B Does Human Observation Affect the Double Slit Experiment Results?

[laymanB]

Let's say we do a double slit experiment with the standard laser/two slit setup with a screen to view the pattern.

1. In the first run, we just turn on the laser and observe the screen with our unaided eyes. We see an interference pattern consistent with the wave-like nature of light, correct?

2. In the second run, we place a video camera on the screen and start recording while we observe the laser light between the source and the two slits with our unaided eyes. When we view the recording from the camera we still see an interference pattern on the screen, correct?

3. In the final run, we place a video camera on the screen and start recording while we observe the laser light between the two slits and the screen with our unaided eyes. When we view the recording from the camera we still see an interference pattern on the screen, correct?

Would not this experiment show that the unaided human eye and brain (or consciousness if you want to use that term) have no effect on the laser light displaying wave-like behavior as opposed to particle-like behavior?

 

[PeterDonis]

The answer to all three of your questions is "yes, correct".

Would not this experiment show that the unaided human eye and brain (or consciousness if you want to use that term) have no effect on the laser light displaying wave-like behavior as opposed to particle-like behavior?

No, because your human eye and brain still has to look at the recording from the video camera and observe that it shows an interference pattern. So a person who wanted to claim that only human consciousness collapses the wave function (or affects the wave-like vs. particle-like behavior of the light, or whatever) could still claim that, until the human looked at it, the video camera recording was actually in a superposition and had not yet collapsed.

A better response to this kind of claim (i.e., a claim that only human consciousness can collapse the wavefunction, etc.) is that, since it is inherently untestable (I've just explained how it can explain away any possible test result you can imagine), it's not a scientific claim.

 

[Khashishi]

How do you intend to observe the laser light before it hits the screen? In practice, there's some dust in the air which will cause the laser to scatter and be visible. Is that what you are talking about?

 

[laymanB]

How do you intend to observe the laser light before it hits the screen? In practice, there's some dust in the air which will cause the laser to scatter and be visible. Is that what you are talking about?

I hadn't really thought about that. Can't you see some laser beams with the unaided eye?

 

[laymanB]

The answer to all three of your questions is "yes, correct".

No, because your human eye and brain still has to look at the recording from the video camera and observe that it shows an interference pattern. So a person who wanted to claim that only human consciousness collapses the wave function (or affects the wave-like vs. particle-like behavior of the light, or whatever) could still claim that, until the human looked at it, the video camera recording was actually in a superposition and had not yet collapsed.

A better response to this kind of claim (i.e., a claim that only human consciousness can collapse the wavefunction, etc.) is that, since it is inherently untestable (I've just explained how it can explain away any possible test result you can imagine), it's not a scientific claim.

But would not the recording on the video camera then show two marks on the screen consistent with particle-like behavior, even if the result was delayed until I watched the video.

 

[Khashishi]

Of course you can see the beam if you stick your eye in the beam. But you can't see the beam just looking at it from the side (except the parts that scatter on dust).

 

[laymanB]

Of course you can see the beam if you stick your eye in the beam. But you can't see the beam just looking at it from the side.

Then if we had some fine particles in the air, like dust, in which to scatter the laser to view it from other angles, would this compromise our experiment?

 

[Khashishi]

The dust helps you see the laser but it doesn't give you any which way information for individual photons hitting the screen, since the photons that hit the dust don't make it to the screen. You will see a double slit pattern.

 

[DrChinese]

I hadn't really thought about that. Can't you see some laser beams with the unaided eye?

You can certainly see laser light that reflects in your direction from dust or gas or other particles the light encounters. But otherwise you would not see that light unless it was going straight into your eyes.

 

[PeterDonis]

would not the recording on the video camera then show two marks on the screen consistent with particle-like behavior

Why? Particle-like behavior only happens if you have something at each slit that records which slit the light went through. Your description of the setup didn't include that, so I assumed it wasn't there. Putting it there changes the experiment.

 

[laymanB]

I am just trying to sort out what seems to be the claim that "observing" or "looking" at the laser light is enough to change the behavior from wave-like to particle-like. Some of the more reputable QM physicists that I have watched on different programs seem to be saying that experimental results change if we "look" or "observe". But if the "photons or waves" or whatever you want to call them in this superposition state never make it to your eyes, you have not "observed" them. So how do you "observe" a "photon or whatever" without interacting with it, and changing it's momentum or position?

