Simulation hypothesis

• 1. Introduction

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  The simulation hypothesis, the claim that everyday reality might be a computer simulation with humans as constructs inside it, produced a startling number from Elon Musk in 2016. In an interview that year, he said he believed there was "a one in billion chance we're in base reality." Musk, the CEO of Tesla and SpaceX, is one of many people who have taken the idea seriously.

  Philosophers have argued over it at length. Physicists have attacked it, and a few have proposed experiments to detect it. Writers of science fiction have turned it into stories and films.

  This documentary asks where the notion came from, and how a philosopher shaped it into a formal argument. It asks what that argument actually claims, and what its sharpest critics say. It also asks whether any experiment could detect a simulation, and what the possibility would mean for someone living under it.

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• 2. Dreams and Demons

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  Around the 4th century BC, the Chinese philosopher Zhuangzi described a dream in which he was a butterfly, flitting about and pleased with itself. In Burton Watson's 1968 translation, he wakes as solid, unmistakable Zhuangzi but cannot say whether he is a man who dreamt of a butterfly or a butterfly dreaming of a man. He insists there must be some distinction between the two, and calls it the Transformation of Things.

  Zhuangzi was far from the only thinker to distrust appearances. Indian philosophy has the idea of Maya. In ancient Greece, Anaxarchus and Monimus compared existing things to a scene-painting, and said they resemble the impressions of sleep or madness. Aztec philosophical texts theorized that the world was a painting, or a book written by the Teotl.

  Gnosticism is the scholars' label for a group of religious movements, and many of them shared a darker theme. Reality as we experience it was the work of a lesser and possibly malevolent deity, and humanity should seek to escape it. In the Western tradition, Plato's allegory of the cave pictured human beings as chained prisoners who cannot see reality.

  René Descartes gave these doubts a formal shape with his evil demon, and later writers added variations such as the brain in a vat. In Meditations on First Philosophy, he wrote that "there are no certain indications by which we may clearly distinguish wakefulness from sleep." From there he concluded that "It is possible that I am dreaming right now and that all of my perceptions are false."

  David Chalmers, writing in 2003, observed that the dream hypothesis comes in two forms. In the first, a person is dreaming right now, so many beliefs about the world are wrong. In the second, the person has always been dreaming, so the perceived objects exist, but only in imagination. Both the dream argument and the simulation hypothesis count as skeptical hypotheses.

  Ludwig Wittgenstein, in On Certainty, called such hypotheses unsinnig, meaning nonsensical, because they doubt the very knowledge needed to make sense of them. Bertrand Russell took a milder line. He granted that the dream hypothesis is not a logical impossibility, but said common sense, simplicity and inference to the best explanation count against it. Neither objection was written with software in mind, and neither says what happens when the dreamer is replaced by a program.

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• 3. The Trilemma

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  In 2003, the philosopher Nick Bostrom asked readers to suppose that forecasts of enormous future computing power are correct. Later generations, he reasoned, might use such machines to run detailed simulations of their forebears, and a great many of them. If the simulations are fine-grained enough, and a widely accepted position in the philosophy of mind is right, the simulated people would be conscious. Most minds like ours would then belong to simulations rather than to the original race.

  Bostrom named his result the simulation argument, and it takes the form of a trilemma. Despite the name, it does not directly argue that humans live in a simulation. It says one of three unlikely-seeming propositions is almost certainly true. First, the fraction of human-level civilizations that reach a posthuman stage, able to run high-fidelity ancestor simulations, is very close to zero. Second, the fraction of posthuman civilizations interested in running such simulations is very close to zero. Third, the fraction of all people with our kind of experiences who live in a simulation is very close to one.

  The arithmetic behind the third option is simple. A technologically mature posthuman civilization would command enormous computing power. Even a tiny share of ancestor simulations, meaning recreations so faithful that the simulated ancestor could not tell them from reality, would yield more simulated ancestors, called Sims, than there were actual ancestors. Bostrom adds anthropic reasoning: if the third proposition is the true one, humans are almost certainly living in a simulation.

  Bostrom's own reading of the first two options is blunt. If the first is true, he wrote, "we will almost certainly go extinct before reaching posthumanity." If the second is true, advanced civilizations must converge so strongly that virtually none contains anyone who both wants to run ancestor simulations and is free to do so.

  He sees no strong argument for choosing among the three. "In the dark forest of our current ignorance, it seems sensible to apportion one's credence roughly evenly between (1), (2), and (3)," he wrote. He noticed that people who hear the argument often call one option obvious, yet different people pick different ones.

