We challenge the common belief that if there is no absolute time parameter in physics, a quantum system described without reference to an external clock can be assumed to be in a stationary state of its Hamiltonian. We present a time-reparameterisation invariant quantum evolution law, which for a given initial condition predicts the same trajectory in state space as the Schrödinger equation, except that for nontrivial trajectories it does not predict the speed at which the trajectory is traversed. The solutions of this evolution law are all time-reparameterised solutions of the Schrödinger equation. We show how the predictions of the Schrödinger equation are recovered relative to an internal clock in this framework. In contrast to the Page-Wootters formalism or Dirac's quantisation of the Hamiltonian constraint, here the global sate is not stationary. We discuss the assumptions leading to the common conclusion that the state can be taken stationary and suggest that they need to be revisited.
We challenge the common belief that if there is no absolute time parameter in physics, a quantum system described without reference to an external clock can be assumed to be in a stationary state of its Hamiltonian. We present a time-reparameterisation invariant quantum evolution law, which for a given initial condition predicts the same trajectory in state space as the Schrödinger equation, except that for nontrivial trajectories it does not predict the speed at which the trajectory is traversed. The solutions of this evolution law are all time-reparameterised solutions of the Schrödinger equation. We show how the predictions of the Schrödinger equation are recovered relative to an internal clock in this framework. In contrast to the Page-Wootters formalism or Dirac's quantisation of the Hamiltonian constraint, here the global sate is not stationary. We discuss the assumptions leading to the common conclusion that the state can be taken stationary and suggest that they need to be revisited.
This paper establishes the Quantum Temporal Non-Constructibility Theorem (QTNT): the external parameter t appearing in the Schrödinger equation carries no operational meaning for an observer embedded within a closed quantum system. Working strictly within the standard Hilbert-space formalism, using only Stone’s theorem, POVM measurement theory, and unitary evolution, the paper proves that all observable quantum data (POVM probabilities, conditional statistics, and multi-time correlators) are invariant under smooth, strictly monotonic reparametrizations t' = f(t), provided the Hamiltonian transforms covariantly as H' = H (dt/dt'). The theorem shows that internal observers, who have access only to reduced states and subsystem POVMs, cannot detect or reconstruct the external Schrödinger-time parameter. Observable histories depend solely on the geometric orbit traced by the state in projective Hilbert space, not on the particular parameter used to traverse it. As a result, Schrödinger-time functions as a gauge label rather than a measurable physical quantity. The analysis provides an operational reinterpretation of Pauli’s theorem, clarifies the correlational nature of quantum clocks, and completes the temporal non-constructibility program alongside its classical and relativistic counterparts. Appendices supply supporting derivations from Stone’s theorem, Pauli’s theorem, quantum clocks, explicit examples, and the path-integral formulation. Note. The corresponding classical and relativistic results are developed in the companion papers The Temporal Non-Constructibility Theorem in Classical Dynamical Systems (Zenodo, DOI: 10.5281/zenodo.17753466) and Temporal Non-Constructibility in General Relativity (Zenodo, DOI: 10.5281/zenodo.17765323) respectively.
The basic tenet of the present work is the assumption of the lack of external and fixed time in the Universe. This assumption is best embodied by general relativity, which replaces the fixed space-time structure with the gravitational field, which is subject to dynamics. The lack of time does not imply the lack of evolution but rather brings to the forefront the role of internal clocks which are some largely arbitrary internal degrees of freedom with respect to which the evolution of timeless systems can be described. We take this idea seriously and try to understand what it implies for quantum mechanics when the fixed external time is replaced by an arbitrary internal clock. We put the issue in a solid, mathematically rigorous framework. We find that the dynamical interpretation of a quantum state of a timeless system depends on the employed internal clock. In particular, we find that the continuous spectra of well-known dynamical observables like the position of a free particle on the real line may turn discrete if measured in unusual clocks. We discuss the meaning of our result for attempts at quantization of global gravitational degrees of freedom.
Because the time parameter in the Schr\"odinger equation is not observable, energy apparently obeys a superselection rule in the same sense that charge does. That is, observables must all commute with the Hamiltonian and hence be stationary. This means that it is consistent with all observations to assume that any closed system such as the Universe is in a stationary state. We show how the observed dynamic evolution of a system can be described entirely in terms of stationary observables as a dependence upon internal clock readings.
Physical systems are usually assumed to evolve relative to an external time parameter, which is problematic because in quantum theory that parameter is not a physical observable. Page and Wootters (1984) solved this by proposing that the Universe is in a stationary state, eliminating the need for the external time parameter. Instead, their model contains an isolated subsystem, a clock, with which other subsystems are entangled, making the latter appear to evolve relative to different states of the clock. While this resolves the problem of the time parameter, the assumption that the clock is isolated prevents it from being measured, as this requires an interaction with another system. We prove that the clock can be measured while preserving the core features of the Page-Wootters construction. We also discuss clock synchronization.
In quantum mechanics it is usual to represent physical reality as a vector in Hilbert space at a particular time, with evolution being governed by Schrodinger's equation which involves an externally imposed time parameter. This leads to difficulties if one wishes to regard the universe as a quantum mechanical system, because there should no time external to such a system. The approach in this paper is to represent the totality of reality as a vector in Hilbert space. The author shows how time evolution follows, where time now is defined in terms of the states of a quantum mechanical clock which is part of the system. Rather than the correlation between the clock states and the states of the rest of the system arising because both are governed by an imposed law involving an external time parameter, it is seen that this correlation is of the separation-independent Einstein-Podolsky-Rosen type. The total reality vector, which incorporates the whole history of the system, is shown to be a zero-energy eigenstate of the system Hamiltonian. He discusses systems of finite and infinite lifetime, and is able to answer the question: what was the state before the initial state? He concludes that the quantum mechanical system of this paper is a reasonable representation of the observed universe.
Does Time Tick in the Schrödinger Equation? is a short conceptual note about a distinction that is entirely standard in quantum mechanics and yet remarkably easy to lose in ordinary explanatory language. The time-dependent Schrödinger equation contains a temporal parameter t, but t is not itself a clock, an observable, a physical substance, or a causal agent. It orders and metrically distinguishes members of a parameterized state family and becomes operationally temporal through correlation with physical clocks. The Hamiltonian generates the evolution; the parameter indexes it. This paper is emphatically not an attempt to teach physicists how to suck eggs. Physicists already know that t is conventionally a parameter. The problem is that, when quantum mechanics is discussed informally, even technically sophisticated accounts repeatedly slip from “the state is parameterized by time” into language suggesting that time itself flows, acts, drives the state, produces phase change, or creates physical effects. The note uses reparameterization, stationary energy eigenstates, relative phase, entanglement, and the distinction between ordering, metric parameter, and physical clock process to expose this category shift. No modification of quantum mechanics is proposed. The aim is simply to keep the ontology no less disciplined than the mathematics: clocks tick, Hamiltonians generate, and the symbol t indexes their correlation. For further information about the ENSO Framework, please contact Eric Needham:ensotheory1@gmail.com