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Comment by yohanatan | Hacker News Reader

> * You can't just simulate 'observable areas'. Everything needs to be simulated.

Is that an assertion? Do you have something to back up that claim? [I think if this is not backed up then your first point falls as well].

> * An infinite loop does not end, even in an infinitely powerful computer

What do infinite loops have to do with anything? Presumably the creators of the simulation would be skillful enough to avoid them.

> * A fun calculation from the ZFS folks: To fully populate a 128bit filesystem (ie permute all combinations) you a lot of energy. So much energy you could boil the world's oceans,

Once again, who said anything has to be 'fully populated'?

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I think the assertion that you have to simulate the whole thing is somewhat self-obvious, and I'm not physicist, but I'll try.

Take the "window" of the Earth. Without simulating the rest of the solar system, there is no way to account for the gravity effects of the larger planets, the energy from the sun, the occasional impacts from asteroids, the high-profile fly-bys of comets like Halley's, etc. Without simulating the rest of the universe, what would the simulated-astronomers on your simulated-Earth be looking at when they peer into their telescopes? What happens when you fast-forward far enough into the "future" that the Milky Way merges with Andromeda?

You can't simply hand-wave these things away by saying, "well, we would ray-trace the things that are observable" because you have no way of knowing what is observable without simulating the whole rest of the universe too. It's either simulate the whole thing, or your simulation is very limited and inaccurate.

If you are mainly interested, in what happens with people, accuracy of the rest of simulation is not important, you only need to simulate the parts people see with the resolution they can see, you even can have lots of inconsistencies as long as they are not reproducible.

the problem is the world is chaotic: any approximation, no matter how small, will cause divergence very quickly. (This is actually the main problem with the simulation: how did they get the exact initial conditions and physical constants?).

I think because of the laws of causality you can't just simulate a tiny bit of the universe and expect to get accurate results because everything has a sphere of influence expanding at the speed of light.

For instance astronomers observe phenomenons that occurred in galaxies far far away. If in the simulated universe you just bound yourself to simulate, say, the solar system then such events wouldn't occur exactly in the same way. We wouldn't find the same bodies at the same position. That in turn would make the path of the simulation diverge significantly from our reality.

Think for instance what would happen if the constellations weren't the same in the sky. All astrology would be different. It might seem like a minor change but in the course of centuries that would probably amount to a big change.

That being said since the story postulates that the computer has infinite processing power and storage you can just leave out this bit and the story still makes sense, you just assume that it simulates the entire universe at all time.

Take a look at Hash Life, it changed my perspective on this specific point.

Hash Life still simulates everything, it just optimizes for similar patterns. While it does seem a reasonable optimization for simulating an entire universe (although why would you bother if you have unlimited processing power?) I don't think it's a good parallel to what the parent was suggesting.

In Hashlife you still need to have your entire universe in the hash tree, you can't take a bit of the pattern, only simulate for X generations without considering outside influence and expect to get the same results as a full simulation.

Right, so take it a bit further. You don't have to simulate everything that there isn't an observer on since you can create a temporal boundary between two areas.

So, translating that into physics:

1. You know you are only going to simulate for the next, say, day. 2. Take all the photons, xrays, etc. heading towards earth and continue to simulate them, but freeze everything outside of a reasonable distance (in hash life, this isn't arbitrary, it is perfectly defined). 3. Continue simulation for that sub area only.

It is not known with certainty that very small actions across very large distances have any effect at all. It's possible that, just like you're literally seeing a past version of a galaxy, you're also literally seeing a less precise version of the galaxy.

But how can you decide what's going to have an influence and what will be without effect if you don't simulate everything?

If tomorrow we receive a transmission from a form of life that lived thousands of years ago thousands of light years away from us it's going to change a lot of things. You can't ignore that.

You can only simulate a small part of the universe if you can simulate correctly what happens at the boundaries of the region. Otherwise it "contaminates" the simulation inwards at the speed of light. if you want to simulate what will happen on earth for the next year you at least need to know the full state of the universe in a sphere one light year in radius, or at least that's my understanding.

I think these concepts somewhat overlap with the "Holographic principle" although I might be mistaken, I'm way out of my depth: https://en.wikipedia.org/wiki/Holographic_principle

That's why us being in a simulation is one solution to fermi's paradox.

This is merely a question of fidelity. One can imagine a simulator which produces low-fidelity everything and only increases fidelity gradually in isolated regions as the human observers peer at those regions. And, there is nothing to prevent results being continuously retroactively computed (just in time) as new 'discoveries' are made.

> * An infinite loop does not end, even in an infinitely powerful computer

What do infinite loops have to do with anything? Presumably the creators of the simulation would be skillful enough to avoid them.

That would be this:

"""

But it was still pretty exciting stuff. Holy Zarquon, they said to one another, an infinitely powerful computer? It was like a thousand Christmases rolled into one. Program going to loop forever? You knew for a fact: this thing could execute an infinite loop in less than ten seconds. Brute force primality testing of every single integer in existence? Easy. Pi to the last digit? Piece of cake. Halting Problem? Sa-holved.

"""

That's a mathematical joke/commentary on the nature of infinity and how people think it's just a really really large number.

At least I hope that's what it is.

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