What is Walpert's multiverse approach to simulation?

David Walpert's paper suggests that if we are simulated, our universe could be one of many, and one universe could simulate another if certain compatibility conditions between physical laws are met. This is framed as a way to formalize the idea using a multiverse concept.

Why is the Planck scale mentioned in this context?

The Planck scale is cited as a fundamental physical limit that could constrain how a larger universe might be simulated, implying you might have to reduce detail or particle counts when simulating another universe.

Can one universe simulate itself?

Yes, according to the discussion, one universe could simulate itself if the laws of nature are sufficiently reducible, allowing the information to be compressed.

What is the embedding space mentioned in the video?

The embedding space is the broader ‘space’ in which a programmer could be running a simulation of our universe, i.e., the space of possible parent realities that host our simulation.

Has anyone proven we can't be in a simulation?

Some physicists have argued there could be bounds if the laws of nature are computable, but the video notes this is disputed and not universally accepted, with debates about category errors and mathematical conflation.

What is the current takeaway about the simulation hypothesis?

The video suggests the idea is 'back from the dead' but remains unclear in terms of what it would imply physically or practically.

The Simulation Hypothesis Gets Scientific Backing

Sabine Hossenfelder

Science & Technology5 min read7 min video

Mar 7, 2026|127,226 views|6,668|1,609

hossenfelder science with sabine science humour science news sabine science news science humor physics simulation simulation theory computer science physics news simulation hypothesis

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Key Moments

TL;DR

Simulation hypothesis gains scientific framing via multiverse ideas and embedding-space concepts; debate continues.

Key Insights

A formal framework: David Walpert’s multiverse approach treats a simulated universe as potentially being run by a higher-level universe, turning a philosophical idea into something with computability-style criteria.

Compatibility properties: For one universe to simulate another, the laws of nature must satisfy specific compatibility conditions, and with sufficiently reducible laws, even a universe could simulate itself.

Upper-level perspective: If simulations are possible, physics may shift to studying the embedding space—the universe in which a programmer runs the simulation—potentially guiding toward a new kind of theory of everything.

Practical limits: Realistic simulations face computational constraints (e.g., Planck-scale limits, scale dependence), raising questions about feasibility and how much detail must be omitted or compressed.

Critiques and nuance: Arguments based on computability and incompleteness have resurfaced, with debates about whether maths truly maps to physical reality or if those arguments miss essential aspects of what a simulation would entail.

Future of physics: Even with questions unresolved, this formalism is seen as a promising direction for theoretical development, suggesting a potential reframe of how we seek a theory of everything.

FROM PHILOSOPHY TO SCIENCE: WHY NOW

The video opens by noting a notable shift in the conversation around the simulation hypothesis: once mainly the purview of philosophers and physicists who dismissed it as vague, there is now a push to frame it within a more rigorous, testable context. The narrator points to rapid advances in artificial intelligence and computer-generated world models as practical demonstrations that complex, self-contained universes can be imagined and explored by nonhuman agents. He cites DeepMind’s genie-like world models as a catalyst for imagining that our own universe could be a high-fidelity world model running inside a more powerful substrate. This framing nudges the idea away from abstract speculation toward questions about what makes a computation “physical” and what the rules governing a simulated cosmos must look like. The discussion then transitions to a concrete theoretical contribution by computer scientist David Walpert, who uses the multiverse as a scaffolding to analyze whether one universe could simulate another, thereby pushing the debate into the realm of computability and formal properties of physical laws. In short, the video argues that the simulation hypothesis deserves scientific scrutiny because it can be formalized and tested against a set of well-defined criteria.

A FRAMEWORK FOR COMPUTATION: WHAT MAKES A REALISTIC SIMULATION?

Central to Walpert’s approach is the idea that a simulation can be analyzed through the lens of a multiverse: if our universe is simulated, then it is a “part” of a larger computation performed by some upper-level cosmos. This reframes the question from asking whether a universe could be simulated to asking what properties the laws of physics in the simulated and simulating universes must share to be compatible. Walpert emphasizes compatibility requirements—conditions under which one universe can reliably reproduce another’s dynamics without collapsing the simulation’s internal consistency. He even entertains the provocative possibility that a universe could simulate itself if the information needed to describe its entire state can be compressed into a smaller, reducible set of rules. The speaker cautions that, while the framing is promising for computer science, it remains less developed from a physics perspective, signaling a productive but incomplete bridge between disciplines.

EMBEDDING SPACE AND THE THEORY OF EVERYTHING

A striking proposal in the discussion is to shift the target of inquiry from the microstructure of our universe to the embedding space—the conceptual ‘space’ in which a programmer or higher civilization runs a potential simulation. This perspective invites a reinterpretation of physics as a study of how an outer computational substrate imposes structure on the inner universe. Such a shift could, in principle, align with attempts to unify disparate physical laws by identifying the overarching rules that govern the simulator’s capabilities rather than focusing solely on the detailed laws inside the simulated realm. The narrator suggests that embracing the embedding-space viewpoint might illuminate pathways toward a theory of everything that explains why our universe behaves as it does, by clarifying which aspects of reality must be true in the higher-level framework to produce the observed mathematics and dynamics inside our cosmos.

LIMITS, CRITICISMS, AND THE ROLE OF MATHEMATICS

The discussion acknowledges substantial pushback centering on computability and mathematical logic. A prior claim—supported by theorems related to incompleteness—argued that if the laws of nature are computable, then certain observational bounds should exist, which allegedly would preclude a fully real simulation. The narrator notes subsequent commentary on social media and scholarly archives suggesting that those arguments may conflate mathematics with physical reality or commit category errors. In other words, just because a math-based bound exists in an abstract sense does not necessarily translate into a definitive constraint on whether our universe could be simulated. The video also revisits pragmatic objections: if the universe is not scale-invariant and includes fundamental limits like the Planck scale, any outer simulation would need to deal with compression or omission of detail, complicating the feasibility of massif-scale, multi-level simulations.

WHAT THIS MEANS FOR SCIENCE AND THE FUTURE

Ultimately, the video frames the simulation hypothesis as officially alive again, albeit still inconclusive regarding its implications for physics and cosmology. The formalism offered by Walpert is praised as a promising starting point that could yield insights into a future theory of everything, especially if researchers are willing to expand their focus beyond microstructures to consider embedding-space dynamics. The takeaway is twofold: (1) the hypothesis offers a concrete research program at the intersection of computer science, quantum theory, and cosmology; and (2) the current state of the debate is still unsettled, characterized by healthy skepticism, ongoing critique, and the possibility that new mathematical and physical ideas will reshape how we understand computation, reality, and the cosmos.

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Common Questions

The hypothesis suggests our reality could be a computer simulation run by a more advanced civilization. The video frames this as a scientific question by discussing how computation and physical laws could be realized across universes.