# Weekly Papers on Quantum Foundations (38)

Finding the world in the wave function: some strategies for solving the macro-object problem

Abstract

Realists wanting to capture the facts of quantum entanglement in a metaphysical interpretation find themselves faced with several options: to grant some species of fundamental nonseparability, adopt holism, or (more radically) to view localized spacetime systems as ultimately reducible to a higher-dimensional entity, the quantum state or wave function. Those adopting the latter approach and hoping to view the macroscopic world as grounded in the quantum wave function face the macro-object problem. The challenge is to articulate the metaphysical relation obtaining between three-dimensional macro-objects and the wave function so that the latter may be seen in some sense as constituting the former. This paper distinguishes several strategies for doing so and defends one based on a notion of partial instantiation.

Mechanisms for constrained stochasticity

Abstract

Creativity is generally thought to be the production of things that are novel and valuable (whether physical artefacts, actions, or ideas). Humans are unique in the extent of their creativity, which plays a central role in innovation and problem solving, as well as in the arts. But what are the cognitive sources of novelty? More particularly, what are the cognitive sources of stochasticity in creative production? I will argue that they belong to two broad categories. One is associative, enabling the selection of goal-relevant ideas that have become activated by happenstance in an unrelated context. The other relies on selection processes that leverage stochastic fluctuations in neural activity. At the same time, I will address a central puzzle, which is to understand how the outputs of stochastic processes can nevertheless generally fall within task constraints. While the components appealed to in the accounts that I offer are well established, the ways in which I combine them together are new.

Two notions of holism

Abstract

A simple argument proposes a direct link between realism about quantum mechanics and one kind of metaphysical holism: if elementary quantum theory is at least approximately true, then there are entangled systems with intrinsic whole states for which the intrinsic properties and spatiotemporal arrangements of salient subsystem parts do not suffice. Initially, the proposal is compelling: we can find variations on such reasoning throughout influential discussions of entanglement. Upon further consideration, though, this simple argument proves a bit too simple. To get such metaphysically robust consequences out, we need to put more than minimal realism in. This paper offers a diagnosis: our simple argument seems so compelling thanks to an equivocation. The predictions of textbook quantum theory already resonate with familiar holistic slogans; for realists, then, any underlying reality, conforming to such predictions, also counts as holistic in some sense or other, if only by association. Such associated holism, though, does not establish the sort of specific, robust supervenience failure claimed by our simple argument. While it may be natural to slide to this stronger conclusion, facilitating the slide is not minimal realism per se but an additional explanatory assumption about how and why reality behaves in accordance with our theory: roughly, quantum theory accurately captures patterns in the features and behaviors of physical reality because some underlying metaphysical structure constrains reality to exhibit these patterns. Along with the diagnosis comes a recommendation: we can and should understand one traditional disagreement about the metaphysics of entanglement as another manifestation of a familiar and more general conflict between reductive and non-reductive conceptions of metaphysical theorizing. Such reframing makes clearer what resources reductionists have for resisting the simple argument’s challenge from quantum holism. It also has an important moral for their opponents. Traditional focus on whole-part supervenience failure distracts from a root disagreement about metaphysical structure and its role in our theorizing. Non-reductionists fond of our simple argument would be better off tackling this root directly.

La Bohume

Abstract

This paper critically assesses whether quantum entanglement can be made compatible with Humean supervenience. After reviewing the prima facie tension between entanglement and Humeanism, I outline a recently-proposed Humean response, and argue that it is subject to two problems: one concerning the determinacy of quantities, and one concerning its relationship to scientific practice.

Quantum holism: nonseparability as common ground

Abstract

Quantum mechanics seems to portray nature as nonseparable, in the sense that it allows spatiotemporally separated entities to have states that cannot be fully specified without reference to each other. This is often said to implicate some form of “holism.” We aim to clarify what this means, and why this seems plausible. Our core idea is that the best explanation for nonseparability is a “common ground” explanation (modeled after common cause explanations), which casts nonseparable entities in a holistic light, as scattered reflections of a more unified underlying reality.