 

[laymanB]

Why? Particle-like behavior only happens if you have something at each slit that records which slit the light went through. Your description of the setup didn't include that, so I assumed it wasn't there. Putting it there changes the experiment.

No, you are correct. I did not have such a device in this experiment.

 

[PeterDonis]

I am just trying to sort out what seems to be the claim that "observing" or "looking" at the laser light is enough to change the behavior from wave-like to particle-like.

Then you need to point us at a reputable source (textbook or peer-reviewed paper) that makes the claim, so we can look at what it says. Pop science videos or articles or TV shows don't count. Physicists will say all kinds of things in such venues that they know they can't get away with in a textbook or peer-reviewed paper.

 

[laymanB]

Then you need to point us at a reputable source (textbook or peer-reviewed paper) that makes the claim, so we can look at what it says. Pop science videos or articles or TV shows don't count. Physicists will say all kinds of things in such venues that they know they can't get away with in a textbook or peer-reviewed paper.

Busted. I was hoping that Filipenko, Carroll, Greene, Susskind, and Weinberg from PBS Nova couldn't steer me wrong. Stupid pop-sci misinformation.

 

[laymanB]

if you have something at each slit that records which slit the light went through

How does such a device interact with the "light wave"?

 

[PeterDonis]

How does such a device interact with the "light wave"?

It depends on the device. The key point is that, with such a device present, the outcome of the experiment is now not "what is observed on the screen", but "what is observed on the screen, plus which slit's device registers the passage of a photon".

 

[DrChinese]

How does such a device interact with the "light wave"?

To add to PeterDonis' answer: there are a number of approaches. I like one in which the slits have polarizers in front.

a. Orient the polarizers PERPENDICULAR, and it is possible to identify which slit the light goes through by its polarization. Regardless of whether you check the light's polarization or not, there will be NO interference pattern on the screen.

b. Orient the polarizers PARALLEL, and it is impossible to identify which slit the light goes through by its polarization. There WILL be an interference pattern on the screen.

Obviously, it is a bit difficult to assert that the individual polarizer itself modified the momentum of each light particle so as to collapse the interference. Because there are polarizers present in both a. and b. It is only the relative orientation that is a factor.

 

[laymanB]

It depends on the device. The key point is that, with such a device present, the outcome of the experiment is now not "what is observed on the screen", but "what is observed on the screen, plus which slit's device registers the passage of a photon".

Thanks, I appreciate the responses. Is there a free paper online that you could recommend that discusses the different mechanisms for detection of said photons and what implications the mechanism may have on the nature of the light ray?

 

[laymanB]

To add to PeterDonis' answer: there are a number of approaches. I like one in which the slits have polarizers in front.

a. Orient the polarizers PERPENDICULAR, and it is possible to identify which slit the light goes through by its polarization. Regardless of whether you check the light's polarization or not, there will be NO interference pattern on the screen.

b. Orient the polarizers PARALLEL, and it is impossible to identify which slit the light goes through by its polarization. There WILL be an interference pattern on the screen.

Obviously, it is a bit difficult to assert that the individual polarizer itself modified the momentum of each light particle so as to collapse the interference. Because there are polarizers present in both a. and b. It is only the relative orientation that is a factor.

Fascinating.

 

[DrChinese]

Fascinating.

I think so. Here is a more detail description if you are interested:

http://sciencedemonstrations.fas.ha...-demonstrations/files/single_photon_paper.pdf

 

[laymanB]

Do electrons have polarization also?

 

[Nugatory]

Let's say we do a double slit experiment with the standard laser/two slit setup with a screen to view the pattern.

That's a standard experiment, but not one that has anything to do with quantum mechanics. You shine light through a barrier with two slits, you see an interference pattern on the screen behind the barrier because according to ordinary non-quantum electrodynamics light is a wave and waves interfere behind a barrier with two slits. This is all stuff that was observed early in the 19th century (Young's double-slit experiment) and completely explained by 1861 (Maxwell's equations of classical electrodynamics), more than a half-century before any quantum phenomena had been observed.

To demonstrate quantum effects, you need a source that sends a single photon at a time towards the barrier. Because you're sending a single photon, it's just going to make a dot where it lands on the screen - there's no pattern to be seen. To get a pattern, you use something like a piece of photographic film that records each individual dot as it forms; and then when you're done with the experiment and develop the film you'll see a whole bunch of dots that form some pattern (A google image search for "quantum interference pattern builds up" will find many good examples).

