# A Weinberg on the measurement problem

#### Lord Jestocost

Gold Member
2018 Award
It seems that many scientists have still not become aware that what we call "nature" appears to us first of all through our consciousness.
Maybe, this might be true for matter-of-fact scientists with a limited philosophical background.
But there are others. For example, Bernard d'Espagnat who puts it in the following way: "What we call 'reality' is just a state of mind".

#### vanhees71

Gold Member
I think you are referring to the 'dynamics' of the probabilities $\psi(t) = e^{i\hat{H}t}\psi$. I mean something else.
So what do you mean?

Dynamics means to calculate from the past state of the system (i.e., knowing it for $t<t_0$) the state for all $t>t_0$. QT is even more modest: You only need to know the state at one point in time $t=t_0$ to know it at all times $t>t_0$. So we have a pretty strong form of the causality principle.

Note than this does NOT imply determinism, since determinism means that all observables take determined values at any time, which is not the case in QT since the full preparation of a state does not imply that all observables take determined value, because the physical meaning of the quantum state is probabilistic (and only probabilistic).

#### Elias1960

We must go beyond the metaphysical beliefs of naive realism, to which many physicists seem to adhere.
As long as viable realistic interpretations exist, even "naive" ones, there is no need to give up realism.

#### Elias1960

According to QT the implied randomness of nature is fundamental, i.e., it is not due to lack of knowledge about the value of an observable but even if we have determined the state of a system completely, i.e., determined the values of a complete set of compatible observables, some other observables are indetermined.
According to QT this indeterminism is not due to lack of knowledge of the state but it's a necessary conclusion of the theory.
Sorry, but this is correct only for some particular interpretations of QT. Moreover, it has to be a non-minimal, because there exist deterministic interpretations of QT.
Maybe one day somebody finds a deterministic theory, but then it must explain at least the same phenomenology as QT does,
Sorry, but it already has been found long ago, namely de Broglie-Bohm theory.

#### vanhees71

Gold Member
Well, for me there's one quantum theory, describing all observations of nature correctly. There are not many different theories but just several metaphysical additions, dubbed "interpretations". The only interpretation one needs as a physicist is to relate the formal descriptions of the theory to real-world observations, and that's done with the minimal interpretation.

The only determinstic interpretation which makes some sense in the non-relativistic case is de Broglie-Bohm, but it doesn't add anything to QT as a physical theory since all predictions are by construction equivalent. I've not yet seen a convincing dBB interpretation of relativistic QFT. That's why I consider it incomplete.

#### Elias1960

Well, for me there's one quantum theory, describing all observations of nature correctly. There are not many different theories but just several metaphysical additions, dubbed "interpretations". The only interpretation one needs as a physicist is to relate the formal descriptions of the theory to real-world observations, and that's done with the minimal interpretation.
But the minimal interpretation remains silent about the metaphysical questions which those other interpretations add to QT. It does not reject them in any way.
I've not yet seen a convincing dBB interpretation of relativistic QFT. That's why I consider it incomplete.
I have. D. Bohm, B. Hiley and P. Kaloyerou, An ontological basis for the quantum theory: II - A causal interpretation of quantum fields, Phys. Rep. 144 (1987) 349-375.

#### vanhees71

Gold Member
Science is and should be silent about metaphysical questions. That's both its strength and limitation!

#### DarMM

Gold Member
What does the minimal interpretation say about superpositions which encompass macroscopic degrees of freedom. I don't mean macro dofs alone as they are handled by decoherence, I mean the superposition still present in total system, environment included

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#### A. Neumaier

What does the minimal interpretation say about superpositions which encompass macroscopic degrees of freedom.
(dies -> does)

It says nothing about these themselves since it only interprets ideal measurements of observables defined by selfadjoint operators, nothing else.

But it says that if in such state you measure a spin component you get the right probabilities corresponding to the macroscopic superposition.

#### vanhees71

Gold Member
In the standard interpretation you trace out all details which are not resolved. You can do quantum statistics and quantum-many-body theory in the standard formulation very well and straight-forwardly.

#### DarMM

Gold Member
In the standard interpretation you trace out all details which are not resolved. You can do quantum statistics and quantum-many-body theory in the standard formulation very well and straight-forwardly.
My question was about the total system. What does the fact that the environment considered in total still has interference terms mean in the minimal statistical interpretation?

#### vanhees71

Gold Member
It means that, if you could resolve enough microscopic details you should be able to observe the corresponding interference terms, but it's tough to get such details measured. There are, however, some examples, where macroscopic systems show quantum behavior, like two diamonds with entangled phonon modes.

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