Quantum wave function collapse

[Kessy]

I was wondering if someone with a deeper understanding of this could speak a little on the topic. There are a few details that I've always found a bit vague in conventional explanations about how and when exactly wave function collapse occurs.

Let's start with the classic double slit experiment. If we shoot a stream of electrons at a barrier with two slits in it with a phosphorus coated screen (or similar detector) on the other side, we get a wave interference pattern on the screen. If we then add a device to the barrier to detect the electrons as they pass through the slits, we destroy the interference pattern.

Exactly what sort of detectors are we talking about? I'm guessing something to measure the induced EM field created by the electron passing through the slit? Does it matter what kind of detectors are used? Does it matter if we put a detector on both slits or just one? What if there are three slits?

I've heard some sources say that wave function collapse depends on the particle being observed. I'm a little confused about whether this is talking about a physical interaction or something involving a conscious mind? What exactly qualifies as an observer? My thought about Schrodinger's Cat is, "Doesn't the cat count as an observer?"

Sorry if I'm rambling a bit, but I figure this should be enough to start a conversation.

 

[emperor_of_localgroup]

I was wondering if someone with a deeper understanding of this could speak a little on the topic. There are a few details that I've always found a bit vague in conventional explanations about how and when exactly wave function collapse occurs.

Exactly what sort of detectors are we talking about? I'm guessing something to measure the induced EM field created by the electron passing through the slit? Does it matter what kind of detectors are used? Does it matter if we put a detector on both slits or just one? What if there are three slits?

I've heard some sources say that wave function collapse depends on the particle being observed. I'm a little confused about whether this is talking about a physical interaction or something involving a conscious mind? What exactly qualifies as an observer? My thought about Schrodinger's Cat is, "Doesn't the cat count as an observer?" .

I wish more people participate in this thread, because wavefunction in QM is a fascinating idea. I guess many people just shy away when it comes to QM because the idea of wavefunction is very confusing. When I took QM class (with dinosaurs) I never asked my professor many questions, but now I have so many questions on QM.

Anyway, from what I have read so far, the detector they use is something which does not perturb the path of photon (or electron). But once the detector is placed there is no interference pattern. My one of the big questions here is if the detector can cause the wavefunction to collapse, why not the 'slits' of double slits? Slits are microscopic, they can as well act as detector of photons (electrons). In such case, we should not observe any interference even with 2 slits open. Is there something else going on other than observer theory?

 

[darkmatter4brains]

problem is, nobody really knows the ultimate answer to the question of wave function collapse. I'd highly recommened reading two things

1) Nick Herbert's Quantum Reality - this goes over the wave function collapse and many different viewpoints/interpretations of quantum mechanics - all from a historical to modern day perspective

2) Roger Penrose's latest ideas - it's an attempt, yet to be proven, to bridge the gap between the micro and macro worlds. Also, it talks about the stability of a superposition of states, which ties into wave function collapse

As far as the double slit experiment and interference patterns, the concepts of knowability and unknowability are helpful. With the experiment and it's normal setup, you have no way of knowing which slit the electron goes through and, from the results of the experiment, you have to conclude it went through both! It is unknowable which slit it went through.

Anytime you try to know which slit it went through, no matter what method you use for measuring/detecting that, you will force the electron to make a stand and go through that particular slit, and not the other - the interference pattern will be lost. You essentially forced a collapse of the wave function on that particular slit.

A decent read that goes into examples of this is Brian Green's book The Fabric of Spacetime, or something like that. It's his newest book I think, but I probably don't have the title quite right.

EDIT: I think Penrose ideas, if they turn out to be correct, do get rid of the idea that a conscious obsever is needed for wave function collapse. However, a ways back, John Von Neumann did some sort of mathematical "proof" that the wave function collapse happens at the observer, implying you do need a conscious observer. I put proof in quotation marks, because if it really proved it, the problem of wave function collapse would be settled. Since it's not, I'm guessing the proof wasn't altogether convincing for some reason. Herbert's book covers this too.

 

[captdude]

I know you are asking about the collapse of the wave-form but I thought the Heisenberg uncertainty principle might be worth mentioning here.

