Can a conscious observer collapse the probability wave?

[Fiziqs]

I know very little about QM, so forgive me if this question is a bit difficult to comprehend. I understand that there is some debate about whether a conscious observer is necessary to collapse the wave function. But I was wondering if there was any experimental evidence showing to what degree a conscious observer is actually able to collapse the wave function.

What I mean by this is, as an example, as I understand it, in the double slit experiment it doesn't matter if a device is set up to monitor which slit the particle went through, as long as the information about what the device "saw" is not available to an observer, then the device's observation alone is not sufficient to collapse the wave function. Basically if we turn on the device but do not record the information, then the wave function won't collapse. Turn on a recording device, and the wave function collapses. So is a conscious observer, i.e. a person, a sufficient recording device?

If we set up the double slit experiment so that a person gets a visual indication of which slit the particle went through, and then sees where on the detector the particle was found, is the observer's memory a sufficient recording device. In an experimental setting wouldn't we have to send many thousands of particles through the experiment in order to establish a clear interference, or non-interference pattern? Obviously a person would be unable to remember all the slit and detection information, and lacking that information, what type of pattern should we expect to see? If a mechanical recording device was unable to accurately record the information, wouldn't we expect the interference pattern to remain. Likewise a person might be able to collapse the wave function of one individual particle, but after a large number of particles, shouldn't we expect to see an interference pattern? So even with a person watching, might we not expect an interference pattern in the double slit experiment?

If the conscious observer is the key in collapsing the wave function, then we would expect to see a non-interference pattern, but if the observer is simply a recording device, (and an insufficient one) then wouldn't we still see an interference pattern in the double slit experiment?

To me it seems that the conscious observer is merely a recording device, which is why I ask if there is any experimental evidence indicating that a conscious observer can actually collapse the wave function. Is a conscious observer only able to collapse the wave function of that which they are currently observing, but on a long term scale wouldn't the system revert to a probability wave, as the observer's memory is an unreliable recording device?

As you can tell, I'm confused. Any information that might help me clear this up would be appreciated.

 

[StevieTNZ]

I know very little about QM, so forgive me if this question is a bit difficult to comprehend. I understand that there is some debate about whether a conscious observer is necessary to collapse the wave function. But I was wondering if there was any experimental evidence showing to what degree a conscious observer is actually able to collapse the wave function.

In principle we can determine where collapse occurs. See sources on proper vs improper mixtures.

If the conscious observer is the key in collapsing the wave function, then we would expect to see a non-interference pattern, but if the observer is simply a recording device, (and an insufficient one) then wouldn't we still see an interference pattern in the double slit experiment?

We would see an interference pattern for those particles we weren't able to determine which path each went through.

 

[Cthugha]

What I mean by this is, as an example, as I understand it, in the double slit experiment it doesn't matter if a device is set up to monitor which slit the particle went through, as long as the information about what the device "saw" is not available to an observer, then the device's observation alone is not sufficient to collapse the wave function.

This is unfortunately already incorrect. Any measurement is considered a measurement, irrespective of whether there is a conscious observer taking note of it. There is no experimental evidence suggesting something else.

 

[Fiziqs]

This is unfortunately already incorrect. Any measurement is considered a measurement, irrespective of whether there is a conscious observer taking note of it. There is no experimental evidence suggesting something else.

Forgive the poor wording of my question, but I did not mean to imply that a conscious observer was necessary. Quite the opposite, I was positing that a conscious observer is actually a very poor method of collapsing the wave function, but I should have been clearer.

What I meant to say was, that it is my understanding, that even if there is a device to measure which slit the particle went through, as long as that information is not recorded in some way, then the interference pattern will persist. It's not the observation that matters, it's the recording of the results of the observation that matters.

As such, a person, being a poor recording device, is ill equipped to collapse the wave function. Even if a conscious observer sees which slit the particle went through, that is not enough to collapse the wave function. They must remember which slit the particle went through, and human memory is short-lived and unreliable, making them a poor recording device. So what I was actually implying was that in some sense a conscious observer is a very ineffectual means of collapsing the wave function. It's not the observation that matters, it's the information that matters.


StevieTNZ's answer makes some sense to me:

In principle we can determine where collapse occurs. See sources on proper vs improper mixtures.

We would see an interference pattern for those particles we weren't able to determine which path each went through.

