Are we living in a "base reality" or could all this be a simulation?

It is not often that a comedian gives an astrophysicist goose bumps when discussing the laws of physics. But comic Chuck Nice managed to do just that in a recent episode of the podcast StarTalk.The show’s host Neil deGrasse Tyson had just explained the simulation argument—the idea that we could be virtual beings living in a computer simulation. If so, the simulation would most likely create perceptions of reality on demand rather than simulate all of reality all the time—much like a video game optimized to render only the parts of a scene visible to a player. “Maybe that’s why we can’t travel faster than the speed of light, because if we could, we’d be able to get to another galaxy,” said Nice, the show’s co-host, prompting Tyson to gleefully interrupt. “Before they can program it,” the astrophysicist said,delighting at the thought. “So the programmer put in that limit.”

Such conversations may seem flippant. But ever since Nick Bostrom of the University of Oxford wrote a seminal paper about the simulation argument in 2003, philosophers, physicists, technologists and, yes, comedians have been grappling with the idea of our reality being a simulacrum. Some have tried to identify ways in which we can discern if we are simulated beings. Others have attempted to calculate the chance of us being virtual entities. Now a new analysis shows that the odds that we are living in base reality—meaning an existence that is not simulated—are pretty much even. But the study also demonstrates that if humans were to ever develop the ability to simulate conscious beings, the chances would overwhelmingly tilt in favor of us, too, being virtual denizens inside someone else’s computer. (A caveat to that conclusion is that there is little agreement about what the term “consciousness” means, let alone how one might go about simulating it.)

In 2003 Bostrom imagined a technologically adept civilization that possesses immense computing power and needs a fraction of that power to simulate new realities with conscious beings in them. Given this scenario, his simulation argument showed that at least one proposition in the following trilemma must be true: First, humans almost always go extinct before reaching the simulation-savvy stage. Second, even if humans make it to that stage, they are unlikely to be interested in simulating their own ancestral past. And third, the probability that we are living in a simulation is close to one.

Before Bostrom, the movie The Matrix had already done its part to popularize the notion of simulated realities. And the idea has deep roots in Western and Eastern philosophical traditions, from Plato’s cave allegory to Zhuang Zhou’s butterfly dream. More recently, Elon Musk gave further fuel to the concept that our reality is a simulation: “The odds that we are in base reality is one in billions,” he said at a 2016 conference.

“Musk is right if you assume [propositions] one and two of the trilemma are false,” says astronomer David Kipping of Columbia University. “How can you assume that?”

To get a better handle on Bostrom’s simulation argument, Kipping decided to resort to Bayesian reasoning. This type of analysis uses Bayes’s theorem, named after Thomas Bayes, an 18th-century English statistician and minister. Bayesian analysis allows one to calculate the odds of something happening (called the “posterior” probability) by first making assumptions about the thing being analyzed (assigning it a “prior” probability).

Kipping began by turning the trilemma into a dilemma. He collapsed propositions one and two into a single statement, because in both cases, the final outcome is that there are no simulations. Thus, the dilemma pits a physical hypothesis (there are no simulations) against the simulation hypothesis (there is a base reality—and there are simulations, too). “You just assign a prior probability to each of these models,” Kipping says. “We just assume the principle of indifference, which is the default assumption when you don’t have any data or leanings either way.”

So each hypothesis gets a prior probability of one half,much as if one were to flip a coin to decide a wager.

The next stage of the analysis required thinking about “parous” realities—those that can generate other realities—and “nulliparous” realities—those that cannot simulate offspring realities. If the physical hypothesis was true, then the probability that we were living in a nulliparous universe would be easy to calculate: it would be 100 percent. Kipping then showed that even in the simulation hypothesis, most of the simulated realities would be nulliparous. That is because as simulations spawn more simulations, the computing resources available to each subsequent generation dwindles to the point where the vast majority of realities will be those that do not have the computing power necessary to simulate offspring realities that are capable of hosting conscious beings.

Plug all these into a Bayesian formula, and out comes the answer: the posterior probability that we are living in base reality is almost the same as the posterior probability that we are a simulation—with the odds tilting in favor of base reality by just a smidgen.

