WHAT DO NEWTONIAN FORCES HAVE TO DO WITH THE STANDARD MODEL?

The debate about what there is runs deep in the history of science and is still a central question, in particular for scientific realists who believe that empirically successful scientific theories are approximately true and inform us about what the world is like.

The existence of tectonic plates was contested in geology for a long time, and something similar happened in physics and chemistry with atoms. Eventually, these entities were agreed to exist and they are now central to our understanding of the Earth. But sometimes scientists end up denying the existence of an entity that had been regarded as existing and that had some explanatory value. A fluid-like substance called caloric was posited in the eighteenth century to explain heat exchange, but was eventually dismissed in favour of the kinetic theory of heat. The aether is another famous example. And, to a certain extent, Newtonian gravitational forces have also been explained away by general relativity.

But not even theories that seem extremely well confirmed today are completely safe. Atoms are central to physics and their existence is confirmed in countless ways from many points of view. However, there is also a sense in which atoms are an imperfect description of what there is. A physicist could insist that our description of reality in terms of atoms and forces acting between them is essentially misguided, and all there is are quantum fields. Even more, they could further insist that both these pictures are technically false because only our final theory of quantum gravity will tell us how the world really is. Should we thus give up our belief in atoms because they represent at best an imperfect and only approximately true description of reality? Should we do the same with Newtonian forces?

Our BJPS article starts from the idea that there is a common thread that links these issues about existence in the context of scientific theories. We maintain that the question about the ontology of quantum field theory and the question about the existence of atoms and Newtonian forces are intimately related.

A theory like Newtonian gravitation can be classified as a superseded theory, while atomic theory or even quantum field theories can be classified as special sciences like molecular genetics, in the sense that these theories do not apply in every domain, as they are not the most fundamental physics. We argue that both superseded theories that were once believed to be true and theories of the special sciences can actually be taken as true about certain features in a specific limited domain.

A theory that has a limited range and domain of applicability is often called an ‘effective theory’, and theories belonging to those two categories can be classified as effective theories to a certain degree. The version of scientific realism that takes effective theories seriously as telling us about what exists is called ‘effective realism’. The central idea is that if a theory is correct at least in a limited domain, then we should simply be committed to the existence of the entities posited by such a theory in that domain. As such, we should be ‘effective’ realists about atoms or Newtonian gravitational forces, even if they are not the most fundamental building blocks of reality.

Different distinct versions of effective realism have been proposed in the philosophical literature recently. These diverse accounts help us to make sense of what superseded and special science theories tell us about reality, but also solve several other more specific problems.

Fraser and Vickers ([2025]) develop a version of effective realism about non-relativistic quantum mechanics. This theory is only true in a limited domain, and they also argue that it faces an underdetermination issue: different empirically equivalent versions of non-relativistic quantum mechanics describe reality in completely different ways. To break the underdetermination, we can find a common core to all the different versions of the theories and be effective realists about that description.

Williams ([2019]) and Fraser ([2020]) propose effective realism about quantum field theory as a response to the problems facing realist interpretations in this context. Theories in this family are empirically incredibly successful, but they only apply at certain energy scales and their physical content is debated. Effective realism provides a realist reading of these theories, while granting that they don’t tell us about the fundamental nature of reality.

Robertson and Wilson ([2026]) propose effective realism to deal with the dubious ontological status of the entities belonging to superseded theories, which is the problem related to entities like Newtonian gravity mentioned earlier.

All of these effective realist proposals have been developed largely independently one from another. Our BJPS article brings them together and argues that they all are different sides of the same kind of question. But, most importantly, we also argue that these accounts should be accommodated within a broader approach to scientific realism called ‘structural realism’, a refined version of scientific realism with a long pedigree that enjoys several benefits. Structural realism (in the ontic structural realist version) comes with a very useful pre-assembled package of conceptual tools that help us to better understand all of these disparate contexts. Reframing the effective realist accounts we described here in terms of structural realism improves them.

Structural realism is a sophisticated version of scientific realism. Hence, it is an answer to the question, ‘what do scientific theories tell us about the world?’. According to this view, scientific theories reveal the structure of the world, for example, by telling us about causal or lawlike relations between phenomena. For instance, physics tells us what electrons can do, how they behave, how they are related to other particles, and that they obey the Pauli exclusion principle.

Structural realism was originally motivated by the debate about scientific realism and the historical cases of theory change. Ontic structural realism was further motivated by the nature of quantum physics and general relativity. As already noted, certain entities have been discarded as science has progressed and theories have changed. However, structural realists stress how certain patterns and structures have been retained even after theories have been abandoned. This gives us reason to believe that these theories were right about those features, and the structural realist claim is that those features are the ones we should believe in. They are the things that science is reliable about.

This shows how the structural realism framework connects with the question about the entities of superseded theories. The structural realist can say that the theory of caloric was partially correct and it was partially recovered in successive theories because it captured correctly certain structural features of the world. Our scientific realism is about those types of features and thus, in a way, caloric theory is taken to be approximately true in a restricted sense. The same line of reasoning can be applied to Newtonian gravitation as well.

However, structural realism does not stop there and can be extended as an account of all of the special sciences. In the version of structural realism defended by Ladyman and Ross ([2007]), the view provides a scientific realist account that accommodates the entities of special sciences, including quantum field theory and non-relativistic quantum mechanics, using the notions of real patterns and the scale-relativity of ontology.

A real pattern is something that makes for a simplified description relative to some background ontology. A wave approaching the shore is a real pattern because it can be taken as the basis for predictions and explanations. In more refined terms, real patterns are entities that feature (non-redundantly) in projectable regularities.

Scale relativity is the idea that reality does not come in a hierarchical structure of levels. Rather, we should think of theories as true in a certain restricted domain: non-relativistic quantum mechanics captures certain structural features of reality and is true in a given domain, just as quantum field theory captures different structural features of reality. Those scale-relative structural features captured by these theories are real patterns, and we should believe in their existence as such. In this way, we can also vindicate the existence of the entities of atomic theory. Clearly, this idea is closely related to the notion of ‘effective’ realism discussed here.

Structural realism thus provides a unified picture of the ontology of superseded and special science theories, and unifies those different kinds of effective realism, bringing together Newtonian forces, atoms, and the ontology of quantum field theory. It achieves this via the idea of realism about structure and the notions of real patterns and scale relativity.

And, even more importantly, structural realism is able to clarify in which way those effective realist proposals introduced earlier are truly realist in spirit and not just in letter. In fact, structural realism provides an ontology for effective realists. To the question, ‘effective realist about what?’, we respond, ‘real patterns’.