Quantum metaphysical indeterminacy and worldly incompleteness

Abstract

An influential theory has it that metaphysical indeterminacy occurs just when reality can be made completely precise in multiple ways. That characterization is formulated by employing the modal apparatus of ersatz possible worlds. As quantum physics taught us, reality cannot be made completely precise. I meet the challenge by providing an alternative theory which preserves the use of ersatz worlds but rejects the precisificational view of metaphysical indeterminacy. The upshot of the proposed theory is that it is metaphysically indeterminate whether p just in case it is neither true nor false that p, and no terms in ‘p’ are semantically defective. In other words, metaphysical indeterminacy arises when the world cannot be adequately described by a complete set of sentences defined in a semantically nondefective language. Moreover, the present theory provides a reductive analysis of metaphysical indeterminacy, unlike its influential predecessor. Finally, I argue that any adequate logic of a language with an indeterminate subject matter is neither compositional nor bivalent.

Timelessness and freedom

Abstract

One way that philosophers have attempted to defend free will against the threat of fatalism and against the threat from divine beliefs has been to endorse timelessness views (about propositions and God’s beliefs, respectively). In this paper, I argue that, in order to respond to general worries about fatalism and divine beliefs, timelessness views must appeal to the notion of dependence. Once they do this, however, their distinctive position as timelessness views becomes otiose, for the appeal to dependence, if it helps at all, would itself be sufficient to block worries about fatalism and divine beliefs. I conclude by discussing some implications for dialectical progress.

On quantum entanglement, counterfactuals, causality and dispositions

Abstract

The existence of non-local correlations between outcomes of measurements in quantum entangled systems strongly suggests that we are dealing with some form of causation here. An assessment of this conjecture in the context of the collapse interpretation of quantum mechanics is the primary goal of this paper. Following the counterfactual approach to causation, I argue that the details of the underlying causal mechanism which could explain the non-local correlations in entangled states strongly depend on the adopted semantics for counterfactuals. Several relativistically-invariant interpretations of spatiotemporal counterfactual conditionals are discussed, and the corresponding causal stories describing interactions between parts of an entangled system are evaluated. It is observed that the most controversial feature of the postulated causal connections is not so much their non-local character as a peculiar type of circularity that affects them.

On the Sign of Fermion-Mediated Interactions. (arXiv:2009.08095v1 [cond-mat.mes-hall])

We develop a unified framework for understanding the sign of fermion-mediated interactions by exploiting the symmetry classification of Green’s functions. In particular, we establish a theorem regarding the sign of fermion-mediated interactions in systems with chiral symmetry. The strength of the theorem is demonstrated within multiple examples with an emphasis on electron-mediated interactions in materials.

One from many: Estimating a function of many parameters. (arXiv:2002.02898v2 [quant-ph] UPDATED)

Difficult it is to formulate achievable sensitivity bounds for quantum multiparameter estimation. Consider a special case, one parameter from many: many parameters of a process are unknown; estimate a specific linear combination of these parameters without having the ability to control any of the parameters. Superficially similar to single-parameter estimation, the problem retains genuinely multiparameter aspects. Geometric reasoning demonstrates the conditions, necessary and sufficient, for saturating the fundamental and attainable quantum-process bound in this context.

Proposed measurement of simultaneous particle and wave properties of electric current in a superconductor. (arXiv:2003.14049v2 [quant-ph] UPDATED)

In a microscopic quantum system one cannot perform a simultaneous measurement of particle and wave properties. This, however, may not be true for macroscopic quantum systems. As a demonstration, we propose to measure the local macroscopic current passed through two slits in a superconductor. According to the theory based on the linearized Ginzburg-Landau equation for the macroscopic pseudo wave function, the streamlines of the measured current should have the same form as particle trajectories in the Bohmian interpretation of quantum mechanics. By an explicit computation we find that the streamlines should show a characteristic wiggling, which is a consequence of quantum interference.

Quantum field theory from a quantum cellular automaton in one spatial dimension and a no-go theorem in higher dimensions. (arXiv:2006.08927v2 [quant-ph] UPDATED)

It has been shown that certain quantum walks give rise to relativistic wave equations, such as the Dirac and Weyl equations, in their long-wavelength limits. This intriguing result raises the question of whether something similar can happen in the multi-particle case. We construct a one-dimensional quantum cellular automaton (QCA) model which matches the quantum walk in the single particle case, and which approaches the quantum field theory of free fermions in the long-wavelength limit. However, we show that this class of constructions does not generalize to higher spatial dimensions in any straightforward way, and that no construction with similar properties is possible in two or more spatial dimensions. This rules out the most common approaches based on QCAs. We suggest possible methods to overcome this barrier while retaining locality.

