You might think we have definitive evidence we’re not in a simulation. That’s impossible.
BY DAVID J. CHALMERS
How do you know you’re not in a computer simulation right now? This idea is often known as the simulation hypothesis. The simulation hypothesis says simply: “We are living in a computer simulation.”
What is it to be living in a simulation? As I understand this notion, it’s all about interacting with the simulation. When you’re in a simulation, your sensory inputs come from the simulation, and your motor outputs affect the simulation. You’re fully immersed in the simulation through these interactions.
At the start of The Matrix, Neo’s biological body and brain are in a pod in a nonsimulated world, connected to a simulation somewhere else. In the ordinary spatial sense of “in,” Neo’s brain is not “in” the simulation. However, all his sensory inputs are coming from the simulation, and his outputs are going there, so he’s living in a simulation in the sense that matters. After he takes the red pill, his senses respond to the nonsimulated world, so he is no longer living in a simulation.
We need only set up the right simulation program and watch it go.
I will use the word sim for someone who is in a simulation. There are at least two sorts of sims. First, there are biosims: biological beings outside the simulation (in the spatial sense) and connected to it. Neo is a biosim. So is a brain in a vat, connected to a computer. A simulation that includes biosims is an impure simulation, since it includes elements (the biosims) that aren’t simulated.
Second, there are pure sims. These are simulated beings who are wholly inside the simulation. Most of the people in Daniel F. Galouye’s 1964 novel Simulacron-3, are pure sims. They receive direct sensory inputs from the simulation because they’re part of the simulation. Importantly, their brains are simulated, too. Simulations containing only pure sims may be pure simulations—simulations in which everything that happens is simulated.
There can also be mixed simulations, which contain both biosims and pure sims. Inside the Matrix, the leading characters Neo and Trinity are biosims, whereas the “machine” characters Agent Smith and the Oracle are pure sims. In the 2021 movie Free Guy, the main character, Guy (played by Ryan Reynolds), is a fully digital nonplayer character in a video game, while his in-game partner, Molotov Girl (played by Jodie Comer), is a video game player and designer with an ordinary life outside the game. So Guy is a pure sim, while Molotov Girl is a biosim.
The simulation hypothesis applies equally to pure, impure, and mixed simulations. Occurrences of the simulation idea in science fiction and philosophy are split fairly evenly among them. In the short term, impure simulations will be more common than pure simulations, since we know how to connect people to simulations but we don’t yet know how to simulate people. In the long term, pure simulations may well be more common. The supply of brains for impure simulations is not endless, and in any case hooking them up may be tricky. By contrast, pure simulations are easy in the long term. We need only set up the right simulation program and watch it go.
As biological systems, bodies can be simulated.
Here’s another distinction. The global-simulation hypothesis says that the simulation simulates a whole universe in detail. For example, a global simulation of our universe will simulate me, you, everyone on Earth, planet Earth itself, the whole solar system, the galaxy, and everything beyond. The local simulation hypothesis says that the simulation simulates only a part of the universe in detail. It might simulate just me, or just New York, or just Earth and everyone on it, or just the Milky Way galaxy.
In the short run, local simulations should be easier to create. They require much less computational power. However, a local simulation has to interact with the rest of the world, and that can lead to trouble. In the 1999 movie, The Thirteenth Floor, the simulators simulated only Southern California. When the protagonist tried to drive to Nevada, he encountered signs saying “Road closed.” He kept going, and the mountains morphed into thin green lines. That’s not a good way to design a convincing simulation. If a local simulation is completely local, it cannot properly simulate interaction with the rest of the world…
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