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First of all, I've read this other question Is the universe linear? If so, why? and I'm aiming at a different kind of answer.

Theories like General Relativity or QFT, which are believed to be quite fundamental, are strongly non-linear. However, in the end, both theories must be just low energy limits of an unified theory. So this question arises: Could this unified theory be linear? I'm looking for both a mathematical and a physical answer. In other words, I'd like to know if 1) it is possible to make a linear theory that has non-linear low energy aproximations and if 2) a non-linear universe would make any physical sense in view of the superposition principle.

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    $\begingroup$ Depends on what you mean by linear. QFT is still QM: the Schrödinger equation is linear and the superposition principle applies, but the field equations and interactions are nonlinear. $\endgroup$
    – Javier
    Dec 19, 2016 at 19:59

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To add to John Rennie's excellent answer I would like to note that the linearity of a theory depends of what you are asking of it, especially which observables you are interested in.

One example is the optical response of a 2-level atom (see e.g. https://en.wikipedia.org/wiki/Jaynes–Cummings_model). While being linear in the wavefunctions involved it is non-linear in the field strength of the incoming laser, i.e. would have an amplitude dependent effective refractive index.

(Note: my initial example was complete and utter non-sense. Thanks to Mark Mitchison for pointing that out.)

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  • $\begingroup$ The Lindblad equation is linear! What do you mean? $\endgroup$
    – Mark Mitchison
    Dec 20, 2016 at 13:24
  • $\begingroup$ @MarkMitchison true that, in the sense that the density matrix obeys the superposition principle. I guess I stopped the argument too early (cause there are non-linear effects from tracing out the environment), let me see if I can fix it. My bad. $\endgroup$
    – Wolpertinger
    Dec 20, 2016 at 21:01
  • $\begingroup$ One has to be very careful about what one means by "linear". In quantum mechanics, the state evolves linearly. This is always true, whether in a closed system or an open system interacting with an environment. However, observables may nonetheless obey highly non-linear equations of motion. Unfortunately, your answer seems to mix the two concepts, stating that QM is linear (always true for states) but then in interacting systems the evolution can be non-linear (never true for states, sometimes true for observables). Thus, the basic premise of your answer seems deeply flawed. $\endgroup$
    – Mark Mitchison
    Dec 20, 2016 at 21:34
  • $\begingroup$ For instance, take the given example: the quantum-optical Lindblad master equation describing spontaneous emission of a laser-driven two-level atom into the electromagnetic vacuum. This kind of evolution is as linear as it gets, since here even the observables evolve according to a strictly linear equation of motion, the celebrated Bloch equations. $\endgroup$
    – Mark Mitchison
    Dec 20, 2016 at 21:40
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    $\begingroup$ +1, thanks, this is much improved. Merry Xmas :) $\endgroup$
    – Mark Mitchison
    Dec 22, 2016 at 11:05
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The obvious example is hydrodynamics.

The interactions in a fluid all originate from the interactions between atoms and molecules that are described by quantum mechnics, and QM is as far as we know linear. However the Navier-Stokes equations are (scarily) non-linear and produce all sorts of weird behaviour.

It's an interesting question whether general relativity is a fundamental theory or whether it's an emergent theory based on some more fundamental interactions. The simple answer is that we don't know as there is no evidence either way. The work on this idea is being led by Erik Verlinde.

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    $\begingroup$ Hi John. Did you mean counter example (to the universe being linear)? I suppose also we need to clarify what is meant by 'the universe'. Behavior at all scales? Or just the cosmic scale. I suppose also I would have posed the question "Is the nature of the universe fundamentally non-linear". I believe it is. Sometimes it may appear linear when one squints. $\endgroup$
    – docscience
    Dec 19, 2016 at 18:36
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    $\begingroup$ Isn't it fair to assume GM is an emergent theory? It operates on large scales after all. Why should anyone expect it to describe small scales accurately? (I'm thinking of the law of large numbers as an analogy here.) $\endgroup$
    – user541686
    Dec 19, 2016 at 19:48
  • $\begingroup$ As anticipated by Javier's comment, this answer conflates two completely distinct types of linearity: linear evolution in phase space and linear evolution on Hilbert space, corresponding to different distance metrics. P. C. Spaniel is asking about the former, so "QM is as far as we know linear" is a red herring. The quantum theory of hydrodynamics is simultaneously linear on Hilbert space but nonlinear on phase space, and there's nothing inconsistent or mysterious about this. $\endgroup$
    – Jess Riedel
    May 3, 2017 at 0:13
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First things first, there is no proof that the universe is either. An outstanding question in philosophy is the ontological question of whether the universe is defined by mathematics, or if we created mathematics to understand the universe. Your question only makes sense in the former.

With sufficient feedback, you can create remarkable approximations of non-linear systems with linear systems. Likewise, linearity can be seen in many non-linear systems which operate on a manifold if one looks at small scale.

As for superposition, it only is useful for linear systems, or systems which have reasonable linear approximations. An excellent example appears in electrical engineering with "small signal" models. These are models of how a non-linear system (lots of transistors amplifying currents) can look linear within a small region around a chosen point. Many times that is more than sufficient for what we need.

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