Nowhere does science meet philosophy more profoundly than in quantum physics. Intellectual excitement, and sometimes also frustration, is tangible in the thriving debates about what our best physics is telling us about the world. Much publicised conceptions of the 'many worlds' and 'pilot waves' of quantum physics represent radically different ways of making sense of the theoretical framework that revolutionised 20th century science, and there is no shortage of alternative ideas. It is undeniable that central notions of quantum theory — superposition, entanglement, spin, and the like — capture strange features of the physical world. But which features exactly? Many old controversies surrounding this question are still fiercely debated, fuelled by often unacknowledged presuppositions that are as much philosophical as they are scientific. The unabating uncertainty about what exactly quantum physics is telling us about the world also raises an important issue about the very nature of physics: what kind of knowledge does it provide? 

Science is often regarded as being in the pursuit of knowledge and understanding. Indeed, according to the Science Council, the very definition of science is “the pursuit of knowledge and understanding of the natural and social world following a systematic methodology based on evidence." This definition has its limitations, however, and reflecting on the kind of knowledge that quantum physics provides suggests a different, complementary way of appreciating the grand theoretical edifices of science. How grand? Well, ‘quantum physics’ encompasses a truly enormous set of specific theories, including those of particle physics with its celebrated discoveries of the Higgs boson, quarks and the like, and the image of reality they imbue us with.

In thinking about what kind of knowledge quantum physics offers, it is instructive to begin by going back to one of the most basic philosophical questions: what is knowledge? Although philosophers still disagree how to answer this question, they (almost) universally agree on one thing about knowledge: that knowing something implies that there is a corresponding fact — an objective truth about the world — that is known. If you know some proposition, that proposition is true.

Taking the point to heart that knowledge has to do with corresponding facts is a key to recognising the subtle nature of the kind of knowledge that we can claim of quantum reality. In addition to this, there are two things implicit in the blooming debates in the foundations of quantum physics that one must acknowledge. 

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"Since no particular foundational account is responsible for the tremendous empirical successes of quantum physics, these accounts cannot really tap into the abundant empirical evidence that flows from quantum physics’ myriad applications and predictions."

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First: at the foundational level quantum physics leaves room for surprisingly different ways of connecting its supremely well-functioning theoretical principles to worldly facts that these principles purportedly capture. That is, there are competing foundational accounts — rival quantum theories, really — that say rather different things about what the world is like regarding its quantum aspects, captured by quantum superposition, entanglement, spin, and so on. The extent of disagreement permeating the foundations of quantum physics is indeed surprising, given how extremely successful, thoroughly tested and well-established quantum physics is.