Now we see something that is uniquely quantum mechanical and that has no classical explanation: We send the particles one at a time, and each one makes a single dot on the screen; but when two slits are open the dots form an interference pattern even though classically we'd expect each particle to go through one slit or the other and make a dot behind that slit. Then if we close one slit, or place a detector (such as drChinese's polarizers if we're using photons) we find that the dots don't form an interference pattern. We get either a clump behind each slit (we have a detector in one of the slits) or we get one clump behind one open slit (we closed the other slit).

Either way, conscious observation is pretty much irrelevant. The pattern is in the photographic film whether we develop it and look at it or not.

Do electrons have polarization also?

No, but they have other properties that we can use (electric charge, magnetic moment) to build a detector that will tell us which slit an electron went through. However, this entire question of observing which slit the particle went through is a bit of a red herring - the uniquely quantum mechanical phenomenon is that when both slits are open and there is no detector, the one-particle-at-a-time experiments produce an interference pattern. Not getting an interference pattern isn't surprising, it's what you'd expect out of a single particle moving from source to screen.

 

[PeterDonis]

Do electrons have polarization also?

They have spin, which has similarities to photon polarization, but is not quite the same (because electrons are spin-1/2 fermions and photons are spin-1 bosons).

 

[bhobba]

Busted. I was hoping that Filipenko, Carroll, Greene, Susskind, and Weinberg from PBS Nova couldn't steer me wrong. Stupid pop-sci misinformation.

Susskind? I know his QM book and he doesn't say that.

But overall books, shows, etc about QM written for a lay audience, to try and get some feel for QM across, take liberties with the 'truth'. Come here if you want the correct answer.

Thanks
Bill

 

[Lord Jestocost]

Either way, conscious observation is pretty much irrelevant. The pattern is in the photographic film whether we develop it and look at it or not.

Can you prove this scientifically?

 

[bhobba]

Can you prove this scientifically?

Are you suggesting the chemical reactions that took place on the photographic plate were not there when developed? I don't think a chemist wouldn't believe that. Prove it - of course you can't - but the world would be rather strange if it wasn't like that. Scientists generally believe a tree that falls in a forest when no one is there to listen still makes a noise - its philosophers that argue about such - well I won't mince my words - from a scientific viewpoint - nonsense. I for example could say forces don't move things - every-time something moves it was the shade of Newton that did it and forces made him do it. Prove its not true - of course you can't - but scientists apply common-sense.

Its like my doctor was telling me the other day there was this famous medical practitioner who in virtually every other way was excellent - but he believed in Homeopathy. Its obvious Homeopathy is rot of the first order, but he actually put it to the scientific test and of course found out it was rot. He was then good enough to stop believing in it - but really - why bother - its just so damn obvious.

QM seems to be one of those areas people make the job of understanding much harder than it should be.

Thanks
Bill

 

[laymanB]

Susskind? I know his QM book and he doesn't say that.

To be fair to Susskind, I don't think he said that in these PBS Nova programs. Most of the other scientists that I listed did though. I'm sure the producers of Nova knew they would lose a lot of viewers if they starting talking about definitions of measurements/observations and methodologies used in experiments. It is far more sensational to introduce some philosophical idealism to blow people's minds.

 

[vanhees71]

To be fair to Susskind, I don't think he said that in these PBS Nova programs. Most of the other scientists that I listed did though. I'm sure the producers of Nova knew they would lose a lot of viewers if they starting talking about definitions of measurements/observations and methodologies used in experiments. It is far more sensational to introduce some philosophical idealism to blow people's minds.

The difference beteween philosophers and natural scientists is that the latter try to understand Nature while the former take the results from science and make them as incomprehensible as possible. SCNR.

 

[Lord Jestocost]

Are you suggesting the chemical reactions that took place on the photographic plate were not there when developed? I don't think a chemist wouldn't believe that. Prove it - of course you can't - but the world would be rather strange if it wasn't like that. Scientists generally believe a tree that falls in a forest when no one is there to listen still makes a noise - its philosophers that argue about such - well I won't mince my words - from a scientific viewpoint - nonsense. I for example could say forces don't move things - every-time something moves it was the shade of Newton that did it and forces made him do it. Prove its not true - of course you can't - but scientists apply common-sense.