If I understand it correctly he's saying that we will see an interference pattern for those particles which we cannot remember which slit the particle went through, and a non-interference pattern for those particles which we can remember which slit they went through. That's if I understand what he was referring to when he says proper and improper mixtures.

But I'm not absolutely certain that StevieTNZ's explanation is correct. I'm not sure that we would see any non-interference pattern at all. Unlike a physical/mechanical recording device, the human brain is notoriously unreliable, and even though a conscious observer may be certain that they remember which slit a particular particle went through, there is always some chance that they are mistaken. And it is conceivable that this inherent uncertainty, however small, is enough to introduce interference.

If we agree that the determining factor in collapsing the wave function is whether or not the information is recorded somewhere, then we have to ask ourselves, whether "remembering" something is the same as "recording" something. Does memory actually constitute "recording"? Can the method by which the human brain stores information truly be characterized as "recording" the information? I'm not so certain that it can. What constitutes "recording" the information?

.

 

[Cthugha]

Forgive the poor wording of my question, but I did not mean to imply that a conscious observer was necessary. Quite the opposite, I was positing that a conscious observer is actually a very poor method of collapsing the wave function, but I should have been clearer.

What I meant to say was, that it is my understanding, that even if there is a device to measure which slit the particle went through, as long as that information is not recorded in some way, then the interference pattern will persist. It's not the observation that matters, it's the recording of the results of the observation that matters.

As such, a person, being a poor recording device, is ill equipped to collapse the wave function. Even if a conscious observer sees which slit the particle went through, that is not enough to collapse the wave function. They must remember which slit the particle went through, and human memory is short-lived and unreliable, making them a poor recording device. So what I was actually implying was that in some sense a conscious observer is a very ineffectual means of collapsing the wave function. It's not the observation that matters, it's the information that matters.

Ok, that depends a bit on what you mean by recording something. For example if some measurement device is present, it does not really matter whether it is switched on or off. If a conscious observes "sees" which slit a particle goes through, the measurement and recording has already taken place. If the slit which a particle goes through is measured in some way, that means that some process must take place. Some electron inside some detector changes energy or momentum, some system may heat up, some other particle may be deflected or whatever. This is already a measurement. Anything that changes entropy for example is necessarily equivalent to being an act of recording. Therefore, if an observer watches which path a particle goes through, you must have some process beforehand, which involves light emission which the observer can see. This process will constitute a measurement. By the way it does not matter whether that "recording" is permanent or immediately thrown away. Remembering is not necessary.

 

[San K]

To me it seems that the conscious observer is merely a recording device, which is why I ask if there is any experimental evidence indicating that a conscious observer can actually collapse the wave function. Is a conscious observer only able to collapse the wave function of that which they are currently observing, but on a long term scale wouldn't the system revert to a probability wave, as the observer's memory is an unreliable recording device?

As you can tell, I'm confused. Any information that might help me clear this up would be appreciated.

Interaction (such as detection, measurement) with any device is what causes the wave-function collapse. Human/life consciousness has nothing to do with it. That said, some would not agree with this.

This (i.e. interaction not consciousness causes collapse) can be proved via having a cascade of events (like a domino effect), with no observer involved in the intermediate stages.

Again, some would not agree and could build a logic that the collapse finally took at the consciousness stage, however the rebuttal to is is that an intermediate instrument would have recorded it. The argument could go on till the cows come home...:)

 

[bhobba]

These days decoherence is usually invoked to explain collapse and its actually quite hard to come up with a physical situation where it doesn't occur. Long before it reaches the brain it is virtually 100% sure it has been 'collapsed' by decoherence.

Thanks
Bill

 

[lugita15]

Here's an old post of mine that you may find interesting:

The reason there is still disagreement as to what constitutes measurement is that it makes no experimental difference according to quantum mechanics. The way QM works under the Copenhagen interpretation is that you have to split the world into two parts, the “observer” or measurement device, and the “observed” or the particles you’re measuring.

The measurement device is assumed to behave classically. The particles in the observed system are in a superposition of states described by the wave function which keeps evolving until it interacts with the classical measurement device. The question is where to draw the line. You could consider a photon to be the observed system and an atom to be the measuring device, but you can also consider the photon-and-atom system as in a superposition of states, and take a Geiger counter to be the measurement device. So there is this von-Neumann chain, going from elementary particles to Geiger counters to human beings, and we have to decide where to cut it off.

