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The simulation hypothesis proposes that what humans experience as the world is actually a simulated reality, such as a computer simulation in which humans themselves are constructs.^[1][2] There has been much debate over this topic, ranging from philosophical discourse to practical applications in computing.

The simulation hypothesis, as formulated by Nick Bostrom,^[3] is part of a long tradition of skeptical scenarios. It was presented by Bostrom as not merely a philosophical speculation, but an empirical claim with quantifiable probabilities. The hypothesis has received criticism from some physicists, such as Sabine Hossenfelder who has called it pseudoscience and religion,^[4] and cosmologist George F. R. Ellis, who stated that "[the hypothesis] is totally impracticable from a technical viewpoint", and that "late-night pub discussion is not a viable theory".^[5][6] Versions of the hypothesis have also been featured in science fiction, appearing as a central plot device in many stories and films, such as The Matrix.^[7]

Origins

Human history is full of thinkers who observed the difference between how things seem and how they might actually be, with dreams, illusions, and hallucinations providing poetic and philosophical metaphors. For example, the "Butterfly Dream" of Zhuangzi from ancient China,^[8] or the Indian philosophy of Maya, or in ancient Greek philosophy Anaxarchus and Monimus likened existing things to a scene-painting and supposed them to resemble the impressions experienced in sleep or madness.^[9]

In the Western philosophical tradition, Plato's allegory of the cave stands out as an influential example.

Aztec philosophical texts theorized that the world was a painting or book written by the Teotl.^[10]

René Descartes' evil demon philosophically formalized these epistemic doubts, to be followed by a large literature with subsequent variations like brain in a vat.

Simulation argument

Nick Bostrom's premise:

Many works of science fiction as well as some forecasts by serious technologists and futurologists predict that enormous amounts of computing power will be available in the future. Let us suppose for a moment that these predictions are correct. One thing that later generations might do with their super-powerful computers is run detailed simulations of their forebears or of people like their forebears. Because their computers would be so powerful, they could run a great many such simulations. Suppose that these simulated people are conscious (as they would be if the simulations were sufficiently fine-grained and if a certain quite widely accepted position in the philosophy of mind is correct). Then it could be the case that the vast majority of minds like ours do not belong to the original race but rather to people simulated by the advanced descendants of an original race.

Nick Bostrom's conclusion:

It is then possible to argue that, if this were the case, we would be rational to think that we are likely among the simulated minds rather than among the original biological ones.
Therefore, if we don't think that we are currently living in a computer simulation, we are not entitled to believe that we will have descendants who will run lots of such simulations of their forebears.

— Nick Bostrom, Are You Living in a Computer Simulation?, 2003^[11]

Expanded argument

Bostrom attempted to assess the probability of our reality being a simulation.^[12] His argument states that at least one of the following statements is very likely to be true:

1. Human civilization or a comparable civilization is unlikely to reach a level of technological maturity capable of producing simulated realities, or such simulations are physically impossible to construct.^[12]
2. A comparable civilization reaching aforementioned technological status will likely not produce a significant number of simulated realities (one that might push the probable existence of digital entities beyond the probable number of "real" entities in a Universe) for any of a number of reasons, such as diversion of computational processing power for other tasks, ethical considerations of holding entities captive in simulated realities, etc.^[12]
3. Any entities with our general set of experiences are almost certainly living in a simulation.^[12]
4. Humans are living in a reality in which post-humans have not developed yet, and current humans are actually living in reality.^[12]
5. Humans will have no way of knowing that they live in a simulation because they will never reach the technological capacity to realize the marks of a simulated reality.^[13]

Bostrom's argument rests on the premises that given sufficiently advanced technology, it is possible to represent the populated surface of the Earth without recourse to digital physics; that the qualia experienced by a simulated consciousness are comparable or equivalent to those of a naturally occurring human consciousness, and that one or more levels of simulation within simulations would be feasible given only a modest expenditure of computational resources in the real world.^[12]

First, if one assumes that humans will not be destroyed nor destroy themselves before developing such a technology, and that human descendants will have no overriding legal restrictions or moral compunctions against simulating biospheres or their own historical biosphere, then, Bostrom argues it would be unreasonable to count ourselves among the small minority of genuine organisms who, sooner or later, will be vastly outnumbered by artificial simulations.^[12]