Its like my doctor was telling me the other day there was this famous medical practitioner who in virtually every other way was excellent - but he believed in Homeopathy. Its obvious Homeopathy is rot of the first order, but he actually put it to the scientific test and of found out it was rot. He was then good enough to stop believing in it - but really - why bother - its just so damn obvious.

QM seems to be one of those areas people make the job of understanding much harder than it should be.

Thanks
Bill

From your argumentation, it seems to me that you assume that there exists an objective, observer-independent reality. I have no problem with that, of course, although I have a complete different point of view. But these are mere assumptions which cannot be proven by means of physics. Thus, what I find annoying when following discussions about quantum mechanics (QM) is that sometimes “statements” are made alongside without disclosing the implicit assumptions these “statements” are based upon. What someone assumes or not, has nothing to do with physics. QM provides calculational recipes to predict the probabilities of various directly observed macroscopic outcomes and the used symbols, such as the probability amplitudes, are intellectual tools used by physicists to perform these calculations. That’s all, QM describes a priori no external reality and you will find no answer in QM whether there is something out there or not or whether a conscious observer plays a decisive role or not.

 

[laymanB]

I think so. Here is a more detail description if you are interested:

http://sciencedemonstrations.fas.ha...-demonstrations/files/single_photon_paper.pdf

Thanks for the link. I see in this experimental setup there is a second polarizer which is used as a "quantum eraser". One which makes it impossible to determine which slit the photon went through. Is there an experiment with electrons that utilizes such an eraser?

 

[laymanB]

Thus, what I find annoying when following discussions about quantum mechanics (QM) is that sometimes “statements” are made alongside without disclosing the implicit assumptions these “statements” are based upon.

I understand what you are saying, history has always shown that it is hard to argue with someone who holds to an idealism philosophy. But unless we agree on the rules of language and the law of non-contradiction in logic, you cannot have a meaningful discussion with someone else. I'm sure it's frustrating, as it seems a lot of people on here either take a realist position or don't disclose their position because they know you could argue in circles forever. Like it has been stated many times in these threads and you just said yourself, it is not a question which science can answer.

What frustrates me about these pop-sci shows like Nova is they give you the impression that everyone accepts a non-realist position instead of clearly saying that there are different interpretations of QM, some of which are mutually exclusive, and adopting a certain interpretation has no bearing on the observed facts and the non-intuitive nature of the subatomic world.

 

[DrChinese]

Thanks for the link. I see in this experimental setup there is a second polarizer which is used as a "quantum eraser". One which makes it impossible to determine which slit the photon went through. Is there an experiment with electrons that utilizes such an eraser?

I am not aware of one with electrons, no. There are a variety of interference experiments relating to particles other than light or electrons, but again I am not aware of ones that happen to use this technique. But perhaps another member does...

 

[laymanB]

I am not aware of one with electrons, no. There are a variety of interference experiments relating to particles other than light or electrons, but again I am not aware of ones that happen to use this technique. But perhaps another member does...

Thanks. I was just thinking if you could do something similar with electrons, it would provide further credence to some sort of retrocausality and not some specific effects of the polarization of light. I'm probably thinking about it too simplistically.

 

[PeterDonis]

Can you prove this scientifically?

No; you can't "prove" anything scientifically. You can only accumulate evidence and compare it with the predictions of various theories.

In the case under discussion, the basis for the statement @Nugatory made is that we expect decoherence to occur when the light hits the photographic film--i.e., long before any conscious observation becomes involved in the process. We would only expect consciousness to be required for the pattern to be "there" if no decoherence occurred until whatever-it-was began to interact with the portions of the observer's brain that were involved in consciousness. But there is no known case where that happens: there are always other objects in the causal chain (things like photographic films, detectors, recording instruments, etc.) which will induce decoherence long before the causal chain reaches a conscious observer.

 

[Khashishi]

A photographic film measures the which way information of the photon and encodes it into the chemicals of the film, much like the polarizers measure the which way information and encodes it into the polarization of the photon. So the photon creates a single dot on the film somewhere rather than spreading over the whole film. Now, if the film were carefully destroyed without observing it, theoretically it should be possible to erase the measurement such that the photon is/was retroactively no longer localized to a point on the film prior to destruction. But the film is constantly interacting with the environment, so the information stored on the film spreads like a virus, and in practice it's not possible to erase the measurement. It's hard to build a quantum computer because any interactions with the environment destroys the superposition in the qubits.

