>I remember reading once that the limitation on speed of light could be a performance optimisation.

>Things in superpositions only becoming "set" after observation... that is just lazy evaluation.

I've always thought about it this way also, I'd love to get a source on where you read it so I can see a different take than my own.

Some other things to consider:

1. Planck length is the most granular unit or pixel the computer can measure.

2. The time it takes to move one Planck length at the speed of light is the time of one iteration of the simulation's "main loop".

3. The reason time dilates as you approach the speed of light is because the faster a particle moves, the more the main loop must access that particle, and the more the particle's state may be in a "being processed" lock where it can't be mutated by anything else.

Just some thing I've always thought about when trying to see if I could use code patterns and processes to quantify the behavior of the universe. Of course, I wouldn't say I believe these things as fact or anything, just awesome to think about.

> The reason time dilates as you approach the speed of light is because the faster a particle moves, the more the main loop must access that particle, and the more the particle's state may be in a "being processed" lock where it can't be mutated by anything else.

There's a simpler reason, I think. Any mechanism or "clock" must have moving parts of sorts (atoms, exchange particles like photons, etc.) which means that when functioning or "ticking", some parts move faster than some other parts. But if the mechanism is moving at the maximum speed, it can only be the case that some parts move at c, and others move slower, but these slower parts can never catch up, so the clock disintegrates. It can only remain whole if all parts move at maximum speed, which means it is static: it is frozen in time.

I imagine that given a speed distribution for all components of the mechanism, there would be a smooth time dilation effect as the speed of the whole mechanism increases. I'd have to calculate. In any case, I think time dilation is basically necessary in a system with a maximum speed. I'd expect to see something similar in cellular automata, regarding complex objects that can move at various speeds.

I feel these limitations are pretty central to the way our universe works, and that without them the universe would be very deeply different. You can only "simplify" the world so much before it becomes something else entirely, or loses its interesting properties. For instance, if you use fluid dynamics to simplify the simulation of the ocean, you'll save a lot of computation and it'll look the same, but life won't evolve in it.

> Isolating each "light cone" of local spacetime allows the simulation to be distributed and run on separate compute clusters, because it limits things to local interactions rather than universal ones.

I don't know if that would make a difference. If you split the universe into zones and assign a cluster to each, they still have to interact with each other all the time because of the boundary. Besides, in a universe where interactions can be "universal", what's the performance gain of local algorithms?

> Things in superpositions only becoming "set" after observation... that is just lazy evaluation.

Really, though, what collapses a quantum state is interaction in general. It happens everywhere, all the time, so I don't know if there's actually anything to gain with this "lazy" evaluation.

>Things in superpositions only becoming "set" after observation... that is just lazy evaluation. It's fun to think about.

Lazy evaluation is generally used to defer some expensive computation until later. In this case, though, the pre-measurement state (a superposition of states) is MORE complicated (computationally speaking) than the post-measurement state, which is nominally a single definite state. It's not a good analogy I'm afraid.

How, though? Assume a particle has some Flatten() method that is called only when it interacts with another particle. Until this is called, certain data points (let's say, anything not related to it's movement through space) could be either undefined or out of date.

When this function is called, it causes some algorithm to set up these data points. You can extend this to treat entanglement as copy-on-write style branching, where entangled particles A and B have some subset of their properties stored in the same physical memory, but calling this methods causes writing to those properties, and, thus, when the function is called for particle A, a copy of the properties is made for both A and B, and the evaluation is done for both.

You're assuming that movement properties are special, and that non-movement properties don't affect the environment until they are collapsed. This is not the case -- quantum mechanics treats all quantum properties in the same way. An indeterminate spin can be detected in the same way an indeterminate position can be detected (i.e. indirectly, via diffraction and interference effects). It is true that if particle A and B are entangled so that spin A + spin B = 0, then that may help you optimize your "universe computer", but it doesn't gain you much since any gains will be offset by needing to store the fact that A and B are entangled. Not just that, but you have to take into account the fact that any number of particles can be entangled, with any arbitrary linear combination of properties.

IMO the biggest reason to think the universe isn't a computer is just how darn non-computable quantum mechanics is. The complexity grows at an absurd rate. Even for just the position of two particles you have a 6-dimensional wavefunction with (as far as we know) an infinity of possible values for every (infinitesimal) point in that 6-dimensional state space.

>the biggest reason to think the universe isn't a computer is just how darn non-computable quantum mechanics is

If an intelligence is simulating a universe, they aren't exactly going to be doing it von-neumann style.

That's a good point, but the fact that all Turing machines are equivalent (and the fact that the equivalence is purely mathematical, and does not rely on any laws physics) implies that computers in ANY universe would be similar to ours. At least in terms of what it can compute. Even if the laws of the parent universe made their computers dramatically faster than ours, you would still expect that they would choose our universe's laws of physics so that they were relatively easy to compute, so as to save CPU time. That doesn't seem to be the case -- it takes us a supercomputer to accurately simulate a volume a fraction of the size of a proton. Of course this doesn't prove anything -- it's possible that computers in the parent universe are so ridiculously powerful that even simulating a trillion galaxies is just a drop in the bucket -- but IMO it is suggestive.