What qualifies as an observer in quantum mechanics?

[kostas230]

I'm studying quantum mechanics and I can't seem to understand what qualifies as an observer. Does the "observer" need to be a conscious one? Yes or no and why? Thanks in advance :)

 

[DennisN]

I'm studying quantum mechanics and I can't seem to understand what qualifies as an observer. Does the "observer" need to be a conscious one? Yes or no and why? Thanks in advance :)

No. Observation in quantum mechanics means interaction/measurement with e.g. an instrument in general.

[...] why?

Hmm. Let's try this: You have many small (biological) photodetectors in your eyes, which enables you to detect light. But we can build a lot of other kinds of photodetectors, which obviously are not conscious.

 

[Fiziqs]

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.

No. Observation in quantum mechanics means interaction/measurement with e.g. an instrument in general.

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.

Hmm. Let's try this: You have many small (biological) photodetectors in your eyes, which enables you to detect light. But we can build a lot of other kinds of photodetectors, which obviously are not conscious.

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?

 

[meBigGuy]

Does that mean that the particle hitting a photo detector does not collapse until 20 years later when the data is checked by a human?

 

[DennisN]

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.

I'm not surprised. I guess that's why there are 17+ [URL="https://www.physicsforums.com/insights/fundamental-difference-interpretations-quantum-mechanics/"]interpretations of quantum mechanics[/URL] .

Nevertheless, I don't think that details on interpretations is very useful to a person (post #1) who (which I presumed) has recently started to study QM. That's why I did not bring it up (personally I try to keep a certain distance to discussions on interpretations).

[...] 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.

The notion of wave function collapse originates from the Copenhagen interpretation, and there is no scientific consensus which interpretation is most satisfactory. Yes, there are issues to consider; Measurement in quantum mechanics and Measurement problem.

It does seem logical that there is no need for a conscious observer, but is there evidence?

I take the scientific method very seriously, and just like Carl Sagan I say "extraordinary claims require extraordinary evidence".

So let me put it this way:

Is there any scientific evidence there is any need for a conscious observer?
I'd say there is none whatsoever. The burden of "proof" lies on those who makes the claim; they need to show experiments that demonstrate it. And there are none as far as I know.

My photodetector example was just a little hint to the OP (which the OP btw thanked me for), but here is a somewhat more detailed thought experiment:

Imagine a quantum experiment done by a machine placed on the far side of the Moon - there are obviously no conscious observers there. Then let the results of the experiment be printed out and sent back to me here on Earth by rocket. Am I supposed to believe that when I read the results here on Earth, this retrocausally collapses any wave functions on the far side of the Moon? I can imagine a lot, but that idea is just too extreme for me.

After all, this is science, right?

Well, quantum mechanics is science. Some interpretations of quantum mechanics are, how shall I put it, science with a twist. I personally try to stay away from them .

 

[DennisN]

Does that mean that the particle hitting a photo detector does not collapse until 20 years later when the data is checked by a human?

Good point.

 

[meBigGuy]

There is a whole bunch of good interpretation interpretation in the following thread.
https://www.physicsforums.com/showthread.php?t=709099

 

[bhobba]

An observation is anything capable of leaving a mark here in the common sense macro world. A conscious observer is most definitely NOT required. A measurement apparatus is a device capable of displaying the results of such observations as numbers, or at least allow it to be simply mapped to numbers. QM is a theory about such measuring devices observing quantum systems.

You will find this explained clearly and concisely in my go-to book on QM - QM - A Modern Development by Ballentine:
https://www.amazon.com/dp/9810241054/?tag=pfamazon01-20

Get it, study it and you will understand QM. Pretty soon you will realize a lot of the stuff written in the popular press, such as this idea you need a conscious observer for an observation - is laughable. That's not to say the consciousness causes collapse is not a viable interpretation - it is - but required it certainly aren't. Indeed when you really understand what's going on you probably will, like me, wonder why anyone wants such an extreme view anyway - but that is a discovery you need to make - me telling it to you is not the way to understand such things - you need to think about it yourself.

You will also understand the true basis of much of QM - stuff like Schrodinger's Equation - is symmetry. Once that is understood its a mind blowing revelation about what really lies at the fundamental foundations of physics.

At a more non technical level the following is pretty good as well:
https://www.amazon.com/dp/0918024161/?tag=pfamazon01-20

I read it years ago - didn't have a huge impact on me then until I really thought hard about it and came to understand something called Noethers Theorem - then I realized it was very important - but want to read it again because it really is fundamental - absolutely fundamental.

