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Read this in the past and there's a few things that bother me * It's impossible to simulate a universe of our current resolution, because it would take more ma

by achille 13y ago

Read this in the past and there's a few things that bother me * It's impossible to simulate a universe of our current resolution, because it would take more matter than the original universe. * You can't just simulate 'observable areas'. Everything needs to be simulated. * An infinite loop does not end, even in an infinitely powerful computer * 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, See: * [1] Physical Limits of Computation: http://arxiv.org/pdf/quant-ph/9908043.pdf http://arxiv.org/pdf/quant-ph/9908043.pdf * [2] https://blogs.oracle.com/dcb/entry/zfs_boils_the_ocean_consumes https://blogs.oracle.com/dcb/entry/zfs_boils_the_ocean_consu...

yohanatan 13y ago
> * 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'?
- emil0r 13y ago
  > * 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. """
  - ars 13y ago
    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.
- jhardcastle 13y ago
  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.
  - chr1 13y ago
    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.
    - rcxdude 13y ago
      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?).
- simias 13y ago
  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.
  - 3pt14159 13y ago
    Take a look at Hash Life, it changed my perspective on this specific point.
    - simias 13y ago
      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.
      - 3pt14159 13y ago
        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.
Schiphol 13y ago
> An infinite loop does not end, even in an infinitely powerful computer If the infinitely powerful computer is an accelerating Turing machine[1], wouldn't it end in finite time? [1] Copeland, B. J. (2002). Accelerating turing machines. Minds and Machines, 12(2), 281–300.
cuttle 13y ago
Nothing about the actual computing mechanism introduced in the story is realistic, and that's on purpose. The point of the story is to investigate what would happen if such a thing were possible.
- pbhjpbhj 13y ago
  Isn't it basically a reductio argument for why it's not a possible scenario - there are no [discernible?] infinites in reality (or maybe there are no infinites because it's a simulation and simulating infinites is impossible ;0)> )
qntm 13y ago
> An infinite loop does not end, even in an infinitely powerful computer Sure it does! Run one processor instruction, then run the next one in half the time, then the next one in half as much time again...
- shrikant 13y ago
  See also: https://en.wikipedia.org/wiki/Zeno%27s_paradoxes https://en.wikipedia.org/wiki/Zeno%27s_paradoxes
- dTal 13y ago
  On termination, is the instruction counter odd or even?
  - bulte-rs 13y ago
    both, untill you observe it ;-)
  - gwern 13y ago
    Better ask the folks researching https://en.wikipedia.org/wiki/Hypercomputation https://en.wikipedia.org/wiki/Hypercomputation
bermanoid 13y ago
A closed timelike loop under quantum mechanics is essentially an automatic fixed-point solver, in the same way that a stable quantum state in space is a coherent superposition. So yes, the story is kind of off, but in spirit it's also kind of dead on.
wreegab 13y ago
> You can't just simulate 'observable areas'. Everything needs to be simulated. I can't make sense of this. Whatever is outside the causality sphere is by definition irrelevant.
- chiph 13y ago
  At what point do objects & events outside the causality sphere stop influencing things inside it?
  - deleted 13y ago
    [deleted]
  - delluminatus 13y ago
    Immediately, by definition. By causality sphere he means light cone [0]. If you want to simulate an entity, you only need to simulate the entity and the contents of its light cone. Anything outside the cone could not have a causal impact on the entity without FTL travel. Of course, an entity's light cone is still likely to be quite large, especially if your simulated universe is old. [0]: https://en.wikipedia.org/wiki/Light_cone https://en.wikipedia.org/wiki/Light_cone
    - pdonis 13y ago
      an entity's light cone is still likely to be quite large, especially if your simulated universe is old. And the light cone (more precisely, the past light cone) keeps getting larger, so in fact the "amount of universe" that needs to be simulated increases without bound as time goes on.
      - delluminatus 13y ago
        Some back-of-the-envelope calculation leads me to believe that for a given location in space and time, its past light cone contains 1/8th of the spacetime between the beginning of the universe and the location.
jerf 13y ago
"It's impossible to simulate a universe of our current resolution, because it would take more matter than the original universe." The story is quite clear on this: "a countable infinity of TEEO 9.9.1 ultra-medium-density selectably-foaming non-elasticised quantum waveform frequency rate range collapse selectors and the single tormented tau neutrino caught in the middle of it all" That is, the entire simulation is being run on a single tormented tachyon. As for this not being possible in the real world, well, sure. In the real world all evidence points towards there being limits on the computational capacity of the real universe. In their universe, by construction, they do in fact have access to countably infinite amounts of computation, at which point this is potentially possible. Physics in the story are obviously, by construction, not the same as the ones in the real world. This is, shall we say, a well-established literary move in the field of science fiction. Very, very, very... well established.
- jerf 13y ago
  (Ugh... I remembered it as being a tachyon, so I typed that too. A tau neutrino is of course not a tachyon.)
aufreak3 13y ago
Not to mention that when such an accurate simulation becomes possible, special relativity gets violated - you can know whats happening anywhere before a signal can travel from there to you.

