In 1999, box office hit The Matrix introduced a wider audience to an age-old philosophical problem: how do you know what you're experiencing is real?
In the Wachowskis' Matrix series, humans far in the future have been enslaved by sentient robots, to be used as one of the most inefficient batteries imaginable and somehow power their world. To keep the humans docile, they are hooked up to the Matrix: a computer simulation of life on Earth in the late 1990s. Most humans are unaware of this, though some have realized – or feel something is off – and manage to break free.
It's a fun sci-fi premise, which has been used before and since. But there are those, from respected philosophers to Elon Musk, who believe that it is possible we may be living in a simulated reality. Musk, though no expert on the subject, places the odds that we are living in the "base reality" (or "reality") at billions to one.
One particularly influential idea, put forward by Swedish philosopher and Oxford University Professor Nick Bostrom in his 2003 paper "Are you living in a computer simulation?", is known as the simulation hypothesis.
The simulation hypothesis, in its most basic form, goes like this: if humans (or another species, for cuteness feel free to imagine it's puppies) continue to advance for hundreds, thousands, or even millions of years, it's a pretty safe bet that we will have a lot of computational power at our finger/paw tips. If we were to expand out into the galaxy (or even further) we may harness the power of stars, or possibly even black holes.
With all this energy and computational power, it's likely that at some point our descendants will be curious enough to run "ancestor simulations", using just a tiny fraction of the computing power available to us.
Ancestor simulations is the idea that future generations might have the computing power to run simulations on our forebears, and imbue these simulations with a sort of artificial consciousness. If this has already happened, it would mean the vast majority of people are simulations by the advanced descendants of the original humankind, and if that's the case, it's more rational to assume you are one of the simulations rather than one of the original biological humans.
In his paper, Bostrom proposes three possible scenarios, only one of which can be true:
1) The fraction of human-level civilizations that reach a stage where they are able to run these simulations is very close to zero.
I.e. it's probable we'll get wiped out (by nuclear war, a disaster, or some other horrible discovery) before we reach a point where we are able to perform such feats of computing.
2) The fraction of posthuman civilizations that are interested in running ancestor simulations is very close to zero.
In other words, our species has changed so much by that point that we are no longer interested in running simulations, and no curious individuals have access to the power to create them, or else running these simulations is banned.
3) The fraction of all people with our kinds of experiences that are living in a simulation is very close to one.
If the other two are false, then we are left with option number three: our species will develop the required technology and start creating an incomprehensible number of ancestor simulations (over time). Moreover, the simulations may start running their own simulations, and so on and so forth. This would mean that the vast majority of "people" with experience of living on Earth are inside a simulation, we just don't know it.
What are the odds we are living in a simulation?
Without finding some evidence that we are living in a simulation (which, of course, our simulators could hide from us), it's not an easy question to answer, and has been criticized for being unscientific. People have tried to look at the odds, however, focusing on whether this kind of computation is possible, and how likely it is humans will make it to a stage where we can achieve it.
One astronomer used a Bayesian analysis to try and put a rough number on it – taking into account factors such as diminishing computing power as you move down through layers of simulation – putting it at around 50/50 odds that we are living in a simulation.
However, the closer we get to being able to simulate ancestors, the more likely it becomes that we will run it, and therefore the more likely it is that a simulator did too.
“The day we invent that technology, it flips the odds from a little bit better than 50–50 that we are real to almost certainly we are not real, according to these calculations," Columbia University astronomer David Kipping told Scientific American. "It’d be a very strange celebration of our genius that day.”
Can we find out?
There is some evidence that points to our reality being actual reality. One team, not originally investigating the hypothesis, found evidence that certain quantum mechanics problems cannot be simulated by computers. The team attempted to simulate the Thermal Hall effect, where systems are subjected to extreme magnetic fields, and found that in order to simulate a few hundred electrons it would take more atoms than are contained in the observable universe.
Of course, supporters of simulation theory could argue that the universe above us has far more computational power, or that most of the time the quantum is not simulated (except when we observe it). For these people though, perhaps no evidence that we are living in base reality is enough, as it can always be a clever trick by The Simulator to throw us off the scent.
There are people out there who are trying to test the simulation hypothesis. One team in 2017 proposed using a variation of the double-slit test, to find out when reality is being rendered. Another computer scientist went further, proposing ways in which we might escape.
The most obvious way would be to try and crash the computer, by giving it a paradox it can't resolve, e.g. by killing your own grandfather. Of course, this necessitates simulated time travel, which we might want to put on the back burner. Another would be to crash it by using up too many resources, say by running simulations each running their own simulations until the whole system grinds to a halt.
In his paper, he concluded that the most sensible course for anyone wishing to survive, rather than merely reboot Windows, would be to attract the attention of the creator and convince them to let us into the real world, and also look for signs of glitches in the universe which we could exploit.
"As we currently have no capability to read/write simulation’s source code and do not know if our attempts at social engineering attacks have any impact, our best bet is to investigate the structure of our universe at the smallest possible scale in the hopes of detecting exploitable effects," Roman Yampolskiy wrote, adding that quantum mechanics has plenty of weirdness to it, which would "make a lot of sense" if we saw them as glitches or possible exploits.
"Such anomalies, alone or in combinations have been exploited by clever scientists to achieve what looks like simulation hacking at least in theory and often in later experimentation (ex. modifying the past, keeping cats both dead and alive, communicating counterfactually)."
If we ever did find out that we are living in a simulation, however, it might be best to keep it quiet, in case our creators decided to turn us off.