5 ms·

> And of course N-body systems are even more unpredictable, and the whole of the Universe is an N-body system, so if correct it would mean the end of determinis

> And of course N-body systems are even more unpredictable, and the whole of the Universe is an N-body system, so if correct it would mean the end of determinism. I read this as the systems are, for all intents and purposes, practically unpredictable, not fundamentally unpredictable. Just because we can't measure beyond the plank length doesn't mean there aren't deterministic rules down there. So my take is that the universe could still be deterministic, but that can't be knowable to us since we'd have to be able to peer below the plank length. Maybe I'm reading this wrong.

nine_k 3y ago
"Fundamentally"as in "QM forbids this". If our universe were different, a simpler kind which Laplace dealt with, it could be completely predicable, as theories of 18th century stated.
- simonh 3y ago
  I think an appeal to quantum mechanics is a different argument from the one under discussion, which is based on discrete physics.
  - TheOtherHobbes 3y ago
    It's a different view of the same problem.
  - mannykannot 3y ago
    I'm not sure it is a different argument, given its dependence on the Planck length.
layer8 3y ago
Indeed. The evolution of the universal wave function as described by the Schrödinger equation is perfectly deterministic. If you believe in that as the fundamental description of reality (which also means not believing in wave-function collapse), the result is the many-worlds interpretation of QM. Similarly, hidden-variable theories are deterministic.
- TheOtherHobbes 3y ago
  If hidden variable theories exist, they're non-local. Which is a whole other philosophical problem. As for many worlds - when everything is possible, nothing is explained.
  - layer8 3y ago
    Many-worlds doesn’t mean that everything is possible, it rather means that everything that is possible becomes actual. It is arguably strongest in explanation, in the sense that it relies on the least amount of assumptions (just the Schrödinger equation).
    - BoiledCabbage 3y ago
      > It is arguably strongest in explanation, in the sense that it relies on the least amount of assumptions (just the Schrödinger equation). It really seems like Physicists day this because they want it to be true, not because it's actually true. I completely understand they don't like the idea of non-local hidden variables (I mean programmers don't love them much either) - but the idea that a non- local variable is somehow more complex than an infinitely dividing universe breaking into infinite copies and exploring all possible paths at all possible times relies on fewer assumptions or is simpler is just laughable to me. Maybe I'm just not getting it, but it really seems like a way of redefining the rules of a game until the preferred party wins. "Hey we have this compression scheme that's incredibly fast. It's only a constant time lookup." "Really? How do you do that?" "Well we store and index all possible inputs." Won't some outputs be longer due to pigeonhole principle?" "Well actually in one single encoding it would, but we store an infinite number of different encodings of all possible inputs, meaning in at least one of the encodings it's smaller and just use that one." "So you store infinite variations of infinitely sized data and as a result claim your compression scheme is simpler?" "Yes because when we go to decompress we spawn an infinite number of threads and each thread decompresses by following one of the encodings, and in that thread's view it's just a constant time lookup to store the index which is clearly smaller (so compressed) and constant time to reverse and decompress." "And what about the complexity of the infinite threads with infinite copies of infinite storage?" "Ah we don't count that, we only count the world line of the successful thread." The incredible "simplicity" of Many-Worlds.
derbOac 3y ago
Just to go back to the philosophy thread, this is one of the things that made Kripke famous (relatively speaking, philosophically speaking): a priori "knowability" or predictability is not the same as determinism. You can have a system that is perfectly deterministic, but with outcomes that are a priori unknowable, in the sense of being unpredictable. Kripke didn't use this language, but if the information required to compute the prediction becomes unattainable, either because of capacity or input requirements, you can't make the prediction. I think there's some very abstract computability theorem in computer science that's more recent that came to a similar conclusion. I think that came at it more from the sense of the amount of change in inputs that would pass during the time it took to simulate a system makes it impossible to perfectly simulate things past some point. These aren't examples in physics, but they speak to how determinism per se isn't necessarily the same thing as predictability.
codethief 3y ago
> Maybe I'm reading this wrong. No, you're reading this correctly. GP is misinterpreting the results.

nine_k 3y ago

"Fundamentally"as in "QM forbids this". If our universe were different, a simpler kind which Laplace dealt with, it could be completely predicable, as theories of 18th century stated.

simonh 3y ago
I think an appeal to quantum mechanics is a different argument from the one under discussion, which is based on discrete physics.
- TheOtherHobbes 3y ago
  It's a different view of the same problem.
- mannykannot 3y ago
  I'm not sure it is a different argument, given its dependence on the Planck length.

layer8 3y ago

Indeed. The evolution of the universal wave function as described by the Schrödinger equation is perfectly deterministic. If you believe in that as the fundamental description of reality (which also means not believing in wave-function collapse), the result is the many-worlds interpretation of QM. Similarly, hidden-variable theories are deterministic.

TheOtherHobbes 3y ago
If hidden variable theories exist, they're non-local. Which is a whole other philosophical problem. As for many worlds - when everything is possible, nothing is explained.
- layer8 3y ago
  Many-worlds doesn’t mean that everything is possible, it rather means that everything that is possible becomes actual. It is arguably strongest in explanation, in the sense that it relies on the least amount of assumptions (just the Schrödinger equation).
  - BoiledCabbage 3y ago
    > It is arguably strongest in explanation, in the sense that it relies on the least amount of assumptions (just the Schrödinger equation). It really seems like Physicists day this because they want it to be true, not because it's actually true. I completely understand they don't like the idea of non-local hidden variables (I mean programmers don't love them much either) - but the idea that a non- local variable is somehow more complex than an infinitely dividing universe breaking into infinite copies and exploring all possible paths at all possible times relies on fewer assumptions or is simpler is just laughable to me. Maybe I'm just not getting it, but it really seems like a way of redefining the rules of a game until the preferred party wins. "Hey we have this compression scheme that's incredibly fast. It's only a constant time lookup." "Really? How do you do that?" "Well we store and index all possible inputs." Won't some outputs be longer due to pigeonhole principle?" "Well actually in one single encoding it would, but we store an infinite number of different encodings of all possible inputs, meaning in at least one of the encodings it's smaller and just use that one." "So you store infinite variations of infinitely sized data and as a result claim your compression scheme is simpler?" "Yes because when we go to decompress we spawn an infinite number of threads and each thread decompresses by following one of the encodings, and in that thread's view it's just a constant time lookup to store the index which is clearly smaller (so compressed) and constant time to reverse and decompress." "And what about the complexity of the infinite threads with infinite copies of infinite storage?" "Ah we don't count that, we only count the world line of the successful thread." The incredible "simplicity" of Many-Worlds.

derbOac 3y ago

Just to go back to the philosophy thread, this is one of the things that made Kripke famous (relatively speaking, philosophically speaking): a priori "knowability" or predictability is not the same as determinism. You can have a system that is perfectly deterministic, but with outcomes that are a priori unknowable, in the sense of being unpredictable. Kripke didn't use this language, but if the information required to compute the prediction becomes unattainable, either because of capacity or input requirements, you can't make the prediction. I think there's some very abstract computability theorem in computer science that's more recent that came to a similar conclusion. I think that came at it more from the sense of the amount of change in inputs that would pass during the time it took to simulate a system makes it impossible to perfectly simulate things past some point. These aren't examples in physics, but they speak to how determinism per se isn't necessarily the same thing as predictability.

codethief 3y ago

> Maybe I'm reading this wrong. No, you're reading this correctly. GP is misinterpreting the results.