Von Neumann proved in his famous "Bible" of QM that regardless of where you cut the chain, you would get the same experimental results. But he argued that wherever you cut the chain you have things made out of particles on each side of the cut, so there’s no principled way to place the cut in the middle. So he decided that you should place the cut between the human mind and the human body, because he believed that the mind is non-physical. Hence "consciousness causes collapse" was born. Nowadays, the most popular view is decoherence, where there is no real collapse, it's just that when you have a large number of particles in the environment interacting with the system, the wave function becomes smeared out and looks like it has collapsed. So decoherence gives us a reasonable place to cut the chain, when the number of particles involved reaches a critical number so that interference effect become negligible.

 

[Fiziqs]

Thanks everyone for the input. I will consider each of your points carefully.

lugita15, I distinctly remember reading that post. I actually read many of the other threads. At least the ones that I can understand.

This (i.e. interaction not consciousness causes collapse) can be proved via having a cascade of events (like a domino effect), with no observer involved in the intermediate stages.

San K, can you point me to an experiment demonstrating this phenomena of cascading events?


I guess that I'm going to have to take a closer look at decoherence and what actually constitutes a measurement/interaction. For instance, if I set up the double slit experiment, but all I add is a polarizer after the double slit, so that I interact with the particle, but gain no which-way information, will this cause the wave function to collapse? It seems that in many experiments a complex series of "interactions" is performed without collapsing the wave function. How is it that experimenters can interact with the particle to such an extent without causing the wave function to collapse, if it's interactions that are supposedly causing the collapse? It had always been my understanding that it was only when the which-way information was present that the wave function collapsed.

Anyway, I'll keep thinking about it, and no doubt be back with more dumb questions.

 

[StevieTNZ]

Ok, that depends a bit on what you mean by recording something. For example if some measurement device is present, it does not really matter whether it is switched on or off. If a conscious observes "sees" which slit a particle goes through, the measurement and recording has already taken place. If the slit which a particle goes through is measured in some way, that means that some process must take place. Some electron inside some detector changes energy or momentum, some system may heat up, some other particle may be deflected or whatever. This is already a measurement. Anything that changes entropy for example is necessarily equivalent to being an act of recording. Therefore, if an observer watches which path a particle goes through, you must have some process beforehand, which involves light emission which the observer can see. This process will constitute a measurement. By the way it does not matter whether that "recording" is permanent or immediately thrown away. Remembering is not necessary.

You are just advancing your particular viewpoint on the matter as correct.

Here's an old post of mine that you may find interesting:

Do you not remember the PDF I sent you?

 

[Fredrik]

What I meant to say was, that it is my understanding, that even if there is a device to measure which slit the particle went through, as long as that information is not recorded in some way, then the interference pattern will persist. It's not the observation that matters, it's the recording of the results of the observation that matters.

As such, a person, being a poor recording device, is ill equipped to collapse the wave function. Even if a conscious observer sees which slit the particle went through, that is not enough to collapse the wave function.

No, this is much, much more than what's needed to collapse the wave function. All it takes to effect a collapse is that some kind of record is created. This "record" can be something that for practical purposes is completely useless, like a tiny change in the state of the surrounding air.

If you see something, then the nerve signal from your eye to your brain is a record of what you saw, regardless of whether your brain will store it or not. If what you're looking at is the display of a measuring device, then you can be sure that many other records of the state of the measured object had already been created before the nerve signal.

There are by the way double-slit experiments with large molecules in which the interference pattern looks more and more like the pattern from two single slits, as the density of the surrounding air is increased.

 

[audioloop]

as for decoherence

..The fact that interference is typically very well suppressed between localised states of macroscopic objects suggests that it is relevant to why macroscopic objects in fact appear to us to be in localised states. A stronger claim is that decoherence is not only relevant to this question but by itself already provides the complete answer. In the special case of measuring apparatuses, it would explain why we never observe an apparatus pointing, say, to two different results, i.e. decoherence would provide a solution to the measurement problem of quantum mechanics. As pointed out by many authors, however (e.g. Adler 2003; Zeh 1995, pp. 14–15), this claim is not tenable..

