Can Quantum Mechanics be postulated to exclude humans?

[vanhees71]

The difference is that in classical physics we assume a deterministic behavior, i.e., in classical physics all observables always take determined values, no matter in which state the system is "prepared" in. In quantum theory the state determines, which observables take determined values. The physical content of the quantum state is probabilistic and only probabilistic, i.e., for each observable it gives probabilities for the outcomes of measurements and nothing else. An observable's value is determined if and only if with probability 1 you measure this one value.

 

[WernerQH]

I agree that quantum physics is fundamentally probabilistic. In my view the difference to classical physics is not due to "measurements", but to the actual discreteness of the microscopic processes. Photons can be counted. The continuity of classical physics is lost.

 

[vanhees71]

But not all states of the electrodynamical field have a definite number of photons like thermal radiation and coherent states.

 

[WernerQH]

Agreed. I didn't mean to imply that the number of photons is always definite. The underlying microscopic processes can be non-markovian, and photons are not created or absorbed in an instant. But there is some actual "graininess" that is absent in the classical picture.

 

[Demystifier]

So, where's the difference between classical and quantum mechanics in your opinion?

It depends on the interpretation of QM. In some interpretations it's the absence of clear ontology (most interpretations in the Copenhagen spectrum), in others it's very non-classical ontology (many worlds, GRW), and in those with classical ontology (Bohm, Nelson) it's nonlocality.

 

[EPR]

There is no cut and the difference between QM and CM amounts mostly to brain structure and aspects of perception.

Our perception is not equipped to probe below 1/100th of a millimeter, hence we always observe the classical aspect of QT and never noticed in close to a million years of evolution that reality was quantum.

If we had senses equipped to observe at nm scales, we might have seen earlier that fundamental particles don't move in classical trajectories and generally obey different rules. But we always observe the aggregate of statistics of an enormous amount of particles where the mean probability takes the course of a moving classical object.

 

[gentzen]

That's why I think one should introduce statistical operators as the representants of states early and the pure states as the special case, where the statistical operator is the special case of being a projection operator.

I'm not so sure about which approach to QM should be used first. For quite a while I was thinking, that the most simple approach is to start with some finite-dimensional cases like spin 1/2 (problematic for a first encounter, because it's hard to explain what spin is without having a concept of QM first) or polarization of photons (problematic for a first encounter, because you have somehow to introduce the concept of photons, and this all too easily leads to the very wrong idea that photons were in any sense "particles", but it's in principle doable). I've started by intro QM lectures twice in such a way, but my experience is that this is too abstract. The students don't get a good idea what QM is about.

My experience a long time ago was that lectures don't live in isolation, but work together with the recommended literature and the literature directly available in the university libraries and bookstores around the university. For me, the lectures often served as motivation to lookup the topics in some books, where I might also find answers to related questions that came to my mind while thinking about the concepts.
One trap I fell into especially when studying physics where those book-series like Landau-Lifschitz, Feynman, Thorsten Fließbach, (and many lower quality ones I forgot already or at least don't want to mention) ... The trap was that the series which helped me most during previous lectures was not necessarily the best for a lecture on a different topic. (And of course also that books not part of any series might have been a significantly superious choice to begin with, but I was not mature enough yet to understand that.)

In fact, I still fall into this trap today. I want to learn some statistical physics, and instead of trying to find out which would be good recommended books on that subject, I just try a book from Nolting's series and a book from Fließbach's series. They are forced to introduce the statistical operator, but it remains a second class citizen even here. Here is an excercise from such a book which highlights how bad one can misappreciate the statistical operator:

Try to understand where that excercise commits bad mistakes, and how strongly it communicates that the author prefers pure states. And that is my point: I care less about whether the statistical operator is introduced early, or even introduced at all, but about whether it is introduced well when it is introduced.

 

[vanhees71]

WHAT? Where is this from (Nolting?). $\hat{\rho}$ is a pure state if and only if it's a projection operator, i.e., it must fulfill $\hat{\rho}^2=1$. Also it's obviously not self-adjoint. So the right answer without any need for calculation is that it cannot be a statistical operator to begin with.

 

[physika]

So, where's the difference between classical and quantum mechanics ?

...maybe not so much

Reconstruction of Gaussian quantum mechanics from Liouville mechanics with an epistemic restriction​

https://arxiv.org/abs/1111.5057
https://journals.aps.org/pra/abstract/10.1103/PhysRevA.86.012103

Quantum mechanics as classical statistical mechanics with an ontic extension and an epistemic restriction​

https://arxiv.org/abs/1711.01547
https://www.nature.com/articles/s41467-017-01375-w

.

 

[CelHolo]

I'd say there are various limits where the observer can be essentially removed but they're not the general case.

So for example under strong decoherence the probabilities of the theory begin obeying the rules of classical probability and so can be taken as ignorance of an event occurring independent of measurement.
Similarly the notion of particle is shared across all inertial observers in Minkowski space. Also it is possible, due to the presence of PVMs to prepare a unique/objective state of a system in Minkowski space.

However in more general cases like QFT in curved spacetime you get effects where there are no pure states or PVMs, so it's impossible for two observers to completely remove their priors and arrive at the same state in general and thus one is not able to prepare an "objective" state for a system. In the absence of strong decoherence an event cannot be said to occur independent of measurement and so on.

So I'd say removing the subjective/observer dependent aspects of the theory is a limiting case.