What is the connection between quantum relationality and symmetry-based relationality?

Physics has long been divided into two camps that seem to speak different languages. On one side sits quantum mechanics, the rulebook for the very small, where particles can exist in multiple states at once and where the act of looking at something seems to change what it is. On the other side sits general relativity, the theory of gravity and the very large, where space and time are flexible and where the position of an object only makes sense when compared to something else. For decades, physicists have noticed that both theories rely heavily on the idea of "relationality"—the notion that properties are not absolute facts about the universe, but rather relationships between things. In the world of gravity, an object's location is meaningless without a reference point, like a clock or a landmark. In the quantum world, the state of a particle often depends on who is measuring it. Yet, despite this shared reliance on relationships, the two theories have remained stubbornly separate, and it has been unclear whether their versions of "relationality" are actually the same thing or just a coincidence of vocabulary.

A new paper by Emily Adlam proposes a bold unification: that the relational nature of quantum mechanics is not a strange, unique quirk of the subatomic world, but is actually the exact same phenomenon found in relativity. The author suggests that the reason quantum states seem to change depending on the observer is simply because they describe properties that are inherently relative, just like direction or position in space. In this view, a quantum state is not a description of a particle's absolute reality, but a description of how that particle stands in relation to a specific reference system, much like a compass needle only has meaning when compared to the magnetic field around it. If this idea holds, it could dissolve some of the deepest mysteries of quantum physics, particularly the confusing puzzle of how a single, definite reality emerges from a world of possibilities.

To understand the argument, one must first grasp the difference between two types of physical quantities. In physics, some facts are absolute and unchangeable, while others are "partial." A partial observable is a quantity that can be measured but only makes sense when tied to a reference. For example, saying a car is moving at sixty miles per hour is a partial observable; it is only true relative to the road. The speed itself is not a fixed property of the car in isolation. In contrast, a "complete" observable is a full, self-contained fact, such as the distance between the car and a specific tree at a specific moment. In the theory of relativity, physicists have long known that we can only ever measure partial observables. We cannot measure an absolute speed or an absolute direction; we can only measure how things relate to our measuring tools.

Adlam takes this established fact from relativity and applies it to the quantum realm. She argues that every quantum state we have ever written down or measured describes a partial observable. When a physicist writes a quantum state for a particle's spin, they are not describing an absolute direction the spin is pointing in the universe. Instead, they are describing the spin's orientation relative to a specific axis, which is defined by the measuring apparatus. The paper suggests that the famous "Wigner's Friend" paradox—a thought experiment where two observers disagree on whether a particle has collapsed into a single state or remains in a superposition—arises simply because the two observers are using different reference axes. One observer is describing the particle relative to their own laboratory equipment, while the other is describing it relative to a different frame. There is no contradiction, the author argues, because both are simply reporting different relationships.

The paper builds this case by showing that quantum states of symmetry-variant properties—properties that change when you rotate or shift your perspective—must always be relativized. If you try to describe a quantum state without a reference system, the description becomes a messy, incoherent mixture that loses all the specific information about the particle's state. However, once you introduce a reference system, like a specific direction in space defined by a magnet, the quantum state snaps into a clear, coherent form. The author uses a framework developed by other researchers to demonstrate that it is mathematically consistent to have a situation where the relationship between two systems is a definite, non-superposed fact, while the state of one system relative to the other appears to be in a superposition. This means that the "weirdness" of quantum superposition might not be a feature of reality itself, but a feature of how we describe things relative to a reference point.

This perspective offers a fresh way to look at the measurement problem, the long-standing difficulty of explaining why we see one outcome when a quantum system seems to be in many. In standard quantum mechanics, the transition from a superposition of possibilities to a single result is often described as a "collapse" of the wavefunction, a process that is mathematically abrupt and physically mysterious. Adlam suggests that this collapse is not a sudden, magical event, but a natural consequence of the interaction between a partial observable and a reference system. When a quantum system interacts strongly with a measuring device, the system is no longer just a collection of relative properties; it becomes entangled with the reference frame in a way that breaks the linear rules of quantum evolution. The "collapse" is simply the moment when the approximation of treating the system as independent from the reference frame fails.

The paper does not claim to have solved the measurement problem entirely. It does not provide a new equation that predicts exactly when a collapse happens, nor does it offer a complete model of the physical mechanism that links the relative quantum world to the absolute classical world. Instead, it suggests that the solution lies in recognizing that quantum mechanics applies only to partial observables, while the definite outcomes we see are complete observables. The author argues that because we can only measure partial observables, we have never actually seen a quantum state of a complete observable. The strange behavior of quantum mechanics, including the ability to be in two places at once, applies only to the relative, symmetry-variant quantities. The definite, single outcomes we experience are the invariant, complete facts that emerge when these relative quantities are tied to a reference system.

By proposing that quantum relationality and relativistic relationality are one and the same, the paper bridges a gap that has separated physics for a century. It suggests that the universe is not made of absolute, independent objects, but of relationships. The quantum state is not a snapshot of a particle's soul, but a description of its stance relative to the observer. This view does not require us to abandon the idea of a real, objective world; rather, it reframes that world as a network of connections. The paper concludes that while more work is needed to fully describe how these connections form and break, the path forward is to stop looking for absolute quantum states and start understanding the specific, relational nature of the quantities we can actually measure. In doing so, the paper offers a quiet, logical explanation for why the quantum world seems so strange: it is strange only because we are trying to describe relative things as if they were absolute.