..Unfortunately, naive claims of the kind that decoherence gives a complete answer to the measurement problem are still somewhat part of the ‘folklore’ of decoherence, and deservedly attract the wrath of physicists (e.g. Pearle 1997) and philosophers (e.g. Bub 1997, Chap. 8) alike..

http://plato.stanford.edu/entries/qm-decoherence/#SolMeaPro
The Role of Decoherence in Quantum Mechanics
First published Mon Nov 3, 2003; substantive revision Mon Apr 16, 2012

 

[Fiziqs]

No, this is much, much more than what's needed to collapse the wave function. All it takes to effect a collapse is that some kind of record is created. This "record" can be something that for practical purposes is completely useless, like a tiny change in the state of the surrounding air.

If you see something, then the nerve signal from your eye to your brain is a record of what you saw, regardless of whether your brain will store it or not. If what you're looking at is the display of a measuring device, then you can be sure that many other records of the state of the measured object had already been created before the nerve signal.

I'm sorry, but I fail to see how the surrounding air is going to provide me with any which-path information. I may be completely wrong on this, but it seems to me that the photons interacting with the surrounding air tell me only that a signal was detected, it gives me no information about which slit the signal came from. It takes my brain to combine the information from a multitude of nerve impulses to ascertain the which-path information. One photon, or one nerve, on their own wouldn't constitute a recording device in my opinion.

Now I could set the experiment up so that it would flash a red light for one slit, and a blue light for the other slit, then each individual photon would act as a recording device telling me which path the electron took. In that case the photons themselves would act as a recording device. But absent such a distnction, my brain would become the recording device. But in any case my original premise still remains, the brain is a very poor recording device.

I understand what you're saying, but I just think that your assertion that the air would act as a recording device is in error. Of course I know diddley about photons and optic nerves, so I could well be completely wrong.

.

 

[Fiziqs]

http://plato.stanford.edu/entries/qm-decoherence/#SolMeaPro
The Role of Decoherence in Quantum Mechanics
First published Mon Nov 3, 2003; substantive revision Mon Apr 16, 2012

That link has been on my list of favorite sites for awhile now. I still haven't been able to understand most of it, even as simplistically as it tries to present everything. But I'm working on it. One of these days, I'm going to figure this stuff out.

 

[bhobba]

..Unfortunately, naive claims of the kind that decoherence gives a complete answer to the measurement problem are still somewhat part of the ‘folklore’ of decoherence, and deservedly attract the wrath of physicists (e.g. Pearle 1997) and philosophers (e.g. Bub 1997, Chap. 8) alike..

It attracts the wrath of all physicists? Hardly - not even the majority as far as I can tell. I would say the majority opinion is that expressed by Scholosshauer in his book I am currently studying. It is controversial in some quarters - that's all. I think it is misleading to give the idea it is not generally accepted by the physics community. For example, as pointed out in Scholosshauer book, even Wigner, a proponent of consciousness causes collapse abandoned it when he read some early papers on the decoherence program.

When it is said it does not solve the measurement problem that is true of the measurement problem as usually formulated - but change it to for all practical purposes and it solves that no problemo. If you think not describe the experiment that can tell the difference between a wavefuntion collapse where it collapses into a particular pure state (that's how it is usually formulated) and one where it picks a pure state from a mixed state which is what decoherence does explain. As far as I can tell there is no way to tell the difference - it is simply experimentally indistinguishable from actual collapse.

Issues do remain and further research needs to be done but I think the issues have basically been solved.

Thanks
Bill

 

[Fredrik]

I'm sorry, but I fail to see how the surrounding air is going to provide me with any which-path information.

It's not going to provide you with that information, but the information is there, regardless of whether you can extract it or not.

I may be completely wrong on this, but it seems to me that the photons interacting with the surrounding air tell me only that a signal was detected, it gives me no information about which slit the signal came from.

Right, it doesn't give you that information. But you still get the same interference pattern as if you had put particle detectors at the slits and examined the result after each particle is sent through. The experiment with C70 molecules that I mentioned is discussed in Schlosshauer's decoherence book, section 6.2.

 

[bhobba]

The experiment with C70 molecules that I mentioned is discussed in Schlosshauer's decoherence book, section 6.2.

I am studying that book right now and am very impressed with it - highly recommended.

Thanks
Bill

 

[Fiziqs]

It's not going to provide you with that information, but the information is there, regardless of whether you can extract it or not.

I will admit that I'm a bit dense, but I still don't see how the photons contain any information about which path the electron took. I could understand if I had a photon emitted from the detector at slit "A" that was in some way distinguishable from a photon emitted from the detector at slit "B", but if the photons emitted from slit "A" and slit "B" are identical, then how can they possibly contain any information about which slit they came from?

