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What is the relation between classical from quantum vs measurement problem. On the one hand they seem to be related on the other they seem to be of different nature.

We always see our screens in front of us and not 100 meters away, and we say it is a classical object although the screen is a quantum object in the end and it exists even when we are not looking at it. But the measurement of quantum systems says it only takes reality when "measured". I am confused when people discuss wavefunction collapse in regard as to which problem is actually being addressed and the relation.

EDIT: Since I am not getting any response let me ask a simpler question. Are classical objects considered to be a collapsed wavefunction of the system or the subsystems or what?.

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  • $\begingroup$ It's not the quantum system that takes on "reality" but the measured quantity, which has changed the thermodynamic state of the measurement apparatus. Since in classical mechanics the system state and the measurement can be uniquely identified, a lot of people are making the mistake to assume that this relationship also has to exists in quantum systems. In reality, of course, this relationship does not exist, the theory doesn't pretend that it does and it stops being self-consistent when we try to kludge it on top of an otherwise perfectly competent theory. $\endgroup$ – CuriousOne Jun 12 '15 at 5:30
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Experimental data up to now have established that the underlying level of nature is quantum mechanical, i.e. described by the theory of quantum mechanic.s. This theory makes accurate predictions for dimensions commensurate with h_bar

Classical mechanics describes set ups where h_bar, whose value is of order 10^-34 joulesecond , is essentially zero due to its tiny value.

As a conceptual example take Thermodynamics, which is an elegant and complete theory describing the macroscopic behavior of bulk matter. It has been shown that it emerges from the underlying framework of particles , from statistical mechanics exploring smaller dimensions than the macroscopic where thermodynamics holds, in a consistent mathematically manner.

In a similar way it can be shown that classical fields emerge from the underlying quantum mechanical framework.

Classical many body objects, like this screen, have a collective state function described with a huge number of variables. The density matrix formalism is used to describe the transition from a few body quantum mechanical system where effects of QM are measurable to a classical system.

A density matrix is a matrix that describes a quantum system in a mixed state, a statistical ensemble of several quantum states. This should be contrasted with a single state vector that describes a quantum system in a pure state. The density matrix is the quantum-mechanical analogue to a phase-space probability measure (probability distribution of position and momentum) in classical statistical mechanics.

The quantum mechanical form:

density matrix .

For a system of many quantum mechanical bodies, an operator in matrix form. When dimensions are such that the off diagonal elements are essentially zero ( due to h_bar value) one is describing a classical system.

Are classical objects considered to be a collapsed wavefunction of the system or the subsystems or what?.

The "collapsed" language is not useful here, there is nothing sudden (collapse) there is just large dimensions and large numbers that create the emergence of classical from quantum. It means that the influence on probabilities of molecule 1 on molecule n is so small it can be considered zero when describing a classical object.

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  • $\begingroup$ Have a look at my answer to a similar question here physics.stackexchange.com/q/126019 $\endgroup$ – anna v Jun 12 '15 at 4:20
  • $\begingroup$ Thanks Anna, I suppose you mean decoherance. If that is so, the wiki article says that "it gives the appearance of collapse". Also how is that different from measurement issue, in the end it is an interaction between environment and the quantum isn't it. What I am getting at is that I get conflicting info as to whether decoherence needs consciousness or not. $\endgroup$ – qsa Jun 12 '15 at 16:15
  • $\begingroup$ As far as physics goes, a specific interaction is enough for decoherence. Consciousness discussions do not belong to this site. $\endgroup$ – anna v Jun 13 '15 at 4:10
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The answer to this question depends on if the user thinks there is a wave function collapse. If there is a wave function collapse, the classical object can be thought of as a collapsed wave function, but there are a lot more than just saying if there is a wave function collapse, the classical objects exist because of the collapse.

There is fundamental disagreement between how quantum mechanics translate into classical view (classical physics). Why is classical physics are determinate and exact (chair exists, and it will exist) while the quantum mechanics is complementary and wave-like? When and how does a quantum mechanics translate into classical mechanics? When and where does that transition begin if there is such a transition. Why is quantum mechanics contradicting with classical mechanics? How does quantum nonlocality translate into classical view? How do you explain quantum entanglement in classical view? Why is chair not acting like a wave? These are intricately linked questions on the same subject.

There are many questions that need to be answered to reconcile quantum mechanics to classical mechanics. These are two different pictures of different things that are showing different results. In my humble opinion, all these questions point to the single answer, and that single answer will answer all these questions. At this point, I believe in the theory of everything. It is hard, but I think there is the ultimate theory.

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