by achille 13y ago

yohanatan 13y ago

> * 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'?

emil0r 13y ago
> * 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. """
- ars 13y ago
  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.
jhardcastle 13y ago
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.
- chr1 13y ago
  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.
  - rcxdude 13y ago
    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?).
simias 13y ago
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.
- 3pt14159 13y ago
  Take a look at Hash Life, it changed my perspective on this specific point.
  - simias 13y ago
    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.
    - 3pt14159 13y ago
      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.

Schiphol 13y ago

> An infinite loop does not end, even in an infinitely powerful computer If the infinitely powerful computer is an accelerating Turing machine[1], wouldn't it end in finite time? [1] Copeland, B. J. (2002). Accelerating turing machines. Minds and Machines, 12(2), 281–300.

cuttle 13y ago

Nothing about the actual computing mechanism introduced in the story is realistic, and that's on purpose. The point of the story is to investigate what would happen if such a thing were possible.

pbhjpbhj 13y ago
Isn't it basically a reductio argument for why it's not a possible scenario - there are no [discernible?] infinites in reality (or maybe there are no infinites because it's a simulation and simulating infinites is impossible ;0)> )

qntm 13y ago

> An infinite loop does not end, even in an infinitely powerful computer Sure it does! Run one processor instruction, then run the next one in half the time, then the next one in half as much time again...

shrikant 13y ago
See also: https://en.wikipedia.org/wiki/Zeno%27s_paradoxes https://en.wikipedia.org/wiki/Zeno%27s_paradoxes
dTal 13y ago
On termination, is the instruction counter odd or even?
- bulte-rs 13y ago
  both, untill you observe it ;-)
- gwern 13y ago
  Better ask the folks researching https://en.wikipedia.org/wiki/Hypercomputation https://en.wikipedia.org/wiki/Hypercomputation

bermanoid 13y ago

A closed timelike loop under quantum mechanics is essentially an automatic fixed-point solver, in the same way that a stable quantum state in space is a coherent superposition. So yes, the story is kind of off, but in spirit it's also kind of dead on.

wreegab 13y ago

> You can't just simulate 'observable areas'. Everything needs to be simulated. I can't make sense of this. Whatever is outside the causality sphere is by definition irrelevant.

chiph 13y ago
At what point do objects & events outside the causality sphere stop influencing things inside it?
- deleted 13y ago
  [deleted]
- delluminatus 13y ago
  Immediately, by definition. By causality sphere he means light cone [0]. If you want to simulate an entity, you only need to simulate the entity and the contents of its light cone. Anything outside the cone could not have a causal impact on the entity without FTL travel. Of course, an entity's light cone is still likely to be quite large, especially if your simulated universe is old. [0]: https://en.wikipedia.org/wiki/Light_cone https://en.wikipedia.org/wiki/Light_cone
  - pdonis 13y ago
    an entity's light cone is still likely to be quite large, especially if your simulated universe is old. And the light cone (more precisely, the past light cone) keeps getting larger, so in fact the "amount of universe" that needs to be simulated increases without bound as time goes on.
    - delluminatus 13y ago
      Some back-of-the-envelope calculation leads me to believe that for a given location in space and time, its past light cone contains 1/8th of the spacetime between the beginning of the universe and the location.

jerf 13y ago

"It's impossible to simulate a universe of our current resolution, because it would take more matter than the original universe." The story is quite clear on this: "a countable infinity of TEEO 9.9.1 ultra-medium-density selectably-foaming non-elasticised quantum waveform frequency rate range collapse selectors and the single tormented tau neutrino caught in the middle of it all" That is, the entire simulation is being run on a single tormented tachyon. As for this not being possible in the real world, well, sure. In the real world all evidence points towards there being limits on the computational capacity of the real universe. In their universe, by construction, they do in fact have access to countably infinite amounts of computation, at which point this is potentially possible. Physics in the story are obviously, by construction, not the same as the ones in the real world. This is, shall we say, a well-established literary move in the field of science fiction. Very, very, very... well established.

jerf 13y ago
(Ugh... I remembered it as being a tachyon, so I typed that too. A tau neutrino is of course not a tachyon.)

aufreak3 13y ago

Not to mention that when such an accurate simulation becomes possible, special relativity gets violated - you can know whats happening anywhere before a signal can travel from there to you.