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Sorry, I don't understand this point. If a model of the universe was completely correct, wouldn't it have equivalent complexity to the universe and hence not b

Sorry, I don't understand this point. If a model of the universe was completely correct, wouldn't it have equivalent complexity to the universe and hence not be a model?

jon_richards 8y ago
No? Think of the game of life. Low model complexity, high universe complexity.
- deleted 8y ago
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- digi_owl 8y ago
  I think the term used is emergent complexity. So when you go from particle physics to chemistry, new complexities emerge that can't be explained in the realm of particle physics alone (iirc). sadly certain "sciences" still cling to the idea that they can simply aggregate the results from multiple of their "particles" and get a solution for larger systems.
  - xoa 8y ago
    >So when you go from particle physics to chemistry, new complexities emerge that can't be explained in the realm of particle physics alone (iirc). Correct me if I'm misunderstanding, but I think you're confusing general theory vs practical applicable models here maybe? Yes, fundamental principles of interaction can combine at scale to create new large scale effects, but that doesn't change the fact that they came out of fundamental principles nor that they can't be "explained" via those principles. There is no magic that pops into existence up the chain. The asymmetry of water molecules and the way their electron clouds distribute create all sorts of fascinating effects in bulk water, but they're still directly coming out of physics of course. The issue in practice however is that the level of computing necessary to accurately model reality at scale from fundamentals matches or exceeds actually doing it in reality, and for us rapidly becomes absolutely, utterly infeasible for anything but the simplest systems. "New emergent complexities" absolutely "can be explained" from a correct lower level set of principles, but that doesn't mean we can actually crunch the math at any scale we want. So we need higher level bulk models too, at many levels all the way up, which are good enough to be effective approximations to a given level of accuracy in practical computing time. The low level fundamentals often at bulk average out due to random variance in sufficient quantity and are irrelevant to whatever we care about, so there is no need to do it that expensively (even if we could). But that isn't the same thing as the fundamentals being wrong somehow or not being at the root of everything above.
    - jon_richards 8y ago
      Right. There are roughly 10^15 atoms in a speck of dust. A terabyte is 10^12 bytes. Our inability to simulate does not mean our models are wrong.
mhh__ 8y ago
No. This is false both due to the nature of mathematical modelling in physics, and the ad-hoc nature of the term "complexity" when comparing the universe and the science we use to understand (or, perhaps, only model it). To elaborate on the first point: No measurement can be perfect enough for their to be no uncertainty on it. It is therefore impossible for a model to be completely correct, although it can (obviously) agree with experiment to within current experimental uncertainties - like the standard model is now, so far.
- deleted 8y ago
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mehrdadn 8y ago
I think the answer to your question is that we're not modeling the universe itself, but the laws governing the dynamics of the universe. Hence we don't need to know all information about the universe.
- delhanty 8y ago
  So with the laws, how do we know when we're done? For example, we had Newtonian Mechanics and that looked good for a couple of centuries, but then it turned out that it was too simple and we needed to add more laws.
  - Bjartr 8y ago
    We're so far away from being done it's not an applicable question just yet, but it may not be possible to know if there's something outside our ability to comprehend or perceive, even with sophisticated measuring tools. We would have to settle for merely being able to explain every single phenomena in the observable universe at every scale.
    - graycat 8y ago
      Look up and see a recent essay by the head of the Princeton Institute of Advanced Study where, for my rough summary from rough memory, he explained that physics can have lots of models that all fit all the data the same. So, it can be tough to find the one correct model, the one book of physics, the one way the universe runs, etc. Then see a recent essay at Quanta Magazine that explains that physics has long looked to some largely esthetic concerns especially about symmetry to pick and choose among alternate theoretical explanations -- again my rough summary from my rough memory.
      - fovc 8y ago
        I found an essay by the head of the IAS, published in Quanta Magazine. Could the two essays be one and the same? https://www.quantamagazine.org/there-are-no-laws-of-physics-theres-only-the-landscape-20180604/ https://www.quantamagazine.org/there-are-no-laws-of-physics-...
Bjartr 8y ago
The model isn't as complex as the universe in that sense, but filling out that model with the exact state of the entire universe would be. Just like how a chaotic function can be very simple, but give rise to incredibly complex outputs.
eevilspock 8y ago
Only if the universe is not reducible in any way. But why would a non-reducible universe be so predictable?