Thanks
Bill

 

[bhobba]

Good point.

Yea - it really does make a nonsense of consciousness causes collapse. Computer science would be rendered laughable if true.

Thanks
Bill

 

[Fiziqs]

Does that mean that the particle hitting a photo detector does not collapse until 20 years later when the data is checked by a human?

I know that my previous post gave the impression that I believe that a conscious observer is necessary to collapse the probability wave, but that is actually not the case. My own personal opinion, (and that's all that it is) is that a conscious observer is not necessary to collapse the probability wave. Indeed my view is much in line with what bhobba proposed, that "An observation is anything capable of leaving a mark here in the common sense macro world." I believe that so long as the information about which state a particle is in exists anywhere, then the particle will absolutely be in that state. Simply put, if the information exists, then the state exists.

I don't believe that it matters, whether the information won't be seen until 20 years from now, or whether the event happened on the dark side of the moon, if it left a mark as bhobba says, then it happened. To me, and to most other people, wave collapse occurs based upon the information the environment has, not based upon the information I have.

But the problem is that I can't state this as being categorically true. It's only my opinion, and as quantum physics shows, what's intuitively logical, may not be true, and so the question of just what constitutes an observer is still open for debate. The OP was looking for a yes or no answer. But there isn't a yes or no answer. To claim otherwise wouldn't be completely honest.

And we all want to be completely honest, right?

 

[DennisN]

I know that my previous post gave the impression that I believe that a conscious observer is necessary to collapse the probability wave, but that is actually not the case. My own personal opinion, (and that's all that it is) is that a conscious observer is not necessary to collapse the probability. Indeed my view is [...]

I have no problem whatsoever with this . And I did not get the impression you believed in e.g. "http://en.wikipedia.org/wiki/Quantum_mind%E2%80%93body_problem#.22Consciousness_causes_collapse.22".

The OP was looking for a yes or no answer. But there isn't a yes or no answer. To claim otherwise wouldn't be completely honest. And we all want to be completely honest, right?

Yes. I was short in post #2, but I think I was honest. There is no notion of consciousness in formal quantum mechanics. And - as a sidenote - consciousness is as far as I know a very slippery term. How should it be defined with pure physics - can it be defined with pure physics? I guess the consciousness question is a question mainly for biologists and possibly chemists.

 

[bhobba]

The OP was looking for a yes or no answer. But there isn't a yes or no answer. To claim otherwise wouldn't be completely honest. And we all want to be completely honest, right?

There is a definite no answer is the sense QM requires a conscious observer - it doesn't. There is a definite yes answer in the sense you can't prove consciousness doesn't cause collapse. But consciousness causes collapse does lead to a very weird view of the world - so weird I think anyone that understands its full implications, such as what happens if you replace the conscious observer with a computer (think about it) will reject it.

And I didn't get the impression you were advocating it either - simply putting it out there as a possibility - which it is. The issue though is not that - its some books, especially those of the pop-sci type give the impression its what's QM says - it isn't.

To the OP - don't take my word for it - get the book I mentioned by Ballentine - it really is that good. It's unusual in it gives a full account of foundational issues rather than being a cookbook to solve problems.

Oh - I am assuming you are a serious student - its a serious book for serious students.

Thanks
Bill

 

[Fiziqs]

There is a definite no answer is the sense QM requires a conscious observer - it doesn't.

It's good that we're somewhat in agreement as to the necessity of a conscious observer, but I don't believe that I'm as steadfastly against the idea as others may be. It would help if I had a clearer understanding of what exactly constitutes an observer.

For example, in the double slit experiment, if we put a detector at the slits, such that it measures which slit the particle went through, but we don't attach this detector to any type of data storage device, then the interference pattern will remain. What is it about the detector that keeps it from acting as an observer? Isn't it part of the environment? Perhaps its state has simply become entangled with that of the particle, and the subsequent system is in its own quantum state. But the two do constitute an environment, just not a very complex one. If we add a bunch of mirrors and filters to the setup, but we gain no information by doing so, then the probability wave still won't collapse. Even if we add a data storage device, but we make the data inaccessible, the wave still won't collapse.

So the question becomes, at what point is the environment complex enough to cause the collapse? It may well be that an environment is capable of causing the collapse only when that environment contains a conscious observer. It isn't necessarily required that an observer actually be involved in any way with the measurement, or the subsequent data, but it could very well be a requirement, that the environment at least contain a conscious observer. Show me a collapsed quantum system in which a conscious observer is not a part of that system, and I would be a lot more certain that a conscious observer isn't necessary.

