(alphabetical by author)

Bruno Arderucio Costa: Can Quantum Mechanics Breed Negative Masses?

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It is no secret that quantum mechanics often produces weirdness. While some of its unexpected predictions are seemingly inconsequential, sidestepping confrontation with experiments, others could, in principle, be game-changers. Or couldn’t they? In this poster, we focus on quantum mechanical violations of the classical energy conditions, i.e., reasonable expectations on the classical matter that, for example, prevent one from building a time machine to meet oneself in the past. Without compliance with energy conditions, it may seem intriguing why there aren’t negative masses roaming around us. Based on the Casimir effect, we argue that it may be fundamentally impossible to observe an object with a negative mass, despite the presence of static negative energy densities. We identify the specific elements that outweigh the Casimir energies, making the entire apparatus yield an attractive gravitational force on distant bodies.

Saakshi Dulani: Information, Entropy Bounds, and The Holographic Principle

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For familiar statistical mechanical systems, we expect entropy to scale with volume – more space means more available states. However, Bekenstein-Hawking entropy scales with area, not volume. Assuming that Bekenstein-Hawking entropy is a complete measure over black hole degrees of freedom and that black holes are the most entropic objects in the universe, Bekenstein-Hawking entropy represents a bound on how many degrees of freedom can be packed into a region of spacetime. Susskind creatively dubbed the spherical entropy bound the ‘Holographic Principle,’ which states that the maximum amount of information needed to specify what is happening in the bulk of a system can be encoded on its boundary, much like a 3-D hologram is projected from a 2-D film. This analogy has been further sensationalized by the preliminary successes of AdS/CFT correspondence. Maldacena showed that string-theoretic dynamics in a conformal field theory without gravity can be mapped onto the bulk of a higher-dimensional anti-de Sitter space containing black holes. My goal for this presentation is to analyze the relationship between the Holographic Principle (as motivated by black hole statistical mechanics) and AdS/CFT correspondence. Are they separate or interdependent claims? Does theoretical support for AdS/CFT correspondence strengthen the merits of the Holographic Principle, even if entropy bounds lack ironclad generality and admit of violations? The answers to these questions depend sensitively on the notions of duality and physical equivalence being employed. A philosophically careful treatment of these terms and transparency about technical maneuvers weaken the connection between the Holographic Principle and AdS/CFT correspondence.

Luca Gasparinetti: Tensed or Tenseless Time? A Quietist Cure for Philosophers of Time

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In the philosophical literature, tensed theories of time and physical theor- ies are generally conceived as incompatible. Such incompatibility, rooted in the dichotomy between the manifest and the scientific image of time, raises some anxieties centered on unsolved debates – e.g., tense/tenseless, present- ism/eternalism. Nevertheless, Rovelli (2021) indicates the cure: since time is a multilayered concept. More specifically, there are several facets of the concept of time. None of these notions are false or illusory. All of them are valid but in different contexts.

In this paper, in order to prove the effectiveness of the therapy, I apply Rovelli’s cure, so close to the philosophical quietism of Wittgenstein (1956) and McDowell (1996), to a typical debate in the philosophy of time. Some philosophers following some insights of physical theories (e.g., Putnam 1967, Callender 2008) state that tensed concepts are incompatible with physics. Oth- ers, (e.g., Lucas 1998, Tooley 2008) argue that not only quantum mechanics but also physical theories in general, provide a fertile ground for tensed theories of time. The result is unsatisfactory.

In light of such unsolved debate affected by the anxious dichotomy, I propose a quietist therapy: there is no solution to the problem because it should not be raised. Time is a concept with different facets that have their meaning within a precise context. Applying a concept of time to different contexts is nothing more than a conceptual confusion that leads to pseudo-problems.

Viktoria Kabel: Falling through masses in superposition: quantum reference frames for indefinite metrics

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The current theories of quantum physics and general relativity on their own do not allow us to study situations in which the gravitational source is quantum. I will present a strategy to determine the dynamics of objects in the presence of mass configurations in superposition, and hence an indefinite spacetime metric, using quantum reference frame (QRF) transformations. Specifically, my collaborators and I showed that, under certain conditions, one can use an extension of the current framework of QRFs to change to a frame in which the mass configuration becomes definite. Assuming covariance of dynamical laws under quantum coordinate transformations, this allows to use known physics to determine the dynamics. We applied this procedure to find the motion of a probe particle and the behavior of clocks near the mass configuration, and thus found the time dilation caused by a gravitating object in superposition. Comparison with other models further shows that semi-classical gravity and gravitational collapse models do not obey the covariance of dynamical laws under quantum coordinate transformations. 

Álvaro Mozota Frauca: Taking seriously the problem of time of quantum gravity

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In this paper I raise a worry about the most extended resolutions of the problem of time of canonical quantizations of general relativity. The reason for this is that these resolutions are based on analogies with deparametrizable models for which the problem can be solved, while I argue in this paper that there are good reasons for doubting about these resolutions when the theory is not deparametrizable, which is the case of general relativity. I introduce an example of a non-deparametrizable model and argue that the standard resolutions of the problem of time don’t work for this case. I argue that as general relativity is strongly analogous to this model, one should take seriously the view that the canonical quantization of general relativity doesn’t lead to a meaningful quan- tum theory. Finally, I comment that this has an impact on the foundations of different approaches to quantum gravity.

Ray Pedersen: Why Not Assume Just One Everettian Universe?

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Bare Everettian quantum mechanics (EQM) suggests that for any quantum mechanical process, all possible outcomes obtain. On first inspection, EQM appears to lend itself well to many worlds interpretations. However, this approach requires a metaphysical commitment to a form of modal realism. In response to this concern, I propose a single-world Everettian universe. On my model, the complete branching structure itself is a metaphysically possible world. To solve the problem of probability in a single world where all possible outcomes obtain, I propose that in addition to the mereological concepts of material and temporal parts, branchlike parts exist. I then use a decision theoretic approach to treating probabilities as the credences of a rational agent, where the probability of some outcome corresponds to branch weight. This one-world model thus has the capacity to reproduce the predictions of the Born rule while addressing concerns over the apparent frivolity of many-world interpretations.

Farshid Soltani: The black-to-white hole transition

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Black holes formation and evolution have been extensively studied at the classical level. However, little is known about the end of their lives and about the true nature of the spacetime singularity in their interior, the description of which requires to consider the quantum nature of the gravitational field. A very natural and conservative scenario describing the physics of both regions is the black to white hole transition: the quantum transition of the black hole geometry in the geometry of a white hole. Recent theoretical evidence suggests a scenario in which the black hole horizon undergoes a quantum transition into a white hole horizon and the classical singularity is replaced by a smooth transition of the interior trapped region into an anti-trapped region. I review the evidence supporting this scenario and I discuss how the spin foam formalism can be used to describe the non-perturbative physics of the horizon.

Diana Taschetto:  Classical and Quantum Observables

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According to the so-called algebraic approach, the physical content of a quantum theory lies in its algebra of observables. Hence to identify the observables of a theory. This conditio sine qua non to determine what the world must be like if the theory is to be true of the world; what requirements a quantity must satisfy in order to count as an observable, however, is an open problem. Ambiguities notwithstanding, quantum observables are generally constructed by following out the prescription advanced by Dirac: Replace the Poisson brackets between classical observables by ħ/i times the commutator between the corresponding quantum observables. The purpose of this presentation is to briefly discuss the meaning of the “correspondence” thereby intended, the assumptions involved, its limitations and implications.