I have a personal theory about life, death and reincarnation. I can’t remember when I started thinking of our lives as characters of a videogame, in which somebody in the future is using “lives” as in any typical role-playing game to impersonate us, all of us on this planet. In this vision, we perform actions, travel places, meet other people, according to the rules fixed by the game. When somebody dies, they are “retired” from the game: in some sense, to be transported back home. This idea, admittedly incomplete and prone to several logical fallacies, had the only real value for me of seeing death as “going back home”, a quite comforting thought. Exegetes and eschatologists will dub this idea of mine as yet another transposition of memories from childhood religious education. However, I reckon this idea is not at all new, and it is more likely born out of the influence of classical studies.

In 17th century Europe the Catholic world-view was predominant, and French philosopher Descartes brought up the possibility that maybe we are all deceived by our senses, and the contents of our minds may be imposed by some superior entity, raising the doubt that maybe physical reality is not what we think it is. An idea that had been around in European philosophical thinking at least since the Renaissance revival of Platonism, with the likes of Marsilio Ficino in his Theologia Platonica, who presents the man troubled by the melancholy of life and eager for everything to be a dream, and writes “forsitan non sunt vera quae nunc nobis apparent, forsitan in praesentia somniamus” (maybe the things that appear to us are not real, maybe we are dreaming as we live).

According to Descartes, most of what we know about the external world comes from our sensory experiences, which are in turn interpreted by our mind/brain.  In his view, our sensory experiences happen all just in our minds, meaning that while we naturally accept that external world and events are the cause our inner sensory experiences, these could philosophically be something else. What could we know or learn about the external world without any sensing organs to receive and interpret information?  Descartes therefore hypothesized that it was philosophically possible that an evil demon (the malin génie) was responsible for our sensory experiences of the world.  This demon could be directly and deliberately implanting false sensory and thought patterns in our minds. This “skeptical” view of reality goes indeed way back in the past, such as the famous Pyrrhon of Elis who slammed against a tree while walking because he purported reality to be an illusion. And such a view is not restricted to the Western philosophy, since it is found for example also in ancient Indian thinkers.

However, the modern computing and information theory adds a layer of cybernetic meaning to Descartes’ evil demon, making it a realistic possibility that some alien civilization that developed (in)credibly powerful computers may be simulating our reality now. This has become known in our times as the “simulation hypothesis”. Around 2006, I stumbled into a paper by Nick Bostrom titled Are you living in a computer simulation? [Phil. Quart53, 211 (2003)]. After reading it, I checked Goggle scholar: at that time this paper had exactly 8 citations. Today, about 15 years later, it has passed the 1,200 citations and has spurred endless discussions. Bostrom’s paper develops a seemingly rigorous logical argument, ending up to a seemingly rigorous conclusion that, if it would ever be technologically possible someday in the future to run a whole-world computer simulation, this proves that we must be currently living in a simulated world. However, his demonstration goes by contradicting the argument, therefore he seems in the end rather skeptical on the fact that we, or our posterity, will never have that much computer power available, and therefore we are not living in a simulation.

Of course, reading Bostrom’s paper (especially back in 2006) brings up to mind the 1999 movie The Matrix, and for the movie geeks there are plenty of ancestors to dig in, such as Strange Days (1995), the Japanese anime Akira (1988) and Ghost in the shell (1985), and many of Philip Dick’s stories, such as A scanner darkly (1977, from which a 2006 movie starring the same Keanu Reeves who gave life to Matrix’ hero Neo), Ubik (1969), and chiefly The adjustment team (1954). The Wachowskis, authors and directors of The Matrix, famously required all cast members to read Jean Baudrillard’s philosophy text Simulacra and Simulation, so that they would get the ‘right atmosphere’ to step into the movie plot. Baudrillard as well claimed that our current society has replaced all reality and meaning with symbols and signs, and that human experience is but a simulation of reality. However, what makes Bostrom’s study different from all these philosophical or cinematical musings, is that he tried to put the whole question on a sort of quantitative basis.

Bostrom defines the notion of “posthuman civilization”, as one that would have enough computing power to run hugely many simulations, even while using only a tiny fraction of their resources for that purpose. He then builds three probability functions for the following quantities: fsim, the fraction of all observers with human‐type experiences that live in simulation; fI, the fraction of posthuman civilizations that are interested in running real-life simulations of a previous civilization; fP, the fraction of technological civilizations that survive long enough to reach a posthuman stage. By inserting as realistic as possible estimates of the free parameters, he comes to the conclusion that one of the following probabilistic statements must be true: either fP or fI are practically equal to zero, or fsim is close to one. This can be interpreted as follows: if an evolved society could ever have the technological possibility of running real-life simulations, they most likely would. And since we cannot know for sure how many other alien, more or less advanced civilizations are (or have been) present in our universe, the probability that we are, or even the entire universe is, a simulation is actually 1.