Until then it is perfectly reasonable to argue that a conscious observer is indeed necessary, at least indirectly. It's a whole other debate as to what actually constitutes a conscious observer.

Until you rule it out, even the outrageous is possible.

 

[StevieTNZ]

Does that mean that the particle hitting a photo detector does not collapse until 20 years later when the data is checked by a human?

Could be the case.

 

[DennisN]

It may well be that an environment is capable of causing the collapse only when that environment contains a conscious observer.

I would not even start to go down that path without a proper definition of conscious observer. So, what is your definition of a "conscious observer"?

Until then it is perfectly reasonable to argue that a conscious observer is indeed necessary, at least indirectly.

I don't agree. I do not think it's reasonable that quantum mechanics on the far side of the Moon (or Mars etc - choose any place you like in the Universe) would work differently than quantum mechanics on Earth.

Until you rule it out, even the outrageous is possible.

Possible - well, with a considerable grain of salt. But is it probable? Well, that's certainly up for debate. Extraordinary claims require extraordinary evidence. I also personally like to apply the tools in Carl Sagan's so-called Baloney Detection Kit;

Among the tools:

• Wherever possible there must be independent confirmation of the "facts".
  
• Encourage substantive debate on the evidence by knowledgeable proponents of all points of view.
  
• Arguments from authority carry little weight -- "authorities" have made mistakes in the past. They will do so again in the future. Perhaps a better way to say it is that in science there are no authorities; at most, there are experts.
  
• Spin more than one hypothesis. If there's something to be explained, think of all the different ways in which it could be explained. Then think of tests by which you might systematically disprove each of the alternatives. What survives, the hypothesis that resists disproof in this Darwinian selection among "multiple working hypotheses," has a much better chance of being the right answer than if you had simply run with the first idea that caught your fancy.
  
• Try not to get overly attached to a hypothesis just because it's yours. It's only a way station in the pursuit of knowledge. Ask yourself why you like the idea. Compare it fairly with the alternatives. See if you can find reasons for rejecting it. If you don't, others will.
  
• Quantify. If whatever it is you're explaining has some measure, some numerical quantity attached to it, you'll be much better able to discriminate among competing hypotheses. What is vague and qualitative is open to many explanations. Of course there are the truths to be sought in the many qualitative issues we are obliged to confront, but finding them is more challenging.
  
• If there's a chain of argument, every link in the chain must work (including the premise) -- not just most of them.
  
• Occam's Razor. This convenient rule-of-thumb urges us when faced with two hypotheses that explain the data equally well to choose the simpler.
  
• Always ask whether the hypothesis can be, at least in principle, falsified. Propositions that are untestable, unfalsifiable are not worth much. Consider the grand idea that our Universe and everything in it is just an elementary particle -- an electron, say -- in a much bigger Cosmos. But if we can never acquire information from outside our Universe, is not the idea incapable of disproof? You must be able to check assertions out. Inveterate skeptics must be given the chance to follow your reasoning, to duplicate your experiments and see if they get the same result.

 

[meBigGuy]

Could be the case.

Well, it could also be the case that Fred in Pittsburg actually monitors all quantum interactions and decides which ones collapse.

My personal view is that collapse is a relative view (relational) and that things are as defined as they need to be (relative to other things) to properly correlate. I haven't gotten to decoherence in the susskind lectures yet, so maybe that will change my mind. But I expect not.

 

[bhobba]

if we put a detector at the slits, such that it measures which slit the particle went through, but we don't attach this detector to any type of data storage device, then the interference pattern will remain.

No - it goes away ie regardless of if it is connected to a storage device or not the interference pattern disappears. An observer, detector or whatever you want to call it, is anything capable of leaving a mark here in the macro world. If its a particle detector it will click or flash. The storage thing is just to bring out what kind of a weird view you are led to if you think its consciousness that causes collapse. In modern parlance the detector becomes entangled with the particle and its position becomes localized through one slit or the other so it can no longer interfere.

There is a nice set of lectures by Lenny Susskind that explains it all really well, and the modern take involving decoherence and entanglement:
http://theoreticalminimum.com/courses/quantum-entanglement/2006/fall
'The old Copenhagen interpretation of quantum mechanics associated with Niels Bohr is giving way to a more profound interpretation based on the idea of quantum entanglement. Entanglement not only replaces the obsolete notion of the collapse of the wave function but it is also the basis for Bell's famous theorem, the new paradigm of quantum computing, and finally the widely discussed "many-worlds" interpretation of quantum mechanics originated by Everett.'

