Let me make a few comments from snippets of information made by various posters.
The term coherent in quantum mechanics has a specific meaning namely the state being in a superposition or entangled state.
The objective of quantum computing is to extend the coherence time for as long as possible by isolating the state from the external environment such as using low temperatures and electromagnetic shielding.
Decoherence is a form of wavefunction collapse as it is not possible or even feasible to have total isolation from the environment and is another reason along with post #121 why consciousness does not appear to play a role in wavefunction collapse.
On another subject whether reality is “linear” or “non-linear” depends on the mathematical model used.
At very small scales reality is linear as quantum mechanics is based on linear algebra and Hilbert spaces.
The linearity of the model does not help in our understanding of reality as exemplified with the various physical interpretations of quantum mechanics.
At macro scales the theory of gravity for the universe is general relativity which is a non-linear mathematical model based on tensor analysis and Riemannian geometry.
The linear Newtonian gravity model becomes an approximation for general relativity.
Despite being non-linear general relativity is far better understood compared to quantum mechanics.
In Newtonian gravity mass is a source for gravity, whereas in general relativity it is both mass and the gravitational field which accounts for the non-linearity of the theory.
At local scales general relativity explains Mercury’s orbit which the Newtonian model cannot, and makes accurate predictions for the orbit well into the future.
What distinguishes a linear from a non-linear model are in the differential equations used.
The differential equations for basic non relativistic quantum mechanics and Newtonian gravity are linear and directly solvable whereas for general relativity they are non-linear and are impossible to solve directly.
Despite this, exact solutions have been obtained by making clever assumptions such as the gravitational field having a spherical symmetry.
In more recent times with the aid of supercomputers involving numerical relativity and post Newtonian expansion, approximate solutions approaching an “exact” solution for the non-linear differential equations have resulted in the predicted waveforms of gravitational waves which led to their subsequent discovery.
With regard to the comment "Random" and "Chaos" are code words for "we can't do the math”, chaos is distinctly different from randomness where we can certainly “can do the math”.
In this case the maths involves running numerical integrations as algorithms on computers to find approximate solutions for equations which cannot be solved directly, an example of which is numerical relativity.
Our solar system is a slowly evolving chaotic dynamic system which allows numerical integrations to provide orbital data for planets extending out millions of years into the future.
This is the Lyapunov time which defines of the predictability of the system beyond which the system becomes too chaotic for accurate predictions.
The Lyapunov time for various chaotic dynamic systems are:
SYSTEM | LYAPUNOV TIME |
Pluto's orbit | 20 million years |
Solar System | 5 million years |
Axial tilt of Mars | 1–5 million years |
Orbit of 36 Atalante | 4,000 years |
Rotation of Hyperion | 36 days |
Chemical chaotic oscillations | 5.4 minutes |
Hydrodynamic chaotic oscillations | 2 seconds |
1 cm³ of argon at room temperature | 3.7×10⁻¹¹ seconds |
1 cm³ of argon at triple point (84 K, 69 kPa) | 3.7×10⁻¹⁶ seconds |