I don't see the photons mere existence as evidence of any which-path information. In what way do the photons contain any which-path information?

I will make a point of looking for information about Schlosshauer's decoherence. Hopefully I will be able to find some references. But it is far less likely that I'll understand the references. But I'll try.

Thanks

 

[Cthugha]

You are just advancing your particular viewpoint on the matter as correct.

No, absolutely not. The notion that a typical process which changes entropy is equivalent to a measurement is pretty standard.

I could understand if I had a photon emitted from the detector at slit "A" that was in some way distinguishable from a photon emitted from the detector at slit "B", but if the photons emitted from slit "A" and slit "B" are identical, then how can they possibly contain any information about which slit they came from?

If you use light with a coherence volume smaller than the slit distance, they are automatically distinguishable. This is not a big problem.

 

[Fredrik]

I will admit that I'm a bit dense, but I still don't see how the photons contain any information about which path the electron took. I could understand if I had a photon emitted from the detector at slit "A" that was in some way distinguishable from a photon emitted from the detector at slit "B", but if the photons emitted from slit "A" and slit "B" are identical, then how can they possibly contain any information about which slit they came from?

I don't see the photons mere existence as evidence of any which-path information. In what way do the photons contain any which-path information?

I wasn't talking about photons (we were talking about air molecules), but OK, let's do that. If you manage to make the photons indistinguishable, then you won't be able to use them to obtain which-path information. But the state of the detector is already a record of the which-path information, so the interference pattern will be the one you would get if you only had one slit open each time a particle passes through (slit A open half of those times, and slit B open the other half). It doesn't matter if that information is communicated to you or not.

 

[Fiziqs]

If you use light with a coherence volume smaller than the slit distance, they are automatically distinguishable. This is not a big problem.

I have no reason to doubt that you are correct. Which was of course what I was rather simplistically trying to show with my example of using red and blue lights, if you "mark" the photons in some manner such that they become a recording device, then the observer doesn't need to serve as the recording device. The original OP however was about the ability of a conscious observer to serve as a recording device and collapse the probability wave.

Can the human brain really serve as a recording device? And if they can't, what does that imply about an observer created reality?

In a somewhat related question, what actually happens when a which-path observation is made but not recorded? Does the wave function collapse for an instant in time, only to re-emerge the following instant? And in an even more ridiculous corollary, do the slits themselves actually serve as an measuring device, but their inability to record the information means that the wave function immediately re-emerges? What do these things imply about the state of the "reality" that I see around me?

So many questions, so little time. As you can no doubt tell, I don't know much about this stuff. So I appreciate any and all input. I crave information. I want to know what's happening.

.

 

[Fiziqs]

I wasn't talking about photons (we were talking about air molecules), but OK, let's do that. If you manage to make the photons indistinguishable, then you won't be able to use them to obtain which-path information. But the state of the detector is already a record of the which-path information, so the interference pattern will be the one you would get if you only had one slit open each time a particle passes through (slit A open half of those times, and slit B open the other half). It doesn't matter if that information is communicated to you or not.

I willingly admit that I had to reread this several times to get the gist of it, but I think that I've got it. Funny enough I just asked in my previous post if the slits themselves can act as an observer, so it would not shock me to think that the detectors at slits "A" and "B" could act as observers. But I'm curious to know exactly how they act as a recording device, and furthermore what happens if they can't. Does the wave function collapse at the moment of observation only to immediately re-emerge?

Anyway, thank you for your input, and forgive me if I ask too many questions. I'm just too curious I suppose.

.

 

[San K]

Can the human brain really serve as a recording device? And if they can't, what does that imply about an observer created reality?

sure Fiziqs, why not.

it's about (specific kinds of) interaction with the photon that "collapses the wave function".
it could be anything - an experimental device, polarizer, quarter wave-plate, human eye/retina/hand etc.

as to your previous question about the cascade:

its a simple idea and can be easily replicated. you can imagine any of the domino effect demonstrations

the idea was to just record the photon's (and even add a clock/time-stamp) position, time etc...without any human being watching/being there.

The cascade could be made by having some device (such as a computer, bar code, pendulum, electromagnetic device, Morse code device, hourglass etc.) read of that time and record and you could go on adding more devices to the "cascade", without the presence of any human/life.

