Wave functions of particles vs wave functions of systems (macroscopic entaglement)

benjayk

As I understand it wavefuctions of individual particles are incomplete descriptions of a system because ultimately a wavefunction describes a system and cannot be reduced to individual particles (which would exclude things like entanglement).
So the only way to have a good idea how a system behaves in QM is to make many many precise measurements of a system, to find all the subtle ways in which the wavefunction of the system is not reducible to the wave function of the individual parts (like correlations between measurements of parts).

That by itself is not a problem and makes sense.
The only thing that I don't get is how that is applied in practice. Because here it seems that in most cases only very specific wavefunctions are considered relevant even for macro systems. That is, it is assumed that the wavefunction of the macro behaviour doesn't include much (if any) information that goes beyond the individual wavefunction of the constituent parts.
Sometimes this is justified through our observations about decoherence. But I don't get how that works given that our notion of decoherence mostly describes decoherence of particles or small systems, not decoherence of systems in general (in which case there may be no correlations between individual particles, but still between bigger systems). It seems we just don't know the wavefunction of (remotely) macro systems because this would require *very* difficult to make measurements. Thus we can make no precise statements about its decoherence. From this also follows that we can't make any statements about subtle macro entanglement and phenoma related to it (psi, quantum brain or other quantum processes in nature).

But given that it is mostly taken for granted that practically relevant macro entanglement does not exist (as commonly seen in discussion regarding psi, where it is claimed it is against the laws of QM), either there is an implicit hypothesis at work here (like "the macro wave functions don't involve any large scale correlations") or I don't understand the reasons for excluding such possibilities, or even regarding them as unlikely.

I think I understand the most basic concepts of quantum mechanics, but not much beyond that, so please answer using simple concepts. :)

Jano L.

I do not understand exactly your question, could you restate it? But to the above quote, I think this can be said:

Macroscopic objects are usually well described by much simpler theory - classical theory - and there are views like that of Bohr that say it is misguided to apply quantum-theoretical formalism to such objects.

Of course, even if successful in practice, this abyss between the micro/macro systems is unsatisfactory. If one adopts the view that the QM formalism should apply to any kind of system regardless of its size, there does not seem to be any obstacle mathematically. However, in most situations, one would choose such wave function for the macro system that it does not predict special correlations between its inner parts (no entanglement). This is because, most usually, in practice we do not observe such strange correlations.

Thus their possibility is neglected. Theoretically, this is because the behaviour of macro systems is too complicated and chaotic; fine correlations which would require entangled wave functions do not manifest easily. Things like superfluid helium or superconductors are possibly an exception; here the correlations may be important and the description by wave function seems to be useful even at the macro level.

benjayk

Sorry, I can't express it much more clearly. To sum it up, it doesn't seem to me that what we know about the quantum mechanical behaviour of large objects is even remotely complete and wonder why most scientists seem to believe that quite firmly.

How do we know macroscopic objects are well described by classical theory? We thus far haven't succesfully predicted or modeled the beviour of any complex system successfully with classical physics, and given our knowledge about QM mechanics it doesn't make much sense to postulate that macro reality behaves almost as if it were classical.
We only know that in many of the simplistic experiments that we can do classical mechanics does apply, which doesn't say much at all. It may quite well be that classical mechanics only accurately describes the more superficial and less subtle aspects of how reality works.

But neither do we have the tools to reliably and precisely observe such correlations. This logic would only make sense if we had the tools, but don't find anything. But to find this correlations we would have to make many many very precise measurements and do a extremely accurate statistical analysis on it. A man in the stoneage can't say that atoms, molecules, genes, cells don't exist just because he has no tools to observe it. He just can state that he has no way to make precise statements about what exists at the smaller scale. In the same way at the moment we can't make precise statements on the quantum behaviour of large things, we just know there isn't a lot of entanglement between the individual particles of different things (which is no suprise).

Also, as far as we do have the tools to analyze it, we do find such strange correlations. Parapsychology has found results that are strongly significant many times over. Their results were just not clear enough to convince the scientific community (which strongly expect such things to be non-existent).
There are a lot of things which are very hard to explain now which might make more sense if macroscopic objects behave more quantum like than scientists expect now.
Many processes in nature are incompletely understood and intuitively are not sufficiently adressed by our current classical models or simple QM models (like coherence in photosynthesis or the behaviour of the brain or morphogenesis or evolution) and many reasonable people report things that are hardly possible in our current model (OBEs with veridical perceptions, alien visitations, ghosts), which might well be possible using a model of QM where macroscopic objects are quantum like and sometimes manifest weird and vague but real phenomena.