Epistemologically, it is not impossible for humans to tell whether they are living in a simulation. For example, Bostrom suggests that a window could pop up saying: "You are living in a simulation. Click here for more information." However, imperfections in a simulated environment might be difficult for the native inhabitants to identify and for purposes of authenticity, even the simulated memory of a blatant revelation might be purged programmatically. Nonetheless, should any evidence come to light, either for or against the skeptical hypothesis, it would radically alter the aforementioned probability.^[12]

In 2003, Bostrom proposed a trilemma that he called "the simulation argument". Despite its name, the "simulation argument" does not directly argue that humans live in a simulation; instead, it argues that one of three unlikely-seeming propositions is almost certainly true:

1. "The fraction of human-level civilizations that reach a posthuman stage (that is, one capable of running high-fidelity ancestor simulations) is very close to zero", or
2. "The fraction of posthuman civilizations that are interested in running simulations of their evolutionary history, or variations thereof, is very close to zero", or
3. "The fraction of all people with our kind of experiences that are living in a simulation is very close to one."

The trilemma points out that a technologically mature "posthuman" civilization would have enormous computing power; if even a tiny percentage of them were to run "ancestor simulations" (that is, "high-fidelity" simulations of ancestral life that would be indistinguishable from reality to the simulated ancestor), the total number of simulated ancestors, or "Sims", in the universe (or multiverse, if it exists) would greatly exceed the total number of actual ancestors.

Bostrom goes on to use a type of anthropic reasoning to claim that, if the third proposition is the one of those three that is true, and almost all people live in simulations, then humans are almost certainly living in a simulation.

Bostrom claims his argument goes beyond the classical ancient "skeptical hypothesis", claiming that "... we have interesting empirical reasons to believe that a certain disjunctive claim about the world is true", the third of the three disjunctive propositions being that humans are almost certainly living in a simulation. Thus, Bostrom, and writers in agreement with Bostrom such as David Chalmers, argue there might be empirical reasons for the "simulation hypothesis", and that therefore the simulation hypothesis is not a skeptical hypothesis but rather a "metaphysical hypothesis". Bostrom states he personally sees no strong argument as to which of the three trilemma propositions is the true one: "If (1) is true, then we will almost certainly go extinct before reaching posthumanity. If (2) is true, then there must be a strong convergence among the courses of advanced civilizations so that virtually none contains any individuals who desire to run ancestor-simulations and are free to do so. If (3) is true, then we almost certainly live in a simulation. In the dark forest of our current ignorance, it seems sensible to apportion one's credence roughly evenly between (1), (2), and (3)... I note that people who hear about the simulation argument often react by saying, 'Yes, I accept the argument, and it is obvious that it is possibility #n that obtains.' But different people pick a different n. Some think it obvious that (1) is true, others that (2) is true, yet others that (3) is true."

As a corollary to the trilemma, Bostrom states that "Unless we are now living in a simulation, our descendants will almost certainly never run an ancestor-simulation."^{[11][14][15][16]}

Criticism of Bostrom's anthropic reasoning

Bostrom argues that if "the fraction of all people with our kind of experiences that are living in a simulation is very close to one", then it follows that humans probably live in a simulation. Some philosophers disagree, proposing that perhaps "Sims" do not have conscious experiences the same way that unsimulated humans do, or that it can otherwise be self-evident to a human that they are a human rather than a Sim.^[14][17] Philosopher Barry Dainton modifies Bostrom's trilemma by substituting "neural ancestor simulations" (ranging from literal brains in a vat, to far-future humans with induced high-fidelity hallucinations that they are their own distant ancestors) for Bostrom's "ancestor simulations", on the grounds that every philosophical school of thought can agree that sufficiently high-tech neural ancestor simulation experiences would be indistinguishable from non-simulated experiences. Even if high-fidelity computer Sims are never conscious, Dainton's reasoning leads to the following conclusion: either the fraction of human-level civilizations that reach a posthuman stage and are able and willing to run large numbers of neural ancestor simulations is close to zero, or some kind of (possibly neural) ancestor simulation exists.^[18]

Some scholars categorically reject—or are uninterested in—anthropic reasoning, dismissing it as "merely philosophical", unfalsifiable, or inherently unscientific.^[14]

Some critics propose that the simulation could be in the first generation, and all the simulated people that will one day be created do not yet exist,^[14] in accordance with philosophical presentism.

