This is the tenth and final article in a series exploring the birth of quantum physics.
The world of the very small is like nothing we see in our everyday lives. We do not think of people or rocks being in more than one place at the same time until we look at them. They are where they are, in one place only, whether or not we know where that place is. Nor do we think of a cat locked in a box as being both dead and alive before we open the box to check. But such dualities are the norm for quantum objects like atoms or subatomic particles, or even larger ones like a cat. Before we look at them, these objects exist in what we call a superposition of states, each state with an assigned probability. When we measure many times their position or some other physical property, we will find it in one of such states with certain probabilities.
The crucial question that still haunts or inspires physicists is this: Are such possible states real — is the particle really in a superposition of states — or is this way of thinking just a mathematical trick we invented to describe what we measure with our detectors? To take a stance on this question is to choose a certain way of interpreting quantum mechanics and our take on the world. It is important to stress that quantum mechanics works beautifully as a mathematical theory. It describes the experiments incredibly well. So we are not debating whether quantum mechanics works or not, because we are well past that point. The issue is whether it describes physical reality as it is or whether it does not, and we need something more if we are to arrive at a deeper understanding of how nature operates in the world of the very small.
States of thinking about the quantum world
Even though quantum mechanics works, the debate about its nature is fierce. The subject is vast, and I could not possibly do it justice here. My goal is to give a flavor of what is at stake. (For more details, see The Island of Knowledge.) There are many schools of thought and many nuanced arguments. But in its most general form, the schools line up along two ways of thinking about reality, and they both depend on the protagonist of the quantum world: the famous wavefunction.
In one corner stands those who think that the wavefunction is an element of reality, that it describes reality as it is. This way of thinking is sometimes called the ontic interpretation, from the term ontology, which in philosophy means the stuff that makes up reality. People who follow the ontic school would say that even though the wavefunction does not describe something palpable, like the particle’s position or its momentum, its absolute square represents the probability of measuring this or that physical property — the superpositions that it does describe are a part of reality.
In the other corner stand those who think that