A human/physicist could check the whole setup/cascade later (say next hour/day/week/month). the "cascade/series" serves as "additional proof/doubly sure".

note: one can still argue about human consciousness causing the collapse...;), but the counter to that would be the various events and their domino effects in the universe cannot all be waiting (that's way too unrealistic/complicated) for a human to look at the final event to cause the collapse...because that would stop a lot of the other events in the experimenters' part of the universe.

 

[yuiop]

I read that if we take the classic spilt beam interference experiment like this:

and send photons one at a time, then if we mount the mirrors(M) on sensitive devices that detect the reflection of a photon the interference pattern is destroyed. This destruction of the interference pattern due to the potential to determine "which way" information occurs even if the the mirror deflection devices are not connected to any counting or other recording devices. The mere interaction and potential to record which way" information is sufficient to collapse the probability function. Personally (although I might be wrong) an "observation" of a quantum particle is any interaction between the particle and its surroundings that causes a physical change that could in principle be detected by a human, even if it is not. "Observation" is any physical interaction between the quantum particle and the (coherent?) environment and does not have to include a human observer. That makes the phrase "observation causes collapse" a bit misleading. I think "interaction causes collapse" is a bit more accurate. Observation necessarily involves interaction but not vice versa.

P.S. Bear in mind that the universe evolved for billions of years before sentient observers evolved and presumably quantum principles worked just fine even back then.

 

[San K]

"Observation" is any physical interaction between the quantum particle and the (coherent?) environment and does not have to include a human observer. That makes the phrase "observation causes collapse" a bit misleading. I think "interaction causes collapse" is a bit more accurate. Observation necessarily involves interaction but not vice versa.

i guess...any interaction that causes a change in phase...i.e. from coherent to de-coherent...

there is degree of de-coherency as well. if they are in same phase (i.e. coherent) the interference pattern is clear...as we change the phase slowly the interference pattern gets murkier/muddier

 

[Fredrik]

That makes the phrase "observation causes collapse" a bit misleading. I think "interaction causes collapse" is a bit more accurate. Observation necessarily involves interaction but not vice versa.

But reflection from a mirror is an interaction, isn't it? It's just that the effect on the mirror isn't large enough to create a record of what just happened. So I think it's more accurate to say that an observation is a special kind of interaction, the kind that produces a record of the result (a record that for all practical purposes can be treated classically).

 

[Fiziqs]

Can the human brain really serve as a recording device? And if they can't, what does that imply about an observer created reality?

sure Fiziqs, why not.

Just to be clear, and not to imply that my opinion means anything, but I personally do not believe that a conscious observer is necessary to collapse the wave function. But I do believe that information about the state of the system is absolutely essential. And I also believe that the state of the system is always relative to the observer. (The observer being any other system. It does not need to be conscious) But of course, since I am a conscious observer, it is always going to require a conscious observer (me) to collapse the wave function relative to me. I try to base all of my conclusions purely on logic, and as such I appreciate and seek any information that will confirm or deny my conclusions. I want information. My conclusions are only as good as my information. Unfortunately I often lack the education necessary to interpret the information that I receive, but I'm doing my best. I tend not to put too much confidence in the interpretations of others unless they can logically defend them. But then again, my lack of education can make it difficult to discern a logical interpretation from a completely asinine one, especially if that interpretaion is based solely on mathematics, of which I understand absolutely squat. By the way, I have looked for an experiment with a cascading series of measurements but have been unable to find one, but I'll keep looking. At the moment I have a lot of information to sift through, and it is a difficult and tedious process for me.

As to my question about the human brain being an adequate recording device, it has to do with the idea of available information. In my view a system only collapses relative to an observer to the point that the information concerning the state of that system is available to the observer. I completely accept that if a photon, or an optic nerve, or a brain cell has some means of discerning or recording the which-path information, then that information is theoretically available to the observer, and I accept that that is enough to collapse the wave function. But I do not, as of now, accept that any and all interactions are capable of collapsing the wave function, only those interactions which are capable of providing which-path information. Thus my problem is, how does the photon in my prior example give me which-path information? I do realize, and gave an example of how a photon could be configured to give that information, and in such a case I fully accept that a photon could record which-path information. I do however fail to see how an intervening air molecule or a single neuron or a brain cell could provide which-path information. Could I take that photon, or that neuron to someone else and tell them, this is how I set up the experiment, and this is the photon I got, would that photon then theoretically tell them the which-path information? If not, then it doesn't contain the which-path information. Perhaps even more intriguing, is there a brain cell, or group of brain cells that would give them the which-path information?

