Is This Real Life? Do We Live in a Computer Simulation or Not?

I quite like the idea that we live in a computer simulation. It gives me hope that things will be better on the next level. This simulation hypothesis, as it’s called, has been mostly ignored by physicists, but it enjoys a certain popularity among philosophers and people who like to think of themselves as intellectual. Evidently, it’s more appealing the less you understand physics.

The simulation hypothesis is most strongly associated with the philosopher Nick Bostrom, who has argued that (given certain assumptions I will come to in a moment) pure logic forces us to conclude that we are simulated. Elon Musk is among those who have bought into it. “It’s most likely we’re in a simulation,” he said. And even Neil deGrasse Tyson gave the simulation hypothesis “better than 50-50 odds” of being correct.

The simulation hypothesis annoys me, but not because I’m afraid people will actually believe it. Most people understand that the idea lacks scientific rigor. No, the simulation hypothesis annoys me because it intrudes on the terrain of physicists. It’s a bold claim about the laws of nature that doesn’t pay any attention to what we know about the laws of nature.

Loosely speaking, the simulation hypothesis has it that everything we experience was coded by an intelligent being, and we are part of that computer code. The opinion that we live in some kind of computation in and by itself is not an outrageous claim. For all we currently know, the laws of nature are mathematical, so you could say the universe is really just computing those laws. You may find this terminology a little weird, and I would agree, but it’s not controversial. The controversial bit about the simulation hypothesis is that it assumes there is another level of reality where some being or some thing controls what we believe are the laws of nature, or even interferes with those laws.

The belief in an omniscient being that can interfere with the laws of nature, but that for some reason remains hidden from us, is a common element of monotheistic religions. The difference is that those who believe in the simulation hypothesis argue that they have arrived at their belief by reason. Their line of argumentation usually closely follows Nick Bostrom’s argument, which, in a nutshell, goes like this: if there are (a) many civilizations, and these civilizations (b) build computers that run simulations of conscious beings, then (c) there are many more simulated conscious beings than real ones, so you are likely to live in a simulation.

First of all, it could be that one or both of the premises is wrong. Maybe there aren’t any other civilizations, or they aren’t interested in simulations. That wouldn’t make the argument wrong, of course; it would just mean that the conclusion can’t be drawn. But I will leave aside the possibility that one of the premises is wrong, because I don’t really think we have good evidence for one side or the other.

The point I have seen people criticize most frequently about Bostrom’s argument is that he just assumes it is possible to simulate humanlike consciousness. We don’t actually know that this is possible. However, in this case, it would require an explanation to assume that it is not possible. That’s because, for all we currently know, consciousness is simply a property of certain systems that process large amounts of information. It doesn’t really matter exactly what physical basis this information processing is based on. It could be neurons or transistors, or it could be transistors believing they are neurons. I don’t think simulating consciousness is the problematic part.

The problematic part of Bostrom’s argument is that he assumes it is possible to reproduce all our observations not using the natural laws that physicists have confirmed to extremely high precision but using a different, underlying algorithm, which the programmer is running. I don’t think that’s what Bostrom meant to do, but it’s what he did. He implicitly claimed it’s easy to reproduce the foundations of physics with something else. This is the problematic part of the argument.

To begin with, quantum mechanics features phenomena that are not computable with a conventional computer in finite time. At the very least, therefore, one would need a quantum computer to run the simulation—that is, a computer that works with quantum bits, or q-bits for short, that are superpositions of two states (say, 0 and 1).

But nobody yet knows how to reproduce general relativity and the standard model of particle physics from a computer algorithm running on any type of machine. Waving your hands and yelling “quantum computer” doesn’t help. You can approximate the laws we know with a computer simulation—we do this all the time—but if that were how nature actually worked, we could see the difference. Indeed, physicists have looked for signs that natural laws really proceed step by step, like a computer code, but their search has come up emptyhanded. It’s possible to tell the difference because all known attempts to algorithmically reproduce natural laws are incompatible with the full symmetries of Einstein’s theories of special and general relativity. It’s not easy to outdo Einstein.

This problem exists regardless of what the laws are in the higher-level reality from which the programmer supposedly simulates us. We don’t know any kind of algorithm that would give us the laws we observe, regardless of what that algorithm is running on. If we knew, we’d have found a theory of everything.

A second issue with Bostrom’s argument is that, for it to work, a civilization needs to be able to simulate a lot of conscious beings, and these conscious beings will themselves try to simulate conscious beings, and so on. While you can imagine simulating a single brain with its inputs only, in this case the conclusion we are likely to live in a simulation because there are more simulated than real brains wouldn’t work. You actually need a lot of brains. But this means you have to compress the information we think the universe contains because otherwise your simulations will run out of disk space quickly. Bostrom therefore has to assume it’s somehow possible to not care much about the details in some parts of the world where no one is currently looking, and just fill them in in case someone looks.

Again, though, he doesn’t explain how this is supposed to work. What kind of computer code can actually do that? What algorithm can identify conscious subsystems and their intention and then quickly fill in the required information without ever producing an observable inconsistency? That’s a much more difficult issue than Bostrom seems to appreciate. Not only does it assume that consciousness is computationally reducible, for otherwise you couldn’t predict where someone is about to look before they look, but also you can in general not just throw away physical processes on short distances and still get the long distances right.

Global climate models are an excellent example. We don’t currently have the computational capacity to resolve distances below something like 10 kilometers (6 miles) or so. But you can’t just discard all the physics below this scale. This is a nonlinear system, so the details from short scales leave a mark on large scales—butterflies causing tornadoes and so on. If you can’t compute the short-distance physics, you have to at least suitably replace it with something. Getting this right even approximately is a big headache. And the only reason climate scientists do get it approximately right is that they have observations they can use to check whether their approximations work. If you have only a simulation, like the programmer in the simulation hypothesis, you can’t do that.

That’s my issue with the simulation hypothesis. Those who believe it make big assumptions, maybe unknowingly, about what natural laws can be reproduced with computer simulations, and they don’t explain how this is supposed to work. But finding alternative explanations that match all our observations to high precision is really difficult. I should know—it’s what we do in the foundations of physics.

Maybe you’re now rolling your eyes because, come on, let the nerds have some fun, right? And, sure, some part of this conversation is just intellectual entertainment. But I don’t think popularizing the simulation hypothesis is entirely innocent fun. It’s mixing science with religion, which is generally a bad idea, and, really, I think we have better things to worry about than that someone might pull the plug on us.

In summary, the simulation hypothesis isn’t a serious scientific argument. This doesn’t mean it’s wrong, but it means you’d have to believe it because you have faith, not because you have logic on your side.

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Excerpted from Existential Physics: A Scientist’s Guide to Life’s Biggest Questions by Sabine Hossenfelder. Copyright © 2022. Available from Viking, an imprint of Penguin Publishing Group, a division of Penguin Random House, LLC.