What qualifies as an observer in quantum mechanics?

[Ken G]

But here's the problem. When they say "the perception of single “outcomes” is likely to be explainable through decoherence effects in the neuronal apparatus", how does it help answer the following question:
What determines which outcome results from an experiment that must yield one of several outcomes?
I see no resolution of that question, anywhere in that paper.

 

[StevieTNZ]

I don't think we can ever know what determines whether +1 or -1 (for example, spin) will result (note that is the outcome +1 over -1, not that one of them becomes real). I don't think we can rid any future theory of randomness, unless we discover hidden variables that allow us to predict with certainty either +1 or -1.

 

[Ken G]

And note we had a similar problem in classical mechanics, predicting weather and so on. But at least we had a theory whereby we could (erroneously) imagine that the weather was actually determined, we just couldn't ever get precise enough information to "know what nature knows about itself." Of course it was always a fantasy that nature really had that information stored in some memory bank somewhere, but we could imagine it. Now, we don't have that any more, we can't even imagine it unless we go beyond our theories and choose an interpretation that allows that.

 

[marksesl]

I don't really mean to imply that a conscious observer is absolutely necessry to collapse the probability wave, but I do have a couple of problems with your answer.

Here you defined observation as interaction/measurement, which can sometimes leave people with the wrong impression, that interaction alone is enough to collapse the probability wave, which it absolutely isn't. Only measurement collapses the probability wave, and then only for the property for which the state of the particle is thus known. Thus in the double slit experiment you could interact with, and measure the particle, until the cows come home, you could knock the heck out of it, but if none of those measurements gives you which path information, then the interference pattern isn't going anywhere. The probability wave ain't going to collapse for just any old measurement, it's got to be specific. The particle somehow seems to know what you're measuring, but not only that, it also seems to know what you may indirectly learn from that measurement. So while the observer may not need to be a conscious one there appears to be more going on here than merely, I measure it, it collapses.

While it's true that we can easily build a photodetector to "see" the particle in question, in what way is this evidence that such a detector could collapse the probability wave? The last time that I checked, every biological photodetector is connected to a biological brain, and who's to say that it's not the latter that actually collapses the probability wave? Is there indisputable evidence that detection alone collapses the probability wave?

It does seem logical that there is no need for a conscious observer, but is there evidence? After all, this is science, right?

Yes, there is evidence because the position of the particle need not even be known. One need only configure a mechanism that distinguishes one slit from the other to lose the interference pattern, but no knowledge of where the particle is, need exist. So, no one can say a human observation is necessary.

 

[Ken G]

However, note the importance of the potentially different meanings of "collapse." Some might take that word to mean "put the system in a mixed state", i.e., "destroy correlations between different eigenstates of some operator." That is certainly what decoherence does, and it does it independently of the presence of any information in a conscious brain.

But that is not the meaning of "collapse" that is required by the measurement problem, because that is no problem at all. The second meaning, the important one, is "take the system from a state where it could have multiple values for a given measurement, and realize only one value." Note how that meaning of "collapse" is much trickier, and is very hard, or impossible, to achieve without a conscious brain.

Indeed, I would say there is an issue of where lies the burden of proof here. You hold that we should regard all occurrences as objectively independent of human perception until such a time that it can be demonstrated that the occurrence requires human perception. That is an impossible standard, because no experiment ever could look like "comes out A if no human perception is ever involved." On the other hand, since all physics experiments ever done did involve human perception, it is easy to argue that if all experiments have property A, then the burden of proof is on those who would claim that A is not an essential property of a physical outcome.

 

[marksesl]

I read you to mean that decoherence can happen independently of an observer, but just what state out of all possible states a wave "collapses" to requires an observer. That seems philosophically valid, and the rest certainly presents a pause for thought. If all experiments are done by a human, how can one ever know if human perception is not a catalyst for the outcomes. There can be no such thing as a specific event unless experienced by a mind.

 

[Nugatory]

If all experiments are done by a human, how can one ever know if human perception is not a catalyst for the outcomes.

We cannot, at least through the methods of empirical science.

However, the assumption that human perception is a necessary catalyst leads to a number of conclusions that most people find wildly unsatisfying. "Is the moon still there when nobody is looking?" tends to draw an automatic "yes of course" from most people most of the time.

 

[marksesl]

This is getting at exactly the crux of the issue that I'm exploring, and many people see a connection between wave-function collapse in the double slit experiment and the nature of reality around us. I take the position while we do not cause decoherence in the double slit experiment, the results are ambiguous (not settled) until known to a mind, which I believe is what Ken G was getting at. Reality is consummated in the mind. Would the moon or anything else really exist if there were no minds to perceive them at all, to distinguish those happenings from an infinite domain of other possible happenings? Even in the double slit, just what position a probability wave collapses to, out of infinite possibilities, may indeed require a mind to perceive it, even if the more general idea of decoherence does not. This gets really weird, I know, so I'm going to bed now. Thanks for your input.