Enrico Fermi in Argentina and his Lectures in Buenos Aires, Cordoba and La Plata. (arXiv:2003.13368v4 [physics.hist-ph] UPDATED)

Authors: Alessandro De AngelisJose M. Kenny

In 1934 Enrico Fermi accepted an invitation to deliver lectures in Argentina, Brazil and Uruguay. He arrived in Buenos Aires on July 30, lectured in Buenos Aires, Cordoba, La Plata and Montevideo, and then moved on August 18 to Sao Paulo via Santos and Rio de Janeiro; he traveled back from Rio to Naples on September 1st. His visit had a large resonance, and halls were crowded despite the fact that he lectured in Italian. The University of Buenos Aires recorded his five lectures and transcribed them in Spanish. They contain the first public presentations of the theory of beta decay and of the works on artificial radioactivity started by the via Panisperna group, but are not included in Fermi’s Collected Works edited by the Accademia dei Lincei in Rome and by the University of Chicago, although listed in the Bibliography. In this paper we present the transcription of Fermi’s five lectures in Buenos Aires, a summary of the lecture in La Plata and an extended summary of the lecture in Cordoba, translating them in English for the first time.

Two Dogmas of Dynamicism. (arXiv:2006.11400v2 [physics.hist-ph] UPDATED)

Authors: James Owen Weatherall

I critically discuss two dogmas of the “dynamical approach” to spacetime in general relativity, as advanced by Harvey Brown [Physical Relativity (2005) Oxford:Oxford University Press] and collaborators. The first dogma is that positing a “spacetime geometry” has no implications for the behavior of matter. The second dogma is that postulating the “Strong Equivalence Principle” suffices to ensure that matter is “adapted” to spacetime geometry. I conclude by discussing “spacetime functionalism”. The discussion is presented in reaction to and sympathy with recent work by James Read [“Explanation, geometry, and conspiracy in relativity theory”(20??) Thinking about Spacetime. Boston: Birkauser].

Spin networks and the cosmological constant in the AdS/CFT correspondence. (arXiv:2009.07843v1 [gr-qc])

Authors: Carlos Silva

We propose a generalization of the recently proposed holographic duality between spin networks and superstrings, and show that it can provide a possible solution to the cosmological constant problem.

New Test of Weak Equivalence Principle Using Frequency Comparisons of High-precision Clocks. (arXiv:2009.06945v2 [gr-qc] UPDATED)

Authors: ChengGang QinChengGang Shao

The Einstein Equivalence Principle carries a pivotal role in the understanding theory of gravity and spacetime. In its weak form, namely, weak equivalence principle (WEP), it implies the universality of free fall. Currently, a WEP test of relativistic form is blank in the experiments. In this work, we propose a novel scheme for the test of the WEP using frequency measurements. Our proposal consists of the comparison of high-precision clocks, comoving with the freely falling frame. In the presence of WEP violation, described by the E$\ddot{\text{o}}$tv$\ddot{\text{o}}$s parameter $\delta$, we demonstrate the feasibility to measure the potential changes of clock rates. In contrast to the traditional tests, measuring the difference on E$\ddot{\text{o}}$tv$\ddot{\text{o}}$s parameter between two materials of different compositions, our proposal allows for measuring the E$\ddot{\text{o}}$tv$\ddot{\text{o}}$s parameter for a single “test” body. Therefore, it potentially opens up a new window for the tests of the WEP. By searching for a daily variation of frequency difference between strontium optical clocks connected by optical fiber links, we obtain the upper limit of E$\ddot{\text{o}}$tv$\ddot{\text{o}}$s parameter about the Earth $\delta_{\text{E}}=(0.3\pm0.9)\times10^{-4}$.