  The argument carries a corollary in Bostrom's words: "Unless we are now living in a simulation, our descendants will almost certainly never run an ancestor-simulation." He also allowed that evidence could in principle turn up. A window might pop up reading "You are living in a simulation. Click here for more information." A program could purge even the memory of such a message, but any real evidence would radically alter the odds.

  Bostrom claims his case goes beyond the ancient skeptical hypothesis, because he sees interesting empirical reasons to believe a disjunctive claim about the world. Chalmers agrees, and the two describe the simulation hypothesis as a metaphysical hypothesis rather than a skeptical one.

  The argument rests on three premises. The populated surface of the Earth can be represented without recourse to digital physics. The qualia of a simulated consciousness are comparable to those of a natural human mind. And simulations nested inside simulations would cost the real world only a modest expenditure of computing resources. The second premise, about consciousness, is the one that draws the most scrutiny.

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• 4. Minds as Computation

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  Computationalism is a theory in the philosophy of mind which says that cognition is a form of computation. It matters here because a simulation containing conscious people needs minds that can be built from computation. Physical systems can already be simulated to some degree of accuracy. If computationalism is correct and artificial consciousness poses no obstacle, a simulated reality becomes theoretically possible.

  Philosophical zombies mark the sticking point. The relationship between cognition and the felt qualities of experience is disputed, so consciousness might require a vital substrate that a computer cannot provide. Simulated people would then behave appropriately while experiencing nothing. That would undermine Bostrom's argument, since humans cannot be simulated consciousnesses if consciousness cannot be simulated. The skeptical hypothesis would survive, because people could still be vatted brains living in a simulated environment.

  Philosopher Barry Dainton answered by modifying the trilemma. He replaced ancestor simulations with neural ancestor simulations. These range from literal brains in a vat to far-future humans given induced, high-fidelity hallucinations that they are their own distant ancestors. Every philosophical school can agree that such experiences would be indistinguishable from unsimulated ones. Even if computer Sims are never conscious, his reasoning forces a choice. Either very few civilizations reach the posthuman stage and run many neural simulations, or some kind of ancestor simulation exists.

  Some theorists pair the view that consciousness is computation with mathematical realism, also called radical mathematical Platonism. On that picture, a Platonic realm or ultimate ensemble contains every algorithm, including those that implement consciousness. Consciousness could then run on any platform. Roboticist Hans Moravec, who also discussed mind uploading and speculated that our reality might be a simulation made by future intelligences, argued for a kind of Platonism in which every object, a stone included, implements every possible computation.

  One further claim concerns the senses. Virtual reality, it has been suggested, lets a participant experience only three: sight, sound and optionally smell. A full simulated reality would offer all five, including taste and touch.

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• 5. Universe as Code

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  In 1969, Konrad Zuse published Calculating Space, a book on automata theory. In it he proposed that the universe is fundamentally computational, a concept that became known as digital physics.

  The physicist Wheeler was the first to observe the universe and its workings as the ebb and flow of information. Two views of the world followed. One proposes that the universe is itself a quantum computer. The other proposes that the system running the simulation is distinct from the simulated universe.

  Quantum-computing specialist Dave Bacon was unimpressed by the first view. He suggested it may be nothing more than an extrapolation, because computation is "the disease of our age" and computers appear everywhere we look. He added that perfectly operating computers are unlikely to exist outside a platonic ideal. He also pointed out that no evidence shows the kind of digitization found in computers, and no prediction from its advocates has been experimentally confirmed.

  In 2012, physicists Silas R. Beane, Zohreh Davoudi and Martin J. Savage proposed a way to test one type of simulation hypothesis. With finite computational resources, a simulated universe would have to divide the space-time continuum into a discrete set of points. That could leave observable traces. Their reasoning is analogous to the small simulations lattice-gauge theorists run today, which build up nuclei from quantum chromodynamics, the theory of strong interactions.

  Among their proposed signatures is an anisotropy in the distribution of ultra-high-energy cosmic rays. If observed, they say, it would be consistent with the simulation hypothesis. In 2017, Campbell and colleagues proposed several further experiments in a paper titled "On Testing the Simulation Theory."

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• 6. The Critics

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  Physicist Sabine Hossenfelder holds that it is physically impossible to simulate the universe without producing measurable inconsistencies. She has called the hypothesis pseudoscience and religion. Cosmologist George F. R. Ellis calls it "totally impracticable from a technical viewpoint" and says "late-night pub discussion is not a viable theory." Some scholars go further and reject anthropic reasoning itself as merely philosophical, unfalsifiable or unscientific.

