What qualifies as an observer in quantum mechanics?

[StevieTNZ]

Observers in formal quantum mechanics need not to be human, biological nor what we call "alive" or "conscious". An instrument will do.

Something to think about:
If I did a quantum experiment and recorded it with camera (without watching it), and put the clip on youtube (something like this), I would surely not believe that the first human who saw the clip collapsed the wave function in my experiment. Youtube is cool, but not that cool .

The problem is QM suggests just that!

 

[DennisN]

The problem is QM suggests just that!

I disagree. Certain types of interpretations suggest that. That's why they are interpretations and not scientific theories on their own. A scientific theory requires repeatable and accurate experiments which supports the theory. So show me an experiment which demonstrates that an "observer" in formal quantum mechanics need to be human, biological, "alive" or "conscious" (and you could try to define consciousness also, if you like). And please tell me if you find any "consciousness" here or here.

 

[Ken G]

It is a perfectly reasonable question, but like many perfectly reasonable philosophical questions, it cannot be answered with the methods of empirical science... So is maybe a bit out of scope for this forum.

It is only philosophical in the same sense that all interpretations of quantum mechanics are philosophical. I agree that QM interpretations have a kind of uncomfortable stance in a science forum, but they dovetail with the common desire to do more than just predict-- we also want to understand.

 

[Ken G]

I have zero idea where you get the idea that QM is only about the outcome of experiments done by humans.

It's about any quantum process that leaves a mark here in the classical world - whether done in an experiment devised by humans or not.

I'd say the issue has more layers than that. It is indeed about everything that leaves a mark, but it is also a tool of the physicist. That is the only place one ever finds quantum mechanics. It would be highly questionable to claim that quantum mechanics happens whether there are physicists or not, that would be like saying the force of gravity happens, before we doubted there is any such thing as a force of gravity, or like saying that the trajectory of a particle happens, before we knew that particles don't really have trajectories. So if quantum processes are however we are choosing to conceptualize them in our current best understanding, then they have no meaning until they are ultimately connected with something that we think we already understand, like the reading of a pointer. That is the sense to which human experiment is always essential, it is essential to the very language of quantum mechanics. It is essential to how we test quantum mechanics, and why we use it, instead of using Greek mythology or some other way of making sense. Bohr said there is no quantum world, but I suspect he might have agreed that there is no classical world either-- there is just what we think we already understand well enough to use as a reference toward what we don't yet understand.

I have posted before about the absurd world view you are led to if you don't do that - but even beyond such considerations I know of no textbook on QM that presents such a view - Ballentine certainly doesn't.

But there I would argue that he certainly does. He does as soon as he takes as understood the basic axioms of human experience. This is always the starting point of physics-- if mathematics starts with "imagine two points and a line connecting them", physics starts with "imagine doing a measurement."

 

[Ken G]

And please tell me if you find any "consciousness" here or here.

But that's shooting fish in a barrel. When you look at those links, what you see are words. Do you imagine that you understand those words? Why do you think you understand those words? As you ponder those questions, you will certainly come smack into an encounter with your own experiences, and where are those experiences registered anyway? What is it about you that has allowed you to have those experiences, and thereby make you think you understand the meanings of the words in those charts? The point is, you always have to start with something you think you already understand, and use that to understand the rest. What gives you that starting point is very definitely your intelligence, your consciousness, and how they make sense of your experiences. So if those are all the starting points of all physics, we really shouldn't be so surprised that we occasionally bump into them again, well down the road.

 

[DennisN]

It would be highly questionable to claim that quantum mechanics happens whether there are physicists or not, that would be like saying the force of gravity happens, before we doubted there is any such thing as a force of gravity, or like saying that the trajectory of a particle happens, before we knew that particles don't really have trajectories.

Do you imply that all mainstream astrophysics, cosmology and physics in general before life appeared on Earth would be highly questionable?

This discussion should really be about the original post #1, which I believe has been answered in this thread;

• No, an "observer" in quantum mechanics need not to be conscious. That's not what "observer" in quantum mechanics mean.
  
• Yes, there are different interpretations of quantum mechanics.

But now this thread is quickly going down the philosophy drain, I am afraid :zzz:.

 

[bhobba]

It would be highly questionable to claim that quantum mechanics happens whether there are physicists or not, that would be like saying the force of gravity happens, before we doubted there is any such thing as a force of gravity, or like saying that the trajectory of a particle happens, before we knew that particles don't really have trajectories.