Since the argument was formulated in such terms, simulation hypothesis ‘supporters’ have appeared everywhere, including the coffee bar next to your place. Elon Musk is their fearless leader (https://www.youtube.com/watch?v=xBKRuI2zHp0), but also famous scientists like Neil deGrasse Tyson supported the idea that “it is hard to argue that we are not living in a simulated world”. Experimental tests have been proposed [S Beane et al., Constraints on the universe as a numerical simulation, Eur. Phys. J. A 50, 148 (2014); T Campbell et al, On testing the simulation theory, Int J Quantum Found 3, 78 (2017)], however none has yet been practically realized. However, to most physicists such an idea sounds outright far from reality. Many have spoken publicly against it, such as Frank Wilckzek, Paul Davies, David Deutsch, Sabine Hossenfelder. I think that part of the issue stems from the modern views of the universe as an information reservoir, the ‘holographic universe’ model strongly advocated by string theorists like Leonard Susskind. In this model of reality, the three-dimensional world of ordinary experience, galaxies, stars, planets, houses, cats and people, is a hologram, that is an image of reality projected on a distant two-dimensional surface. This representation was inspired by black hole thermodynamics, which conjectures that the maximal entropy of a region of the universe scales with its radius squared, and not cubed as might be expected from classical physics arguments.

Unfortunately, the whole of modern physics seems at fault when it comes to ‘explaining’ the universe, despite the impressive amount of experimental measurements that piecewise confirm all our physical theories tested in our Earth-based laboratories. It is hard to admit that even our best current theory of the origins of the universe makes little sense, when scrutinized with a little more critical vision. It goes as follows. In the very beginning there should be something called quantum foam. It is barely there, and cannot even be said to occupy space because there is no space or time yet. It just is. Until something pops in, precisely the right way, and out of that infinitesimal instability the entire universe instantly big-bangs into existence. Inflation made it happen, and the inflaton should be the (new) “god’s” particle that produces this hiccup in the foam, still undiscovered and ready for the next, Higgs-like quest. But imagine, instead, someone sitting at a desk booting up their computer. This is the quantum-foam stage, the computer turned on but existing in a state of suspended anticipation. Then our desk person moves the mouse to a file called, say, KnownUniverse.mov, and double-clicks it. This is the emergence of the inflaton. It’s the tiny zzzt that launches the program into execution.

The universe around us looks very homogeneous on cosmological scales, and simulation supporters often claim this as one evidence for a simulated reality. “How was it possible” CERN physicist Giulio Tonelli asks in his book Genesis: the story of how everything began, “that all the most remote corners of the universe, distant from each other by billions of light years, had agreed among themselves to attain exactly the same temperature at precisely the moment when scientists on a small planet in an anonymous solar system of an unremarkable galaxy had decided to take a look at what was happening around them?” Moreover, if the simulation is being run on some futuristic computer, shouldn’t it leave some artifacts of its existence within the simulated world it is running? (Now, you do remember the famous scene in Matrix when Neo notices the glitch in the simulation, with the black cat going twice through the door.) Fouad Khan goes as far as suggesting that the finite speed of light we observe as an insurmountable bound in our current universe, could be a limit imposed by the algorithms, something like the clock frequency of the computer that is running the entire simulation (https://www.scientificamerican.com/article/confirmed-we-live-in-a-simulation/).

I would observe that we humans tend to shape our vision of the universe according to the technology that is most widely available at any given time. In the ancient world, technology was restricted to instruments to measure distances and angles, and the universe was seen as a static arrangement of spheres with the Earth at the center. Then, in the Middle Ages and Renaissance the top of technology was the measure of time, and our vision of the universe evolved into that of a clockwork, with Newtonian gravity rigorously explaining the motion of planets and stars. In the 19th century the steam machines were leading the industrial revolution, and thermodynamics reshaped our vision of the universe into one of stars as burning furnaces, with the entropy increase according to the Second Law as the limit, and thermal death of the universe as the inescapable conclusion. No wonder that today, in the digital age, we want to see the universe as a giant computer, and the most intimate fabric of the reality as made of bits of information. Then, I just wonder what would come next.

But… what if we really were living in a simulation, and somebody decides to turn it off, maybe for fun, or just because he had a bad day at work? Then, I suggest you to read a popular novel recently appeared in France (most evidently, another labour of our claustrophobic Covid-19 age), L’Anomalie, written by the mathematician-turned-writer Hervé Le Tellier, who got the Prix Goncourt 2020 for his endeavour.

Are you living in a computer simulation?

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