Thermality Versus Objectivity: Can They Peacefully Coexist?

Under the influence of external environments, quantum systems can undergo various different processes, including decoherence and equilibration. We observe that macroscopic objects are both objective and thermal, thus leading to the expectation that both objectivity and thermalisation can peacefully coexist on the quantum regime too. Crucially, however, objectivity relies on distributed classical information that could conflict with thermalisation. Here, we examine the overlap between thermal and objective states. We find that in general, one cannot exist when the other is present. However, there are certain regimes where thermality and objectivity are more likely to coexist: in the high temperature limit, at the non-degenerate low temperature limit, and when the environment is large. This is consistent with our experiences that everyday-sized objects can be both thermal and objective.

Magnetic Guiding with Permanent Magnets: Concept, Realization and Applications to Nanoparticles and Cells

The idea of remote magnetic guiding is developed from the underlying physics of a concept that allows for bijective force generation over the inner volume of magnet systems. This concept can equally be implemented by electro- or permanent magnets. Here, permanent magnets are in the focus because they offer many advantages. The equations of magnetic fields and forces as well as velocities are derived in detail and physical limits are discussed. The special hydrodynamics of nanoparticle dispersions under these circumstances is reviewed and related to technical constraints. The possibility of 3D guiding and magnetic imaging techniques are discussed. Finally, the first results in guiding macroscopic objects, superparamagnetic nanoparticles, and cells with incorporated nanoparticles are presented. The constructed magnet systems allow for orientation, movement, and acceleration of magnetic objects and, in principle, can be scaled up to human size.

Probing the nature of black holes: Deep in the mHz gravitational-wave sky

Black holes are unique among astrophysical sources: they are the simplest macroscopic objects in the Universe, and they are extraordinary in terms of their ability to convert energy into electromagnetic and gravitational radiation. Our capacity to probe their nature is limited by the sensitivity of our detectors. The LIGO/Virgo interferometers are the gravitational-wave equivalent of Galileo’s telescope. The first few detections represent the beginning of a long journey of exploration. At the current pace of technological progress, it is reasonable to expect that the gravitational-wave detectors available in the 2035-2050s will be formidable tools to explore these fascinating objects in the cosmos, and space-based detectors with peak sensitivities in the mHz band represent one class of such tools. These detectors have a staggering discovery potential, and they will address fundamental open questions in physics and astronomy. Are astrophysical black holes adequately described by general relativity? Do we have empirical evidence for event horizons? Can black holes provide a glimpse into quantum gravity, or reveal a classical breakdown of Einstein’s gravity? How and when did black holes form, and how do they grow? Are there new long-range interactions or fields in our Universe, potentially related to dark matter and dark energy or a more fundamental description of gravitation? Precision tests of black hole spacetimes with mHz-band gravitational-wave detectors will probe general relativity and fundamental physics in previously inaccessible regimes, and allow us to address some of these fundamental issues in our current understanding of nature.

Dark Matter as dark dwarfs and other macroscopic objects: multiverse relics?

First order phase transitions can leave relic pockets of false vacua and their particles, that manifest as macroscopic Dark Matter. We compute one predictive model: a gauge theory with a dark quark relic heavier than the confinement scale. During the first order phase transition to confinement, dark quarks remain in the false vacuum and get compressed, forming Fermi balls that can undergo gravitational collapse to stable dark dwarfs (bound states analogous to white dwarfs) near the Chandrasekhar limit, or primordial black holes.

Must an Ontology for Quantum Theories Contain Local Beables?

This chapter considers and responds to the objection that a wave function in a high-dimensional space cannot ultimately constitute the low-dimensional macroscopic objects of experience. It discusses two forms this objection takes: one based on the putative fact that our evidence for quantum theories consists of low-dimensional objects, and another based on the putative fact that quantum theories are about low-dimensional objects, that they have primitive ontologies of local beables. Even admitting that there may be something straightforward and comprehensible about the fundamental ontologies for quantum theories proposed by the wave function realist, the philosophers who raise these objections see a problem with these ontologies in that they cannot serve as the constitutive foundation for the world as we experience it. And this undermines the promise of wave function realism to serve as a framework for the interpretation of quantum theories.

The Causal Role of Macroscopic Objects

This chapter considers and critiques some strategies for solving the macro-object problem for wave function realism. This is the problem of how a wave function understood as a field on a high-dimensional space may come to make up or constitute the low-dimensional, macroscopic objects of our experience. It is first noted that simply invoking correspondences between particle configurations and states of the wave function will not suffice to solve the macro-object problem, following issues noted previously by Maudlin and Monton. More sophisticated strategies are considered that appeal to functionalism. It is argued that these functionalist strategies for recovering low-dimensional macroscopic objects from the wave function also do not succeed.

Finding the Macroworld

This chapter proposes a solution to the macro-object problem for wave function realism. This is the problem of how a wave function in a high-dimensional space may come to constitute the low-dimensional, macroscopic objects of our experience. The solution takes place in several stages. First, it is argued that how the wave function’s being invariant under certain transformations may give us reason to regard three-dimensional configurations corresponding symmetries with ontological seriousness. Second it is shown how the wave function may decompose into low-dimensional microscopic parts. Interestingly, this reveals mereological relationships in which parts and wholes inhabit distinct spatial frameworks. Third, it is shown how these parts may come to compose macroscopic objects.

Realization of a complete Stern-Gerlach interferometer: Toward a test of quantum gravity

The Stern-Gerlach effect, found a century ago, has become a paradigm of quantum mechanics. Unexpectedly, until recently, there has been little evidence that the original scheme with freely propagating atoms exposed to gradients from macroscopic magnets is a fully coherent quantum process. Several theoretical studies have explained why a Stern-Gerlach interferometer is a formidable challenge. Here, we provide a detailed account of the realization of a full-loop Stern-Gerlach interferometer for single atoms and use the acquired understanding to show how this setup may be used to realize an interferometer for macroscopic objects doped with a single spin. Such a realization would open the door to a new era of fundamental probes, including the realization of previously inaccessible tests at the interface of quantum mechanics and gravity.