Amazing stuff!
"When one applies the powerful tools of quantum gravity to an entire universe—not just black holes—a glaring paradox appears: in certain “observer-free” models of a closed cosmos, the universe seems to admit only a single possible quantum state. This conclusion, first raised by Juan Maldacena, Carl P. Feinberg Professor in the School of Natural Sciences, and several collaborators in the School—including Member Ahmed Almheiri; Raghu Mahajan, Member (2019–20) and Visitor (2017–19); and Ying Zhao, Member (2018–21)—challenges one of physics’s deepest ideals: the hope of an objective, observer-independent description of the universe. ..."
"... But almost 30 years ago, a landmark paper ..., showed that difficult string theory calculations could sometimes be sidestepped and carried out using familiar concepts from particle physics instead. The catch is that this approach only works if the universe has an unusual “anti-de Sitter” geometry. An anti-de Sitter universe has a boundary, often illustrated to resemble a tin can. Remarkably, everything that happens inside the can, from colliding particles to spinning black holes, is revealed by shadows on the can’s outer boundary. It’s as if the 3D universe inside were equivalent to an image on a flat screen, a concept physicists call holography. ...
The problem is that we don’t live in an anti-de Sitter tin-can cosmos. The nature of the universe’s expansion implies that it has no boundary. No matter how far you travel, you will never hit an edge. ...
In 2024, ... began to work on the problem of how to put an observer into a closed universe. ... thought of the observer as introducing a new kind of boundary: not the edge of the universe, but the boundary of the observer themself. When you consider a classical observer inside a closed universe, all the complexity of the world returns, Zhao and her collaborators showed.
The ... paper came out at the beginning of 2025, around the same time that another group came forward with a similar idea. ...
If the idea holds up, using the subjective nature of the observer as a way to account for the complexity of the universe would represent a paradigm shift in physics. Physicists typically seek a view from nowhere, a stand-alone description of nature. ..."
Cosmic Paradox Reveals the Awful Consequence of an Observer-Free Universe "Encouraged by successes in understanding black holes, theoretical physicists are applying what they’ve learned to whole universes. What they’re finding has them questioning fundamental assumptions about how physics ought to be done."
The Large N Limit of Superconformal Field Theories and Supergravity (preprint, open access)
Quantum mechanics and observers for gravity in a closed universe (preprint, open access)
The gravitational path integral from an observer's point of view (preprint, open access)
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