What qualifies as an observer in quantum mechanics?

[bhobba]

The wikipedia article states "Decoherence occurs when a system interacts with its environment in a thermodynamically irreversible way." Doesn't observation always causes decoherence? Isn't decoherence always caused by some sort of "observation"? Can you say that observation is any interaction that causes decoherence? How about we say that observation consists of any therodynamically irreversable interaction with a quantum system?

BTW, I went through this same struggle with the concept of "knowing which slit" vs observing at one slit but then I realized you could do the same observation at the other slit and still have interference, so I gave up. Maybe I'll have to do the same with observation.

I just don't like the fuzziness of "leave a mark" and "macro world" and "here". Seems one can do better.

Welcome to the club - that's one of the issues with Copenhagen.

Decoherence doesn't always count as an observation in the Copenhagen sense eg large molecules are decohered to be in a position eigenstate by just a few photons - it isn't leaving any kind of mark - but because it is now behaving in a classical manner by having a (near - it can't be exact or its momentum will be totally unkown) definite position in a sense it has been observed.

My view is observation should be replaced by decoherence - but you will get a big argument about that and I do not want this thread to degenerate into that. Its simply my view - make of it what you will.

Thanks
Bill

 

[Prathyush]

My view is observation should be replaced by decoherence - but you will get a big argument about that and I do not want this thread to degenerate into that. Its simply my view - make of it what you will.

Thanks
Bill

Measurement involves Decoherence, But That does not mean all decoherence process will have a useful record stored somewhere.

 

[meBigGuy]

I'm not quite sure how to interpret your view "observation should be replaced by decoherence". Do you mean that observation causes decoherence and decoherence means observation has occured, or something else.

If one accepts your view, can one replace the word decoherence with the word observation in the sentence "Decoherence occurs when a system interacts with its environment in a thermodynamically irreversible way." when speaking of quantum systems.

 

[meBigGuy]

Measurement involves Decoherence, But That does not mean all decoherence process will have a useful record stored somewhere.

What are some examples?

 

[bhobba]

Any record by definition invokes the fact that it will seen

I agree with everything you said.

Just a small point - I suspect you mean CAN be seen. For example if a particle detector flashes even if no one was there a record occurred.

Its like in Schrodinger's Cat. Even if no one opens the box the cat will be alive or dead (in Copenhagen). Bohr never had the idea of an observer created reality - his ideas were in fact a lot more subtle that than that, and I personally admit defeat me entirely eg:
http://plato.stanford.edu/entries/qm-copenhagen/
'Bohr was definitely neither a subjectivist nor a positivist philosopher, as Karl Popper (1967) and Mario Bunge (1967) have claimed. He explicitly rejected the idea that the experimental outcome is due to the observer. As he said: “It is certainly not possible for the observer to influence the events which may appear under the conditions he has arranged” (APHK, p.51). Not unlike Kant, Bohr thought that we could have objective knowledge only in case we can distinguish between the experiential subject and the experienced object.'

Sorry to be pedantic about it but I believe this is an important point as far as the usual Copenhagen is concerned.

It is of zero concern for my ignorance ensemble interpretation - but that has others issues.

Thanks
Bill

 

[bhobba]

Measurement involves Decoherence, But That does not mean all decoherence process will have a useful record stored somewhere.

Of course.

That's why I believe it should replace the concept of measurement. For example, in the example I gave of a few photons decohering large molecules like for example Buckyballs so that it has a reasonably well defined position in a sense it has been observed - no mark was left - but in a sense it has been observed.

Thanks
Bill

 

[meBigGuy]

A decohering interaction process is only useful as a measurement when you can access the record. To access this information is not necessarily possible.

Sorry, I missed this post.

Is the mathematics of the effect of decoherence different between a interaction that leaves "accessable" information vs one that is not accessable? I don't see how the encoding of the information changes the significance of the fact that a change in state of the initial entity occurred. Why is my ability or inability to decode the change a factor at all? It has effectively been measured by the system with which it is now entangled.

 

[bhobba]

I'm not quite sure how to interpret your view "observation should be replaced by decoherence". Do you mean that observation causes decoherence and decoherence means observation has occured, or something else. If one accepts your view, can one replace the word decoherence with the word observation in the sentence "Decoherence occurs when a system interacts with its environment in a thermodynamically irreversible way." when speaking of quantum systems.

You are getting a bit confused with terminology. Exactly what an observation is, in Copenhagen, is a bit slippery - as you have found.

Now what decoherence does is this - it singles out a preferred basis and the state is a mixed state in that preferred basis. This is unambiguous and I would say an observation has occurred when that happens ie the system is in one of those preferred basis states. It isn't really - and this is the key point those that do not believe decoherence solves the measurement problem harp on - and they are correct. However it OBSERVATIONALLY is exactly the same as that and for decocherence afficioadoes like me that's good enough - it has given the APPEARANCE of wavefunction collapse and we are happy with that.

But I do not want to discuss that because it can get quite heated and it has been thrashed out many many times. I will simply give the following link that explains what's going on and leave it at that:
http://philsci-archive.pitt.edu/5439/1/Decoherence_Essay_arXiv_version.pdf

Thanks
Bill

 

[meBigGuy]

I'm just to the point of beginning to understand the density matrix view so it will be a bit before I can understand what you mean by "it isn't really" (in one of those preferred basis states) and how that is controversial. It seems like that is actually a good thing, in that "fuzziness" hasn't entirely gone out of the system.

 

[bhobba]

What are some examples?

In practice, and this occurs all the time around us, most examples of decoherence cause objects to be in a mixed state of definite position - this is associated with the Coulomb like interaction of photons etc. For example even a few stray photons from the CMBR is enough to decohere a dust particle and for all practical purposes behave classically. This is explained in, for example, Schlosshauer's text:
https://www.amazon.com/dp/3540357734/?tag=pfamazon01-20

One of the issues here is this is all backed up by deeply technical detail that really needs to be understood first. It means guys like me need to go though that first which makes it a slow slog.

Thanks
Bill

 

[Dale]

Closed pending moderation