4 ms·

The laws of physics are computable; why would a sufficiently large computer not be capable of simulating a universe?

rendall 3y ago
You make an unwarranted logical leap, there. Some laws of physics are computable given sufficient simplifying constraints. It does not follow that a large enough computer therefore can simulate the Universe.
- vidarh 3y ago
  You're making the assumption that our universe 1) is the universe, 2) is as computationally complex as the universe, 3) that all, or any, of our universe exists outside of simulated minds, or a simulated mind, 4) that time passes and that we exist as more than a single moment with memories of a non-existent past. In other words, we have no basis for saying our universe exists, that we have physical existence, that anyone but ourselves exist, or that we exists in time.
  - rendall 3y ago
    > In other words, we have no basis for saying our universe exists, that we have physical existence, that anyone but ourselves exist, or that we exists in time. Sure, I will grant that. This does not warrant a leap from we don't actually know what the Universe is to therefore definitely simulation.
    - vidarh 3y ago
      No, the more convincing argument for that is the prevalence of computation in the universe. Given the extreme prevalence of computation, in a universe large enough it'd be surprising if there weren't simulations. In an infinite universe, it'd be surprising if there weren't infinite simulations of universes just like ours. There only need to be two for it to be more likely we're in a simulation. There are lots of unknowns there we can't quantify very well, but unless the universe is firmly finite in both time and space, I find it increasingly difficult to see a way around simulation in some form or other. Especially because the threshold for us to find ourselves in an apparent simulated universe might well be the barest sliver of simulation: One mind, a single moment. Even assuming we one day try to run a large simulation, I'd expect we before then will have run huge number of simulations of minds for brief slivers of time to try to figure out if we get it right. It seems likely that if we're in a simulation, we're even more likely to be in a brief, constrained simulation rather than a full blown universe simulation. That of course doesn't mean we definitely are - like the Fermi paradox it's more question of "why don't we have a better answer?"
      - rendall 3y ago
        > In an infinite universe, it'd be surprising if there weren't infinite simulations of universes just like ours. I disagree. Physical laws hold throughout the Universe - or, that is the consensus null hypothesis anyway. In even an infinite Universe, assuming it is infinite, there are limits to that which is possible, and it is not yet demonstrated to my satisfaction that a computer could even theoretically simulate a Universe. That's leaving aside my fundamental objection that the space of what the Universe can possibly be is infinitely vast compared to the singular assertion that it's a simulation. In other words, that this is the correct view is exceedingly unlikely simply because it has been thought of. Imagine that I generate a random complex number of arbitrary precision and ask you to guess it. It could be `0.46451453499412721134961376996+5.275644069844024487293819737719i` for example. It's my view that you have a far, far higher chance of guessing that complex number than we humans do of coming up with a simple, correct cosmology. > the prevalence of computation in the universe I also disagree that it's certain that computation or even mathematics are universal or fundamental. I suspect that these both are based in a very human outlook and perspective. Their apparent universality is an illusion deriving from our inability to see outside of our human perspective. > In other words, we have no basis for saying our universe exists, that we have physical existence, that anyone but ourselves exist, or that we exists in time While true, I'm not sure what this has to do with the simulation hypothesis. Really, given this premise, the Universe could be anything at all.
breuleux 3y ago
I suppose it could in theory, but it probably wouldn't do so very well. In general, you can't simulate physics on physics faster than physics, so non-approximate simulation of the universe would probably run several orders of magnitude slower than the universe does. You would have to approximate massively, but that means a lot of things that are physically possible in the universe would become impossible in the simulation, especially systems that require a lot of precision and optimal behavior such as computers. Honestly, even if you simplified everything to the max except for conscious agents in the simulation, it's not even clear to me that we could simulate human brains faster than actual human brains.
- vidarh 3y ago
  > In general, you can't simulate physics on physics faster than physics You assume that the physics of the simulation are equivalent to the physics outside it. We simulate simpler environments than our own all the time. Maybe the constraints of our universe are the way they are because it reflects the tradeoffs required to be able to simulate it given the conditions the simulation runs in.
  - breuleux 3y ago
    Maybe, but that's not interesting. It is possible that the physics inside are different from the physics outside, but that tells us absolutely nothing of value about whether we are in a simulation or not. If we do assume that an observer cannot differentiate the simulation from the outside world, then we can make an interesting argument: if the rules of physics in some universe entails that this universe will simulate many other universes with the same apparent rules, then an observer of these rules is probably simulated regardless of what the root universe happens to be. This is the only way you can ground the probability calculation. A lot of things are possible. Not many things are probable.
    - vidarh 3y ago
      For an observer to be unable to differentiate the simulation from the outside world if put in the simulation is a very low bar, and the one that matters. It's a low bar because you don't need to presume the simulated entity gets the benefit of the passage of time. For it to be likely for us to be in a simulation, it's sufficient for it to be likely for the total runtime of "simulated us" to be higher than "real us", and our only knowledge of the passage of time are the momentary memories and sensory inputs right now, and we can't validate their truthfulness. In other words: you don't need to simulate any physics to prevent a mind from differentiating. You just need to limit the scope of your simulation to a sufficiently narrow slice of time. Personally, I tend to think that if simulations become widespread, test runs, debugging and development, research runs, or application specific runs of small slices are likely to far outpace full scale simulations in runtime, and so if simulations are probable, I'd be inclined to think we're most likely in a (very) short-lived, scope-limited simulation in a horrible groundhog day like setting.

