Foundational puzzles on quantum universality: From Wigner's friend to black holes
This thesis investigates foundational puzzles in quantum theory by extending Wigner's friend scenarios to derive new no-go results on nonclassicality and free choice, and by unifying these arguments with black hole information paradoxes to strengthen cloning and firewall contradictions while analyzing detector responses near superposed black holes.
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Modern physics rests on two towering pillars: quantum theory, which governs the behavior of the very small, and general relativity, which describes the curvature of space and time caused by massive objects. For over a century, scientists have tried to unite these two frameworks into a single theory of everything, yet they remain stubbornly separate. The tension is most visible in two specific puzzles. The first concerns the act of measurement itself. In the quantum world, particles can exist in a blur of multiple possibilities at once, a state known as superposition. Yet, when we look at them, they snap into a single, definite state. It is unclear why this happens, or even what counts as a "look." The second puzzle involves black holes, the cosmic traps where gravity is so strong that nothing escapes. When these objects evaporate over time, they seem to destroy the information about what fell inside them, which violates a fundamental rule of quantum physics that says information can never be lost.
A new thesis by Laurens Walleghem, a researcher at the University of York, dives deep into these contradictions. The work does not offer a final solution to the mystery of the universe, but rather sharpens the questions we must ask. It explores what happens when we take the rules of quantum mechanics seriously enough to apply them to the observers themselves. Imagine a scientist inside a sealed laboratory measuring a particle. To the scientist, the particle has a definite result. But to an outside observer watching the entire laboratory from the street, the scientist and the particle are still in a blur of possibilities until the door is opened. This scenario, known as Wigner's friend, has long been a thought experiment. Walleghem takes it further, combining it with black hole physics to see if the contradictions become unavoidable.
The first part of the research refines a famous argument known as the Frauchiger–Renner paradox. This paradox involves multiple observers and super-observers reasoning about each other's measurements. Previous versions of this argument relied on a specific, somewhat fragile setup where a contradiction only appeared if certain rare outcomes occurred. Walleghem constructed a stronger version using a specific arrangement of three particles. In this new setup, the contradiction arises in every single run of the experiment, not just the lucky ones. The result is a powerful no-go theorem: it proves that you cannot simultaneously hold three beliefs that seem perfectly reasonable. You cannot believe that quantum theory applies to everyone and everything, that every measurement has one absolute truth that everyone agrees on, and that observers can use the standard rules of probability to predict what others will see. The work suggests that if we want to keep quantum theory universal, we must accept that the "facts" of an experiment might not be absolute for everyone. Instead, what is true for one observer might be relative to their perspective.
The second part of the thesis turns its gaze to the cosmos, specifically to black holes. Here, the researcher combines the logic of the observer paradoxes with the physics of black holes to create new, sharper versions of the cloning and firewall paradoxes. The cloning paradox asks how information can be both inside a black hole and radiating out of it without being copied, which is forbidden. The firewall paradox suggests that the smooth edge of a black hole might actually be a wall of fire. Walleghem shows that these paradoxes are not just quirks of a specific theory but are deep inconsistencies in how we currently describe the universe. By treating the black hole and the observer as quantum systems that can be in superposition, the research demonstrates that no theory can satisfy all our standard assumptions about how information and gravity interact without running into a logical wall.
To move beyond just pointing out these contradictions, the thesis performs a detailed calculation involving a particle detector placed near a black hole that is in a superposition of two different locations. This is a highly theoretical setup, imagining a black hole that is simultaneously at two different distances from a detector. The researcher used a mathematical tool called a quantum reference frame to describe this situation, essentially shifting the viewpoint so that the detector is stationary and the black hole is the one in a fuzzy, dual location. The goal was to see how the detector would respond to this strange spacetime.
The calculation revealed something specific about the nature of the black hole's mass. In previous studies, researchers had found that if a black hole is in a superposition of different masses, a detector would register sharp, distinct peaks in its response, which some interpreted as evidence that black hole mass comes in discrete chunks, or quanta. Walleghem's new calculation, however, looked at a black hole in a superposition of positions but with a single, fixed mass. In this case, the sharp peaks disappeared entirely. The detector's response was smooth and lacked those distinct spikes. This finding supports the idea that the sharp peaks seen in other scenarios are indeed a signature of mass quantization, not just a general feature of any quantum black hole. It suggests that the "fuzziness" of the black hole's location does not create the same dramatic signal as the "fuzziness" of its weight.
The work concludes by emphasizing that these paradoxes are not merely mathematical games but indicators that our current understanding of reality is incomplete. The research does not claim to have solved the black hole information problem or the measurement problem. Instead, it acts as a rigorous stress test, showing exactly where our current assumptions break down. By proving that certain combinations of beliefs lead to logical impossibilities, the thesis forces physicists to reconsider the nature of reality, the role of the observer, and the behavior of gravity at the quantum level. It suggests that the universe may be more relational than we thought, where facts are not absolute truths waiting to be discovered, but outcomes that depend on who is asking the question.
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