These probabilities would change dramatically if humans created a simulation with conscious beings inside it, because such an event would change the chances that we previously assigned to the physical hypothesis. “You can just exclude that [hypothesis] right off the bat. Then you are only left with the simulation hypothesis,” Kipping says. “The day we invent that technology, it flips the odds from a little bit better than 50–50 that we are real to almost certainly we are not real, according to these calculations. It’d be a very strange celebration of our genius that day.”

The upshot of Kipping’s analysis is that, given current evidence, Musk is wrong about the one-in-billions odds that he ascribes to us living in base reality. Bostrom agrees with the result—with some caveats. “This does not conflict with the simulation argument, which only asserts something about the disjunction,” the idea that one of the three propositions of the trilemma is true, he says.

But Bostrom takes issue with Kipping’s choice to assign equal prior probabilities to the physical and simulation hypothesis at the start of the analysis. “The invocation of the principle of indifference here is rather shaky,” he says. “One could equally well invoke it over my original three alternatives, which would then give them one-third chance each. Or one could carve up the possibility space in some other manner and get any result one wishes.”

Such quibbles are valid because there is no evidence to back one claim over the others. That situation would change if we can find evidence of a simulation. So could you detect a glitch in the Matrix?

Houman Owhadi, an expert on computational mathematics at the California Institute of Technology, has thought about the question. “If the simulation has infinite computing power, there is no way you’re going to see that you’re living in a virtual reality, because it could compute whatever you want to the degree of realism you want,” he says. “If this thing can be detected, you have to start from the principle that [it has] limited computational resources.” Think again of video games, many of which rely on clever programming to minimize the computation required to construct a virtual world.

For Owhadi, the most promising way to look for potential paradoxes created by such computing shortcuts is through quantum physics experiments. Quantum systems can exist in a superposition of states, and this superposition is described by a mathematical abstraction called the wave function. In standard quantum mechanics, the act of observation causes this wave function to randomly collapse to one of many possible states. Physicists are divided over whether the process of collapse is something real or just reflects a change in our knowledge about the system. “If it is just a pure simulation, there is no collapse,” Owhadi says. “Everything is decided when you look at it. The rest is just simulation, like when you’re playing these video games.”

To this end, Owhadi and his colleagues have worked on five conceptual variations of the double-slit experiment, each designed to trip up a simulation. But he acknowledges that it is impossible to know, at this stage, if such experiments could work. “Those five experiments are just conjectures,” Owhadi says.

Zohreh Davoudi, a physicist at the University of Maryland, College Park, has also entertained the idea that a simulation with finite computing resources could reveal itself. Her work focuses on strong interactions, or the strong nuclear force—one of nature’s four fundamental forces. The equations describing strong interactions, which hold together quarks to form protons and neutrons, are so complex that they cannot be solved analytically. To understand strong interactions, physicists are forced to do numerical simulations. And unlike any putative supercivilizations possessing limitless computing power, they must rely on shortcuts to make those simulations computationally viable—usually by considering spacetime to be discrete rather than continuous. The most advanced result researchers have managed to coax from this approach so far is the simulation of a single nucleus of helium that is composed of two protons and two neutrons.

What if your thoughts and everything you perceive are nothing but bits in a computer simulation designed to satisfy the curiosity of scientists with capabilities far beyond anything known to human beings?

Maybe you’re thinking, “I already saw that movie.” Or, “What’s the point in speculating on some abstract philosophical theory that we can never test anyway?” Or maybe you just think it all sounds pretty far-fetched.

But some philosophers are taking this idea, called the “simulation argument,” very seriously. Physicists have gone even further, suggesting that we might even be able to detect evidence that confirms it, if we know where to look.

In 2003, philosopher Nick Bostrom of the University of Oxford made the first rigorous exploration of the simulation argument. The simulations he considered are different from those in movies like “The Matrix,” in which the world is simulated but the conscious minds are not—that is, where biological human beings with human brains interface with the simulated world. In Bostrom’s simulations, human consciousness is just another figment of the simulation.

Bostrom assumes that the human mind is substrate-independent : that human consciousness isn’t strictly dependent on the biological brain itself, and that if we could physically replicate that brain in sufficient detail in another form (such as within a computer) it would also have the subjective experience of consciousness. The replication doesn’t have to be perfect. It just has to be good enough that the replicated being has a human-like subjective experience (a “mind”). An advanced civilization with sufficient computing power to pull this off would be classified as “posthuman.”

