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The laws of physics have an extremely rich structure. The more fundamental you go, the more complex it becomes (e.g. Quantum Mechanics is more complex (no pun) than Newtonian mechanics). This introduces a lot of arbitrariness about the fundamental structure of the universe, as in, "why this very specific structure"?

The anthropic principle is one of the crazy metaphysical theories which explains this by introducing an uncountable number of infinite universes, so that all structures (whatever that means) are supposed to be realised.

Are there some other theories about why the structure is the way it is? It need not be a final explanation, but just a different way to think about it. Thanks

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Yes. There are theories that are metaphysical that explain the structure of the universe. One famous philosophical theory is the many world interpretation of quantum mechanics. From WP:

The many-worlds interpretation (MWI) is a philosophical position about how the mathematics used in quantum mechanics relates to physical reality. It asserts that the universal wavefunction is objectively real, and that there is no wave function collapse. (emphasis mine)

You'll note that the highlighted words are all philosophical in nature. 'Reality', 'objectivity', 'real', and 'there is' are all ontological primitives of one sort or another. MWI is NOT scientific explanation, because nothing about the hypothesis is TESTABLE, and that is to say nothing can be disconfirmed, and therefore the explanation offered is in the same vein as Russell's teapot. From WP:

Russell's teapot is an analogy, formulated by the philosopher Bertrand Russell (1872–1970), to illustrate that the philosophic burden of proof lies upon a person making empirically unfalsifiable claims, rather than shifting the burden of disproof to others... Russell specifically applied his analogy in the context of religion.1

Any explanation which offers some sort of reality above and beyond our reality, be it religious or secular philosophy, is inherently unfalsifiable, because it carries claims of existence outside of the closed causality of our physical universe. Such claims are metaphysically speculative and empirically untenable. Read Metaphysical Explanation (SEP) for more insights about what is happening when we make claims about reality.

It should be noted that metaphysical speculation is part of the project of modern science. One simply cannot pull rabbits from the hat for scientific theories, and such theories often start as speculation. It is the processes of the various sciences then to shape such speculation into scientifically rigorous hypothesis and the subject to scientific evidentialism. The logical positivists tried and failed to eliminate metaphysical speculation from science as it was eventually and successfully argued that all observation is inherently subjective. Thus observation sentences and confirmation as philosophical strategies are ultimately circumscribed by theory-ladenness.

For a better explanation, see physicist Sabine Hossenfelder's video on multiverses on YT.

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  • Thanks for the links. The Many Worlds is an answer to why the structure we observe is the way it is, but Many worlds does this by assuming a bigger structure which is a very specific structure in itself because it obeys the Schrodinger eqn and has the very particular Lagrangian of the standard model (let's say). I get that the true question here is impossible to answer, but I'm looking for new ways to think about. One commenter mentioned "Plato's realm of becoming"
    – Ryder Rude
    Dec 8, 2023 at 16:49
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    I see. Forget Plato. He is a scientific ignoramus. Start with Mathematical Explanation (SEP), Brouwer's Intuitionism in Mathematics (SEP), and Lakoff's cognitive science on the origins of mathematics.
    – J D
    Dec 8, 2023 at 17:21
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The immediate answer is that the models we have for predicting reality are the way that they are (really: work the way that they work) because they are approximations to a more fundamental model or models for predicting reality that we don't have yet, but which we know will reduce(1) to general relativity and the Standard Model in the limit as some parameters(2) tend towards certain values. Theories which might one day qualify as such a model are broadly referred to as theories of quantum gravity. Figuring out what this more fundamental, more general, and more predictively sound model should be like is a necessary precondition to asking meaningful questions about why it has the (hitherto unknown) characteristics that it has. Hence physical cosmology and the search for a model of quantum gravity are overlapping fields.

  1. output identical predictions to; having a structure that can be proved using mathematics to be identical under certain circumstances to the structure of the governing equations of...

  2. mathematical objects which represent one or more measurable observables

The anthropic principle makes no testable predictions so it is not a theory or part of a theory in the scientific sense. It is a tautology: we are in the set things, therefore the set of things has the property of including us. There's nothing wrong with tautologies, and they are of inestimable use to science because they set the boundaries within which predictions can possibly be true: if you don't conform to all tautologies, your argument is fallacious. They just aren't science on their own.

Multiple universes including many-worlds cannot be science by itself. We cannot make testable predictions about regions we can never interact with. Since a universe is everything that affects measurement, "other universe" may not even be a coherent concept, except insofar as we arbitrarily decide that our universe is the region of all-that-affects-measurement delimited in one direction by the Big Bang. Depending on how hard you lean into your intuition that "other universe" just has to mean something, a belief in multiple universes may either be an incoherent nonsense-belief or an article of faith.

Multiple universes in the context of their relation to our universe can be science, insofar as guesses about those universes can allow us to make predictions about our universe because of a speculative shared or sequential causal relationship. Whether guesses about how those universe are now means anything or not about those universes is not clear, but they can mean something about ours.

For instance: we might hypothesize that universes have such character that the values of a certain yet-unmeasured fundamental constant are distributed randomly between universes over the known range that allows for oxygen to be produced in significant quantities by stellar fusion if we vary that parameter without significantly varying other parameters. We then guess that that parameter can vary over that range, and that it can do so without falling afoul of the anthropic principle, such that we constitute a random selection from that range. We can then predict that the value in our universe will fall within one standard deviation of the expectation value of that range. We can then go out and measure that fundamental constant, which may conform to the prediction or not. If it does, we have weak evidence for our hypothesis. If it doesn't, we have weak evidence against it. We can argue about whether our hypothesis actually means anything about universes other than our own, but we have defined a principle, correlated it to a prediction, done a measurement, and either confirmed or disconfirmed our principle, so we have done science.

Here are some things about cosmology that are almost certainly true, even though we don't know which way they are true.

  • time is fundamental and the present is real, or time is emergent and the present is arbitrary
  • if time is fundamental and the present is real, locality is emergent and more fundamental concepts of shape, size, and place must be very, very weird
  • if time is real, new universes either can form, or they can't (otherwise it doesn't mean anything to be new outside of the context of a particular observer in a particular universe)
  • if new universes can form, their formation can be random, or stochastic or deterministic according to their properties
  • if new universes form according to their properties, universes evolve by natural selection such that most new universes eventually have the properties that form more new universes
  • the real fundamental laws of physics (not just our models, but the reasons why those models work) change over space, time, or both, or they don't
  • there are laws that determine the real laws of physics, and laws that determine those, and so on, in infinite recursion; or the recursion stops or circles back on itself somehow
  • the real laws of physics are exempt from the principle of no unreciprocated action, or the reality pushes back somehow on the laws that describe it
  • the principle of identity of indistinguishables is true, or we are missing something very important about the way logic works
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The anthropic principle has been formulated originally without reference to the multiverse hypothesis.

Namely: If the physical conditions were a bit different, then there would be no intelligent life in the universe. And no-one could pose a coresponding question.

In my opinion, both the anthropic principle as well as the concept of a multiverse are still on the level of speculative answers to the OP's question. Currently there is no accepted scientific answer.

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  • This other version of the anthropic principle is a tautological sentence.
    – Ryder Rude
    Dec 8, 2023 at 16:46

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