On the Hardness of Detecting Macroscopic Superpositions. (arXiv:2009.07450v2 [quant-ph] UPDATED)

Authors: Scott AaronsonYosi AtiaLeonard Susskind

When is decoherence “effectively irreversible”? Here we examine this central question of quantum foundations using the tools of quantum computational complexity. We prove that, if one had a quantum circuit to determine if a system was in an equal superposition of two orthogonal states (for example, the $|$Alive$\rangle$ and $|$Dead$\rangle$ states of Schr\”{o}dinger’s cat), then with only a slightly larger circuit, one could also $\mathit{swap}$ the two states (e.g., bring a dead cat back to life). In other words, observing interference between the $|$Alive$\rangle$and $|$Dead$\rangle$ states is a “necromancy-hard” problem, technologically infeasible in any world where death is permanent. As for the converse statement (i.e., ability to swap implies ability to detect interference), we show that it holds modulo a single exception, involving unitaries that (for example) map $|$Alive$\rangle$ to $|$Dead$\rangle$ but $|$Dead$\rangle$ to -$|$Alive$\rangle$. We also show that these statements are robust—i.e., even a $\mathit{partial}$ ability to observe interference implies partial swapping ability, and vice versa. Finally, without relying on any unproved complexity conjectures, we show that all of these results are quantitatively tight. Our results have possible implications for the state dependence of observables in quantum gravity, the subject that originally motivated this study.

Determinism and general relativity

Smeenk, Chris and Wuthrich, Christian (2020) Determinism and general relativity. [Preprint]

Can we make sense of relational quantum mechanics?

Ruyant, Quentin (2018) Can we make sense of relational quantum mechanics? Foundations of Physics, 48 (4). pp. 440-455. ISSN 1572-9516

The physical limits of computation inspire an open problem that concerns abstract computable sets X⊆N and cannot be formalized in the set theory ZFC as it refers to our current knowledge on X

Kurpaska, Sławomir and Tyszka, Apoloniusz (2020) The physical limits of computation inspire an open problem that concerns abstract computable sets X⊆N and cannot be formalized in the set theory ZFC as it refers to our current knowledge on X. [Preprint]

Dissipation-Time Uncertainty Relation

Author(s): Gianmaria Falasco and Massimiliano Esposito

We show that the entropy production rate bounds the rate at which physical processes can be performed in stochastic systems far from equilibrium. In particular, we prove the fundamental tradeoff ⟨S˙e⟩T≥kB between the entropy flow ⟨S˙e⟩ into the reservoirs and the mean time T to complete any process …

[Phys. Rev. Lett. 125, 120604] Published Wed Sep 16, 2020

Relativity without miracles

Sus, Adán (2020) Relativity without miracles. [Preprint]

Symmetry breaking and the emergence of path-dependence

Desmond, Hugh (2017) Symmetry breaking and the emergence of path-dependence. Synthese, 194 (10). pp. 4101-4131. ISSN 0039-7857

Reeh-Schlieder, space-time foam, and the implications for neuroscience

McCabe, Gordon (2020) Reeh-Schlieder, space-time foam, and the implications for neuroscience. [Preprint]

One time, two times, or no time?

Wuthrich, Christian (2020) One time, two times, or no time? [Preprint]

The Dynamical Renaissance in Neuroscience

Favela, Luis H. (2020) The Dynamical Renaissance in Neuroscience. [Preprint]

Is determinism completely rejected in the standard Quantum Mechanics?

Bahreini, Jafar and Razmi, Habibollah and Monfared, Mahdi (2020) Is determinism completely rejected in the standard Quantum Mechanics? [Preprint]

From Time Asymmetry to Quantum Entanglement: The Humean Unification

Chen, Eddy Keming (2020) From Time Asymmetry to Quantum Entanglement: The Humean Unification. [Preprint]

Weak-to-strong transition of quantum measurement in a trapped-ion system

Nature Physics, Published online: 14 September 2020; doi:10.1038/s41567-020-0973-y

A weak-to-strong quantum measurement transition has been observed in a single-trapped-ion system, where the ion’s internal electronic state and its vibrational motion play the roles of the measured system and the measuring pointer.

A Tale of Two Nortons

Skeels, Patrick (2020) A Tale of Two Nortons. In: UNSPECIFIED.