Until you rule it out, even the outrageous is possible.

Make no mistake - its a valid explanation - but the world view you are led to if you accept it is very very bizarre and totally unnecessary.

The great mathematical physicist, Eugine Winger, was one of the high priests of consciousness causes collapse. The reason was Von Neumann's famous analysis that showed the collapse could be put anywhere and the only real place that was different is the consciousness of an actual organic observer - so that's where he put it. But over the years progress was made, especially in the area of decoherence, and what that showed is its likely the best place to put the collapse - right after decoherence. When Wigner heard of some early work on decoherence by Zurek he realized the consciousness thing was no longer required and abandoned it.

I also want to add, and to be very clear about it, decoherence does NOT solve the measurement problem to everyones satisfaction. Without going into he details, it merely gives the APPEARANCE of wave-function collapse, the exact meaning of which you need to investigate the detail to understand. If you want to do that check out:
http://philsci-archive.pitt.edu/5439/1/Decoherence_Essay_arXiv_version.pdf

That paper reaches the correct conclusion it leaves the central problem untouched - which is true - the debate about if it solves the measurement problem is if that central problem, issue, or whatever you want to call it, is worth worrying about in light of what decoherence does do - I don't believe it is - but opinions vary and you will find a number of, sometimes heated, discussions about it on these forums.

Thanks
Bill

 

[bhobba]

My personal view is that collapse is a relative view (relational) and that things are as defined as they need to be (relative to other things) to properly correlate. I haven't gotten to decoherence in the susskind lectures yet, so maybe that will change my mind.

That's a perfectly valid interpretation, just in the light of decoherence overkill because once decoherence occurs all observers agree. Its examined in the link I gave before (see section 3.2.3):
http://philsci-archive.pitt.edu/5439/1/Decoherence_Essay_arXiv_version.pdf

Modern versions, such as Zureck's version, takes all this into account and leads to a very clean interpretation:
http://arxiv.org/pdf/0707.2832v1.pdf

Thanks
Bill

 

[meBigGuy]

That doesn't say anything about what could be significant lengths of time during which observers don't agree. It seems to just define that away. I'll try to say what I am thinking, but I'll probably screw it up. If one thinks of entanglement as any interaction that allows continued superposition (correlation), you could imagine that two entangled (correlated) entities see each other as a definite state, but an external entity still "sees" (actually, doesn't see) an indefinite system until it interacts. And this state of different relative views is stable.

 

[bhobba]

That doesn't say anything about what could be significant lengths of time during which observers

Its provable to be on time scales that even with modern equipment you can't detect. It has been possible to rig it so it occurs slower to actually observe it - and that has been done - ie decoherence has been observed - but that is a very special experimentally designed setup for that purpose - it never occurs in practice.

If you want to hold to the relational view I suggest a version similar to Zurec's in the paper I linked to - it takes into account modern developments since Rovelli proposed it.

Thanks
Bill

 

[StevieTNZ]

I personally subscribe to the consciousness 'causes collapse' view, and agree that decoherence is nowhere near to solving the measurement problem.

I quote 'causes collapse' as I don't believe the laws of QM don't change, thus when we see a macroscopic object in a definite state, it really is still a superposition but we are privy to seeing it in a definite state.

 

[bhobba]

I personally subscribe to the consciousness 'causes collapse' view, and agree that decoherence is nowhere near to solving the measurement problem.

Yes, the issue is controversial and having been involved in a few discussions about it know no definite answer will be reached.

Best I think to point to good literature on it an get people to make up their own mind.

Thanks
Bill

 

[DennisN]

Best I think to point to good literature on it an get people to make up their own mind.

Yes, I think so too.

I'm personally in a confused superposition of ensemble+decoherence and relational QM (I also like the Susskind lectures, by the way). But to be honest, I don't think very much about interpretations, so I guess I'm sort of agnostic. But I do not disrespect those who like to discuss interpretations, I often read such threads, it can be interesting.

 

[ktx49]

my opinion is that it comes down to information theory and is related to entropy.

it seems to me that extracting information from a system(regardless if its "observed") and lowering its entropy is key to the measurement problem and is not tied to consciousness or anything else for that matter...instead any process, interaction, or phenomenon that ends up lowering the entropy of the quantum system will suffice.

 

[Fiziqs]

No - it goes away ie regardless of if it is connected to a storage device or not the interference pattern disappears. An observer, detector or whatever you want to call it, is anything capable of leaving a mark here in the macro world. If its a particle detector it will click or flash.