 

[DennisN]

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 .

 

[Ken G]

I read you to mean that decoherence can happen independently of an observer, but just what state out of all possible states a wave "collapses" to requires an observer. That seems philosophically valid, and the rest certainly presents a pause for thought. If all experiments are done by a human, how can one ever know if human perception is not a catalyst for the outcomes. There can be no such thing as a specific event unless experienced by a mind.

Right. For example, one take a many-worlds view, and say that a closed system begins and ends in a pure state, but if the closed system can generate consciousness, it may generate incoherent consciousnesses that each perceive "one universe", but they really only perceive the sector of the pure state that is mutually coherent with that consciousness. In a situation like that, which requires an understanding of consciousness to either accept or refute, we could certainly hold that the closed system evolves unitarily as per quantum mechanics, but that the perception of experimental outcomes is indeed a product of consciousness. That would be essentially Wigner's position that consciousness is responsible for wavefunction collapse, yet with zero "mystical" elements involved-- unless you count many-worlds quantum mechanics as already mystical.

 

[Ken G]

"Is the moon still there when nobody is looking?" tends to draw an automatic "yes of course" from most people most of the time.

Yet it is easy to argue that a universe with no minds in it has not the least reason to attribute any meaning to the phrase "the Moon." In a very real sense, "the Moon" does not exist in a universe like that, a universe like that is just what it is, with no need to describe itself or partition itself in any way.

 

[meBigGuy]

Yet it is easy to argue that a universe with no minds in it has not the least reason to attribute any meaning to the phrase "the Moon." In a very real sense, "the Moon" does not exist in a universe like that, a universe like that is just what it is, with no need to describe itself or partition itself in any way.

You need to read some basic philosophy. Try "The problems of philosophy" by Bertrand Russell. It is available on the web. It explains objectivism. What is "real" what can we "know", what "exists" independent of us?

How you can read the Schlosshauer paper and not relate to classical existence independent of consciousness is beyond me. It is well explained. Start at chapter VI. One can make up all these weird interpretaions, but they don't explain anything that needs explaining beyond people wanting to know why and making up answers (that can't be tested) to feel good about questions they can't answer (yet). What data do we have that isn't fully explaind by QM, a minimal interpretation, and dechoherence, but is explained by some exotic interpretation. None, obviously, or it wouldn't just be an interpretation.

As soon as you make consciousness the basis of existence or reality you are in mumbo-jumbo land. You can play that game but it will never get you anywhere meaningful.

 

[Ken G]

You need to read some basic philosophy. Try "The problems of philosophy" by Bertrand Russell. It is available on the web. It explains objectivism. What is "real" what can we "know", what "exists" independent of us?

I don't think you understand what I'm saying. There are (at least!) two very different meanings to the phrase "exists outside of us," yet people imagine that statement has a clear interpretation. It does not. The two very different meanings I have in mind are:
1) Exists outside of us in the sense that our presence would make no difference if we were a "fly on the wall." This is the usual meaning people give when they talk about trees falling in woods and making noise, but it is a meaning that has little to do with quantum mechanics. Quantum mechanics is about the difficulty of being a "fly on the wall", difficulties that philosophers who don't know quantum mechanics have most likely never even dreamed of.
2) Exists outside of us in the sense that the words could mean something without any intelligences to convey them meaning. When one recognizes this different meaning of "exists outside of us", the one relevant to quantum mechanics, one can see that even stating "a tree falls in the woods" has already begged the question of existence.

How you can read the Schlosshauer paper and not relate to classical existence independent of consciousness is beyond me.

I notice you did not answer the question I posed. Let's try it again: how does that paper account for the fact that I will perceive a single outcome if I do an experiment? Until you can answer that, you have not even scratched the surface of the "measurement problem."

As soon as you make consciousness the basis of existence or reality you are in mumbo-jumbo land. You can play that game but it will never get you anywhere meaningful.

To me, this just sounds like the "mumbo jumbo" objection, closely related to the "navel gazing" objection. They are fallback positions when the ability to answer the question that I just posed is found lacking. But don't shoot the messenger, the fact that we have a perfectly reasonable question on the table, and cannot answer it, is not a fact that can be ducked with the "mumbo jumbo" objection.

 

[Nugatory]

the fact that we have a perfectly reasonable question on the table, and cannot answer it, is not a fact that can be ducked with the "mumbo jumbo" objection.

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.

 

[bhobba]

IIf all experiments are done by a human, how can one ever know if human perception is not a catalyst for the outcomes.

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 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 for example certainly doesn't.

Thanks
Bill

 

[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.