David Albert (Columbia University)
The arrow of time: from the whole universe to local systems
There is a long-cherished hope, which has its origins in the work of Boltzmann, that all that we are going to need to do, in order to account for all the of the differences there are between the past and the future, is to add to the fundamental time-reversal-symmetric dynamical laws, and to the standard statistical-mechanical probability-measure over the space of possible fundamental physical states, a simple postulate – a so-called “past-hypothesis” – about the initial microstate of the universe as a whole. And there are various widespread and perennial sorts of puzzlement about how a hope like that can even seriously be entertained – puzzlements (that is) about how it is that the macrocondition of the universe 15 billion years ago, all by itself, can even imaginably be up to the job of explaining so much about the feel, today and on earth, of the passing of time. I want to try to alleviate those puzzlements here. I will begin with a number of very general observations – and then, by way of illustration, I will present a new and detailed analysis of how it is that a simple pendulum clock invariably arranges to turn its hands clockwise in the temporal direction that points away from the Big Bang.
Valia Allori (Northern Illinois University)
Time for pancakes
According to the standard account of time reversal, namely the account which is found in physics books, a time reversal transformation involves a temporal operator T that, when acting on a sequence of states, it inverts the order with which states happen, and suitably changes the state as to make the theory time reversal invariant. This approach may be dubbed ‘symmetry first approach, because it imposes symmetries on the theory: the changes in the states are a consequence of requiring the theory to be time reversal invariant. Some (Albert, Callender) find this view unjustified: we discover a theory has a given symmetry, on the basis of the theory’s ontology, not the other way around. So, they propose the so-called ‘pancake account’ of time reversal: T inverts the order of the state but does nothing else. Consequently, since there are no obvious independent reasons for the state to changes as T prescribes to preserve time reversal symmetry, then the theory is not time reversal invariant.
In this paper I wish to defend the pancake account against some objections recently raised by Roberts. Moreover, I wish to propose an alternative account, which aims at retaining the best of both approaches: the T operator changes the order of the states, it leaves the state unaffected (like the pancake account), but also makes the theory time reversal invariant (like the standard account).
Craig Callender (University of California-San Diego)
What real quantum theory teaches us about time reversal
It is well-known that Schrödinger wanted to interpret the quantum wavefunction ψ as representing a continuous distribution of charge traveling through three-dimensional space. What is less appreciated is that Schrödinger also originally desired that his wavefunction be represented by a real-valued function and not a complex one. My talk introduces this 4th-order real wave equation, the first published time dependent Schrödinger equation, and asks what insight it provides into quantum time reversal invariance. I show that it nicely resolves the tension between the Albert-Callender “pancake” understanding of time reversal and the conventional wisdom in quantum theory.
Elise Crull (The City College of New York)
Temporal Entanglement, Entangled Time, Entangled Processes
Analyses of entanglement in both physics and philosophy have almost exclusively focused on quantum correlations between spatially separated systems, e.g. experimental violations of Bell’s inequalities. Of course such experiments, occurring as they do within spacetime, suggest there is a temporal component to quantum correlations. The most work done to date along these lines concerns the Leggett-Garg inequalities, which are often touted as the temporal analogue to Bell. Recent critical studies convincingly argue that Leggett-Garg tests fail as such, and anyway the focus of this literature is decidedly not temporal entanglement. Thus, the first part of my talk aims to shine further light (or better: shine light from a different dimension) on the fiction of “separable” systems by considering more carefully the temporal aspect of entanglement relations.
Temporal entanglement understood this way – as mere appreciation of the fact that entanglement relations span not just space but also time – is not the most thrilling, perhaps. In the second part of my talk I will suggest that two additional, far more interesting types of temporal entanglement are in the offing. The first is that time itself might be entangled – call this “entangled time”. Discussion here will focus on the fascinating paper of Aharanov et al. from 2014, “Every moment of time a new universe.” The second is that temporally ordered processes might be entangled – call this “entangled processes”. This phenomenon is being intensely studied at present by indefinite causal structure research programs. Though experimental and theoretical results in this arena have not yet shown anything decisive regarding quantum causal structure, I think they are on to something regarding quantum temporal structure.