I must politely disagree with this statement. Although the way that you have phrased it does make it technically correct, but it is not the situation that I was attempting to describe. Specifically, in the highlighted sentence you essentially described turning the environment into a storage device. A particle detector that clicks or flashes, obviously has the potential to leak information into the environment. Making the environment the storage device. But if our detector does not leak information into the environment, then it will of its own accord be unable to introduce decoherence.

If the detector alone introduced decoherence, than the various iterations of the double slit experiment would be pointless, as the detector would always introduce decoherence into the system. As it is however, the detector is not capable of introducing decoherence unless it is set up to do so. This is such a simple point to understand, that I have to question your objectivity on this subject.

We are however in agreement as to our view that anytime the environment acts as an information storage device, it alone should be capable of introducing decoherence, but this is by no means proven. It may well be that a system must always contain a conscience observer for decoherence to occur. Not that the conscious observer must be actively involved in the observation, but that it must at least be a part of the system.

I realize that you disagree with this point of view, but it's not at all bizarre or any more speculative than any other interpretation. The facts fit this interpretation just as well as they do any other.

 

[DennisN]

This is a good and healthy discussion, I think . I leave the decoherence discussion to bhobba, since I think bhobba is more up-to-date with decoherence than me.

But concerning

I realize that you disagree with this point of view, but it's not at all bizarre or any more speculative than any other interpretation.

In my opinion it gets bizarre when you think of what it implies;

It may well be that a system must always contain a conscience observer for decoherence to occur. Not that the conscious observer must be actively involved in the observation, but that it must at least be a part of the system.

So let me try this again;

I would not even start to go down that path without a proper definition of conscious observer. So, what is your definition of a "conscious observer"?

Is it a human? A human that is awake or sleeping? Are animals included? Are all animals included? Awake or sleeping? Are plants included? Bacteria? Viruses? Is a computer that can store data conscious? Is a plain paper that can store data conscious? Does quantum mechanics work differently on Earth than in other places?

What is the definition of a "conscious observer"? My point is that without a proper definition of consciousness I don't see the point in talking about conscious observers at all.

 

[ktx49]

I'd define consciousness as the attachment of subjective experience/"feelings" while processing information...I'd venture and say that any biological form that can process information about its environment is "conscious" to some degree. Personally I think we as human beings do not like to think about the implications of this and try to define consciousness as the ability to relay these feelings of subjective experience, which IMO is pretty naive.

however, I still don't think its relevant for this discussion.

just because we are limited to seeing and experiencing the world through our consciousness doesn't mean its fundamental to physical processes.

 

[DennisN]

I'd define consciousness as the attachment of subjective experience/"feelings" while processing information...however I still don't think its relevant for this discussion. just because we are limited to seeing the world through our consciousness doesn't mean its fundamental to physical processes.

I agree. And that is one of the points I am trying to make .

 

[kostas230]

I found this article that may be relevant

http://journalofcosmology.com/JoC16pdfs/25_Post%20-%20Copy.pdf

 

[StevieTNZ]

http://www.sciencedaily.com/releases/2013/09/130909092835.htm

A new article discussing temperature as a possible mechanism in which the classical world arises. I suspect although the quantum system has its temperature spread (or increased), it still acts quantum but -appears- classical. That is the numerous answers I've received from various Physics Professors - a system that is not cooled still is quantum in nature; the effects are just harder to detect.

 

[meBigGuy]

All this consciouness stuff arises out of misconception that consciouness arises out of something more than chemical (biological) complexity. I don't see how the complexity of the interacting elements can change the results of interaction.

Things are as defined as they need to be to correlate with their surroundings. And, they will always correlate. Entanglement doesn't involve "spooky action". It isn't "cause-effect". It's that the results will always correlate and the alternatives never existed. I'm not allowed to post a link to the spooky socks thought experiment that makes this clear.

 

[bhobba]

All this consciouness stuff arises out of misconception that consciouness arises out of something more than chemical (biological) complexity.

Of course I do not hold to the consciousness causes collapse interpretation but physicts (and here I mean really top notch physicists and mathematical physicists like Wigner and Von Neumann that are not fooled by a simple semantic confusion about what an observation is that is easy to fall into by reading the more elementary tratments) were not driven to this without due evidence. The evidence was detailed in Von Neumann's classic Mathematical Foundation's of Quantum Mechanics. He did the first fully quantum analysis of the measurement process and showed the collapse can be placed anywhere - there was no reason to chose one place over another. You keep tracing it back and the only thing different is the consciousness of an organic observer - of course you have the issue of defining precisely what counts as that - but that is another issue. That is why they did it - that is what drove them to it. It has all sorts of very very weird implications especially with regard to modern technology and computer science - but computer science and associated technology wasn't really around then so many of the issues were not that apparent - they were there - but not just as easily seen.