What is the probability, then, that we ourselves are simulated minds?

To calculate the probability that a randomly-selected human-like mind—let’s call it “you”—is a simulation, you would divide the number of simulated minds by the total number of all human-like minds (both simulated and non-simulated, or ”real”).



It might seem that there’s no way to make sense of these quantities, but keep in mind that the “simulated minds” in this case come from a posthuman civilization running detailed simulations of its own past. The total number of “simulated minds” will be a multiple of the “real minds” of the humans that existed before they reached posthuman status. This multiple will be the average number of simulations run by the society (although this argument doesn’t rule out multiple human-like societies existing, if you’re a fan of the somewhat-similar Drake equation). So if you divide both the numerator and denominator by the number of “real minds” (even though we have no idea what that number is), you reach the following:



Now we get to play with the numbers to see what happens. If the total number of simulations is very small, the ratio is very small. But if the total number of simulations is very large, the ratio will be close to one. Arguing from some reasonable assumptions about what drives the number of simulations, Bostrom explains that we can expect at least one of three scenarios:

1. The fraction of civilizations that survive to the posthuman level is very small.
2. The fraction of posthuman civilizations interested in running simulations is very small.
3. The probability that you are a simulated mind is a simulation is very high.

If options 1 and 2 are correct, we can relax: we’re probably real. But if you think that many civilizations do survive to become “posthuman,” and that many of those posthuman civilizations are indeed interested in running simulations, then option 3—that you are just a computer simulation—becomes a serious probability. It’s hard to tell, of course, as we have no direct experience with posthuman civilizations and their preferences, and there are a number of philosophical objections worthy of debate: Why would any posthuman civilization want to do this? Are Bostrom’s assumptions actually reasonable, or do they miss something vital? Is substrate-independence true, or is it impossible to replicate a human mind?

Can physics offer any insight here? English cosmologist John D. Barrow addressed this question in a 2007 essay published in the book “Universe or Multiverse?,” in which he argued that the simulations might have limits. Even if posthuman simulators “have a very advanced knowledge of the laws of Nature, it’s likely they would still have an incomplete knowledge of them,” wrote Barrow. Any flaws or gaps in this knowledge “would of course be subtle and far from obvious, otherwise our ‘advanced’ civilization wouldn’t be too advanced.”

If these gaps exist, as Barrow reasons, the result would be either glitches in the working of reality, or update “patches” to fix a glitch before it causes a problem. (Recall that in “The Matrix,” local changes to the Matrix caused déjà vu.) These patches could result in changes, over time, to the laws of nature. Barrow concludes:

[…] if we live in a simulated reality we should expect occasional sudden glitches, small drifts in the supposed constants and laws of Nature over time, and a dawning realization that the flaws of Nature are as important as the laws of Nature for our understanding of true reality.

An un-peer-reviewed 2012 attempt at a more rigorous physical analysis of the situation, “Constraints on the Universe as a Numerical Simulation” by physicists Silas R. Beane, Zohreh Davoudi, and Martin J. Savage, reached the conclusion that “in principle there always remains the possibility for the simulated to discover the simulators.” Their specific prediction was that there might be limitations on cosmic ray energy levels if we live in a simulation. However, the above statement might be overstating their own case: rather than discovering the simulators, if we find that cosmic rays violate these limitations, we will have instead disproved that we’re in a simulation!

This paper also predicts why our posthuman descendants might want to simulate our universe: to test out string theory. Currently, string theory is overpowered by a vast landscape of possible versions of string theory, and scientists haven’t figured out which one might describe our universe. Detailed simulations would allow posthuman races to test hypotheses about these universes, ruling out possible versions of string theory to zero in on the one that describes their real universe.

If this is the case, Barrow must be right that even the simulators are working from incomplete knowledge. A civilization that is simulating the universe to explore the string theory landscape must not know everything about the laws of physics, and therefore we can reasonably expect gaps and flaws in the simulation.

Indeed, these ideas suggest that our entire known universe is itself only a small part of a grand experiment to understand the most fundamental mysteries of the universe. And for the scientifically-minded among us, many may find that a worthy purpose to our simulated creation.