What is Physics For? Why Classical Physics is not the Limit of Quantum Mechanics

For over a century, the most successful theory in the history of science has been quantum mechanics. It is the set of rules that governs the behavior of the smallest things in the universe, from atoms to the light they emit. It predicts experimental results with such precision that it is often called the most accurate theory ever created. Yet, despite this triumph, scientists still cannot agree on what the theory actually means. The mathematics works perfectly, but the story it tells about reality remains a subject of fierce debate. At the heart of this confusion is a simple question: how does the strange, fuzzy world of the very small connect to the solid, predictable world we see around us? The standard answer has long been that our everyday world is simply a special case of the quantum world, a version that appears when things get big enough. This idea suggests that classical physics, the science of everyday objects, is just a limit of quantum mechanics, much like how a flat map is a simplified version of a curved globe.

A new paper challenges this comfortable assumption. A team of physicists and philosophers argues that the deep puzzles we face in quantum mechanics were not created by the theory itself. Instead, they suggest these puzzles were hiding in plain sight within classical physics all along. The authors propose that the difficulty in understanding quantum mechanics is not because it breaks the rules of the old world, but because it forces us to confront assumptions we made about the old world without ever realizing we were making them. They argue that the idea of a completely separate, objective reality that exists independently of anyone looking at it is a philosophical choice, not a scientific fact. By examining the history of physics and the nature of measurement, the researchers show that the divide between the observer and the observed was always a blind spot, and quantum mechanics simply made that blind spot impossible to ignore.

The paper begins by looking at how scientists usually think about the relationship between different theories. In many areas of physics, a newer, more complex theory contains an older one as a special case. For example, the theory of relativity, which deals with very fast speeds, includes the older laws of motion as a limit for slow speeds. It is natural to assume the same is true for quantum mechanics: that the classical world of solid objects is just what happens when quantum effects become too small to notice. The authors acknowledge that this view is useful in many practical situations. However, they point out that unlike other examples, there is no single, universal way to turn quantum mechanics into classical physics. More importantly, they argue that the problem is not just mathematical; it is about how we interpret what the theories are telling us about reality.

In classical physics, it has always been easy to imagine the world as it is. We assume that a rock has a specific position and speed whether we are looking at it or not. We assume that the observer is separate from the object being observed, like a person watching a car drive by. The paper argues that this separation is an illusion. The authors trace this idea back to the 18th-century philosopher Immanuel Kant, who pointed out that we can never know the world exactly as it is in itself, independent of our minds. We only know the world as it appears to us through our senses and our methods of inquiry. In classical physics, this philosophical problem was largely ignored because the technology of the time allowed scientists to pretend that their measurements did not disturb the objects they were studying. They could act as if they were seeing the world from a "view from nowhere," a perfect, objective perspective that belonged to no one in particular.

Quantum mechanics shattered this illusion. In the quantum world, the act of measuring something changes it. You cannot measure the position of a particle without affecting its speed, and you cannot observe a system without interacting with it. The paper highlights that this is not a technical glitch or a limitation of our instruments; it is a fundamental feature of nature. The researchers argue that this does not mean the quantum world is magical or that classical physics is wrong. Instead, it means that the classical idea of a reality that exists completely apart from us was never as solid as we thought. The "blind spot" of objectivity was always there; we just didn't see it until the quantum world forced us to look.

The authors explore two main ideas to support this view. The first is the split between the subject (the observer) and the object (the thing being observed). In classical physics, we assume the object exists with its own properties before we measure it. In quantum physics, the properties of a system often do not exist in a definite state until a measurement is made. The paper suggests that this is not a new discovery, but a clarification of an old truth. Even in classical physics, the properties we measure are a result of the interaction between the object and the observer. We just used to think we could subtract the observer's contribution and get the "pure" object. Quantum mechanics shows us that we cannot. The observer is part of the system, and the reality we describe is co-created by the interaction.

The second idea concerns how we represent the world. We often think of scientific models as pictures or maps that mirror reality. We imagine that a theory is a faithful copy of the world, like a photograph. The paper argues that this is a misunderstanding. Scientific models are not mirrors; they are tools. They are designed to help us make predictions and navigate the world, not to provide a perfect snapshot of reality as it exists in isolation. The authors point out that even in classical physics, the mathematical tools we use are abstract and do not look like the things they describe. For instance, the way we describe the motion of a planet using complex mathematical spaces does not mean the planet is actually moving through those invisible spaces. The model is a way of organizing our experience, not a direct window into a hidden reality.

To illustrate how this shift in thinking works, the paper looks at two specific interpretations of quantum mechanics that embrace these ideas. One is the work of Fritz London and Edmond Bauer from 1939, who used a branch of philosophy called phenomenology to argue that the observer and the observed are inseparable. They suggested that the quantum formalism already contains a theory of how we relate to the world, one that rejects the idea of a detached, objective reality. The other example is a modern interpretation called QBism. This view treats quantum mechanics not as a description of an external world, but as a tool for an agent to manage their expectations about what they will experience. In this view, the "reality" of a quantum state is not something out there waiting to be found; it is a guide for action, specific to the person using it. Both of these approaches reject the idea that classical physics is just a limit of quantum mechanics in the traditional sense. Instead, they suggest that quantum mechanics invites us to revise our understanding of what classical physics was always trying to say.

The paper also draws a parallel to the field of metrology, the science of measurement. For a long time, metrologists believed that every measurement was an attempt to find a single, hidden "true value" that existed independently of the measurement process. They thought that if they could just reduce their errors enough, they would find this perfect value. In recent decades, however, this view has changed. Modern metrology now treats measurement results as probability distributions that reflect our best knowledge given the information we have. The "true value" is no longer seen as a hidden treasure to be dug up, but as a concept that helps us organize our data. The authors argue that this shift happened in metrology without any help from quantum mechanics, proving that the philosophical problems of objectivity and representation were already present in classical science. Quantum mechanics simply brought these issues to the surface.

Ultimately, the paper concludes that the tension between classical and quantum physics is not a sign that the quantum world is broken or that classical physics is obsolete. It is a sign that our philosophical assumptions about how science works need to be updated. The authors argue that we should stop trying to force quantum mechanics to fit into the old box of a detached, objective reality. Instead, we should accept that our knowledge of the world is always a partnership between the observer and the observed. This does not mean that science is subjective or that reality is made up. It means that the reality we can know is the one that emerges from our interaction with the world. By letting go of the idea of a "view from nowhere," we can see that quantum mechanics does not break with classical physics; it reveals the hidden depths of the classical world that were always there, waiting to be understood.

The implications of this view extend beyond the laboratory. The paper suggests that clinging to the idea of a detached, objective reality can lead to a disconnect between the scientific world and the human experience. When we treat our scientific models as perfect mirrors of a reality that exists apart from us, we risk forgetting that these models were built by humans for human purposes. We risk creating a world of abstract equations that feels alien and irrelevant to the life we actually live. By recognizing that our knowledge is always situated and that the observer is part of the story, we can bridge the gap between the world of science and the world of human experience. The paper does not offer a new theory of physics or a new set of equations. Instead, it offers a new way of thinking about the theories we already have. It suggests that the mystery of quantum mechanics is not a puzzle to be solved by finding a hidden mechanism, but a call to re-examine the very foundations of how we understand reality.