However since then a lot of work has been done on a fully quantum theory of measurement and it is now known there is a place that is different - that is just after decoherence has occurred. It now looks like the logical place to put the collapse - especially considering observationally it looks exactly the same as if collapse had occurred at that point.

Like I have mentioned a number of times in relation to this issue, Wigner, when he heard about some the early work on decoherence by Zurek, realized the reasons for consciousness causes collapse was no longer required and abandoned it.

Thanks
Bill

 

[StevieTNZ]

Like I have mentioned a number of times in relation to this issue, Wigner, when he heard about some the early work on decoherence by Zurek, realized the reasons for consciousness causes collapse was no longer required and abandoned it.

Thanks
Bill

Surely Wigner was aware decoherence doesn't solve the measurement problem, so abandoning his, what I call, excellent idea, wasn't required.

I refer you to physicist Stephen Barr's book "Modern Physics ad Ancient Faith" pages 240-241.

 

[stevendaryl]

Surely Wigner was aware decoherence doesn't solve the measurement problem, so abandoning his, what I call, excellent idea, wasn't required.

I refer you to physicist Stephen Barr's book "Modern Physics ad Ancient Faith" pages 240-241.

It depends on what you mean by "the measurement problem". I think there are three different "measurement problems":

1. As far as empirically consequences of measurement, the most striking one is the fact that after a measurement, there can be no interference terms among alternatives. This is completely explained by decoherence.
2. The second effect of measurement is the disappearance of all alternatives (among values for observables) except one. The difficulty here is that it's not clear whether this is a problem or not, because there is no way to observe the multiple alternatives. It's sort of like a coin: both heads and tails are present, but it's not possible to see both at the same time.
3. The final aspect of the "measurement problem" is, to me, the question of how a macroscopic object such as a detector can have a "preferred basis". Such a preferred basis is needed to be able to say that a detector "measures particle spin" or whatever it is that it measures. It's possible that decoherence can say something about this, as well.

 

[malreux]

Responding as if you'd asked 'what's the measurement problem?'

I'm studying quantum mechanics and I can't seem to understand what qualifies as an observer. Does the "observer" need to be a conscious one? Yes or no and why? Thanks in advance :)

Hi, a lot of comments generated by your question are circling the 'measurement problem' territory. Although the linked preprint is old (2007), I think it's an excellent introduction / summary of the whole territory. Please note the author is an Everettian, but this doen't intrude on his excellent paper:

http://arxiv.org/abs/0712.0149

PS Please note the preprint doesn't include any substantive discussion of consciousness, more the question of if a term such as 'observer' needs to be / is desirable as a primitive in physics.



M

 

[bhobba]

Surely Wigner was aware decoherence doesn't solve the measurement problem, so abandoning his, what I call, excellent idea, wasn't required.

Well maybe like many he thought it did FAPP. I am not going to get into discussions about it again - they go nowhere and can get quite heated. Here is a fair minded article about it I have posted before - anyone can form their own view - we know yours:
http://philsci-archive.pitt.edu/5439/1/Decoherence_Essay_arXiv_version.pdf

Mine is the ignorance or ensemble interpretation:
'Ignorance interpretation: The mixed states we find by taking the partial trace over the environment can be interpreted as a proper mixture. Note that this is essentially a collapse postulate'

But there are a plethora to choose from - and consciousness causes collapse is just one of them.

Thanks
Bill

 

[stevendaryl]

Mine is the ignorance or ensemble interpretation:
'Ignorance interpretation: The mixed states we find by taking the partial trace over the environment can be interpreted as a proper mixture. Note that this is essentially a collapse postulate'

Does "can be interpreted as a proper mixture" means "you can pretend that it is a proper mixture, and not get in trouble"?

