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It wasn't the first program I wrote, or even a program I wrote, but in middle school a friend wrote a 3-body integrator in BASIC (sun, earth, moon). That singl

by dekhn 3y ago

It wasn't the first program I wrote, or even a program I wrote, but in middle school a friend wrote a 3-body integrator in BASIC (sun, earth, moon). That single 20 line program shaped my entire world view for a long time (decades), implying to me that we could, if we had powerful enough computers, simulate all sorts of things... even entire universes (which was also an idea that I explored with cellular automata). It's not a particularly helpful worldview and can often be harmful if you're working with complex systems, or systems that require more than o(n(log(n)) per step, or any number of other real-world problems scientists face. Many years later I was impressed at how well astronomy packages work (IE, "it's time T at local L, what is the angle of the sun in the sky?") and stumbled across this paper by Sussman: https://web.mit.edu/wisdom/www/ss-chaos.pdf https://web.mit.edu/wisdom/www/ss-chaos.pdf which shows some pretty serious work on future prediction of solar system objects.

forgotpwd16 3y ago
>simulate all sorts of things... even entire universes You also assumed that chaos is a measurement problem. You could simulate entire universe if you knew the initial conditions sufficiently enough. There were two nice recent papers[1][2] that showed in order to predict some orbits you'll need an accuracy less of Planck length or else some systems are fundamentally unpredictable. [1]: https://arxiv.org/abs/2002.04029 https://arxiv.org/abs/2002.04029 [2]: https://arxiv.org/abs/2311.07651 https://arxiv.org/abs/2311.07651
- dekhn 3y ago
  I'd love to see convincing evidence that we could simulate the universe using only standard physical laws. IIUC we don't have a way to do that or reliably say whether it's possible. It's also not that interesting a problem because it's so impractical.
  - actionfromafar 3y ago
    But it would be interesting to see if we could simulate a universe and observe if it in any way resembled ours. Even if the simulated universe was much "smaller". (It would of course have to be.)
    - dekhn 3y ago
      I think you'd need to be a few rungs up the Kardashev scale to even contemplate this.
      - actionfromafar 3y ago
        That may be ... plus we don't really know what to make of the observations around us, with dark matter and stuff.
bufferoverflow 3y ago
> if we had powerful enough computers, simulate all sorts of things In reality, we're having a hard time precisely simulating even two atoms interacting, with all the quantum effects, diffraction, gravity (however minuscule), etc. Our universe is surprisingly detailed. 64-bit floats aren't even close enough to precisely simulate real world. What's the precision of the mass of an electron? What's the precision of its coordinates or motion vectors? Maybe plank length for coordinates, maybe not. What about acceleration? Can it be arbitrarily low? An electron's gravitational field from a billion light years away should theoretically affect us (in time).
- xvector 3y ago
  The assumption in your comment is that any of this is real to begin with and logic isn't being short-circuited in our brains to make everything "check out" even if it doesn't. If you simulate a universe with cube blocks from Minecraft, it doesn't matter as long as your users think the simulation is real. And since you are simulating their consciousness, you can easily short circuit the train of thought that would cause doubt, or that would attempt logic, etc., so they truly believe their Minecraft cube world is incomprehensibly detailed down to the atoms and galaxies in the sky. They'd happily go on the whiteboard, and prove their theories with math like 2+2=5 and everyone would agree because they literally couldn't disagree - they would feel in their hearts and minds that this is perfectly correct. There's nothing to say that's not happening now. In fact, this is how I see most advanced civilizations performing simulations. The compute savings would be immense if you could just alter user consciousness as opposed to simulating an actual universe.
  - genrilz 3y ago
    I always find skepticism like this to be really interesting, since in the end we could always be getting fooled by the Deus Deceptor or something. That being said, let me take a stab at being anti-skeptical for the fun of it. I work around people who do "Computational Chemistry", which is basically running quantum physics calculations. These tend to be done in order to either understand the properties of materials, or to understand the reasons why reactions happen. The results are more advanced materials and better performing reactions. An early and famous example of such technology is the laser. A more typical modern example would be searching for Zeolite catalysts which have particular properties, or trying to create surface coatings which protect implants from being eaten by the immune system, or on which ice cannot freeze. Basically, I believe the advanced calculations to be correct because they lead to things which are (eventually) used in daily life.

by dekhn 3y ago

forgotpwd16 3y ago

>simulate all sorts of things... even entire universes You also assumed that chaos is a measurement problem. You could simulate entire universe if you knew the initial conditions sufficiently enough. There were two nice recent papers[1][2] that showed in order to predict some orbits you'll need an accuracy less of Planck length or else some systems are fundamentally unpredictable. [1]: https://arxiv.org/abs/2002.04029 https://arxiv.org/abs/2002.04029 [2]: https://arxiv.org/abs/2311.07651 https://arxiv.org/abs/2311.07651

dekhn 3y ago
I'd love to see convincing evidence that we could simulate the universe using only standard physical laws. IIUC we don't have a way to do that or reliably say whether it's possible. It's also not that interesting a problem because it's so impractical.
- actionfromafar 3y ago
  But it would be interesting to see if we could simulate a universe and observe if it in any way resembled ours. Even if the simulated universe was much "smaller". (It would of course have to be.)
  - dekhn 3y ago
    I think you'd need to be a few rungs up the Kardashev scale to even contemplate this.
    - actionfromafar 3y ago
      That may be ... plus we don't really know what to make of the observations around us, with dark matter and stuff.

bufferoverflow 3y ago

> if we had powerful enough computers, simulate all sorts of things In reality, we're having a hard time precisely simulating even two atoms interacting, with all the quantum effects, diffraction, gravity (however minuscule), etc. Our universe is surprisingly detailed. 64-bit floats aren't even close enough to precisely simulate real world. What's the precision of the mass of an electron? What's the precision of its coordinates or motion vectors? Maybe plank length for coordinates, maybe not. What about acceleration? Can it be arbitrarily low? An electron's gravitational field from a billion light years away should theoretically affect us (in time).

xvector 3y ago
The assumption in your comment is that any of this is real to begin with and logic isn't being short-circuited in our brains to make everything "check out" even if it doesn't. If you simulate a universe with cube blocks from Minecraft, it doesn't matter as long as your users think the simulation is real. And since you are simulating their consciousness, you can easily short circuit the train of thought that would cause doubt, or that would attempt logic, etc., so they truly believe their Minecraft cube world is incomprehensibly detailed down to the atoms and galaxies in the sky. They'd happily go on the whiteboard, and prove their theories with math like 2+2=5 and everyone would agree because they literally couldn't disagree - they would feel in their hearts and minds that this is perfectly correct. There's nothing to say that's not happening now. In fact, this is how I see most advanced civilizations performing simulations. The compute savings would be immense if you could just alter user consciousness as opposed to simulating an actual universe.
- genrilz 3y ago
  I always find skepticism like this to be really interesting, since in the end we could always be getting fooled by the Deus Deceptor or something. That being said, let me take a stab at being anti-skeptical for the fun of it. I work around people who do "Computational Chemistry", which is basically running quantum physics calculations. These tend to be done in order to either understand the properties of materials, or to understand the reasons why reactions happen. The results are more advanced materials and better performing reactions. An early and famous example of such technology is the laser. A more typical modern example would be searching for Zeolite catalysts which have particular properties, or trying to create surface coatings which protect implants from being eaten by the immune system, or on which ice cannot freeze. Basically, I believe the advanced calculations to be correct because they lead to things which are (eventually) used in daily life.