The cosmologist Sean M. Carroll argues that the simulation hypothesis leads to a contradiction: if humans are typical, as it is assumed, and not capable of performing simulations, this contradicts the arguer's assumption that it is easy for us to foresee that other civilizations can most likely perform simulations.^[19]

Physicist Frank Wilczek raises an empirical objection, saying that the laws of the universe have hidden complexity which is "not used for anything" and the laws are constrained by time and location – all of this being unnecessary and extraneous in a simulation. He further argues that the simulation argument amounts to "begging the question," due to the "embarrassing question" of the nature of the underlying reality in which this universe is simulated. "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?"^[20]

Brian Eggleston has argued that the future humans of our universe cannot be the ones performing the simulation, since the simulation argument considers our universe to be the one being simulated.^[21] In other words, it has been argued that the probability that humans live in a simulated universe is not independent of the prior probability that is assigned to the existence of other universes.

Arguments, within the trilemma, against the simulation hypothesis

Some scholars accept the trilemma, and argue that the first or second of the propositions are true, and that the third proposition (the proposition that humans live in a simulation) is false. Physicist Paul Davies uses Bostrom's trilemma as part of one possible argument against a near-infinite multiverse. This argument runs as follows: if there were a near-infinite multiverse, there would be posthuman civilizations running ancestor simulations, which would lead to the untenable and scientifically self-defeating conclusion that humans live in a simulation; therefore, by reductio ad absurdum, existing multiverse theories are likely false. (Unlike Bostrom and Chalmers, Davies (among others) considers the simulation hypothesis to be self-defeating.)^[14][22]

Some point out that there is currently no proof of technology that would facilitate the existence of sufficiently high-fidelity ancestor simulation. Additionally, there is no proof that it is physically possible or feasible for a posthuman civilization to create such a simulation, and therefore for the present, the first proposition must be taken to be true.^[14] Additionally there are limits of computation.^[11][23]

Physicist Marcelo Gleiser objects to the notion that posthumans would have a reason to run simulated universes: "...being so advanced they would have collected enough knowledge about their past to have little interest in this kind of simulation. ...They may have virtual-reality museums, where they could go and experience the lives and tribulations of their ancestors. But a full-fledged, resource-consuming simulation of an entire universe? Sounds like a colossal waste of time." Gleiser also points out that there is no plausible reason to stop at one level of simulation, so that the simulated ancestors might also be simulating their ancestors, and so on, creating an infinite regress akin to the "problem of the First Cause".^[24]

In 2019, philosopher Preston Greene suggested that it may be best not to find out if we are living in a simulation, since, if it were found to be true, such knowing might end the simulation.^[25]

Greene's suggestion is similar to Douglas Adams' humorous idea presented in his 1979 novel The Hitchhiker's Guide to the Galaxy: that if anyone in the Universe should actually work out 'The Meaning of Life, the Universe and Everything', it would instantly disappear and be immediately replaced with something "even more complex and inexplicable".

Economist Robin Hanson argues that a self-interested occupant of a high-fidelity simulation should strive to be entertaining and praiseworthy in order to avoid being turned off or being shunted into a non-conscious low-fidelity part of the simulation. Hanson additionally speculates that someone who is aware that he might be in a simulation might care less about others and live more for today: "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".^[26]

Besides attempting to assess whether the simulation hypothesis is true or false, philosophers have also used it to illustrate other philosophical problems, especially in metaphysics and epistemology. David Chalmers has argued that simulated beings might wonder whether their mental lives are governed by the physics of their environment, when in fact these mental lives are simulated separately (and are thus, in fact, not governed by the simulated physics).^[27] Chalmers claims that they might eventually find that their thoughts fail to be physically caused, and argues that this means that Cartesian dualism is not necessarily as problematic of a philosophical view as is commonly supposed, though he does not endorse it.^[28] Similar arguments have been made for philosophical views about personal identity that say that an individual could have been another human being in the past, as well as views about qualia that say that colors could have appeared differently than they do (the inverted spectrum scenario). In both cases, the claim is that all this would require is hooking up the mental lives to the simulated physics in a different way.^[29]

