Nothing exists on its own
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.
A new ontology for RQM
Now, with MBT in mind, let us introduce a new way to conceive objects in RQM. According to Rovelli, a physical system can be characterized “by a family of yes/no questions that can be meaningfully asked of it”, where such questions are measurements that can be performed on physical observables, i.e., properties, attributable to the system under consideration. An observer O may ask a set of potentially infinite questions Q1, Q2, …, Qn to the system s, obtaining the string(e1, e2, …, en) (1)
where each ei;represents a specific answer. Nonetheless, RQM postulates that “there is a maximum amount of relevant information that can be extracted from a system”. Hence, Rovelli says, a complete description of a physical system s is given in terms of the string[e1, e2, …, ek] (2)
with k < n , which is a subset of (1). Because in RQM information about systems is obtained through interactions, and questions are measurements on the system s performed by some observer O, (2) contains O’s knowledge about s. Clearly, this string represents the description of s relative to O; indeed, another observer P may ascribe to s a different list of properties/values.
Since in RQM physical systems are defined via a specific set of observables, it is then reasonable to characterize them as mereological bundles of qualities. In this way, RQM can be provided with a property-oriented ontology in which objects are defined straightforwardly, for in MBT they are reduced to properties. However, given that the values of the properties of quantum systems in Rovelli’s theory are observer-dependent, we can define the objects of RQM as mereological bundles of properties whose values depend on the perspective from which they are observed.
More precisely, one should say that there is a set of inherent properties characterizing a certain species of particles—such as mass, charge, spin, etc.—which are not observer-dependent, so that their value remains constant. On the contrary, the values of extrinsic properties (such as energy, position, momentum, etc.) are observer-dependent and change relative to specific observers. Additionally, in virtue of contextuality and the algebraic structure of QM, not all observables associated with a certain system can have definite values.
To give an example, in RQM, an electron is characterized by inherent properties such as mass, charge, and spin-1/2, and extrinsic properties such as momentum, energy, angular momentum, and position. These latter have relational and contextual values which depend on the specific interactions of the particle under consideration with different observers.
Conclusion
Elaborating on Paul’s MBT, we saw in this piece that RQM is compatible with an ontology of properties. This fact has interesting consequences, since it shows that quantum objects need neither be thought of as bare particulars nor as Aristotelian substances. On the contrary, following Hume’s tradition, we can define them as bundles of qualities, where intrinsic properties characterizing a certain species of particles have constant values, extrinsic qualities take definite values relative to particular observers, and not every observable defining a system can have a definite value in virtue of the quantum formalism. Additionally, from our perspective on RQM, we can properly speak about material objects in motion and interacting in spacetime. Hence, we can enrich and clarify its event ontology, making it a full-fledged realist interpretation of quantum theory.
Let’s step back from the technical details for a moment and consider what this picture is really telling us. We live our lives taking for granted that objects exist. This seems an obvious fact. The chair you are sitting on, the phone in your hand, the body you inhabit, the most solid and unquestionable features of reality. But what Rovelli’s quantum mechanics suggests is that this intuition, while practically indispensable, is not the deepest truth about our world. Underneath the solid, stable objects of everyday experience, there is no hidden substance giving things their identity and independence. There are only properties, interactions, and the relational facts that such interactions generate. What we call an object is not a “thing in itself,” but a bundle of properties that holds together reliably enough at the scales we inhabit to function as a thing. But zoom in far enough, and the thing dissolves into its relationships and qualities.
