The Problem of Time in Perspective

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Talks should be 40min in length and comments 10min.

Day 1 (3 July 2015)

08:30 Registration
09:00 - 10.00 Donald Salisbury (Austin): The Intrinsic Hamilton-Jacobi Dynamics of General Relativity and its Implications for the Semi-Classical Emergence of Time (Slides, 651 Kb) (Preprint @ (Watch the lecture @ LMUcast)
Chair: Karim Thébault
10:00 - 10:30 Coffee
10:30 - 11:30 Henrique Gomes (Perimeter Institute): Timeless Configuration Space and the Emergence of Classical Behavior for Closed Systems (Watch the lecture @ LMUcast)
Chair: Richard Dawid
11:30 - 12:45 Alex Blum (MPIWG): How Quantum Theory Tried to Get Rid of Time and General Relativity Started Looking for it
Comment: Brian Padden (MCMP/LMU) (Slides, 181 Kb)
Chair: Richard Dawid 
12:45 - 14:00 Lunch
14:00 - 15:00 Francesca Vidotto (Radboud): Compact Phase Space, Cosmological Constant, Discrete Time (Slides, 3.2 Mb) (Watch the lecture @ LMUcast)
Chair: Tim Koslowski
15:00 - 16:15 Sean Gryb (Radboud): In Favour of a Schrodinger Evolution for the Universe (Slides, 203 Kb) (Watch the lecture @ LMUcast)
Comment: Antonio Vassallo (Lausanne) (Slides, 108 Kb)
Chair: Tim Koslowski 
16:15 - 16:45 Coffee
16:45 - 17:45 Bianca Dittrich (Perimeter Institute): The Consistent Boundary Formulation: Renormalization and the Flow of Time (Slides, 10.4 Mb) (Watch the lecture @ LMUcast)
Chair: Philipp Hoehn
19:00 Dinner (Cafe Reitschule)

Day 2 (4 July 2015)

09:00 - 10.00 Philipp Hoehn (Perimeter Institute): Relational Dynamics and Chaos (Slides, 833 Kb) (Preprint @ (Watch the lecture @ LMUcast)
Chair: Sean Gryb
10:00 - 10:30 Coffee
10:30 - 11:45 Brian Pitts (Cambridge): Changing Observables in Canonical General Relativity from Hamiltonian-Lagrangian Equivalence (Slides, 4.4 Mb) (Watch the lecture @ LMUcast)
Comment: Sam Fletcher (MCMP/LMU & Minnesota)
Chair: Alex Blum 
11:45 - 12:45 Oliver Pooley (Oxford): First-Class Constraints, Gauge, and the Wheeler–DeWitt Equation (Slides, 1.3 Mb) (Watch the lecture @ LMUcast)
Chair: Alex Blum
12:45 - 14:00 Lunch
14:00 - 15:15 Tim Koslowski (New Brunswick): The Gravitational Arrow of Time
Comment: Dennis Lehmkuhl (Caltech) (Slides, 872 Kb) (Watch the lecture @ LMUcast)
Chair: Sam Fletcher
15:15 - 16:30 Carlo Rovelli (Aix-Marseille): The Conceptual Structure of Quantum Gravity. A Concrete Case of a Calculation: The Tunneling Time of a Hajicek-Kiefer Black-to-White Hole Transition (Slides, 5.4 Mb) (Watch the lecture @ LMUcast)
Comment: Erik Curiel (MCMP/LMU)
Chair: Sam Fletcher 
16:30 - 17:00 Coffee
17:00 - 18:00 Kurt Sundermeyer (FU Berlin & MPIWG): Notions of Time in Physics (Slides, 749 Kb) (Watch the lecture @ LMUcast)
Chair: Karim Thébault
19:00 Dinner (Georgenhof)


Alex Blum: How Quantum Theory Tried to Get Rid of Time and General Relativity Started Looking for it

After their formulation in the first third of the twentieth century, quantum theory and and relativity followed an almost antiparallel trajectory concerning their relation to time. Quantum theory, originally formulated using differential equations with time as an evolutionary parameter, increasingly abandoned the notion of infinitesimal time evolution, replacing it with the calculation of transition matrix elements and the study of scattering "in-out" problems. On the other hand, general relativity, originally formulated in a manifestly covariant fashion with no explicit mention of time, increasingly focused on canonical formulations and the explicit identification of time as an evolutionary parameter. Both developments were driven not only by experimental and calculational demands, but paradoxically also by the desire to comply with the demands of the respective other theory. I will discuss these anti-parallel developments, focusing also on mathematical techniques which played a role in both developments, in particular the path integral

Henrique Gomes: Timeless Configuration Space and the Emergence of Classical Behavior for Closed Systems