 

[ktx49]

It depends on what you mean by "the measurement problem". I think there are three different "measurement problems":

1. The second effect of measurement is the disappearance of all alternatives (among values for observables) except one. The difficulty here is that it's not clear whether this is a problem or not, because there is no way to observe the multiple alternatives. It's sort of like a coin: both heads and tails are present, but it's not possible to see both at the same time.

lets take the coin analogy a step further...

it is possible to "measure" the coin(along a certain "unstable" axis) in a manner so that both outcomes are possible yet neither outcome is actually decided...so with this measurement both alternatives are actually present, however we can only speak of meaningful outcomes in terms of the probabilities of future events. imagine the coin in the world of some 2D flatlanders...they would never be able to see both outcomes at once unless they were able to move into a higher dimensions yet they would clearly be able to determine the 50/50 probability of said coin...yet whenever they actually observe and measure the coin they would only be able to see head or tails.

we could also perform this same sort analogy with a normal dice, or an 8 sided dice, or a 30 sided dice...and again we would only be able to talk about the probabilities of the outcomes...yet for the flatlanders they would only get meaningful observations about ONE SIDE at a time.

lets say anytime they want to measure these dices they have to interact with the object in some manner and this physical interaction causes the unstable coin/dice to "fall" into a definitive state in their dimensions which is then observable. until the coin/dice has been disturbed, its only meaningful to talk about the probabilities.

just realized how this would completely squash many-worlds interpretationss :)

 

[bhobba]

Does "can be interpreted as a proper mixture" means "you can pretend that it is a proper mixture, and not get in trouble"?

That's part of it - but its slightly more subtle than that. Without going through exactly what the go is and generate these long posts that go nowhere (largely because it boils down to semantics about appear, FAPP etc etc), and I have detailed in plenty of other threads, I will simply give the Wikepedia article on it that explains it pretty well:
http://en.wikipedia.org/wiki/Quantum_mind%E2%80%93body_problem
'Decoherence does not generate literal wave function collapse. Rather, it only provides an explanation for the appearance of wavefunction collapse, as the quantum nature of the system "leaks" into the environment. That is, components of the wavefunction are decoupled from a coherent system, and acquire phases from their immediate surroundings. A total superposition of the universal wavefunction still exists (and remains coherent at the global level), but its fundamentality remains an interpretational issue. "Post-Everett" decoherence also answers the measurement problem, holding that literal wavefunction collapse simply doesn't exist. Rather, decoherence provides an explanation for the transition of the system to a mixture of states that seem to correspond to those states observers perceive. Moreover, our observation tells us that this mixture looks like a proper quantum ensemble in a measurement situation, as we observe that measurements lead to the "realization" of precisely one state in the "ensemble".'

Basically my view is wavefunction collapse doesn't actually exist. Its replaced by decoherence that only gives the apperance of collapse. But since its observationally exactly the same as if it collapsed no wonder it looks like its there. Instead of collapse the measurement postulate, in my interpretation, is replaced by a simple ensemble postulate. The measurement postulate is still there - but its form is now benign.

Also my view is only one of a number of interpretations that use dehoherence in its foundations.

Thanks
Bill

 

[patfada]

But if our detector does not leak information into the environment, then it will of its own accord be unable to introduce decoherence.

.

Lets say the detector is connected by a mechanical arm to pencil that records the detection event on a piece of paper i.e storage device = pencil+paper. If the lead on the pencil breaks, does the interference pattern come back?

 

[bhobba]

Lets say the detector is connected by a mechanical arm to pencil that records the detection event on a piece of paper i.e storage device = pencil+paper. If the lead on the pencil breaks, does the interference pattern come back?

Of course it doesn't.

There is a confusion here about leaving a mark in the classical world.

Think of a particle detector that say clicks when a particle is detected. Its not the click that does it - its the appearance here in the classical world. If you look at how a particle detector works and trace back to exactly when it makes its first appearance - that's when an observation is made. In the pencil thing its the movement of the mechanical arm - and if you go back further the currents in the wire controlling it and even further back the electric fields controlling those currents.

This also lays the issue of Copenhagen bare - deciding on when that happens is not spelled out in the interpretation. For that you need a fully quantum theory of measurement. The modern view is it occurs once decoherence has happened, which generally happens very very quickly and before it actually leaves some kind of mark. For example in the particle detector decoherence localizes the particle even before it is detected. That's when the interference pattern disappears.

Thanks
Bill

 

[Fiziqs]

There is a confusion here about leaving a mark in the classical world.

Think of a particle detector that say clicks when a particle is detected. Its not the click that does it - its the appearance here in the classical world. If you look at how a particle detector works and trace back to exactly when it makes its first appearance - that's when an observation is made...

For example in the particle detector decoherence localizes the particle even before it is detected. That's when the interference pattern disappears

Bill, is there any experimental evidence to support this interpretation? Or is it simply theoretical?
When it comes to decoherence any additional evidence is always welcome.