  Physicist Frank Wilczek raises an empirical objection. The laws of the universe contain hidden complexity "not used for anything," and they are constrained by time and location, all of which would be extraneous in a simulation. He also says the argument begs the question. "Okay if this is a simulated world, what is the thing in which it is simulated made out of? What are the laws for that?"

  Cosmologist Sean M. Carroll finds a contradiction. If humans are typical, as the argument assumes, and cannot themselves run simulations, that clashes with the assumption that we can easily foresee other civilizations doing so. Brian Eggleston adds that future humans of our own universe cannot be the simulators, since the argument treats our universe as the simulated one. The probability of being simulated is therefore not independent of the prior probability given to other universes. Other critics, invoking presentism, suggest the simulation might be in its first generation, with its simulated people not yet created.

  Physicist Marcelo Gleiser doubts that posthumans would bother. Being so advanced, they would know enough about their past to have little interest, and might settle for virtual-reality museums. A full simulation of an entire universe, he says, "Sounds like a colossal waste of time." He adds that nothing gives a reason to stop at one level, so simulated ancestors might simulate their own ancestors, creating an infinite regress akin to the problem of the First Cause.

  Physicist Paul Davies accepts the trilemma but argues that the third proposition is false. He uses it against the idea of a near-infinite multiverse. Such a multiverse would contain posthuman civilizations running ancestor simulations, which leads to the self-defeating conclusion that humans live in a simulation. By reductio ad absurdum, existing multiverse theories are then likely false. Other critics note that no current technology could support a high-fidelity ancestor simulation, and that computation has limits, so for now the first proposition must be taken as true.

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• 7. Living Inside One

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  In 2019, philosopher Preston Greene suggested that it may be best never to find out whether we live in a simulation. If the answer were yes, he argued, the knowing itself might end the simulation.

  Economist Robin Hanson asks how a self-interested occupant of a high-fidelity simulation should behave. His answer is to be entertaining and praiseworthy, to avoid being turned off or shunted into a non-conscious, low-fidelity part of the simulation. He also speculates that awareness of the possibility could make someone care less about others and live more for today. In his words, "your motivation to save for retirement, or to help the poor in Ethiopia, might be muted by realizing that in your simulation, you will never retire and there is no Ethiopia."

  Philosophers have also used the hypothesis as a tool for other problems in metaphysics and epistemology. Chalmers imagines simulated beings wondering whether the physics of their environment governs their mental lives, when in fact those lives are simulated separately. They might eventually find that their thoughts fail to be physically caused. He argues this means Cartesian dualism is not necessarily as problematic as commonly supposed, though he does not endorse it.

  Similar arguments have been made about personal identity, where an individual could have been another human being in the past. They have also been made about the inverted spectrum, where colors could have appeared differently than they do. In both cases, the claim is that one only needs to hook the mental lives up to the simulated physics in a different way.

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• 8. Verdicts and Screens

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  Astrophysicist Neil deGrasse Tyson said in a 2018 NBC News interview that he put the odds of the simulation hypothesis being correct at "better than 50-50." He added, "I wish I could summon a strong argument against it, but I can find none." Later, on a StarTalk episode with Chuck Nice, he described an objection from his friend J. Richard Gott, a professor of astrophysical sciences at Princeton University.

  Gott's point is that every high-fidelity simulated universe would share one trait, the ability to produce further high-fidelity simulated universes. Our world cannot do that. So either humans are in the real universe and no simulated universes exist yet, or humans are the last link in a very long chain. Both outcomes make the hypothesis look less probable. Tyson's response was that it "changes my life."

  Fiction got there earlier. Simulacron-3, a 1964 novel by Daniel F. Galouye, explores a computer-simulated city and inspired the screen adaptations World on a Wire in 1973 and The Thirteenth Floor in 1999. The Matrix, also from 1999, popularized the picture of humanity unknowingly living inside a machine-generated reality.

  Overdrawn at the Memory Bank, from 1983 and 1984, follows a protagonist forced into compulsory "doppling" therapy that transfers his consciousness. After a mishap, his mind is kept inside the corporation's central computer. Philip K. Dick's 1966 short story "We Can Remember It for You Wholesale" deals with implanted memories and unstable realities, and became the basis for Total Recall in 1990 and its 2012 remake.

  Theatre has taken up the theme too. Jay Scheib's 2012 play World of Wires was explicitly inspired by Bostrom's simulation argument and by Fassbinder's World on a Wire. In 2025, Italian creative director and producer Giorgio Fazio released Nothing But Simulation, a two-track project tied to the hypothesis and paired with a generative web experience.

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