Carl Hoefer (Universidad de Barcelona)
‘Freedom from inside out’ –Revisited
In a 2002 paper, I offered a novel way of thinking about the compatibility of free will with determinism, one that depended on appealing to the philosopher of physics’ typical understanding of time: as simply one of the 4 dimensions of the Block Universe, albeit an especially interesting and important one. I argued that rejecting the everyday notion of “passage of time”, and of the explanatory privilege that we usually give to past —> future determination as opposed to future —> past determination, allowed one to articulate a novel way of defending free action in a Block world subject to deterministic laws.
The problem is, most of the time these days I no longer believe in the Block, and do believe in the passage of time! But I still believe that human action is (often) free, and that physics poses no genuine threat to our freedom. In my talk, I will explore how the core idea behind “Freedom from the Inside Out” can be modified to be compatible with a metaphysical picture in which time passes, and explanation is not fully time-symmetric.
Hoefer, Carl (2002). “Freedom from the Inside Out”, in C. Callender (ed.), Time, Reality and Experience, Cambridge University Press.
Barry Loewer (Rutgers University)
The Mentaculus Account of counterfactuals
Counterfactuals and subjunctive conditionals are enormously important in philosophy and especially in philosophy of science. They exhibit a temporal arrow closely related to the temporal asymmetries of causation, measurement, knowledge, and decision. My talk describes a new theory of counterfactuals based on the Mentaculus account of statistical mechanics elaborated by David Albert and myself. The Mentaculus is at the basis of all the arrows of time. The new account of counterfactuals replaces similarity among worlds with objective probabilities. I show how various arrows of time can be explained by this account of counterfactuals.
Cristian López (Université de Lausanne)
Time’s direction, why time-reversal doesn’t matter (that much)
Most debates about the direction of time in physics have been marked by the following rationale: if the dynamical laws are time-reversal invariant, then the direction of time is not fundamental (or primitive, or unreal tout court). Or something of the sort. I am unconvinced by this rationale. Not only do I believe that it can only work under many (disputable) assumptions, but also that there are alternative ways to escape from it. In this presentation I look into three aspects in the rationale: (a) some hidden assumptions in the orthodox understanding of time reversal, (b) the assumptions that underpin the conditional in the rationale, and (c) why primitivists about the direction of time should reject the rationale. Indirectly, this presentation aims to clear the way for primitivism about the direction of time.
Bryan Roberts (London School of Economics)
Misdirected arrows of time
Our naïve senses often detect phenomena that appear asymmetric in time when they are not. For example, a book will slide to a stop on a tabletop, and never the reverse. But, when that experience is carefully described in terms of dynamical systems, we find that the description omits degrees of freedom in a way that hides an underlying temporal symmetry. This talk will develop an account of what is required to have a true arrow of time, in the sense that 'time itself' has an asymmetry. I will argue that most of what is commonly referred to as an ‘arrow of time’ fails to be a time asymmetry in this sense.
Ignacio Rojas (Universidad de Buenos Aires/ANID)
The status of time in Rovelli’s Loop Quantum Gravity
Although positions naturally differ substantially on what time is or how it should be understood in the formulation of a theory of quantum gravity, the debate centers on whether these theories should preserve the background-independent character of general relativity or not. Besides, there is a natural division of philosophical approaches: those who consider time to be fundamental and those who consider time can, or should, be eliminated as a fundamental feature of the physical world.
The Loop Quantum Gravity defended by Rovelli adopts the formalism of Constrained Hamiltonian systems to obtain a quantized theory of gravitation, which is commonly interpreted as implying a frozen dynamic of the universe. To recover a notion of evolution and change, but without resorting to a fundamental time, Rovelli introduces a distinction between partial and complete observables.