Sorry, I don't understand this point. If a model of the universe was completely correct, wouldn't it have equivalent complexity to the universe and hence not be a model?

jon_richards 8y ago

No? Think of the game of life. Low model complexity, high universe complexity.

deleted 8y ago
[deleted]
digi_owl 8y ago
I think the term used is emergent complexity. So when you go from particle physics to chemistry, new complexities emerge that can't be explained in the realm of particle physics alone (iirc). sadly certain "sciences" still cling to the idea that they can simply aggregate the results from multiple of their "particles" and get a solution for larger systems.
- xoa 8y ago
  >So when you go from particle physics to chemistry, new complexities emerge that can't be explained in the realm of particle physics alone (iirc). Correct me if I'm misunderstanding, but I think you're confusing general theory vs practical applicable models here maybe? Yes, fundamental principles of interaction can combine at scale to create new large scale effects, but that doesn't change the fact that they came out of fundamental principles nor that they can't be "explained" via those principles. There is no magic that pops into existence up the chain. The asymmetry of water molecules and the way their electron clouds distribute create all sorts of fascinating effects in bulk water, but they're still directly coming out of physics of course. The issue in practice however is that the level of computing necessary to accurately model reality at scale from fundamentals matches or exceeds actually doing it in reality, and for us rapidly becomes absolutely, utterly infeasible for anything but the simplest systems. "New emergent complexities" absolutely "can be explained" from a correct lower level set of principles, but that doesn't mean we can actually crunch the math at any scale we want. So we need higher level bulk models too, at many levels all the way up, which are good enough to be effective approximations to a given level of accuracy in practical computing time. The low level fundamentals often at bulk average out due to random variance in sufficient quantity and are irrelevant to whatever we care about, so there is no need to do it that expensively (even if we could). But that isn't the same thing as the fundamentals being wrong somehow or not being at the root of everything above.
  - jon_richards 8y ago
    Right. There are roughly 10^15 atoms in a speck of dust. A terabyte is 10^12 bytes. Our inability to simulate does not mean our models are wrong.

mhh__ 8y ago

No. This is false both due to the nature of mathematical modelling in physics, and the ad-hoc nature of the term "complexity" when comparing the universe and the science we use to understand (or, perhaps, only model it). To elaborate on the first point: No measurement can be perfect enough for their to be no uncertainty on it. It is therefore impossible for a model to be completely correct, although it can (obviously) agree with experiment to within current experimental uncertainties - like the standard model is now, so far.

deleted 8y ago
[deleted]

mehrdadn 8y ago

I think the answer to your question is that we're not modeling the universe itself, but the laws governing the dynamics of the universe. Hence we don't need to know all information about the universe.

delhanty 8y ago
So with the laws, how do we know when we're done? For example, we had Newtonian Mechanics and that looked good for a couple of centuries, but then it turned out that it was too simple and we needed to add more laws.
- Bjartr 8y ago
  We're so far away from being done it's not an applicable question just yet, but it may not be possible to know if there's something outside our ability to comprehend or perceive, even with sophisticated measuring tools. We would have to settle for merely being able to explain every single phenomena in the observable universe at every scale.
  - graycat 8y ago
    Look up and see a recent essay by the head of the Princeton Institute of Advanced Study where, for my rough summary from rough memory, he explained that physics can have lots of models that all fit all the data the same. So, it can be tough to find the one correct model, the one book of physics, the one way the universe runs, etc. Then see a recent essay at Quanta Magazine that explains that physics has long looked to some largely esthetic concerns especially about symmetry to pick and choose among alternate theoretical explanations -- again my rough summary from my rough memory.
    - fovc 8y ago
      I found an essay by the head of the IAS, published in Quanta Magazine. Could the two essays be one and the same? https://www.quantamagazine.org/there-are-no-laws-of-physics-theres-only-the-landscape-20180604/ https://www.quantamagazine.org/there-are-no-laws-of-physics-...

Bjartr 8y ago

The model isn't as complex as the universe in that sense, but filling out that model with the exact state of the entire universe would be. Just like how a chaotic function can be very simple, but give rise to incredibly complex outputs.

eevilspock 8y ago

Only if the universe is not reducible in any way. But why would a non-reducible universe be so predictable?