It's a question that philosophers have been asking since the dawn of philosophy itself: What's real? Is the world as we perceive it really the world? And how can we know one way or another?
The latest version of this age-old conundrum, popularized in the 1999 sci-fi film "The Matrix," puts things in modern technological terms: Could reality be nothing more than a computer simulation?
This is, technically, merely a theoretical question. No computer around today has the computing power to simulate the entire universe, not even close. But could such a super-super-supercomputer even be possible? Might you be unknowingly lying in a gel-filled pod somewhere with circuits in your head, while an ultra-powerful artificial intelligence feeds off the electrical pulses surging between neurons in your brain?
Thankfully, the plot of "The Matrix" is not only implausible, it's actually impossible. At least, that's according to a pair of quantum physicists, Zohar Ringel and Dmitry Kovrizhin, from the University of Oxford and the Hebrew University in Israel. They crunched the numbers and found that the computing power needed to simulate the universe all the way down to the quantum level would require a memory built from more atoms than there are in the universe itself.
In other words, thanks to the immense complexity of quantum phenomenon, simulating the universe as we currently understand it is fundamentally an intractable problem.
For their experiment, the researchers focused on a particular problem known as a Monte-Carlo simulation (also known as the "thermal Hall conductance" in condensed matter physics, or a "gravitational anomaly" in high-energy physics). Basically, these simulations are computations based on random samples of a system. When applied in quantum physics, they can be useful for making the weird realm of uncertainty found there into something more predictable.
But when researchers ran their simulations, they kept running into a fundamental error. No matter how they crunched the numbers, Monte-Carlo simulations could not model for quantum complexity.
Despite how this might sound, it's actually good news if you're concerned about your life being a computer simulation. It means that there's simply no way for a computer algorithm to possibly make sense of the weirdness that happens on the quantum level.
"Our work provides an intriguing link between two seemingly unrelated topics: gravitational anomalies and computational complexity. It also shows that the thermal Hall conductance is a genuine quantum effect: one for which no local classical analogue exists," explained Ringel, in a press release.
This doesn't mean that the universe can't still be a simulation, but it's not a simulation that can be run on any computer system like we've ever created. That raises the bar significantly for any theorist or Matrix-enthusiast who still wants to hang on the possibility of one day choosing the red pill. In order to keep faith, they'll need to appeal to a technology that is beyond the realm of current imaginability.
But hey, that's what science fiction is for.