Can a conscious observer collapse the probability wave?

Fiziqs

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

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

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 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

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.

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Fiziqs

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.

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San K

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

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

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

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.

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Fiziqs

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

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

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

Fredrik

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

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.

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

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

Thanks

Fiziqs

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

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.