Computationalism

Computationalism is a philosophy of mind theory stating that cognition is a form of computation. It is relevant to the simulation hypothesis in that it illustrates how a simulation could contain conscious subjects, as required by a "virtual people" simulation. For example, it is well known that physical systems can be simulated to some degree of accuracy. If computationalism is correct and if there is no problem in generating artificial consciousness or cognition, it would establish the theoretical possibility of a simulated reality. Nevertheless, the relationship between cognition and phenomenal qualia of consciousness is disputed. It is possible that consciousness requires a vital substrate that a computer cannot provide and that simulated people, while behaving appropriately, would be philosophical zombies. This would undermine Nick Bostrom's simulation argument; humans cannot be a simulated consciousness, if consciousness, as humans understand it, cannot be simulated. The skeptical hypothesis remains intact, however, and humans could still be vatted brains, existing as conscious beings within a simulated environment, even if consciousness cannot be simulated. It has been suggested that whereas virtual reality would enable a participant to experience only three senses (sight, sound and optionally smell), simulated reality would enable all five (including taste and touch).^{[citation needed]}

Some theorists^[30][31] have argued that if the "consciousness-is-computation" version of computationalism and mathematical realism (or radical mathematical Platonism)^[32] are true, then consciousness is computation, which in principle is platform independent and thus admits of simulation. This argument states that a "Platonic realm" or ultimate ensemble would contain every algorithm, including those that implement consciousness. Hans Moravec has explored the simulation hypothesis and has argued for a kind of mathematical Platonism according to which every object (including, for example, a stone) can be regarded as implementing every possible computation.^[33]

In physics

In physics, the view of the universe and its workings as the ebb and flow of information was first observed by Wheeler.^[34] Consequently, two views of the world emerged: the first one proposes that the universe is a quantum computer,^[35] while the other one proposes that the system performing the simulation is distinct from its simulation (the universe).^[36] Of the former view, quantum-computing specialist Dave Bacon wrote:

In many respects this point of view may be nothing more than a result of the fact that the notion of computation is the disease of our age—everywhere we look today we see examples of computers, computation, and information theory and thus we extrapolate this to our laws of physics. Indeed, thinking about computing as arising from faulty components, it seems as if the abstraction that uses perfectly operating computers is unlikely to exist as anything but a platonic ideal. Another critique of such a point of view is that there is no evidence for the kind of digitization that characterizes computers nor are there any predictions made by those who advocate such a view that have been experimentally confirmed.^[37]

Testing the hypothesis physically

A method to test one type of simulation hypothesis was proposed in 2012 in a joint paper by physicists Silas R. Beane from the University of Bonn (now at the University of Washington, Seattle), and Zohreh Davoudi and Martin J. Savage from the University of Washington, Seattle.^[38] Under the assumption of finite computational resources, the simulation of the universe would be performed by dividing the continuum space-time into a discrete set of points, which may result in observable effects. In analogy with the mini-simulations that lattice-gauge theorists run today to build up nuclei from the underlying theory of strong interactions (known as quantum chromodynamics), several observational consequences of a grid-like space-time have been studied in their work. Among proposed signatures is an anisotropy in the distribution of ultra-high-energy cosmic rays that, if observed, would be consistent with the simulation hypothesis according to these physicists.^[39] In 2017, Campbell et al. proposed several experiments aimed at testing the simulation hypothesis in their paper "On Testing the Simulation Theory".^[40]

Advocates

Elon Musk, the CEO of Tesla and SpaceX, firmly believes in the simulation hypothesis.^[41] In a podcast with Joe Rogan, Musk said "If you assume any rate of improvement at all, games will eventually be indistinguishable from reality" before concluding "that it's most likely we're in a simulation."^[42] At various other press conferences and events, Musk has also speculated that the likelihood of us living in a simulated reality or computer made by others is about 99.9%, and stated in a 2016 interview that he believed there was "a one in billion chance we're in base reality."^[41][43]

