2025 Abstracts

"The Born Rule as the origin of probabilities in macroscopic systems: A puzzle for cosmology."

Abstract: Phillips and I have argued that in every example where we use probabilities to account for uncertain physical phenomena those probabilities are fundamentally Born Rule probabilities, even when the phenomena are macroscopic. To illustrate this, in 1) we show how the 50-50 probabilities of the coin flip can be traced to quantum uncertainties in the neurons of the person flipping the coin. We argue that formal ideas such as the “principle of indifference” are meaningless unless they are “physicalized” in this way. The goal of my presentation is to help the group understand and debate these results. Philips and I argued that our results don’t change anything about everyday applications of probabilities (whether by scientists doing laboratory-based technical work, or more intuitively in everyday life), but they have important implications for cosmology A fully quantum treatment of the entire Universe leads to situations where relative probabilities are needed but a Born rule foundation is unavailable (examples appear in readings 2-4). I will sketch some of these at the end of my presentation to stimulate discussions about how predictions can be made under such conditions. I believe this is an important unresolved question for quantum cosmology.

Abstract: I will discuss the idea that gravity emerges in the macroscopic regime as a kind of hydrodynamic limit of some underlying "atomic"-type description, rather than from gravitons. This will be based on a recent paper of mine and collaborators (https://arxiv.org/abs/2502.17575), building on earlier work of Jacobson (https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.75.1260) and Verlinde (https://link.springer.com/article/10.1007/JHEP04(2011)029).

"Historical issues in the world(s) of the very cold"

Abstract: The paper will raise some historical as well as philosophical questions about the macroscopic quantum approaches to the phenomena of cold up to the mid-1940s.

Phenomena in the very cold, are phenomena which have obliged us to re-conceptualize a number of notions that physicists as well as chemists considered unproblematic: the notion of electrical resistance as well as the notion of fluidity. In the process of providing explanations for superconductivity and superfluidity, a long-cherished aspect of the quantum mechanical wave function was, also, re-conceptualized. It became possible to talk about macroscopic quantum phenomena which could be articulated in terms of macroscopic wave function(s). 

A rather emblematic figure in the development of these ideas was Fritz London (1900-1955), who started his career as a philosopher and who together with Walter Heitler (1904-1981) in 1927 solved the problem of the homopolar bond of the hydrogen molecule, where it was shown that the phenomenon was a “pure quantum phenomenon” – much like superconductivity and superfluidity the conceptual development of which, also, owe much to London’s significant contributions. 

A historically remarkable issue was the transition from the notion of “order in momentum space” (which was so crucial for the explanation of superconductivity) to the complementary notion of “condensation in momentum space” (concerning superfluidity). 

After a very short introduction to these phenomena, it would, perhaps be interesting to discuss the historical and philosophical issues related to the macroscopic quantum phenomena and their relations with 

1. The unexpected new phenomena that gave rise to the “wrong” problems (in the case of superconductivity).

2. The adherence to cherished predictions of a theory without any experimental testings (in the case of the Meissner-Ochsenfeld experiment).

3. The reaching of a consensus about which among the different new phenomena was the “truly new” phenomenon. (the different and peculiar behaviors of liquid helium)

4. The differences in the local cultures (the different approaches of Fritz London, Laszlo Tisza, Piotr Kapitza, and Lev Landau).

"A pseudo-historical approach to the theory of superconductivity"

Abstract: I ask the question:What are the fundamental properties of superconductors whcih a theory needs to explain,and what are the basic requirements for such a theory? Using the insights not only of BCS but also of Ginzburg and Landau and of Yang,I attempt to demonstare that our current understanding adequately answers these questions.

"Quantum-like States"

Abstract: If a biological system found an advantage in exploiting quantum function of some sort, how would that be implemented? I will explain why I contend that such function would likely emerge from a specially designed classical system. To that end, I will present in my talk a strategy for constructing a classical system that is associated with a state space endowed with key properties of quantum states. These states are called quantum-like (QL) states. The idea is that a family of graphs is devised that have states serving as robust two-state systems (QL bits) that can be combined into special products that display states emulating the states of interesting quantum systems. Those graphs, in turn, can template construction of relevant classical systems. Now the classical systems are associated with a QL state space. Of course, there is exponential resource cost to do this in practice. Nevertheless, having a concrete mathematical structure for quantum correlations (the graphs) and even a classical construction opens some interesting topics for discussion. The accompanying paper frames and proposes some such possible discussions. 