In this talk, I will explore a timeless interpretation of quantum mechanics of closed systems, solely in terms of path integrals in non-relativistic timeless configuration space. What prompts a fresh look at the foundational problems in this context, is the advent of multiple gravitational models in which Lorentz symmetry is only emergent. In this setting, I propose a new understanding of records as certain relations between two configurations, the recorded one and the record-holding one. These relations are formalized through a factorization of the amplitude kernel, which forbids unwanted 'recoherence' of branches. On this basis, I show that in simple cases the Born rule is consistent with counting the relative density of observers with the same records. Furthermore, unlike what occurs in consistent histories, in this context there is indeed a preferred notion of coarse-grainings: those centered around piece-wise classical paths in configuration space (with a certain radius). Thus, this new understanding claims to resolve aspects of the measurement problem which are still deemed controversial in the standard approaches (but which probably leaves others open...).top

Bianca Dittrich: The Consistent Boundary Formulation: Renormalization and the Flow of Time

I will introduce the consistent boundary formulation which allows to express renormalization flow in a background independent context. I will discuss consequences of this formulation for the Hamiltonian framework and explore in which sense Hamiltonian constraints do actually exists in this context and how this influences the notion of time.



Sean Gryb: In Favour of a Schrodinger Evolution for the Universe

In the canonical formulation of reparametrization invariant systems, time evolution on phase space is generated by a fully constrained Hamiltonian. On the orthodoxy view, the quantum formalism for such systems is constructed through Dirac quantization, which leads to a real, time-independent constraint on the quantum state. The question then remains how to extract a notion of time evolution from this frozen formalism. On one predominant view, the system is to be split in terms of ''partial observables'' -- which may be used as internal clocks -- and a set of ''complete observables'' -- which are understood to evolve in terms of the former. This has led to a controversy around the interpretation of the partial observables within the formalism. In this talk, we will provide a negative argument against the orthodoxy view that clarifies the role that should be played by the partial observables. We then present a proposal for the canonical quantization of reparametrization invariant systems that naturally encodes a genuine notion of time evolution and illustrate how this proposal can be applied to gravity. top

Philipp Hoehn: Relational Dynamics and Chaos

I will discuss fundamental challenges to the standard relational paradigm arising from chaotic

Tim Koslowski: The Gravitational Arrow of Time

The arrow of time appears to always to point in one direction (i.e. we can clearly tell whether a movie is played forward or backward) although the underlying physics is time-reversal symmetric. The most widely accepted explanation for this is that the experienced arrow is the thermodynamic arrow of time (i.e. the direction of entropy growth). This scenario requires the past hypothesis, i.e. an atypical initial condition. We propose an alternative mechanism: The arrow of time is the direction in which the complexity of the universe grows. Gravity generates this arrow of time and creates subsystems with low entropy initial conditions spontaneously. I show this in detail in the Newtonian limit and discuss the extension to cosmological models in GR.
This talk is based on joint work with Julian Barbour and Flavio

Brian Pitts: Changing Observables in Canonical General Relativity from Hamiltonian-Lagrangian Equivalence

Is change missing in classical canonical General Relativity? If one insists on Hamiltonian-Lagrangian equivalence, then there is Hamiltonian change just when there is no time-like Killing vector field. Change has seemed missing partly due to Dirac’s belief that a first-class constraint, especially a primary, generates a gauge transformation. Pons showed that Dirac’s argument stops too soon: working to second order in time brings in first-class secondaries and hence the gauge generator G, a tuned sum of first-class constraints used by Anderson and Bergmann (1951) and recovered by Mukunda, Castellani et al. from the 1980s. I observe that trouble happens immediately: a first-class primary constraint generates an illegal change of initial data in GR, Maxwell and Yang-Mills. Dirac’s subtractive derivation misses it by cancellation; confusion between the electric field E(dA) and canonical momenta p (auxiliary fields in the canonical action \int dt (p \dot{q}-H) also obscures the problem. Dirac’s conjecture that a first-class secondary constraint generates a gauge transformation rests on a false assumption. Looking for gauge symmetries of the canonical action, one finds that the gauge generator G changes the action by at most a boundary term, but an isolated first-class constraint does not. The gauge generator G generates spatio-temporal coordinate transformations (not just spatial ones) for the space-time metric (not just the spatial metric). But are there locally varying _observables_ in canonical General Relativity? Hamiltonian-Lagrangian equivalence guarantees that Hamiltonian observables are equivalent on-shell to Lagrangian observables. (Historically, Lagrangian-inequivalent observables may have arisen within Bergmann’s school due to novel postulation in Bergmann-Schiller 1953.) With first-class constraints exposed as not generating gauge transformations, observables’ Poisson brackets should be taken with the gauge generator G, as noted by Pons, Salisbury and Sundermeyer. Heeding Einstein’s point-coincidence argument excludes primitive point individuation and thus active diffeomorphisms in favor of (4-d) tensor calculus. Kuchař’s unsystematic waiver of the vanishing Poisson brackets condition to permit change has a more principled extension: observables should be internally gauge _invariant_ (0 Poisson bracket with G for Maxwell, Yang-Mills, etc.) but externally gauge _covariant_. Hence the Poisson bracket with the coordinate-changing G should be the Lie derivative, indeed the Lie derivative of a geometric object (on-shell). For GR with no matter gauge group, observables are (on-shell) space-time geometric objects (components in coordinates with a transformation law). Hence the space-time metric and its concomitants (connection, curvature, etc.) are locally varying observables. Questions regarding Legendre projectability when an internal gauge group is also present and regarding the mixed supergravity transformations are noted. Velocity-dependent gauge transformations call for phase space extended by time---“phase space-time”; GR’s Lie derivative is an example. Vacuum GR’s phase space-time has 20 infinity^3 + 1 dimensions and 8 infinity^3 first-class constraints; one should not have expected a reduced phase _space_ description of a theory with many-fingered time. Classical clarity might be of some use in