Another high-profile proponent of the hypothesis is astrophysicist Neil Degrasse Tyson, who said in an NBC News interview that the hypothesis was correct, giving "better than 50–50 odds" and adding, "I wish I could summon a strong argument against it, but I can find none."^[44] However, in a subsequent interview with Chuck Nice on a YouTube episode of StarTalk, Tyson shared that his friend J. Richard Gott, a professor of astrophysical sciences at Princeton University, made him aware of a strong objection to the simulation hypothesis. The objection claims that the common trait that all hypothetical high-fidelity simulated universes possess is the ability to produce high-fidelity simulated universes. And since our current world does not possess this ability, it would mean that either humans are in the real universe, and therefore simulated universes have not yet been created, or that humans are the last in a very long chain of simulated universes, an observation that makes the simulation hypothesis seem less probable. Regarding this objection, Tyson remarked "that changes my life".^[45]

Rizwan Virk, of Massachusetts Institute of Technology is a founder of PlayLabs, and author of the novel, "The Simulation Hypothesis." A story about Virk trying on a virtual reality headset and forgetting he was in an empty room makes him wonder if the real world was created by more tech-savy individuals, other than us. ^[46]

Other uses in philosophy

Dream argument

There is a long philosophical and scientific history to the underlying thesis that reality is an illusion. This skeptical hypothesis can be traced back to antiquity; for example, to the "Butterfly Dream" of Zhuangzi,^[47] or the Indian philosophy of Maya, or in Ancient Greek philosophy Anaxarchus and Monimus likened existing things to a scene-painting and supposed them to resemble the impressions experienced in sleep or madness.^[48]

A dream could be considered a type of simulation capable of fooling someone who is asleep. As a result, Bertrand Russell has argued that the "dream hypothesis" is not a logical impossibility, but that common sense as well as considerations of simplicity and inference to the best explanation rule against it.^[49] One of the first philosophers to question the distinction between reality and dreams was Zhuangzi, a Chinese philosopher of the 4th century BC. He phrased the problem as the well-known "Butterfly Dream," which went as follows:

Once Zhuangzi dreamt he was a butterfly, a butterfly flitting and fluttering around, happy with himself and doing as he pleased. He didn't know he was Zhuangzi. Suddenly he woke up and there he was, solid and unmistakable Zhuangzi. But he didn't know if he was Zhuangzi who had dreamt he was a butterfly or a butterfly dreaming he was Zhuangzi. Between Zhuangzi and a butterfly there must be some distinction! This is called the Transformation of Things. (2, tr. Burton Watson 1968:49)

The philosophical underpinnings of this argument are also brought up by Descartes, who was one of the first Western philosophers to do so. In Meditations on First Philosophy, he states "... there are no certain indications by which we may clearly distinguish wakefulness from sleep",^[50] and goes on to conclude that "It is possible that I am dreaming right now and that all of my perceptions are false".^[50]

Chalmers (2003) discusses the dream hypothesis and notes that this comes in two distinct forms:

• that he is currently dreaming, in which case many of his beliefs about the world are incorrect;
• that he has always been dreaming, in which case the objects he perceives actually exist, albeit in his imagination.^[51]

Both the dream argument and the simulation hypothesis can be regarded as skeptical hypotheses. Another state of mind in which some argue an individual's perceptions have no physical basis in the real world is psychosis, though psychosis may have a physical basis in the real world and explanations vary.

In On Certainty, the philosopher Ludwig Wittgenstein has argued that such skeptical hypothesis are unsinnig (i.e. non-sensical), as they doubt knowledge that is required in order to make sense of the hypotheses themselves.^[52]

The dream hypothesis is also used to develop other philosophical concepts, such as Valberg's personal horizon: what this world would be internal to if this were all a dream.^[53]

Lucid dreaming is characterized as an idea where the elements of dreaming and waking are combined to a point where the user knows they are dreaming, or waking perhaps.^[54]

Modern philosophy

A version of the simulation hypothesis was theorized as a part of a philosophical argument on the part of René Descartes, by George Berkeley (1685–1753) with his "immaterialism" (later referred to as subjective idealism by others)^{[citation needed]}, and later by Hans Moravec.^[33][55][56]

• René Descartes (1596–1650) and his Evil Demon, sometimes also called his 'Evil Genius'^[57]

• Aztec philosophical texts theorized that the world was a painting or book written by the Teotl.^[58]

• Nietzsche, in Beyond Good and Evil chastised philosophers for seeking to find the true world behind the deceptive world of appearances.

