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entropy1
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Is QM deterministic? Specifically, in the case of MWI? Is it time reversable?
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The second question is not at all related to the first.entropy1 said:Is QM causal? Specifically, is it time reversible?
Yes. I probably should replace that by "determinism".A. Neumaier said:The second question is not at all related to the first.
entropy1 said:Is QM deterministic? Specifically, in the case of MWI? Is it time reversable?
Maybe you can go back in time and rephrase the question to test this ;)?entropy1 said:Yes. I probably should replace that by "determinism".
Well, I guess the question is so bad, that it'd better not be answered. So QM is deterministic. There.WWGD said:Maybe you can go back in time and rephrase the question to test this ;)?
It's interpretation dependent.entropy1 said:Is QM deterministic?
MWI is deterministic.entropy1 said:Specifically, in the case of MWI?
It's interpretation dependent. MWI is time reversible (up to certain effects of weak interactions, which are irrelevant in the present context).entropy1 said:Is it time reversable?
What if the state is deterministic too?vanhees71 said:Determinism: All observables take determined values, independent of the state of the system. This is obviously not the case in QT, but the state describes the probabilities for measurement results of observables. There's no state, where all possible observbles of a system take a determined value.
These are special cases only, for systems that have a specific notion of state.vanhees71 (citing Schwinger) said:Causality: The state of a system of time t is determined by the state at previous times. In QT it's even the more strict causality being "local in time", i.e., the state at time t is determined by the state at one previous point in time.
That was I thinking: if QM is causal, then you could write this causality logically as A -> B. But if A -> B, then NOT-B -> NOT A. So that seems close to retrocausality to me! If the universe is causal, then it can be retrocausal to!A. Neumaier said:In short: effects appear later than their causes. This is a necessary condition for any meaningful use of physics.
This is nonsense! Causality has nothing to do with logical implication. The latter is indepedent of any notion of time.entropy1 said:That was I thinking: if QM is causal, then you could write this causality logically as A -> B. But if A -> B, then NOT-B -> NOT A. So that seems close to retrocausality to me! If the universe is causal, then it can be retrocausal to!
Sure, I referred to the special case this question was about, i.e., quantum theory. The only question is, why you think it's special? Because there's no satisfactory quantum theory of gravity?A. Neumaier said:These are special cases only, for systems that have a specific notion of state.
The general principle of causality is that changes in a system lead to changes in the responses only at later times, in every admissible frame of reference. In short: effects appear later than their causes. This is a necessary condition for any meaningful use of physics.
1. For quantum gravity we do not even know whether there is a deterministic dynamics for the state.vanhees71 said:Sure, I referred to the special case this question was about, i.e., quantum theory. The only question is, why you think it's special? Because there's no satisfactory quantum theory of gravity?
Ad 1. because of black hole thermodynamics, quantum gravity is possibly intrinsically open, hence covered by 2. rather than 1.vanhees71 said:Ad 1. Yes, so we don't know, whether there's a physical description of quantum gravity possible yet.
Ad 2. Sure, if you choose to describe open systems, often a stochastic description can provide a sufficient effective description. Here however we choose not to describe the complete dynamics (for whatever reason; in this case it's because of complexity) and substitute it by some stochastic description. This does of course not imply that the fundamental laws of physics are acausal or stochastic.
Ad 3. What has this to do with the question whether QT is causal or not?
But you predict the results of measurements using a simplified description based on the system alone, and not the fundamental laws for large pieces of matter made of the the standard-model particles. Because then you would end up with the measurement problem in its full difficulty.vanhees71 said:a measurement is just described by the fundamental laws as any other interaction. After all a measurement device is nothing else than a piece of matter made of the the standard-model particles as any other thing around us.
But to show that the effective description is in agreement with the fundamental laws you need at least one truly closed system (i.e., a universe) into which the whole experimental setting is embedded. Without this, there is nothing to which the fundamental description applies that could be used as a starting point for deriving the effective theory. The measurement problem appears at this level.vanhees71 said:Sure, I use effective descriptions of measurement devices and experimentalists use these descriptions even to build them. That's, however no contradiction to the statement that this effective description is just an approximation of the underlying "microscopic" dynamics which one cannot resolve anyway and which you don't need to resolve. To describe how a silicon-pixel detector works you don't need to describe the detailed interactions of a particle hitting ##\mathcal{O}10^{24}## silicon atoms let alone the quarks, gluons, leptons etc. particles these are made of on a more fundamental level.
How can this be if there is no underlying "microscopic" dynamics? The latter would be the dynamics of the whole universe, the only truly closed quantum system, hence the only quantum system to which Schrödinger's equation applies exactly. But according to your often stated view, the latter is not a valid quantum system...vanhees71 said:That's, however no contradiction to the statement that this effective description is just an approximation of the underlying "microscopic" dynamics which one cannot resolve anyway and which you don't need to resolve.
These are tiny systems consisting of beams of particles during the time they are sill unobserved. But their detectors are huge and never even approximately closed.vanhees71 said:Obviously the systems we use to test fundamental physics are closed to a good enough approximation.
It is a matter of principle, not of the ability to resolve it in practice. Laplace's deterministic universe could also not be resolved in practice.vanhees71 said:Of course there's microscopic dynamics, described by a Hamiltonian including all relevant degrees of freedom and all relevant interactions. These cannot and need not be resolved and that's why we use effective theories to describe the relevant macroscopic degrees of freedom. They alone are of course never building a closed system, but you get a description that is accurate enough to describe the relevant dynamics. E.g., the motions of planets around the Sun in our solar System are well-described by classical Newtonian point mechanics. You don't need to solve the problem of Suns and planets as consisting of the particles of the Standard Model, which is neither possible nor useful in any way.
The closed system is the universe including its unobservable parts.vanhees71 said:The observable universe is not closed either if you define it in such a strict way. In this sense you may conclude nothing in nature satisfies the Schrödinger equation exactly.
Now physics is not mathematics, and all natural laws, the Schrödinger equation one of them, are always valid up to the accracy they are empirically successful, and the Schrödinger equation is very successful within its range of applicability (much of atomic, molecule, and condensed-matter physics).
How could this ever be done without treating the universe as a quantum system?vanhees71 said:to find a consistent description of all phenomena (including a quantum description of gravity)
Therefore we can also expect to work successfully with a quantum model of the universe without having ever observed it as a whole. The thermal interpretation gives it a workable interpretation without weird, counterintuitive features.vanhees71 said:We have a classical model of the universe without having ever observed it as a whole.
Sure, we can model the universe as a whole without having observed the whole universe.vanhees71 said:You are always referring to the "universe as a whole" not I ;-)).
MWI stands for Many-Worlds Interpretation. It is a theory within quantum mechanics that suggests the existence of multiple parallel universes, each with a different version of reality.
Yes, MWI is considered a deterministic interpretation of quantum mechanics. This means that it suggests that the outcome of any given event is determined by the initial conditions and the laws of physics, rather than being random or probabilistic.
In MWI, superposition is interpreted as the coexistence of multiple parallel universes in different states. This means that all possible outcomes of an event exist in different universes simultaneously.
MWI is a highly debated topic among scientists, and there is no consensus on its validity. Some scientists find it to be a compelling interpretation of quantum mechanics, while others find it to be overly speculative and lacking in empirical evidence.
MWI differs from other interpretations, such as the Copenhagen interpretation, in that it does not involve the collapse of the wave function. Instead, it suggests that all possible outcomes of an event exist in different parallel universes, rather than being collapsed into a single reality.