That's pretty vague. They may not manifest easily in our current model, but why can't subtle entanglement be inherent to macro objects, just like it is inherent to very small objects?

Jano L.

You seem to think that we should bring the quantum-theoretical formalism into the theory of more casual, non-microscopic phenomena, even to areas outside the scope of physics (parapsychology). There were and still are such attempts by some people, like talking about the wave function of the large body, even Universe, or conscience performing wave packet reduction, quantum processes in brain or plants, etc. Trying to apply quantum-theoretical ideas outside their original scope is not a bad thing a priori, but to my knowledge these attempts did not bring any significant improvement in our understanding of those things/topics.

In fact, there is a good reason not to extend quantum theory automatically to higher scales.
Quantum theory, in its present form as in its beginnings, cannot be claimed to be a self-contained consistent theory of nature. It is still, after hundred years, subject to severe criticisms and the debates over what part of the theory is essential are going on as ever. This is quite strong evidence that the theory is not entirely right yet and contains many imperfections.

On the other hand, theories of classical physics, in their domain of application, are in a much better position; they are generally much more clear than quantum theory and do not have as many problems of principle. Large part of these theories (mechanics+electrodynamics) provide definite answers, which is much more valuable than what quantum theory would provide.

Perhaps it can. Do you have some example where you think it would be so?

StevieTNZ

I must admit I was a bit puzzled by what exactly you were asking - I will attempt to comment hoping it is of use to you.

We can get away with wave functions that describe individual particles, without the need to address the entanglement, where appropriate. Although there is entanglement with other systems, it would be apparent if what we describe the particle as, and experimental results, if there were any deviations from the wave function of the individual system.

Decoherence applies to a system (in the case macro) + environment. Decoherence arises because we cannot specify the wave function of the environment as accurately as we would like. Hence we get a density matrix, which appears (but incorrectly) the systems are in a mixed state.

Indeed there is macro-entanglement.

benjayk

Perhaps pretty much everywhere? Macro structure formation could be intrinsically linked to a field operating on the macro level.
I haven't yet seen a way to derive macro structures from our current quantum laws describing particles, so why would think they are sufficient? I don't believe that we can take that for granted, it seems like an almost magical thing.
A concrete example with quite a bit of research behind it is telepathy. It is naturally explained via macro entanglement and the evidence for it is quite robust (not to speak of the evidence in less controlled conditions).

Would it really be that apparent? We have no ability to bridge micro and macro behaviour (the macro models are far too complicated on the micro level) and thus it seems it is quite hard to spot divergences between QM model and reality on the macro level.
Unsuspected correlations on the macro level could have many potential causes (experimental error, bias in our judgment of what correlations are likely to occur, etc...) and in many cases we simply have no clue what kind of macro structures would be predicted from QM.

bohm2

Most scientists hold the position that that there is little credible scientific evidence for telepathy. And why do you think entanglement would have anything to do with telepathy? I don't see how a hidden/private quantum signal existing between entangled particles/systems can do that. There are a few papers suggesting that perhaps entanglement/quantum physics may play some role in brain processes, that may help explain mental efficacy and consciousness, unity/subjectivity but even this stuff doesn't appear to be taken too seriously by most physicists, for various reasons (e.g. brain is too hot, decoherence, etc.). This is not to say, that there aren't some papers challenging this view, especially more recently.

Quantum Approaches to Consciousness
http://plato.stanford.edu/entries/qt-consciousness/

The Importance of Quantum Decoherence in Brain Processes
http://arxiv.org/pdf/quant-ph/9907009v2.pdf

StevieTNZ

Subject an entangled pair of photons to a measurement. In this example, each goes through a quarter wave plate orientated at 22.5 degrees, followed by a polarising beam splitter orientated in the H/V axis. The entangled pair of photons initially is described as
|H>|V> - |V>|H>
Once the photons reach the PBS and subsequent detectors, we have entanglement between the detectors (due to the linearity of the Schrodinger equation).