I realize that this line of reasoning is getting a bit absurd, but my major concern about the brain is, can it store the information? To me this is an important factor because this would determine whether the wave function collapse is enduring, or temporary. Yes, I do believe that a wave function once collapsed will revert to its former state if the information is lost. This line of reasoning leads to some very profound questions regarding the character of consciousness, and the relationship between the mind and material reality, but I really don't want to go there right now, and forgive me for blathering on like an idiot.

There are so many things that I'm still trying to figure out. Like what type of interaction is necessary to collapse the wave function, and collapse it relative to whom or what? Right now I'm trying to get a clearer picture about the differences between a matrix and an ensemble. I realize that this is elementary to 99% of the people here, but I'm still working on understanding these things. I also realize that I not only look like an idiot, I am an idiot. But everybody starts out as an idiot, right.

So if anyone has any information that might help me get a handle on this I would really appreciate it. Opinions are welcome too, but will of course be taken with a grain of salt, but rarely dismissed entirely.

yuiop, I'm just now contemplating your post, give me a bit to think about it.

.

 

[Fiziqs]

I read that if we take the classic spilt beam interference experiment like this:

and send photons one at a time, then if we mount the mirrors(M) on sensitive devices that detect the reflection of a photon the interference pattern is destroyed. This destruction of the interference pattern due to the potential to determine "which way" information occurs even if the the mirror deflection devices are not connected to any counting or other recording devices. The mere interaction and potential to record which way" information is sufficient to collapse the probability function. Personally (although I might be wrong) an "observation" of a quantum particle is any interaction between the particle and its surroundings that causes a physical change that could in principle be detected by a human, even if it is not. "Observation" is any physical interaction between the quantum particle and the (coherent?) environment and does not have to include a human observer. That makes the phrase "observation causes collapse" a bit misleading. I think "interaction causes collapse" is a bit more accurate. Observation necessarily involves interaction but not vice versa.

P.S. Bear in mind that the universe evolved for billions of years before sentient observers evolved and presumably quantum principles worked just fine even back then.

Some very quick questions. In the diagram, what is the purpose of putting the beam splitter at BS2, and what exactly do the labels "TR+RT" and "TT+RR" signify. Is there a non-interference pattern at both detectors, or is the data combined?

Dumb questions I know. Perhaps if there is a description of the experiment at Wikipedia I could get the details there. Does this experiment have a name that I could Google?

Thanks

P.S. I realize that TR+RT and TT+RR refers to the paths of the particles, but why are they in that combination?

 

[Fredrik]

Some very quick questions. In the diagram, what is the purpose of putting the beam splitter at BS2, and what exactly do the labels "TR+RT" and "TT+RR" signify.

T=transmitted
R=reflected
TR+RT = "this beam consists of a) photons that were transmitted at BS1 and reflected at BS2, and b) photons that were reflected at BS1 and transmitted at BS2".

Without BS2, D1 would only detect photons that were reflected at BS1, and D2 would only detect photons that were transmitted at BS1. BS2 recombines the beams, making it impossible to tell if an individual photon that's detected at either D1 or D2 was transmitted or reflected at BS1.

BS2 also ensures that by carefully adjusting the lengths of the T and R paths, we can choose what percentage of the total number of photons will end up at D1.

 

[StevieTNZ]

But reflection from a mirror is an interaction, isn't it? It's just that the effect on the mirror isn't large enough to create a record of what just happened. So I think it's more accurate to say that an observation is a special kind of interaction, the kind that produces a record of the result (a record that for all practical purposes can be treated classically).

FAPP, but not in principle? There lies the clue.

 

[Fredrik]

And I also believe that the state of the system is always relative to the observer. (The observer being any other system. It does not need to be conscious) But of course, since I am a conscious observer, it is always going to require a conscious observer (me) to collapse the wave function relative to me.

It sounds like you're confusing collapse with correlation, or in a slightly different terminology, measurement with pre-measurement. I started writing an explanation, but I realized that it would take too long. To understand these things, you will have to study measurements in a book, e.g. chapter 9 of Ballentine. It would also help to make sure that you understand density matrices, and the difference between "pure" and "mixed" states. Then you could read something about decoherence theory. (No one said that these things would be easy).