You make an unwarranted logical leap, there. Some laws of physics are computable given sufficient simplifying constraints. It does not follow that a large enough computer therefore can simulate the Universe.

vidarh 3y ago
You're making the assumption that our universe 1) is the universe, 2) is as computationally complex as the universe, 3) that all, or any, of our universe exists outside of simulated minds, or a simulated mind, 4) that time passes and that we exist as more than a single moment with memories of a non-existent past. In other words, we have no basis for saying our universe exists, that we have physical existence, that anyone but ourselves exist, or that we exists in time.
- rendall 3y ago
  > In other words, we have no basis for saying our universe exists, that we have physical existence, that anyone but ourselves exist, or that we exists in time. Sure, I will grant that. This does not warrant a leap from we don't actually know what the Universe is to therefore definitely simulation.
  - vidarh 3y ago
    No, the more convincing argument for that is the prevalence of computation in the universe. Given the extreme prevalence of computation, in a universe large enough it'd be surprising if there weren't simulations. In an infinite universe, it'd be surprising if there weren't infinite simulations of universes just like ours. There only need to be two for it to be more likely we're in a simulation. There are lots of unknowns there we can't quantify very well, but unless the universe is firmly finite in both time and space, I find it increasingly difficult to see a way around simulation in some form or other. Especially because the threshold for us to find ourselves in an apparent simulated universe might well be the barest sliver of simulation: One mind, a single moment. Even assuming we one day try to run a large simulation, I'd expect we before then will have run huge number of simulations of minds for brief slivers of time to try to figure out if we get it right. It seems likely that if we're in a simulation, we're even more likely to be in a brief, constrained simulation rather than a full blown universe simulation. That of course doesn't mean we definitely are - like the Fermi paradox it's more question of "why don't we have a better answer?"
    - rendall 3y ago
      > In an infinite universe, it'd be surprising if there weren't infinite simulations of universes just like ours. I disagree. Physical laws hold throughout the Universe - or, that is the consensus null hypothesis anyway. In even an infinite Universe, assuming it is infinite, there are limits to that which is possible, and it is not yet demonstrated to my satisfaction that a computer could even theoretically simulate a Universe. That's leaving aside my fundamental objection that the space of what the Universe can possibly be is infinitely vast compared to the singular assertion that it's a simulation. In other words, that this is the correct view is exceedingly unlikely simply because it has been thought of. Imagine that I generate a random complex number of arbitrary precision and ask you to guess it. It could be `0.46451453499412721134961376996+5.275644069844024487293819737719i` for example. It's my view that you have a far, far higher chance of guessing that complex number than we humans do of coming up with a simple, correct cosmology. > the prevalence of computation in the universe I also disagree that it's certain that computation or even mathematics are universal or fundamental. I suspect that these both are based in a very human outlook and perspective. Their apparent universality is an illusion deriving from our inability to see outside of our human perspective. > In other words, we have no basis for saying our universe exists, that we have physical existence, that anyone but ourselves exist, or that we exists in time While true, I'm not sure what this has to do with the simulation hypothesis. Really, given this premise, the Universe could be anything at all.

breuleux 3y ago

I suppose it could in theory, but it probably wouldn't do so very well. In general, you can't simulate physics on physics faster than physics, so non-approximate simulation of the universe would probably run several orders of magnitude slower than the universe does. You would have to approximate massively, but that means a lot of things that are physically possible in the universe would become impossible in the simulation, especially systems that require a lot of precision and optimal behavior such as computers. Honestly, even if you simplified everything to the max except for conscious agents in the simulation, it's not even clear to me that we could simulate human brains faster than actual human brains.

vidarh 3y ago
> In general, you can't simulate physics on physics faster than physics You assume that the physics of the simulation are equivalent to the physics outside it. We simulate simpler environments than our own all the time. Maybe the constraints of our universe are the way they are because it reflects the tradeoffs required to be able to simulate it given the conditions the simulation runs in.
- breuleux 3y ago
  Maybe, but that's not interesting. It is possible that the physics inside are different from the physics outside, but that tells us absolutely nothing of value about whether we are in a simulation or not. If we do assume that an observer cannot differentiate the simulation from the outside world, then we can make an interesting argument: if the rules of physics in some universe entails that this universe will simulate many other universes with the same apparent rules, then an observer of these rules is probably simulated regardless of what the root universe happens to be. This is the only way you can ground the probability calculation. A lot of things are possible. Not many things are probable.
  - vidarh 3y ago
    For an observer to be unable to differentiate the simulation from the outside world if put in the simulation is a very low bar, and the one that matters. It's a low bar because you don't need to presume the simulated entity gets the benefit of the passage of time. For it to be likely for us to be in a simulation, it's sufficient for it to be likely for the total runtime of "simulated us" to be higher than "real us", and our only knowledge of the passage of time are the momentary memories and sensory inputs right now, and we can't validate their truthfulness. In other words: you don't need to simulate any physics to prevent a mind from differentiating. You just need to limit the scope of your simulation to a sufficiently narrow slice of time. Personally, I tend to think that if simulations become widespread, test runs, debugging and development, research runs, or application specific runs of small slices are likely to far outpace full scale simulations in runtime, and so if simulations are probable, I'd be inclined to think we're most likely in a (very) short-lived, scope-limited simulation in a horrible groundhog day like setting.