[simonh]

My main problem with the simulated world argument is complexity. Take the Billiard Ball example[0]. This means to accurately simulate the universe you can't really get away with approximations. Under close enough scrutiny discrepancies in the simulation are discernible, and we can scrutinize it at the subatomic level. But to simulate the observable universe, how big would your computer need to be? How slowly would the simulation run relative to the simulator's real-time? It just doesn't stack up.

The only way to do it would be to fake it by generating the appearance of a thorough simulation rather than the reality of one. In which case the arguments put forward for wanting to perform a real simulation - to simulate history and so forth - break down because you'd only be emulating the appearance of it not simulating it.

The only way out of this I can see is if the universe containing the simulator were vastly more complex than ours, such that in comparison our universe would be trivial to simulate. But then why would they do it? Our universe would be nothing like theirs. In principle this is possible, but it massively reduces the chances that our world is a simulation because only a subset, and quite possibly a vanishingly small subset, of possible universes would be capable of hosting the simulation. Possibly fewer universes that there are universes like ours. At which point the odds of ours being a simulation collapse.

[0] http://www.anecdote.com/2007/10/the-billiard-ball-example/

[metafunctor]

Here's a thought that ends up with exactly the opposite view...

Does someone really need to build a computer that carries out the simulation for a universe to be “real”? If there is a set of rules defining a universe, one can say that the universe already exists without having to simulate it.

The same goes for universes that are capable of life that can simulate other universes nested inside them. And, indeed, universes nested three times, four times, all the way to infinity.

There number of nested universes is a much larger infinity that the number of non-nested "root" level universes. Thus, picking a universe at random (ours), the probability that it is a simulation is 1.

[simonh]

>Does someone really need to build a computer that carries out the simulation for a universe to be “real”? If there is a set of rules defining a universe, one can say that the universe already exists without having to simulate it.

Marvin Minsky made this argument and it's a compelling one. But rather than meaning all universes are simulated, it means it doesn't matter whether they are or not because for all possible universes they will exist as root universes and as simulations within more complex universes and there's no meaningful distinction between those. It's not the argument I'm making though.

[AnimalMuppet]

I absolutely agree. But... the quantum mechanical "it doesn't really have a state until you observe it" could make sense as the simulation optimizing by skipping the parts that nobody can see.

[simonh]

Except if that were the case the quantum system could have a determined state, it just doesn't. But the Kochen-Specker theorem[0] proves that it cannot have a determined state. Quite possibly in any universe, though I'm not sure if that's been proven yet.

[0] https://en.wikipedia.org/wiki/Kochen%E2%80%93Specker_theorem

[XorNot]

We already run simulators of our universe with vastly lower resolution then reality, and they do tell us useful things about it.

[goatlover]

We also make maps of the Earth. They tell us useful things, like how to get from point A to point B. But nobody mistakes a map of the subways in New York for riding a subway.

[AnimalMuppet]

Sure, but that isn't simonh's point. Those simulations aren't very good. There's nobody living in those simulations that thinks that they're alive, let alone that is capable of creating a working computer out of the material in that simulation.

[simonh]

Right, and the Billiard Ball Example shows what you need to do to even just simulate a game of billiards. To do so accurately you have to simulate every elementary particle in the observable universe. That's how complex and interconnected the universe is.

[XorNot]

Arguing that present technology is insufficient doesn't prove that all future technology would be insufficient. The trajectory of improvement rather suggests the opposite.

[simonh]

It's not a matter of technology, it's a matter of complexity. By definition a computer capable of simulating every elementary particle in the universe would have to be many orders of magnitude more complex (in crude terms 'bigger') than the universe itself, even assuming ideal technology with mathematically perfect efficiency.

[sa_su_ke_75]

and we create the simulations with the rules of the "real world" that we know. So we can study also the interactions of the rules that we know. But not what we don't know of out universe.