Oliver Pooley: First-Class Constraints, Gauge, and the Wheeler–DeWitt Equation

Recently, Pitts (2014) has argued that the claim that first-class constraints generate gauge transformations (hereafter “orthodoxy”) fails even in electromagnetism, which is standardly taken to illustrate its correctness. Independently, Barbour and Foster (2008) have argued that a key presupposition of the primary argument for orthodoxy (due to Dirac, 1964) is not satisfied in the important case of reparameterization-invariant theories. In assessing these claims, one needs to distinguish between, (i) transformations that relate points of phase space that represent the same instantaneous state and (ii) transformations that map curves on phase space to curves that represent the same history. Pitts shows that arbitrary first-class constraints fail to generate transformations of type (ii), but leaves untouched the orthodox position concerning (i). Barbour and Foster show that we have no reason to regard transformations generated by Hamiltonian constraints as type (i), but that leaves open that they may be regarded as transformations of type (ii). I will discuss whether the latter possibility allows one to reconcile applying Dirac’s constrained quantization procedure to the Hamiltonian constraint and interpreting the Wheeler–DeWitt wavefunction as representing genuine

Carlo Rovelli: The Conceptual Structure of Quantum Gravity. A Concrete Case of a Calculation: The Tunneling Time of a Hajicek-Kiefer Black-to-White Hole Transition

Rivers of ink have flown on the basic conceptual structure of quantum gravity -a theory where we expect the notions of classical spacetime, particles, fields, energy and momentum to require substantial revision-. I discuss a specific solution to these questions and apply concretely it to a physical calculation, the tunneling time of a Hajicek-Kiefer black-to-white hole transition. This is a quantum gravitational effect that might have some chance to be actually observable, or could have even been already observed in the "Fast Radio Bursts" observed by the Arecibo and Parkes

Donald Salisbury: The Intrinsic Hamilton-Jacobi Dynamics of General Relativity and its Implications for the Semi-Classical Emergence of Time

The quantization of the general theory of relativity is notoriously difficult, in particular on account of the underlying general covariance and the consequent appearance of constraints in the classical Hamiltonian theory. The notion of time in the quantum theory is especially troubling since differing ideas of time suggest themselves depending on the quantum rules that are employed and the interpretations given to time in the classical theory itself. I will address the problem of time from a perspective in which constraints are implemented in a Hamilton-Jacobi framework through the use of intrinsic coordinates. The canonical approach is especially suited for this task. The decisive result is that the problem of time is even greater than one might have expected; there are arbitrarily many equally valid and possibly inequivalent time choices that one can introduce in this manner, all involving the use dynamical variables that are invariant under the action of the four-dimensional diffeomorphism-induced group as described in Pons, Salisbury, and Sundermeyer, Phys. Rev. (2009), 084015. I will review a Kuchař-inspired, but fully diffeomorphism covariant, classical Hamiltonian approach to general relativity in which spacetime scalar phase space variables are introduced that can serve as intrinsic coordinates. There corresponds to each choice a constraint which can be converted (as originally proposed by Asher Peres for conventional variables) into an Einstein-Hamilton-Jacobi (EHJ) equation. The choice of intrinsic coordinates is rendered simple in terms of these new variables, as are the choices in the new intrinsic EHJ equation. Indeed, the resulting intrinsic dynamics follows immediately from the EHJ equation. No Lagrangian is obtained, and this might be expected given that spacetime scalars must depend on time derivatives of the metric and their introduction into the Einstein action would result in the appearance higher derivative contributions. To each EHJ equation there corresponds a Wheeler-DeWitt quantum equation with its own emergent time. I will begin to examine possible quantum implications of the existence of distinct emergent intrinsic times.

Kurt Sundermeyer: Notions of Time in Physics

In an attempt to understand slogans such as "The End Of Time", "Forget About Time", "Time Reborn", "Time Remains", I started to write an essay about the various notions of time in physics (with a glimpse to philosophy) from classical mechanics to quantum gravity. By this curiosity-driven motivation I realized that there are not only various notions of time, but that there are diverse problems with time and problems of time. In my talk I will present the structure and inputs of this still unfinished treatise, and I expect to receive valuable feedback from the competent participants of this

Francesca Vidotto: Compact Phase Space, Cosmological Constant, Discrete Time

We study the quantization of geometry in the presence of a cosmological constant, using a discretiza- tion with constant-curvature simplices. Phase space turns out to be compact and the Hilbert space finite dimensional for each link. Not only the intrinsic, but also the extrinsic geometry turns out to be discrete, pointing to discreetness of time, in addition to space. We work in 2+1 dimensions, but these results may be relevant also for the physical 3+1 case.