Details to come

"Quantum Singularity Resolution and Superpositions of the Cosmological Constant"

Abstract: A fascinating and yet under-explored question in the foundations of physical theory relates to the status of the distinction between constants of nature and constants of motion. A plausible line of argument, dating at least as far back as Poincaré, suggests that the distinction may be a fluid one depending upon our limited epistemic vantage point. Poincaré suggested that there may be circumstances where a constant is initially understood to be ‘essential’ (i.e. a constant of nature) but by knowing more we later understand it to be ‘accidental’ (i.e. a constant of motion). This foundational question takes on a more physical character in the context of quantum theories where constants of motion and constants of nature are treated entirely differently. Whereas superpositions of the first can correspond to pure states, the second are subject to superselection rules. In the context of the problem of time in quantum gravity this difference in treatment of constants, when considered in the context of the cosmological constant, corresponds to the difference between a timeless Wheeler-DeWitt type equation subject to a big bang singularity and a dynamic Schrödinger equation with, in simple cosmological models at least, a unitary cosmic bounce. This cosmological model, first studied in the context of unimodular cosmology by Unruh and Wald (1989), was demonstrated to be unitary and non-singular in Gryb and Thébault (2018), cf. Gielen and Menéndez-Pidal (2022). Furthermore, recent work by Gielen and Menéndez-Pidal (2025) has shown that the approach can be extended towards black hole singularity resolution. In this talk I will review the formal and conceptual foundations of these ideas with a particular focus on the connection to the novel approach to the problem of time developed in Gryb and Thébault (2024).

Gielen, S. and L. Menéndez-Pidal (2022). Unitarity, clock dependence and quantum recollapse in quantum cosmology. Classical and Quantum Gravity 29 (7)
Gielen, S. and L. Menéndez-Pidal (2025). Black hole singularity resolution in unimodular gravity from unitarity. Physical Review Letters 134(10), 101501.
Gryb, S., & Thébault, K. P. (2018). Superpositions of the cosmological constant allow for singularity resolution and unitary evolution in quantum cosmology. Physics Letters B, 784, 324-329.
Gryb, S. and K. P. Y. Thébault (2024). Time Regained : Volume 1 : Symmetry and Evolution in Classical Mechanics, Volume 1. Oxford University Press.
Unruh, W. G. and R. M. Wald (1989). Time and the interpretation of canonical quantum gravity. Physical Review D 40(8), 2598.  

Details to come.

"Black Holes Decohere Quantum Superpositions"

Abstract: If a massive body is put in a quantum superposition of spatially separated states, the mere presence of a black hole in the vicinity of the body will eventually destroy the coherence of the superposition. This occurs because, in effect, the gravitational field of the body radiates soft gravitons into the black hole, allowing the black hole to harvest "which path'' information about the superposition. A similar effect occurs for quantum superpositions of electrically charged bodies. The effect is very closely related to the memory effect and infrared divergences in ordinary scattering theory. The talk will discuss these decoherence effects of black holes.

"Finding the Quantum in Biology"

Abstract: The development of quantum mechanics 100 years ago transformed both physics and chemistry, providing a new understanding of the microscopic behavior of atoms and molecules. Questions were also asked about the implications of quantum mechanics for biology, leading to analysis of the structure and stability of biological systems within the framework of quantum and statistical mechanics. A second era of quantum biology began with the development of lasers in the 1960s, ushering in a new generation of dynamical experiments that could probe the very short time scales relevant to atomic and molecular motion. Ultra-fast spectroscopies led to a renaissance of interest in quantum dynamical effects in biology. Today, advances in quantum sciences and nanotechnology are bringing tools of advanced quantum optics and of quantum information science and associated technologies to selectively probe complex biological systems. In this talk I shall present a brief overview of biological phenomena that are currently believed to involve non-trivial dynamical quantum effects and then address the role of quantum phenomena in the initial stages of photosynthesis and how we can access these with quantum light sources.

"Decoherence by Warm Horizons"

Abstract: Recently Danielson, Satishchandran, and Wald (DSW) have shown that quantum superpositions held outside of Killing horizons will decohere at a steady rate. This occurs because of the inevitable radiation of soft photons (gravitons), which imprint an electromagnetic (gravitational) "which-path'' memory onto the horizon. Rather than appealing to this global description, an experimenter ought to also have a local description for the cause of decoherence. One might intuitively guess that this is just the bombardment of Hawking/Unruh radiation on the system, however simple calculations challenge this idea -- the same superposition held in a finite temperature inertial laboratory does not decohere at the DSW rate. In this work we provide a local description of the decoherence by mapping the DSW set-up onto a worldline-localized model resembling an Unruh-DeWitt particle detector. We present an interpretation in terms of random local forces which do not sufficiently self-average over long times. Using the Rindler horizon as a concrete example we clarify the crucial role of temperature, and show that the Unruh effect is the only quantum mechanical effect underlying these random forces. A general lesson is that for an environment which induces Ohmic friction on the central system (as one gets from the classical Abraham-Lorentz-Dirac force, in an accelerating frame) the fluctuation-dissipation theorem implies that when this environment is at finite temperature it will cause steady decoherence on the central system. Our results agree with DSW and provide a complementary local perspective.