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How do physicists control model parsimony, so that the number of particles doesn't keep growing indefinitely? Would it be reasonable to think of the "true model

How do physicists control model parsimony, so that the number of particles doesn't keep growing indefinitely? Would it be reasonable to think of the "true model" of the universe as having infinite complexity, and we are trying to approximate it by adding features and interactions stepwise? And then some of the non-standard models (ex. string theory) tear much of it down and start over?

_Nat_ 7y ago
Looks like you have some good questions for [SE.Physics](https://physics.stackexchange.com/ https://physics.stackexchange.com/)! @knzhou's profile shows that they're a member there. Short version's probably that we don't quite have enough data to compellingly argue for excessively elaborate descriptions. I mean, yeah, every time there's any little observation -- even one that's not anywhere near statistically significant -- there's technically a tiny bit of information in it that could, in principle, be used to better inform a model than a model that excludes it. But at current, folks don't tend to care; too much mental/computational overhead for too little payoff. Cows in Physicsland aren't spherical because physicists couldn't conceive of more precise descriptions, but rather because non-spherical cows were too much work.
knzhou 7y ago
Your question could deserve a book-length treatment, but one quick thing I can say is that the system is self-correcting. The more bells and whistles you add onto the Standard Model, the less anybody else will want to build upon it. And complexity doesn't rise steadily over time. Because of the LHC results, many models these days are a lot simpler (at least to me) than 10 years ago. It is true that in addition to bolting on correct features, you occasionally need syntheses that simplify the whole thing, like how the zoo of mesons and baryons were explained by quarks. But at the moment, we're very far from having that problem...
perlgeek 7y ago
There is no formal process for assessing the complexity of a theory or an interpretation, which leads to some interesting disagreements. Consider the Many-Worlds Interpretation[0] of quantum mechanics. Its proponents say you need just a pretty small assumption (the Universe branches under certain conditions), and its critics say assumption is a huge deal, because it postulates an astronomically high (erm, cosmologically high) number of Universes that we can't observe. [0]: https://en.wikipedia.org/wiki/Many-worlds_interpretation https://en.wikipedia.org/wiki/Many-worlds_interpretation > Would it be reasonable to think of the "true model" of the universe as having infinite complexity, and we are trying to approximate it by adding features and interactions stepwise? Nobody knows. So far, we have very successful theories (general relativity, quantum field theory) that rely on a small number of equations with a small number of terms each. That gives hope that at least the laws that govern the Universe are not infinitely complex, but we cannot really know for sure.

_Nat_ 7y ago

Looks like you have some good questions for [SE.Physics](https://physics.stackexchange.com/ https://physics.stackexchange.com/)! @knzhou's profile shows that they're a member there. Short version's probably that we don't quite have enough data to compellingly argue for excessively elaborate descriptions. I mean, yeah, every time there's any little observation -- even one that's not anywhere near statistically significant -- there's technically a tiny bit of information in it that could, in principle, be used to better inform a model than a model that excludes it. But at current, folks don't tend to care; too much mental/computational overhead for too little payoff. Cows in Physicsland aren't spherical because physicists couldn't conceive of more precise descriptions, but rather because non-spherical cows were too much work.

knzhou 7y ago

Your question could deserve a book-length treatment, but one quick thing I can say is that the system is self-correcting. The more bells and whistles you add onto the Standard Model, the less anybody else will want to build upon it. And complexity doesn't rise steadily over time. Because of the LHC results, many models these days are a lot simpler (at least to me) than 10 years ago. It is true that in addition to bolting on correct features, you occasionally need syntheses that simplify the whole thing, like how the zoo of mesons and baryons were explained by quarks. But at the moment, we're very far from having that problem...

perlgeek 7y ago

There is no formal process for assessing the complexity of a theory or an interpretation, which leads to some interesting disagreements. Consider the Many-Worlds Interpretation[0] of quantum mechanics. Its proponents say you need just a pretty small assumption (the Universe branches under certain conditions), and its critics say assumption is a huge deal, because it postulates an astronomically high (erm, cosmologically high) number of Universes that we can't observe. [0]: https://en.wikipedia.org/wiki/Many-worlds_interpretation https://en.wikipedia.org/wiki/Many-worlds_interpretation > Would it be reasonable to think of the "true model" of the universe as having infinite complexity, and we are trying to approximate it by adding features and interactions stepwise? Nobody knows. So far, we have very successful theories (general relativity, quantum field theory) that rely on a small number of equations with a small number of terms each. That gives hope that at least the laws that govern the Universe are not infinitely complex, but we cannot really know for sure.