 

[DrChinese]

Thanks. I was just thinking if you could do something similar with electrons, it would provide further credence to some sort of retrocausality and not some specific effects of the polarization of light.

Retrocausality is quite a controversial topic around here. It's a subject for a different thread, but there are better experiments that tend to provide evidence of such. Check out entanglement swapping, including the variations where the swap occurs after detection.

 

[laymanB]

theoretically it should be possible to erase the measurement such that the photon is/was retroactively no longer localized to a point on the film prior to destruction.

Are you saying that the math of QM would predict this behavior under certain conditions?

 

[Khashishi]

Are you saying that the math of QM would predict this behavior under certain conditions?

I'm not sure if quantum eraser is something that can really be expressed in terms of math. In QM, normal evolution of a system is given by a unitary operation (the Hamiltonian) and a measurement is given by a projection operation. There's no way to really "undo" a projection operation in mathematics. It doesn't seem possible using matrix mathematics, since matrices are associative and the projection operation has no inverse. The solution is that projection operators probably aren't the correct way to handle measurements and are only introduced for convenience.

 

[PeterDonis]

I'm not sure if quantum eraser is something that can really be expressed in terms of math.

Sure it can; it's just the reverse of a unitary operator.

There's no way to really "undo" a projection operation in mathematics.

Yes, but any "quantum eraser" scenario can be modeled without requiring any projection operations. (Your statement quoted here explains why.)

 

[vanhees71]

I am not aware of one with electrons, no. There are a variety of interference experiments relating to particles other than light or electrons, but again I am not aware of ones that happen to use this technique. But perhaps another member does...

I guess better chances to find experiments with particles rather than photons concerning the fundamental foundations of QT are with neutrons rather than electrons.

 

[Lord Jestocost]

No; you can't "prove" anything scientifically. You can only accumulate evidence and compare it with the predictions of various theories.

In the case under discussion, the basis for the statement @Nugatory made is that we expect decoherence to occur when the light hits the photographic film--i.e., long before any conscious observation becomes involved in the process. We would only expect consciousness to be required for the pattern to be "there" if no decoherence occurred until whatever-it-was began to interact with the portions of the observer's brain that were involved in consciousness. But there is no known case where that happens: there are always other objects in the causal chain (things like photographic films, detectors, recording instruments, etc.) which will induce decoherence long before the causal chain reaches a conscious observer.

“Resort here to decoherence does not really help, for in the best case (where convergence is rapid) the trace over environmental degrees of freedom only tells us that something seems to happen from a certain perspective. Thus, decoherence can sometimes help understand why it seems to us that things happen, but it does not address the question as to why anything does in fact happen (if in fact it does).”

Arthur Fine in: “ELEGANCE AND ENIGMA - The Quantum Interviews” by Maximilian Schlosshauer (editor)

 

[vanhees71]

That something is happen when observing something is a tautology. Maybe philosophers have a problem to accept that something is happen. For normal people and even physicists it's a well-known fact. SCNR. ;-).

 

[bhobba]

“Resort here to decoherence does not really help, for in the best case (where convergence is rapid) the trace over environmental degrees of freedom only tells us that something seems to happen from a certain perspective. Thus, decoherence can sometimes help understand why it seems to us that things happen, but it does not address the question as to why anything does in fact happen (if in fact it does).”

Well that basically seems correct, but not the way I would say it. The issue is how does an improper mixed state become a proper one. That's the issue - but no-one here has been saying anything else.

What the argument is, is if its even an issue, I don't believe it is, but have zero issues with those that do.

As Schlosshauer says this is the central issue decoherence has revealed - colloquially - why do we get any outcomes at all.

I don't know Fines full view on QM in detail, but the above isn't really any different to what those here say. As far as I can see all he is saying is he isn't happy with 'something seems to happen' ie FAPP collapse. He is not the only one here or elsewhere.

Thanks
Bill

 

[bhobba]

That something is happen when observing something is a tautology.

That's right. I really don't see the issue. Does a theory explain everything? It can't - there is always some assumption that is just assumed. All anyone complaining about QM is saying is they don't like the standard assumptions - that's all Einstein didn't like - he still believed in QM (at the end after he tried - and failed - to find holes in it) - he just thought it incomplete ie some deeper theory was lurking about. I don't agree, but its hardly a new idea or a big revelation.

Thanks
Bill

 

[bhobba]

From your argumentation, it seems to me that you assume that there exists an objective, observer-independent reality. I have no problem with that, of course, although I have a complete different point of view.

