Physicists have long known that quantum objects behave nothing like the solid, independent things of everyday experience. But the implications run deeper than strange behaviour. Carlo Rovelli's Relational Quantum Mechanics suggests that quantum systems have observer-dependent properties—what they are depends on their interactions with other systems. Drawing on a tradition running from Hume to contemporary metaphysics, philosopher Andrea Oldofredi explores this novel relational perspective on our world, arguing that objects are not the fundamental furniture of reality.

Quantum Mechanics is arguably one of the most successful theories in the history of science, for its predictions are confirmed by countless experiments, making it a cornerstone of contemporary physics. However, a century after its inception, the theory still challenges our classical worldview, offering a counterintuitive description of nature at microscopic scales. Contrary to classical mechanics, where objects are individually distinguishable and possess well-defined attributes at all times, QM speaks about indistinguishable systems with indeterminate properties, superposed states, and non-local interactions. Unsurprisingly, then, questions concerning its ontology, i.e., what fundamentally exists, are still vividly discussed to this day.

Despite its empirical success, however, physicists and philosophers alike enquire whether QM should be considered a true description of the natural world, because this theory is affected by conceptual conundrums and formal difficulties (e.g., the measurement problem). To address such issues, new quantum interpretations emerged from the 1950s. Among the many existing alternatives, here we consider a widely discussed framework that turns thirty this year, Carlo Rovelli’s Relational Quantum Mechanics (RQM).

RQM is motivated by Rovelli’s work in loop quantum gravity, where spacetime is not a substance existing per se, but rather it emerges from a dynamic network of relations, providing a relational perspective of it.

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Similarly, RQM holds that the notion of an observer-independent state of a quantum system withers away, in favor of a relational view. Indeed, the state of a system is meaningfully defined only with respect to another system, which plays the role of an external observer:

A quantum mechanical description of a certain system (state and/or values of physical quantities) cannot be taken as an “absolute” (observer-independent) description of reality, but rather as a formalization, or codification, of properties of a system relative to a given observer. Quantum mechanics can therefore be viewed as a theory about the states of systems and values of physical quantities relative to other systems. [...] Therefore, I maintain that in quantum mechanics, “state” as well as “values of a variable”—or “outcome of a measurement”— are relational notions in the same sense in which velocity is relational in classical mechanics.

Moreover, the ubiquitous presence of relational concepts in physics, from classical mechanics to cosmology, provides further reasons for RQM. For example, the classical notion of velocity makes sense only relative to a given reference frame; even the passage of time is relational, being different for an observer on Earth compared to one located on the edge of a black hole. We can also experience relational phenomena in our everyday life simply by looking at the night sky: observing the light emitted by a star very far from us, we are looking at events that happened in the past. Reality, then, is very different from the perspective of the observed star and from ours. The main novelty of this approach is to apply such a relational character of physics to the quantum formalism.

Remarkably, RQM rejects the idea that an absolute reality for diverse observers may characterize the same system differently without generating contradictions.

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