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Routledge eBooks

Time and Quantum Mechanics

Valia Allori

Published Sep 2, 2025• 0 citations
Publisher

Abstract

In this chapter, I discuss time in nonrelativistic quantum theories. Within an instrumentalist theory like von Neumann's axiomatic quantum mechanics, I focus on the meaning of time as an observable quantity, on the idea of time quantization, and whether the wavefunction collapse suggests that there is a preferred temporal direction. I explore this last issue within realist quantum theories as well, focusing on time reversal symmetry, and I analyze whether some theories are more hospitable for time travel than others.

DOI
10.4324/9781003495611-37

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In this chapter, I discuss time in nonrelativistic quantum theories. Within an instrumentalist theory like von Neumann's axiomatic quantum mechanics, I focus on the meaning of time as an observable quantity, on the idea of time quantization, and whether the wavefunction collapse suggests that there is a preferred temporal direction. I explore this last issue within realist quantum theories as well, focusing on time reversal symmetry, and I analyze whether some theories are more hospitable for time travel than others.
Time is used in non-relativistic quantum theory as a classical concept. On the one hand, time is the parameter entering Schrödinger's equation and measured by an external laboratory clock. But it could be considered that time also plays the role of an observable when a given event occurs (e.g., when a particle crosses through a potential barrier), and one would expect that like every observable, it should be represented in the theory by an operator whose properties are predictors of the outcome of time measurements made on physical systems. Several time operators have been proposed in different publications, but still, there is not a widely accepted consensus. That subject, among others concerned with time in quantum mechanics, is analyzed in the present chapter, where we focus on the analysis of time-reversal symmetry in non-relativistic quantum systems. The search for symmetries in Physics helps to study the structure of theories. Briefly speaking, a symmetry is a transformation that does not change the structure of the theory. We start with the analysis of Schrödinger’s Equation verifying that for obtaining a time-symmetric description of the dynamics of a particle, the operation of time-reversal must include conjugation represented by the action of an antiunitary operator acting over a wave function. In particular, we analyze quantum systems whose evolution is determined by a set of external parameters varying cyclically, discussing how even though these parameters return to their initial values, the state of the system acquires a geometric phase, which depends on the path followed by the external parameters in the parameter space and which in general can be associated with the lack of the time-reversal symmetry of the system.
Many believe that quantum mechanics makes the world hospitable to the tensed theory of time. Quantum mechanics is said to rescue the significance of the present moment, the mutability of the future and possibly even the whoosh of time’s flow. It allegedly does so in two different ways: by making a preferred foliation of spacetime into space and time scientifically respectable, and by wavefunction collapse injecting temporal ‘becoming’ into the world. The aim of this paper is to show that the reasoning underlying these claims is wishful thinking. Against the first claim I develop what I call the “coordination problem” for tensers. The upshot of this problem is that if tensers escape the threat of relativity, they do so only by embracing conflict with the branch of physics they believed saved them, quantum mechanics. I then step back from the fray and examine some methodological issues, concluding that scientific methodology will always be “against” tenses as they are currently conceived. The Appendix deals with the confused tangle of issues linking wavefunction collapse to an open future.
In the previous chapters, we have made an imaginary travel in a ship which we might call “time” for analyzing how this concept is present in the physical theories which describe our world and the diverse interactions which make possible its existence as we perceive it under our notion of reality. We have traveled from the submicroscopic scale—requiring a quantum description— up to the cosmological dimensions where gravity acquires a crucial role distorting the space-time of the special relativity theory. In this chapter, we discuss some of the most recent publications related to physical theories about time, starting with the cellular automation interpretation of quantum mechanics (QM) formulated by Gerard’t Hooft, where information loss is considered as the cause of the time-reversal asymmetry. Then we continue with the thermal time hypothesis of Carlo Rovelli, who asserts that it is possible to formulate classical mechanics in a way in which the time variable is treated on equal footing as other physical variables, and not considered as a special and independent variable. Afterward, we analyze theories which propose quantization of time, and then continue with the discussion of proposed time operators for non-relativistic and relativistic QM. At the end of that section, we discuss theories related with time in quantum gravity and then we briefly explore in the physics of time travel.
This dissertation is about the sense in which the laws of quantum theory distinguish between the past and the future. I begin with an account of what it means for quantum theory to make such a distinction, by providing a novel derivation of the meaning of "time reversal." I then show that if Galilei invariant quantum theory does distinguish a preferred direction in time, then this has consequences for the ontology of the theory. In particular, it requires matter to admit "internal" degrees of freedom, in that the position observable generates a maximal abelian algebra. I proceed to show that this is not a purely quantum phenomenon, but can be expressed in classical mechanics as well. I then illustrate three routes for generating quantum systems that distinguish a preferred temporal direction in this way.
This chapter introduces the quantum mechanics. It explores the formulation in terms of the operator calculus. The chapter shows how nonrelativistic quantum kinematics emerges from the general theory. It discusses how physical quantities are related to symmetries through unitary transformations, and consider translations. The chapter also introduces the statistical operator and the density matrix. It introduces the consistent history interpretation of quantum mechanics. The true core of quantum mechanics is the rule for assigning the amplitude to an event that can be effected in indistinguishable ways. The chapter examines the presentation of Feynman's space-time approach since it offers the most intuitive formulation of quantum mechanics. A physical quantity is said to be conserved if it for any state of the system stays constant in time. In quantum mechanics this therefore means that its probability distribution in any state is independent of time.
The self-adjoint operator of time in nonrelativistic quantum mechanics is found within the approach where the ordinary Hamiltonian is not taken to be conjugate to time. Quantization of the re-expressed Liouville equation with the total Hamiltonian, consisting of the part that is a conventional function of coordinate and momentum and the part that is conjugate to time, is considered. The von Neumann equation with quantized time is found and discussed from the point of view of exact time measurement.