But the formalism basically assumes that anyway by speaking of the outcomes of observations which are things that happen in 'objective, observer-independent reality..' I think its possible to be a lot more subtle than that and cast doubt on such an assumption, but what I can't figure is why bother? The resulting world view would be rather complex and for what gain? Still - yes that is an assumption, but hardly a controversial one.

Thanks
Bill

 

[vanhees71]

That's right. I really don't see the issue. Does a theory explain everything? It can't - there is always some assumption that is just assumed. All anyone complaining about QM is saying is they don't like the standard assumptions - that's all Einstein didn't like - he still believed in QM (at the end after he tried - and failed - to find holes in it) - he just thought it incomplete ie some deeper theory was lurking about. I don't agree, but its hardly a new idea or a big revelation.
Bill

One must not forget, in my opinion, that physics is about the "real world", and I mean this in a very naive way. We humans experience this real world with our senses first and by learning a lot over some millenia of progress (and as an optimist I think the overall development of mankind has to be seen as a lot of progress indeed) we can even refine and extend our senses with measurement devices built based on the knowledge in terms of empirical evidence as much as in terms of models and theories gained before. It is obvious and not a matter of natural science that we are able to measure (more and more accurately with technological progress) things that are way beyond the realm of what's "detectable" by our senses, among them the atomistic structure of matter on several scales of length and energy, fortunately at these different scales describable by more or less accurate simplifying models, like much atomic and condensed-matter physics in terms of non-relativistic QT, which is simpler than relativistic QFT but leading us also to the latter more refined models and theories due to failures of the older models. More accurately put we discover the ranges of validity of our various models in making progress in science by observations and experiments.

This shows that the scientific method, including quantum theory in all its facets, works pretty well: We can formulate a quite abstract mathematical theory that let's us predict the behavior of many things around us, including the measurement devices used to explore the natural world further. By finding discrepancies between models and observations we also see that what we measure is not just driven by our subjective inventions but that we can gain indeed objective knowledge and adapt our models to the new findings, maybe leading to even better measurements and even better refinements.

It's simply a fact that we can measure things in the microscopic regime, where QT is applicable in its various levels of sophistication, and about parts of it (non-realtivistic QT, e.g.) we know the boundaries of validity, for some we don't. We also know that our theories are by far not complete, because there's the problem to find a consistent theory that unites general relativity with quantum (field) theory. We even don't know, whether such a theory exists at all and how it might manifest itself in terms of observable facts. Maybe that's the reason that we haven't yet found any successful theory of this kind, but we can hope to find one day with some observation where GR really fails and where quantum modeling of the gravitational interaction is really necessary. Maybe then we also get a clever idea how to resolve this puzzle.

On the other hand, asking philosophical questions like "why are there definite outcomes when measuring an observable" is pretty fruitless and tautological. It's simply working, because our models are built based on observational facts. That's the strength of the natural sciences compared to more speculative kinds of knowledge like philosophy: It is evidence based knowledge from centuries of careful observations of nature. There are a lot of failures on the way to gain this knowledge, wrong ideas (e.g., the ether of pre-relativistic models of electromagnetism), but they are corrected sooner or later by objective empirical evidence.

 

[laymanB]

Is not the de Broglie wavelength for matter inversely proportional to the particle's momentum? If so, what effect does this have for macroscopic particles with large masses? How does the wavelength of a massive macroscopic object moving slowly in a surface of the Earth frame compare to the wavelength of a light subatomic particle like an electron in the same frame?

I will bring this back around to the topic of the thread. I just need some more basic understanding first.

 

[Lord Jestocost]

One must not forget, in my opinion, that physics is about the "real world", and I mean this in a very naive way.

I am glad to see that you are saying "in my opinion".

 

[Lord Jestocost]

In the case under discussion, the basis for the statement @Nugatory made is that we expect decoherence to occur when the light hits the photographic film--i.e., long before any conscious observation becomes involved in the process.

What you finally expect is the following: Decoherence occurs when light hits the photographic film which itself is expected to be already in a decohered state.

 

[Ivanovich62]

they are waves of probability, you cannot see them. they do not exist in any meaningful way. if you try to see them, you force wave function collapse. if you want an everyday analogy, you cannot see the result of a die roll before rolling the die, and once you have rolled, you cannot undo it and turn it back into probabilities.