Is the whole universe one big computer program? Nobody really knows. But if you this week you might be excused in thinking we've got it all sorted out. Many news outlets reported on a in the journal Science Advances as proof that our universe is not a simulation. However, the truth is a little more complicated.
For decades, researchers have been trying to use computers to simulate quantum systems. If scientists could learn how to create functional systems of quantum particles, it would be much easier to study how they behave. In particular, if scientists can simulate a quantum computer using a standard computer, there would be no need to build actual quantum computers in the first place. This would save a lot of people a lot of time.
Now, chances are we can't simulate complex quantum computers with regular computers. There's a reason we've spent years trying to build real-life quantum computers, after all. But it's difficult to prove that quantum computers are really necessary, which is what the study's authors were trying to do. So the researchers took an easier path, simply proving that one of the most common techniques for simulating quantum particles—quantum Monte Carlo—is not enough to simulate an entire quantum computer.
This is an important result, and it means there's a good chance that simulating a quantum computer is completely impossible, which is what scientists suspected all along. If we're going to study quantum mechanics using quantum computers, we're going to have to actually go and build one. Which, again, we were doing anyway, but it's nice to be sure we're not wasting our time.
So what does this have to do with the universe being a simulation? The short answer: nothing, which is the point. That wasn't the question the researchers set out to solve, so to claim they answered it by showing that our universe is far too complex to be simulated is a bit presumptuous.
"It's not even a scientific question," says Zohar Ringel, the lead author of the paper. "Who knows what are the computing capabilities of whatever simulates us."
But the long answer is a bit more complicated. The researchers were studying simulations using a classical computer with a quantum Monte Carlo algorithm. If whatever might simulate us is limited to that tech, then this paper proves there's no way they could do it.
"Once you make several assumptions—which you actually have to make if you want to turn a philosophical question into a scientific question—[our research] does have some bearing on [the simulation question]," says Ringel.
Of course, those assumptions are that aliens advanced enough to simulate entire universes would be limited to our level of computing technology. This might be the case—quantum Monte Carlo in particular has a good chance of being the most efficient possible algorithm for simulating quantum systems—but it's just as likely that advanced aliens would have figured out how to build proper quantum computers and design a better algorithm.
So at the end of the day, we're right back where we started. Is the universe a simulation? Maybe, as long as certain specific—and perhaps unlikely—conditions are met. But the very nature of the question ensures that we'll never know the technology of the aliens that may or may not be simulating us. There's a pretty good chance we'll never find out if our universe is "real" or not, so it's probably not worth worrying about. The universe is real to us, and that's all that matters.
Updated with quotes and additional information.