# Weekly Papers on Quantum Foundations (15)

From a microscopic inertial active matter model to the Schr\”odinger equation. (arXiv:2204.03018v1 [cond-mat.soft])

Field theories for the one-body density of an active fluid, such as the paradigmatic active model B+, are simple yet very powerful tools for describing phenomena such as motility-induced phase separation. No comparable theory has been derived yet for the underdamped case. In this work, we introduce active model I+, an extension of active model B+ to particles with inertia. The governing equations of active model I+ are systematically derived from the microscopic Langevin equations. We show that, for underdamped active particles, thermodynamic and mechanical definitions of the velocity field no longer coincide and that the density-dependent swimming speed plays the role of an effective viscosity. Moreover, active model I+ contains the Schr\”odinger equation in Madelung form as a limiting case, allowing to find analoga of the quantum-mechanical tunnel effect and of fuzzy dark matter in the active fluid. We investigate the active tunnel effect analytically and via numerical continuation.

New Directions in the Search for Dark Matter. (arXiv:2204.03085v1 [hep-ph])

The identification of the nature of dark matter is one of the most important problems confronting particle physics. Current observational constraints permit the mass of the dark matter to range from $10^{-22}$ eV – $10^{48}$ GeV. Given the weak nature of these bounds and the ease with which dark matter models can be constructed, it is clear that the problem can only be solved experimentally. In these lectures, I discuss methods to experimentally probe a wide range of dark matter candidates.

Non-universality of free fall in quantum theory. (arXiv:2204.03279v1 [gr-qc])

We show by respecting the Einstein equivalence principle and general covariance in quantum theory that wave-function spreading rules out universality of free fall, and vice versa. Assuming the former is more fundamental than the latter, we gain a quantitative estimate of the free-fall non-universality, which turns out to be empirically testable in atom interferometry.

Is the dynamical quantum Cheshire cat detectable?. (arXiv:2204.03374v1 [quant-ph])

We explore how one might detect the dynamical quantum Cheshire cat proposed by Aharonov et al. We show that, in practice, we need to bias the initial state by adding/subtracting a small probability amplitude (field’) of the orthogonal state, which travels with the disembodied property, to make the effect detectable (i.e. if our initial state is $|\uparrow_z\rangle$, we need to bias this with some small amount $\delta$ of state $|\downarrow_z\rangle$). This biasing, which can be done either directly or via weakly entangling the state with a pointer, effectively provides a phase reference with which we can measure the evolution of the state. The outcome can then be measured as a small probability difference in detections in a mutually unbiased basis, proportional to this biasing $\delta$. We show this is different from counterfactual communication, which provably does not require any probe field to travel between sender Bob and receiver Alice for communication. We further suggest an optical polarisation experiment where these phenomena might be demonstrated in a laboratory.

Is the dynamical quantum Cheshire cat detectable?. (arXiv:2204.03374v1 [quant-ph])

Authors: Jonte R. HanceJames LadymanJohn Rarity

We explore how one might detect the dynamical quantum Cheshire cat proposed by Aharonov et al. We show that, in practice, we need to bias the initial state by adding/subtracting a small probability amplitude (field’) of the orthogonal state, which travels with the disembodied property, to make the effect detectable (i.e. if our initial state is $|\uparrow_z\rangle$, we need to bias this with some small amount $\delta$ of state $|\downarrow_z\rangle$). This biasing, which can be done either directly or via weakly entangling the state with a pointer, effectively provides a phase reference with which we can measure the evolution of the state. The outcome can then be measured as a small probability difference in detections in a mutually unbiased basis, proportional to this biasing $\delta$. We show this is different from counterfactual communication, which provably does not require any probe field to travel between sender Bob and receiver Alice for communication. We further suggest an optical polarisation experiment where these phenomena might be demonstrated in a laboratory.

Physics is organized around transformations connecting contextures in a polycontextural world. (arXiv:2204.03452v1 [physics.hist-ph])

The rich body of physical theories defines the foundation of our understanding of the world. Its mathematical formulation is based on classical Aristotelian (binary) logic. In the philosophy of science the ambiguities, paradoxes, and the possibility of subjective interpretations of facts have challenged binary logic, leading, among other developments, to Gotthard G\”unther’s theory of polycontexturality (often also termed ‘transclassical logic’). G\”unther’s theory explains how observers with subjective perception can become aware of their own subjectivity and provides means to describe contradicting or even paradox observations in a logically sound formalism. Here we summarize the formalism behind G\”unther’s theory and apply it to two well-known examples from physics where different observers operate in distinct and only locally valid logical systems. Using polycontextural logic we show how the emerging awareness of these limitations of logical systems entails the design of mathematical transformations, which then become an integral part of the theory. In our view, this approach offers a novel perspective on the structure of physical theories and, at the same time, emphasizes the relevance of the theory of polycontexturality in modern sciences.

Time Evolution in Quantum Cosmology. (arXiv:2204.03043v1 [gr-qc])

The quantum description of time evolution in non-linear gravitational systems such as cosmological space-times is not well understood. We show, in the simplified setting of mini-superspace, that time evolution of this system can be obtained using a gauge fixed path integral, as long as one does not integrate over proper time. Using this gauge fixed action we can construct a Hamiltonian in the coherent – or classical – state basis. We show that by construction the coherent states satisfy the classical dynamical equations of General Relativity. They do not satisfy the Hamiltonian constraint. A consequence of this is that the Wheeler-DeWitt equation should not be satisfied in quantum gravity. Classical states have a natural non-trivial time evolution since they are not eigenstates of the Hamiltonian. A general feature of the unconstrained quantum theory of gravity is the prediction of a pressureless dark matter component of either sign energy density in the classical universe which may lead to novel phenomenology.

The Structuralist Approach to Underdetermination

Lee, Chanwoo (2022) The Structuralist Approach to Underdetermination. [Preprint]

What is it like to be a chimpanzee?

Abstract

Chimpanzees and humans are close evolutionary relatives who behave in many of the same ways based on a similar type of agentive organization. To what degree do they experience the world in similar ways as well? Using contemporary research in evolutionarily biology and animal cognition, I explicitly compare the kinds of experience the two species of capable of having. I conclude that chimpanzees’ experience of the world, their experiential niche as I call it, is: (i) intentional in basically the same way as humans’; (ii) rational in the sense that it is self-critical and operates with logically structured causal and intentional inferences; but (iii) not normative at all in that it does not operate with “objective” evaluative standards. Scientific data do not answer philosophical questions, but they provide rich raw material for scientists and philosophers alike to reflect on and clarify fundamental psychological concepts.