Is the universe inherently random, as quantum mechanics predicts? - by Mark John Fernee - Quora Question Review

This document contains a review of the answer by Mark John Fernee on the question in Quora: "Is the universe inherently random, as quantum mechanics predicts? "
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• The text in italics is copied from the article.
  
• Immediate followed by some comments
  

Contents

• 1. Answer Review

Reflection

• Reflection 1 - Question Review

1. Answer Review

Actually, quantum mechanics does not predict that the universe is inherently random. Afterall, what does this actually mean? If something is inherently random, it means the state of the system cannot be predicted by the state of the system that preceded it.

If you want discuss this issue, you must at least first define what is meant with the state of a system, i.e. the universe.
This defintion must include that the universe exists at present and that the whole of the universe is constantly changing or to express differently that the state of the universe is constantly changing i.e. the position of all objects considered.
Only when you accepts this, it is in principle possible to predict the state of a system in the future based on observations in the past.
The problem is that such predictions are not very accurate. In fact in this context the earth is considered as a round object of only one element, mixture or mass. A different problem is that if you want to predict the future you must describe the mathematics used. Newton's Law is too simple because it assumes that the force of gravity acts instantaneous, which is not the case.
My own interpretation is if you want to use GR that it is too complex to calculate the positions of all the planets around the Sun. I have the same impression if you want to use GR to calcualte the position of the stars around Sagittarius A*.

That's not what we observe.

Classical physics is deterministic.

First you need to define what Classical physics is.

This principle of determinism is encapsulated in the Lagrangian formulation of classical mechanics. This works well for most dynamical systems.

Also this requires a clear definition of classical mechanics and dynamical systems.

How then do we move to a quantum dynamics that is inherently random? How can quantum randomness lead to classical determinism?

The answer is that the underlying dynamics of quantum theory is deterministic. The randomness only appears with respect to measurement outcomes. However, measurements are not described by a physical theory. They are only described using metaphysical axioms in order that the entire theory provides an effective descriptive framework. In other words, the measurement process is not part of the physical theory. This is called the measurement problem.

This can be understood as an effective interface between a non-local theory and local detection, which requires certain approximations. It is these approximations that result in probabilistic outcomes. Mind you, approximations are not physical processes!

This means we have globally deterministic dynamics investigated using local interactions that result in probabilistic outcomes. That's why, on average we recover classical determinism.

The underlying determinism of quantum theory is exemplified by the development of quantum algorithms that result in deterministic outputs, such as Shor's factoring algorithm.

Is there a possibility of some fundamental physical randomness underlying the theory? Of course there is, subject to solving the measurement problem. A recent paper addressing the interface between quantum theory and classical gravity has shown that if gravity is a classical theory, then there must be fundamental stochasticity in the metric. This theory does solve the measurement problem, as it is the interface with gravity that induces the collapse of the quantum wavefunction. Furthermore, this theory is testable in principle. If its predictions are verified, then we may actually be able to say that there is some fundamental randomness in nature.

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