The Heisenberg uncertainty principle states that position and momentum, cannot simultaneously be known to any real precision. The more precisely one property is measured, the less precisely the other can be measured. (the more you know the position of a particle, the less you can know about its velocity, and the more you know about the velocity of a particle, the less you can know about its position)
This means that it is impossible to determine both the position and velocity of an electron or any other particle with any sort of accuracy or certainty. The Heisenberg uncertainty principle is not about the limitations of a researcher's ability to measure a system, but a statement about the nature of the system itself as described by the equations of quantum mechanics.

I would suggest reading about Bose–Einstein condensate experiments that were correctly predicted years ago by the extre,me implications of this property of QM

 

[Kessy]

LOL, honestly, I'm not asking for someone to explain the ultimate reality behind quantum mechanics. I'm asking what exactly the experimentally determined parameters are for when wave function collapse occurs and when it doesn't. Does it matter what kind of detector you use? What if a detector is present but not turned on? Or if the detector is turned on but has no way to output its data? How about if the detector sends its data to a computer that is programed to immediately overwrite its RAM after each datapoint is received? And I'm completely serious in asking if a cat counts as an observer. Suppose we do an experiment where the detector's data is sent to an setup that has a hopper full of cat treats with two dispensers and two bowls. After so many detections on a slit, the corresponding dispenser drops out a cat treat, making sure that the cat and only the cat can ever determine which dispenser it came out of. And if cats count as observers, what about other living things? Mice? Insects? Cephelapods? I do hope bacteria don't count as observers, that could have some real implications for conditions here on earth, considering how omnipresent bacteria are.

Errr, okay, that may be taking the idea into slightly silly territory, but still, the point remains.

 

[darkmatter4brains]

LOL, honestly, I'm not asking for someone to explain the ultimate reality behind quantum mechanics. I'm asking what exactly the experimentally determined parameters are for when wave function collapse occurs and when it doesn't.

Well, like I said, we don't know. But, as I said above, Penrose may be honing in on some of this.

Does it matter what kind of detector you use? What if a detector is present but not turned on? Or if the detector is turned on but has no way to output its data? How about if the detector sends its data to a computer that is programed to immediately overwrite its RAM after each datapoint is received? And I'm completely serious in asking if a cat counts as an observer. Suppose we do an experiment where the detector's data is sent to an setup that has a hopper full of cat treats with two dispensers and two bowls. After so many detections on a slit, the corresponding dispenser drops out a cat treat, making sure that the cat and only the cat can ever determine which dispenser it came out of. And if cats count as observers, what about other living things? Mice? Insects? Cephelapods? I do hope bacteria don't count as observers, that could have some real implications for conditions here on earth, considering how omnipresent bacteria are.

Errr, okay, that may be taking the idea into slightly silly territory, but still, the point remains.

I think I addressed most of this above. Just think about the concepts of knowability and unknowability. It doesn't matter what detector, method etc. You can't know the unknowable ;-)

The oberserver question is a good one. Again, if Penrose is right, I believe that gets rid of the necessity of a conscious observer (including cats, mice, etc)

 

[darkmatter4brains]

I think I addressed most of this above. Just think about the concepts of knowability and unknowability. It doesn't matter what detector, method etc. You can't know the unknowable ;-)

Another way to think about this ... is along the lines of wave-particle duality. One would never ask at what specific point a wave exists at in space, because a wave by definition is something that is spread out in space, it's not localized. This is how the probability wave of an electron looks and, as a wave, going through both slits at the same time is no problem. But, the minute you ask what slit did it go through, your asking about something localized, and by taking measurements at this or that slit, you evoke the particle like nature of the electron and force it to go through one or the other of the slits. It's the wave nature of the electron that goes through both, but the measurement process at a specific locale brings out the particle like nature and forces the electron to take a stand and go through that specific slit - the interference pattern is then lost.

So, you cannot know the unknowable ( at what specific point is a wave located at - makes no sense) and by taking measurement the wave like nature is "destroyed" in favor of the particle like nature.

I'm speaking real loosely here and taking some liberties, but hopefully it makes the point clearer.

 

[yevaud]

Exactly what sort of detectors are we talking about?

OK, you asked for it.

Two-Slit Interference

Fabry-Perot Interferometer