Thanks

 

[bhobba]

Bill, is there any experimental evidence to support this interpretation? Or is it simply theoretical? When it comes to decoherence any additional evidence is always welcome

That's why its called an interpretation and not a theory. Its experimentally equivalent to the mathematical formalism - which is basically an interpretation with minimal interpretative aspects - there are a number like that eg Copenhagen and the Ensemble interpretation - the difference being their interpretation of the state which is slightly different in each - but no need to go into it here - the Wikipedia article explains it fairly well:
http://en.wikipedia.org/wiki/Copenhagen_interpretation

Since decoherence follows from the principles of QM every interpretation has it - they just differ in the importance they ascribe to it. Decoherence normally occurs so quickly its hard to detect, but some experiments have been performed under special conditions to slow it down and it has been observed. But that proves nothing since its in all interpretations of QM.

Thanks
Bill

 

[stevendaryl]

Lets say the detector is connected by a mechanical arm to pencil that records the detection event on a piece of paper i.e storage device = pencil+paper. If the lead on the pencil breaks, does the interference pattern come back?

I think that the appearance of mixed states from pure states doesn't require decoherence to understand. Suppose you have a system in state |\psi\rangle = \alpha |\psi_1\rangle + \beta |\psi_2\rangle Now, you let it interact with another system described by state |\varphi\rangle. Then the composite state |\Psi\rangle = |\psi\rangle \otimes |\varphi\rangle will make a transition:

(|\psi\rangle \otimes |\varphi \rangle) \rightarrow \alpha (|\psi_1'\rangle \otimes |\varphi_1\rangle) + \beta (|\psi_2'\rangle \otimes |\varphi_2\rangle)

To see interference effects, you have to disentangle the two systems. When the two systems are small (a few particles), this is possible, but when they are large, it's not.

To me, the only thing that decoherence adds to the story is that there is always two systems interacting, since every particle interacts with the environment (the electromagnetic field).

 

[Prathyush]

Measurement is not different from an entanglement of the recording device to the state of the Particle. There are limitations on how the interaction Hamiltonian can be written, which restricts the possibilities of measurement. How the record is later seen is hardly relevant to the question any more. All future records made, will be entangled with the past records. it is always possible to reverse quantum measurement in principle, Provided all the copies of the record are collected and appropriately evolved using a suitable Hamiltonian. In this process all copies of the record are destroyed. Thus there is no records of the state left in the universe, only the state itself. Only in this condition will measurement be reversible.

 

[StevieTNZ]

I think that the appearance of mixed states from pure states doesn't require decoherence to understand. Suppose you have a system in state |\psi\rangle = \alpha |\psi_1\rangle + \beta |\psi_2\rangle Now, you let it interact with another system described by state |\varphi\rangle. Then the composite state |\Psi\rangle = |\psi\rangle \otimes |\varphi\rangle will make a transition:

(|\psi\rangle \otimes |\varphi \rangle) \rightarrow \alpha (|\psi_1'\rangle \otimes |\varphi_1\rangle) + \beta (|\psi_2'\rangle \otimes |\varphi_2\rangle)

To see interference effects, you have to disentangle the two systems. When the two systems are small (a few particles), this is possible, but when they are large, it's not.

Much like the quantum eraser experiment.

 

[ktx49]

so it would not be adequate to describe "measurement" as any exchange of information between the environment and a quantum system that lowers the entropy of said system?

 

[bhobba]

so it would not be adequate to describe "measurement" as any exchange of information between the environment and a quantum system that lowers the entropy of said system?

Its simply when a quantum system leaves a mark here in the macro world. Nothing deep about it other than philosophical waffle such as if a tree fell in the forest and no one heard it did it make a sound. Just apply a bit of common sense eg in a particle detector it's when a flash or a click happens. If that aspect has been disabled something must have triggered it - then that is when it makes its appearance unless it's like say the mark on a photographic plate when a photon hits - that's the first place its makes its appearance.

Thanks
Bill

 

[ktx49]

so its not useful to think about measurement in terms of information theory?

(PS, don't be offended by my ignorance here, I'm learning so much about QMs just by asking you guys these sort of noob questions...thanks!)

 

[bhobba]

so its not useful to think about measurement in terms of information theory?

(PS, don't be offended by my ignorance here, I'm learning so much about QMs just by asking you guys these sort of noob questions...thanks!)

Of course it is. But information in what? Think about it.

Thanks
Bill