B Does Human Observation Affect the Double Slit Experiment Results?

[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.

 

[Ivanovich62]

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.

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.

DB wavelentth = h / p. so 1g moving 1m/s will be around 10^-31 m. that is about 1 / 10^21 th the diameter of an atom. i am not sure that size has any real meaning.

an electron traveling same speed has DB wavelength of about 1 mm

 

[PeterDonis]

Decoherence occurs when light hits the photographic film which itself is expected to be already in a decohered state.

In the sense that the film is continually "decohering itself" due to the strong interactions between its atoms, yes, this is a reasonable statement. Or, to put it another way, there are a very large number of uncontrolled degrees of freedom in the film, that serve to quickly decohere anything with which it interacts--including itself, or light that happens to strike it.

 

[laymanB]

DB wavelentth = h / p. so 1g moving 1m/s will be around 10^-31 m. that is about 1 / 10^21 th the diameter of an atom. i am not sure that size has any real meaning.

an electron traveling same speed has DB wavelength of about 1 mm

Thanks.

So if we have massive, macroscopic objects with extremely tiny wavelengths does this have any implications for the math of QM like the Schrödinger equation and what the probabilities mean for these macroscopic objects? What kind of input does the Schrödinger equation accept for macroscopic objects? Does the macroscopic object have to be treated as a collection of tiny particles or can it be input into the equation as one massive particle?

 

[Ivanovich62]

Thanks.

So if we have massive, macroscopic objects with extremely tiny wavelengths does this have any implications for the math of QM like the Schrödinger equation and what the probabilities mean for these macroscopic objects? What kind of input does the Schrödinger equation accept for macroscopic objects? Does the macroscopic object have to be treated as a collection of tiny particles or can it be input into the equation as one massive particle?

i believe you reach the point where there are so many quantum states possible that it can be considered to be continuous, rather than discrete, so classical methods are adequate to describe the system.

planck constant is tiny. if you consider an electron, which has mass and energy of a similar order of magnitude, trapped in a finite well, you will get several discrete results. if you consider, say a grain of dust, you will get billions upon billions of discrete results.

 

[laymanB]

i believe you reach the point where there are so many quantum states possible that it can be considered to be continuous, rather than discrete, so classical methods are adequate to describe the system.

I guess that's part of my point. If QM is a more fundamental theory for describing reality, fine. I understand that an underlying assumption of most physicists is that the laws of nature should be scale invariant and work in all reference frames. This idea has led to some of the greatest discoveries in trying to reconcile the physics and come up with a theory of everything. QM has been very successful in unifying three of the four fundamental forces.

But if it only gives you infinitesimally small margins of unknowability for macroscopic objects, then who cares what we think the mathematical model means in "reality". Maybe the mathematical model with its basis in complex numbers and abstract configuration space has no real meaning or correlation to something physical. I just don't see how it logically follows that having a probabilistic nature of QM on the subatomic scales means that this probabilistic nature applies to macroscopic objects in any meaningful way. We can tell you where comets are going to be in thousands, if not millions of years. We can predict when the Andromeda galaxy will collide with ours.

Just like special and general relativity was a giant leap in understanding light, gravity, and cosmology, we understand that Newton's Laws are still more than adequate to put spacecraft into orbits and that most of the effects are negligible unless you are dealing with particles at relativistic velocities. Sure, relativity has made some of our technology possible like GPS but it doesn't logically follow, like some have abused the theory, to say that everything is relative, ergo, no objective truth. I hope that QM can be used to develop technology. But to say that because we can't determine the momentum or position of an electron or photon simultaneously until it is decohered from its superposition, therefore objective reality does not exist or everything is unknowable seems to be a giant step towards absurdity.

 

[Ivanovich62]

I guess that's part of my point. If QM is a more fundamental theory for describing reality, fine. I understand that an underlying assumption of most physicists is that the laws of nature should be scale invariant and work in all reference frames. This idea has led to some of the greatest discoveries in trying to reconcile the physics and come up with a theory of everything. QM has been very successful in unifying three of the four fundamental forces.

But if it only gives you infinitesimally small margins of unknowability for macroscopic objects, then who cares what we think the mathematical model means in "reality". Maybe the mathematical model with its basis in complex numbers and abstract configuration space has no real meaning or correlation to something physical. I just don't see how it logically follows that having a probabilistic nature of QM on the subatomic scales means that this probabilistic nature applies to macroscopic objects in any meaningful way. We can tell you where comets are going to be in thousands, if not millions of years. We can predict when the Andromeda galaxy will collide with ours.

Just like special and general relativity was a giant leap in understanding light, gravity, and cosmology, we understand that Newton's Laws are still more than adequate to put spacecraft into orbits and that most of the effects are negligible unless you are dealing with particles at relativistic velocities. Sure, relativity has made some of our technology possible like GPS but it doesn't logically follow, like some have abused the theory, to say that everything is relative, ergo, no objective truth. I hope that QM can be used to develop technology. But to say that because we can't determine the momentum or position of an electron or photon simultaneously until it is decohered from its superposition, therefore objective reality does not exist or everything is unknowable seems to be a giant step towards absurdity.

without the probabilistic nature of quantum theory, we wouldn't have micro processors, so clearly it does impact on the real world, and clearly, it works. i don't see why you need to go further than that. what does it matter what it really means, as long as it gets results.

 

[laymanB]

without the probabilistic nature of quantum theory, we wouldn't have micro processors, so clearly it does impact on the 'real' world

That’s great. I hope they can find more uses for it. I’m just saying that it does not logically follow that it has to affect our epistemology.

You never wake up in the morning and wonder if your micro processor is still going to be in your computer.

 

[laymanB]

i don't see why you need to go further than that. what does it matter what it really means, as long as it gets results.

Agreed

 

[Lord Jestocost]

In the sense that the film is continually "decohering itself" due to the strong interactions between its atoms, yes, this is a reasonable statement. Or, to put it another way, there are a very large number of uncontrolled degrees of freedom in the film, that serve to quickly decohere anything with which it interacts--including itself, or light that happens to strike it.

Quoting Ruth E. Kastner (‘Einselection’ of pointer observables: The new H-theorem?) regarding decoherence:

“It is often claimed that unitary-only dynamics, together with decoherence arguments, can explain the ‘appearance’ of wave function collapse, i.e, that Schrodinger’s Cat is either alive or is dead. This however is based on implicitly assuming that macroscopic systems (like Schrodinger’s Cat himself) are effectively already ‘decohered,’ since the presumed phase randomness of already-decohered systems is a crucial ingredient in the ‘derivation’ of decoherence.”

As a physicist, I would say: It seems that the decoherence program might be based on a simple circular reasoning fallacy.

 

[bhobba]

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

Of course.

Any interpretation in accord with the formalism,.even those some consider weird - like consciousness causes collapse - is scientifically valid.

What me and Vanhees try to stress is there is no need to go down paths like that - but if you want to of course you can. QM when viewed using minimalist interpretations like the Ensemble is much more 'common sense' and less weird than some try to present it.

Thanks
Bill