It is nothing more than a moral prejudice that truth is worth more than semblance; it is, in fact, the worst proved supposition in the world.... Why might not the world which concerns us⁠—be a fiction?^[59]

Relation to solipsism

Skeptical arguments have historically played a role in the evolution of philosophical discussion, particularly in the fields of ontology, metaphysics, the theory of knowledge and the philosophy of science. The fallibility of perception, knowledge and thought have been explored employing several arguments.^[60] Solipsist scenarios, a common ground of debate in these fields, are extreme cases prompting these dilemmas for further discussion.

The brain in a vat is another related philosophical thought experiment, which also explores epistemological questions about what knowledge humans can obtain about the world that they observe.

In popular culture

Science fiction has highlighted themes such as virtual reality, artificial intelligence and computer gaming for more than fifty years.^[61]

Simulacron-3 (1964) by Daniel F. Galouye (alternative title: Counterfeit World) tells the story of a virtual city developed as a computer simulation for market research purposes, in which the simulated inhabitants possess consciousness; all but one of the inhabitants are unaware of the true nature of their world. The book was made into a German made-for-TV film called World on a Wire (1973) directed by Rainer Werner Fassbinder. The film The Thirteenth Floor (1999) was also loosely based on this book. "We Can Remember It for You Wholesale" is a short story by American writer Philip K. Dick, first published in The Magazine of Fantasy & Science Fiction in April 1966, and was the basis for the 1990 film Total Recall and its 2012 remake. In Overdrawn at the Memory Bank, a 1983 television film, the main character pays to have his mind connected to a simulation.^{[citation needed]}

The same theme was repeated in the 1999 film The Matrix, which depicted a world in which artificially intelligent robots enslaved humanity within a simulation set in the contemporary world. The 2012 play World of Wires was partially inspired by the Bostrom essay on the simulation hypothesis.^[62]

The 2012 visual novel Danganronpa 2: Goodbye Despair is set in a simulated reality known as the Neo World Program, which in this instance simulates a class trip to Jabberwock Island which, while initially peaceful, turns into a "killing game" involving the students in the simulation killing each other and trying to not be found guilty. Similarly, 2022's Anonymous;Code explores the idea of the world being a simulation, with an infinite or near-infinite number of "world layers" of simulations running inside other simulations. The main problem with this system is that in some of these "world layers", both above and below the one the characters find themselves living in, the Year 2038 Problem has not been solved, dooming the world to end on January 19, 2038 at 3:14:07 am UTC. The characters have to hack all the way into the highest world layer, the real world that the player lives in, to synchronize all the world layers and solve the Year 2038 problem in all of them.

The 2014 episode of the animated sitcom Rick and Morty, "M. Night Shaym-Aliens!", demonstrates a low-quality simulation that attempts to trap the two titular protagonists, but because the operation is less "realistic" than typically operated "reality", it becomes obvious.

A 2017 episode of the long-running British science fiction series Doctor Who titled "Extremis" features a simulated version of the Twelfth Doctor and his companions. A secret Vatican document describes the truth about the simulated reality by inviting its reader to choose any series of numbers at random. The document lists the same numbers on the next page since the simulated program cannot produce a truly random event. The simulation is finally revealed to be a practice world for aliens intent on real-world domination.

The 2022 Netflix epic period mystery-science fiction 1899 created by Jantje Friese and Baran bo Odar tells the unfinished story of a simulation scenario in which multiple persons find themselves in a circumstance of multiplicities and simultaneities. The storyline involves an amnesia, seemingly to protect the integrity of the simulation, as suggested would be necessary by the philosopher Preston Green.^[25]