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.Is QM causal? Specifically, is it time reversible?
Yes. I probably should replace that by "determinism".The second question is not at all related to the first.
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 ;)?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.Maybe you can go back in time and rephrase the question to test this ;)?
It's interpretation dependent.Is QM deterministic?
MWI is deterministic.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).Is it time reversable?
What if the state is deterministic too?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.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!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.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?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.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.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.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.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....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.Obviously the systems we use to test fundamental physics are closed to a good enough approximation.