That's my point. Although for conceptualization purposes we see talk about an ideal measurement, observation or whatever you want to call it expressed vividly in terms of ideal experiments eg a preparation procedure then some kind of measurement apparatus that gives a reading - its simply that - a conceptualization. Quantum effects make there appearance around us all the time independent of such shenanigans.

Thanks
Bill

 

[meBigGuy]

@KenG -- really, read Bertrand Russell's "The Problems of Philosophy". It gives you a solid framework for the issues you are raising. Totally outside the world of QM.

As for the question I didn't answer, I consider it obvious that it is a question no one has an answer for. I felt the paper expressed that well in section 6. It's not a dumb question in that it expresses the limits of our knowledge, but expecting a scientific answer is a "dumb" expectation.

As to the original question of what entails observation, I keep trying to pin that down differently than bhobba's consistent "leaves a mark here in the macro world". Saying "macro world" implies there is some other world, when it is a continuum and saying "here" implies a distinct "there". Can we say that observation consists of any therodynamically irreversable interaction and leave it at that? Or is that wrong somehow?

 

[bhobba]

No, an "observer" in quantum mechanics need not to be conscious. That's not what "observer" in quantum mechanics mean.

Absolutely. There seems to be some confusion about the issue. But no textbook I am aware of says anything of the sort. There is semantic confusion in that they often use the word observation and people naturally equate that with an actual observer, but Dirac, Ballentine and others, in the better textbooks, are VERY careful to ensure it does NOT imply an actual observer, nor if you think about it should it.

This whole confusion is in part Von Neumann's fault, he showed the cut could be placed anywhere - and guess what - there are those that want to place it at an actual organic observer - ignoring it can be placed anywhere.

Thanks
Bill

 

[bhobba]

Can we say that observation consists of any therodynamically irreversable interaction and leave it at that? Or is that wrong somehow?

They may or may not leave a mark.

Look, the founders of QM were not fools, they were not driven to this view of QM as the theory of outcomes of observations, measurements etc MANIFESTING themselves (leaving a mark or whatever words you want to use) without due reason. You can't define your way out of the issue as a thermodynamicly irreversible interaction - if it was that easy it would have been done ages ago.

As I have pointed out, and will continue to point out, the key issue, the rock bottom issue, is how does this world where such things manifest, which is composed of quantum stuff, the very stuff this theory wants to explain, emerge. It does not logically invalidate or contradict the theory - but it is a blemish that would be better off corrected.

Thanks
Bill

 

[meBigGuy]

I don't dispute the theory as outcomes of observations. I'm just quibbling about the words for what constitutes an observation. What thermodynamically irreversible interaction would not constitute an observation? Would not, in fact, leave a mark in your "macro world"?

 

[meBigGuy]

Looking at this page http://en.wikipedia.org/wiki/Quantum_decoherence, where does the concept of "here in the macro world" fit in.

 

[bhobba]

I don't dispute the theory as outcomes of observations. I'm just quibbling about the words for what constitutes an observation. What thermodynamically irreversible interaction would not constitute an observation? Would not, in fact, leave a mark in your "macro world"?

The further jumbling up of a mixture of two gasses. In fact most thermodynamicaly irreversible interactions do the opposite of leave a mark - which is generally an increase in a systems order - thermodynamics usually goes the other way - it decreases order - entropy increases - chaos is coming.

An observation has a particular meaning - it means something you can assign a number to because that's what quantum operators deal with.

You are missing the point - what you are trying to do is pin down in a precise way something that is a bit vague to begin with. That is one of the issues here - in any given situation you can usually figure out what leaving a mark is - but precisely defining it is another matter. This is one of the advantages of decoherence - the mixture that results is unambiguous.

Thanks
Bill

 

[meBigGuy]

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.

 

[Prathyush]

Decoherence is the Physics of forgetting the past. The Phenomenon of decoherence is Independent of measurement. While measurement process involves decoherence(Because past history of the particle is lost), every process where decoherence happens does not involve measurement.(Such as a putting a free particle prepared in a pure state in contact with thermal bath)
And Information Conservation and Unitarity imply that The information about the state is in some way or the other contained in the entangled system. A decohering interaction process is only useful as a measurement when you can access the record. To access this information is not necessarily possible.

Also note that decoherence does not Necessarily invoke a large environment, even a single Bit is capable of decoherence.(Consider the CNOT Gate)

Any record by definition invokes the fact that it will seen at a later time, And any act of seeing involves further entangles between the instrument used to see the record (be it a computer or a human or anything) and the state of the system. I believe this is what Feynman called amplification. Thus there is never a breakdown of unitarity. Only classical correlations are meaningful. Also note that any act of seeing of the record does not disturb the system further.

 

[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]

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