist in terms of their personal philosophy.

A key aspect of quantum theory, which lends itself more to such an instrumentalist interpretation, is the statistical nature of the theory. Since the predictions of quantum mechanics are probabilistic, it means that statistical samples are required to test its predictions. For this reason, it is quite reasonable to interpret the mathematics as nothing more than a description of the statistical samples observed in the laboratory.


Crude Analogy

Let’s imagine we have a theory which predicts the probability of coloured balls being drawn from a bag*, with a probability of 0.75 that a red ball is drawn and 0.25 for a yellow ball to be drawn. If we draw only one ball from the bag and it is yellow, that doesn’t help us to confirm or refute our theory. Instead, we have to draw a statistical sample large enough that we can see if the probabilistic predictions are accurate.

Let’s say we draw 10,000 balls and 7,500 are red while 2,500 are yellow. We would have increased confidence that our theory is correct, but you might be wondering what this has to do with scientific realism?

Intuitively, we are all imagining the different coloured balls in the bag (realism) in the given ratio, with each ball simply being drawn out of the bag to reveal the pre-existing ratio.

However, we then find out that this cannot be correct, because someone has shown us that, when we work on the assumption that the balls already have their colours in the bag, the probability of pulling a red ball out of the bag is actually 0.666, while the probability of drawing a yellow ball is 0.333. So, simply applying our classical realist view to the “balls” in the bag doesn’t work.

Now, some very clever people come along and show us different ways in which we can still have a realistic view of the balls in the bag, involving spontaneous colourisation or something which guides our hand to pick the appropriate coloured balls.

However, some others say, I don’t think those pictures are correct. In fact, I think the theory simply predicts the ratio of red balls to yellow balls and doesn’t imply that there are “balls” in the bag at all. That doesn’t mean there is nothing in the bag, just that the mathematical objects we use don’t represent ontological objects.

These latter theorists cannot be accused of not doing science nor can they be accused of not explaining the obesrved data. What they might be accused of is, to paraphrase EPR, not providing a complete description of the physical reality, including what is inside the bag and how it results in coloured balls being drawn.

* The thought experiment isn’t meant to replicate a test of Bell’s inequality.


Just for clarity, the nuance I am referring to is the distinction between explaining the observed data – in the barest sense of the term “explain” – and that of explaining the process by which the data comes to be observed. I think most of us, including physicists (if they are being honest with themselves), want both. So, in discussions with instrumentalists it can be useful to bear this in mind and, when pushing them for an “explanation”, be sure to emphasise that we are looking for a “complete description of the physical reality”.


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