Can a conscious observer collapse the probability wave?

1. Jun 25, 2012

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.

Last edited: Jun 25, 2012
2. Jun 25, 2012

StevieTNZ

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.

3. Jun 26, 2012

Cthugha

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.

4. Jun 26, 2012

Fiziqs

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:

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?

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5. Jun 26, 2012

Cthugha

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.

6. Jun 26, 2012

San K

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....:)

7. Jun 26, 2012

Staff: Mentor

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

8. Jun 26, 2012

lugita15

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

9. Jun 26, 2012

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.

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.

10. Jun 26, 2012

StevieTNZ

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

Do you not remember the PDF I sent you?

11. Jun 26, 2012

Fredrik

Staff Emeritus
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.

12. Jun 26, 2012

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

Last edited: Jun 26, 2012
13. Jun 26, 2012

Fiziqs

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.

.

Last edited: Jun 26, 2012
14. Jun 26, 2012

Fiziqs

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 gonna figure this stuff out.

Last edited: Jun 26, 2012
15. Jun 26, 2012

Staff: Mentor

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

Last edited: Jun 26, 2012
16. Jun 26, 2012

Fredrik

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

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.

Last edited: Jun 26, 2012
17. Jun 26, 2012

Staff: Mentor

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

Thanks
Bill

18. Jun 26, 2012

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

19. Jun 27, 2012

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.

20. Jun 27, 2012

Fredrik

Staff Emeritus
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.