The aim of this work is, first, to analyze Rovelli’s interpretive strategy regarding the definition of evolution of physical systems within a formalism that seems to exclude it and, secondly, to argue that this same interpretive strategy would show that the metaphysical consequences regarding the nature and status of time in the physical world are not as clear or direct as Rovelli claims.
Daniel Sudarsky (Universidad Nacional de México)
A possible account for the special initial state that seems to be behind the universe's entropic arrow of time
As it is well known, the entropic arrow of time points towards a set of rather special initial conditions in the remote past. One possibility is that such situation was the result of a special “Law of Initial Conditions” that stands apart, but at the same level of fundamentality as the rest of the laws of nature. There are another possibilities, which would have such situation resulting from the fundamental dynamics encoded in other physical laws. We will discuss a couple of possibilities of that kind, and compare their overall explicative power as they connect with what seems as separate aspects of physics.
Nick Huggett and Karim Thébault (University of Illinois Chicago and University of Bristol)
On the emergence of temporal structure in Wheeler-DeWitt Cosmology
Wheerler-DeWitt cosmologies are quantum models of the universe in which the wavefunction obeys a time independent equation. The standard approach to the recovery of a minimal notion of temporal structure from such an equation is through appeal to a relational concept of evolution that can be formally implemented via the process of de-parametrization. In de-parametrization, a sub-set of the physical degrees of freedom of the model play the role of `clocks' that parametrize the evolution of other degrees of freedom.
A specific approach to the recovery of temporal structure in Wheeler-DeWitt cosmology is due to Kiefer and Zeh. This approach implements a de-parametrization scheme whereby the gravitational degrees of freedom are used as a clock for the matter degrees of freedom. Remarkably, under this de-parametrization scheme and a particular choice of boundary conditions, the gravitational degrees of freedom can be argued to play the role not only of a time parameter, but also of a directed temporal structure. Time and its arrow can thus be understood to emerge from a quantum dynamics without any extrinsic temporal structure.
The success of the Kiefer and Zeh approach depends upon two crucial assumptions. First, in order for the gravitational degrees of freedom to play the role of internal clocks their coupling to the matter degrees of freedom must be neglected to first order under a Born-Oppenheimer type approximation. Second, the boundary condition applied must be isolated as an “initial”' condition. In this paper we will evaluate to cogency of each of these assumptions in the context of critical arguments relating to ``temporal double standards" due to Price (1997) and Chua and Callender (2021).
Francesca Vidotto (University of Western Ontario/Rotman Institute of Philosophy)
Thermodynamics without time
Our fundamental theories, i.e., the quantum theory and general relativity, are invariant under time reversal. Only when we treat system from the point of view of thermodynamics, i.e. averaging between many subsystem components, an arrow of time emerges. The relation between thermodynamic and the quantum theory has been fertile, deeply explored and still a source of new investigations. The relation between the quantum theory and gravity, while it has not yet brought an established theory of quantum gravity, has certainly sparkled in depth analysis and tentative new theories. On the other hand, the connection between gravity and thermodynamics is less investigated and more puzzling. I review a selection of results in covariant thermodynamics, such as the construction of a covariant notion of thermal equilibrium by considering tripartite systems. I discuss how such construction requires a relational take on thermodynamics, similarly of what happens with the quantum theory and in gravity.
David Wallace (University of Pittsburgh)
Thermodynamics with and without reversibility
I provide a general analysis of thermodynamics in the control-theory/resource-theory framework, focusing on the interplay between (i) the level of coarseness of an agent’s control of a system, and (ii) their level of information about the system. I demonstrate that even in the reversible limit where an agent has arbitrarily fine control of a system, and arbitrarily good initial information about its microstate, it is not possible to violate the Second Law. Actual irreversibility results as this limit is replaced by something more realistic.