 

[Fiziqs]

I read that if we take the classic spilt beam interference experiment like this:

and send photons one at a time, then if we mount the mirrors(M) on sensitive devices that detect the reflection of a photon the interference pattern is destroyed. This destruction of the interference pattern due to the potential to determine "which way" information occurs even if the the mirror deflection devices are not connected to any counting or other recording devices. The mere interaction and potential to record which way" information is sufficient to collapse the probability function. Personally (although I might be wrong) an "observation" of a quantum particle is any interaction between the particle and its surroundings that causes a physical change that could in principle be detected by a human, even if it is not. "Observation" is any physical interaction between the quantum particle and the (coherent?) environment and does not have to include a human observer. That makes the phrase "observation causes collapse" a bit misleading. I think "interaction causes collapse" is a bit more accurate. Observation necessarily involves interaction but not vice versa.

P.S. Bear in mind that the universe evolved for billions of years before sentient observers evolved and presumably quantum principles worked just fine even back then.

yuiop, I have been doing my best to find a reference of some kind to the experiment you describe, but I guess that my Googling skills just aren't up to the task.

I did manage to find a recent thread on this topic here at PF, but no reference to the experiment having actually been carried out.

https://www.physicsforums.com/showthread.php?t=589870

If you have a source reference I would greatly appreciate it.

Thanks

 

[Fiziqs]

It sounds like you're confusing collapse with correlation, or in a slightly different terminology, measurement with pre-measurement. I started writing an explanation, but I realized that it would take too long. To understand these things, you will have to study measurements in a book, e.g. chapter 9 of Ballentine. It would also help to make sure that you understand density matrices, and the difference between "pure" and "mixed" states. Then you could read something about decoherence theory. (No one said that these things would be easy).

No need to write a long and complicated answer. Just the idea that I might be confusing collapse with correlation is enough to give me a new direction to go in. And the concept of pre-measurement was new to me too.

It doesn't take a long and complicated answer to pique my interest. You don't have to give me the answers, just show me where to dig, and I'll find the answer.

So thanks

P.S. I wouldn't want it to be easy.

 

[Fredrik]

Just the idea that I might be confusing collapse with correlation is enough to give me a new direction to go in.

I'm looking at these two concepts now, and it seems to me that the distinction is less significant than I had realized before. So maybe I was wrong to suggest that it's important to distinguish between them. I do however stand by my comments about what to read. Ballentine's chapter 9 explains von Neumann's treatment of measurements. For a brief glimpse of that, see this section of the Wikipedia article on measurements in QM. To go beyond that, you need to study decoherence, e.g. the book by Schlosshauer and the review articles by Zurek. To do that, you will need to understand terms like "reduced density matrix".

The term "premeasurement" is used in that Wikipedia article, but it doesn't seem to be used in the books I own. So it may be a non-standard term.

 

[yuiop]

yuiop, I have been doing my best to find a reference of some kind to the experiment you describe, but I guess that my Googling skills just aren't up to the task.

I did manage to find a recent thread on this topic here at PF, but no reference to the experiment having actually been carried out.

https://www.physicsforums.com/showthread.php?t=589870

If you have a source reference I would greatly appreciate it.

Thanks

I am afraid I do not have a reference to an actual Mach–Zehnder interferometer experiment that includes mirrors that can actually detect the the reflection of a single photon. As far as know it is just a thought experiment and the prediction is just hypothetical. It is in some ways similar to the quantum bomb detector experiment, which I think you would find interesting to read up on. The physicsforums discussion on the subject that you found seems to fairly good and worth a read.

But reflection from a mirror is an interaction, isn't it? It's just that the effect on the mirror isn't large enough to create a record of what just happened. So I think it's more accurate to say that an observation is a special kind of interaction, the kind that produces a record of the result (a record that for all practical purposes can be treated classically).

I agree that reflection from a mirror is an interaction and clearly the thought experiment raises the issue of degree of interaction and the effect of the interaction on decoherence or collapse. For a normally mounted mirror, the inertia of the mirror is so great that any movement of the mirror due to deflection of a photon would be negligible and essentially undetectable especially if there is any background noise present. One has to wonder if there is some slight loss of the interference pattern even in this case, with a gradually increasing loss of interference with increasing sensitivity of the mirror to photon deflection or if there is a threshold value with a binary switch from interference plus no which way detection to no interference plus which way detection? There seems to be some hint in the literature of a grey scale with "weak measurements" that allow detection of which way information while still preserving an interference pattern, but I am not clear on this and it seems to violate some quantum principle.