Comment on Carlo Rovellis "An argument against the realistic interpretation of the wave function"

Rovelli’s argument against the realistic interpretation of quantum states and in favor of an ontology of quantum events is refuted.

💡 Research Summary

In this comment, H. Dieter Zeh critically examines Carlo Rovelli’s recent claim that the time‑reversal asymmetry of quantum measurements undermines a realistic (ontic) interpretation of the wave function. Rovelli argues that, because a measurement collapses the wave function in a direction that cannot be reversed, the wave function cannot be a real physical entity. Zeh shows that this conclusion rests on an inappropriate modeling of the measurement process and on a neglect of the thermodynamic arrow that governs any macroscopic apparatus.

First, Zeh points out that Rovelli’s “text‑book” description of a measurement treats the system alone, ignoring that the measuring device is itself a quantum system embedded in an environment. Real measurements are irreversible because the apparatus starts in a low‑entropy state and, through interaction with a large environment, increases entropy. This entropy increase provides the physical time arrow that makes the collapse appear irreversible. In a fully ontic picture the apparatus and environment must also be described by the universal wave function; the apparent asymmetry of collapse is then just the macroscopic manifestation of the underlying unitary dynamics combined with special initial conditions.

Second, Rovelli’s criticism that the coexistence of two dynamical laws—the unitary Schrödinger evolution and the non‑unitary collapse—contradicts realism is misplaced. Zeh argues that collapse is not an ad‑hoc addition but the effective description of a genuinely irreversible process: the entanglement of the measured system with the apparatus and the environment (decoherence). Decoherence spreads the quantum information into uncontrollable degrees of freedom, producing effectively distinct “branches” of the wave function. The branching is asymmetric in time because it relies on a cosmic low‑entropy, low‑entanglement initial condition (the Big‑Bang state). Thus the global wave function can remain ontic and obey the time‑symmetric Schrödinger equation (or even the timeless Wheeler–DeWitt equation), while the observed arrow of time emerges from boundary conditions.

Zeh also addresses Rovelli’s proposal of a stochastic space‑time event ontology. He argues that all experimentally verified quantum phenomena, including rapid decoherence, are continuous processes governed by the time‑dependent Schrödinger equation; there is no empirical need to postulate fundamental “events”. The branching of the wave function is a dynamical approximation that becomes relevant only from the perspective of a particular observer (the “frog’s perspective”). Consequently, the event‑based picture is unnecessary and stems from a misinterpretation of Rovelli’s Eq. (4).

Regarding Bohmian mechanics, Zeh agrees with Rovelli that Bohm trajectories are merely “pointers” indicating which branch of the wave function an observer inhabits; the other branches continue to exist in any no‑collapse interpretation. This reinforces the view that the wave function’s reality does not depend on the existence of hidden‑variable trajectories.

In summary, Zeh’s refutation rests on three pillars: (1) the thermodynamic arrow of the measuring apparatus, (2) the role of decoherence and the requirement of a low‑entropy cosmic initial condition, and (3) the recognition that collapse, while formally non‑unitary, can be understood as an effective description of an irreversible entanglement process within a fully unitary framework. When these aspects are taken into account, the wave function can be consistently interpreted as a real, global entity, and the apparent time‑asymmetry of measurements does not constitute a logical obstacle to realism.