Outputs
Quantum Stress Tensor Fluctuations and the Limits of Semiclassical Gravity
Alejandro Perez
I will analyze the expectation value of the energy-momentum tensor and its fluctuations in quantum field theory on curved spacetimes. A generally accepted condition for the conceptual consistency of semiclassical gravity is that the fluctuations of the energy momentum tensor remain small compared to its expectation value. We study the renormalization of both the energy-momentum tensor ⟨Tab(x)⟩ren and the fluctuation tensor ⟨Tab(x)Tcd(x)⟩ren for suitably well behaved states, using the operator product expansion for a free scalar field on a fixed curved background. We show that states (usually referred to as “squeezed vacua”)—arising naturally in black hole evaporation and in inflationary cosmology—fail to satisfy the natural semiclassicality criterion. I will also show how no-faster-than-light communication implies the existence of states for which the energy conditions for ⟨Tab(x)⟩ren fail as well as positivity of the renormalized quantum uncertainties. Based on arXiv:2512.17789
Quantum Stress Tensor Fluctuations and the Limits of Semiclassical Gravity
Alejandro Perez
I will analyze the expectation value of the energy-momentum tensor and its fluctuations in quantum field theory on curved spacetimes. A generally accepted condition for the conceptual consistency of semiclassical gravity is that the fluctuations of the energy momentum tensor remain small compared to its expectation value. We study the renormalization of both the energy-momentum tensor ⟨Tab(x)⟩ren and the fluctuation tensor ⟨Tab(x)Tcd(x)⟩ren for suitably well behaved states, using the operator product expansion for a free scalar field on a fixed curved background. We show that states (usually referred to as “squeezed vacua”)—arising naturally in black hole evaporation and in inflationary cosmology—fail to satisfy the natural semiclassicality criterion. I will also show how no-faster-than-light communication implies the existence of states for which the energy conditions for ⟨Tab(x)⟩ren fail as well as positivity of the renormalized quantum uncertainties. Based on arXiv:2512.17789
Quantum reference frames, subsystem relativity, and their implications for open quantum systems
Viktoria Kabel
As we describe the world, we almost invariably partition it into subsystems. Restricting to isolated subsystems can be extremely useful in simplifying the physical description. However, this partitioning into subsystems has limitations. First of all, realistic physical systems - in particular gravitational subsystems - are not completely isolated. The study of open quantum systems takes this into account by including the interaction of the system with inaccessible environmental degrees of freedom. However, there is a more fundamental problem: in the presence of symmetries, the partition into subsystems itself may not be absolute. This is the case when studying quantum reference frames (QRFs). While there are several different approaches to modelling such reference frames, they all agree on a fundamental insight: the way we partition the Hilbert space into subsystems - the tensor product structure - depends on the choice of QRF. This subsystem relativity underlies many interesting features of QRFs, such as the frame-dependence of entanglement, entropy, and coherence. Since the study of open quantum systems relies fundamentally on a split into “system” and “environment”, it is natural to ask how our description of these systems, including processes such as decoherence, is affected by a change of QRF. After introducing the general idea of subsystem relativity, I will discuss how and under which conditions changes of QRF lead to a frame-induced decoherence rate and in what way this can affect gravitationally induced decoherence in a simple toy model. Based on arXiv:2607.05578
Quantum reference frames, subsystem relativity, and their implications for open quantum systems
Viktoria Kabel
As we describe the world, we almost invariably partition it into subsystems. Restricting to isolated subsystems can be extremely useful in simplifying the physical description. However, this partitioning into subsystems has limitations. First of all, realistic physical systems - in particular gravitational subsystems - are not completely isolated. The study of open quantum systems takes this into account by including the interaction of the system with inaccessible environmental degrees of freedom. However, there is a more fundamental problem: in the presence of symmetries, the partition into subsystems itself may not be absolute. This is the case when studying quantum reference frames (QRFs). While there are several different approaches to modelling such reference frames, they all agree on a fundamental insight: the way we partition the Hilbert space into subsystems - the tensor product structure - depends on the choice of QRF. This subsystem relativity underlies many interesting features of QRFs, such as the frame-dependence of entanglement, entropy, and coherence. Since the study of open quantum systems relies fundamentally on a split into “system” and “environment”, it is natural to ask how our description of these systems, including processes such as decoherence, is affected by a change of QRF. After introducing the general idea of subsystem relativity, I will discuss how and under which conditions changes of QRF lead to a frame-induced decoherence rate and in what way this can affect gravitationally induced decoherence in a simple toy model. Based on arXiv:2607.05578
Relational path integral, effective actions, and quantum frame covariance in gravity
Philipp Höhn
Relational path integral, effective actions, and quantum frame covariance in gravity
Philipp Höhn
Pluralism, Perspectivalism, and Relationality: from Feminist Epistemology to Today’s Physics
Francesca Vidotto
Pluralism, Perspectivalism, and Relationality: from Feminist Epistemology to Today’s Physics
Francesca Vidotto
Space-time deterministic graph rewriting
Marin Costes
Space-time deterministic graph rewriting
Marin Costes
The perspective-neutral take on quantum reference frames and gravitational entropies
Julian De Vuyst
The perspective-neutral take on quantum reference frames and gravitational entropies
Julian De Vuyst
Observer-dependent entropy in quantum reference frames
Anne Catherine de la Hamette
Observer-dependent entropy in quantum reference frames
Anne Catherine de la Hamette
Bringing it down to Earth: Quantum gravity from black holes to the lab.
Francesca Vidotto
Bringing it down to Earth: Quantum gravity from black holes to the lab.
Francesca Vidotto
The classical gravitational case for relational realism
Daniele Oriti
The classical gravitational case for relational realism
Daniele Oriti
The Perspectives of Non-Ideal Quantum Reference Frames
Sébastien C. Garmier
The Perspectives of Non-Ideal Quantum Reference Frames
Sébastien C. Garmier
The Conceptual and Mathematical Structure of Quantum Theory, June 22 - June 25
Lucien Hardy
The Conceptual and Mathematical Structure of Quantum Theory, June 22 - June 25
Lucien Hardy
Parity erasure: a foundational principle for indefinite casual order
Zixuan Liu
Parity erasure: a foundational principle for indefinite casual order
Zixuan Liu
Picturing causality and localisability on non-factor subsystems
Octave Mestoudjian
Picturing causality and localisability on non-factor subsystems
Octave Mestoudjian
Unitary causal decompositions via lattice theory
Tein van der Lugt (joint work with Robin Lorenz)
Unitary causal decompositions via lattice theory
Tein van der Lugt (joint work with Robin Lorenz)
Testing Spacetime Quantum Superpositions in the Laboratory
Sougato Bose
Large quantum fluctuations of the energy momentum tensor and the limits of semiclassical gravity
Alejandro Perez
Large quantum fluctuations of the energy momentum tensor and the limits of semiclassical gravity
Alejandro Perez
Quantum reference frames in QFT and gravity
Robin Simmon
Quantum reference frames in QFT and gravity
Robin Simmon
How unitaries encode the trade-off between causal order and locality
Nasra Daher Ahmed
How unitaries encode the trade-off between causal order and locality
Nasra Daher Ahmed