Quantum Eraser and Its Implications

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  • #76
Cthugha
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Then look at Len M's last post, to see the necessity of philosophy in science-- if one's goal is to understand one's science. Of course, if one is a "shut up and calculate" type, then that is the only time one can place a firewall successfully between physics and the philosophies that invented physics. But frankly, I've met many who claimed they believed in "shutting up and calculating", but none who ever really did. We all want to understand our calculations.
Understanding in the meaning you use is not the task of science. It is the task of mapping (intersubjective) experimental results to a predictive model of how stuff works and minimizing the number of false predictions. Of course many want to "understand" the meaning at a deeper level. This is, however, a philosophical question, not a scientific one and it is good practice to keep this difference in mind.

Yes, and that's exactly what I'm talking about. Shannon entropy is about information, and information is very much in the mind of the physicist. That is the path for seeing how closely connected is the whole concept of entropy, and the way we process information.
Your tornado example is a classical one. Entropy is clearly more important in a scenario where a system having one state is linked to a system having many degenerate states of the same energy (like an excited atom and the vacuum).

And even more than that, I mean that the whole concept of a "system" that could be in an eigenstate in the first place is an idealization of our conscious minds. We have chosen what we care about, and found a way to predict it, but reality would have to see what we are doing as hopelessly naive.
Does it? To be honest I do not really care. As long as the predictions are ok, the physics is ok, too. By the way I also feel pretty bored by the constant discussions whether CI, MWI, BM, Ithaca or any interpretation of qm is better than the other. As long as the predictions do not differ, that is not a scientific question. I see the point that some people may get some inspiration from one certain interpretation and that is fine. However, I do not see any scientific importance beyond that.

That's where it started, and you answered that already. I'm saying that if we are going to talk about the role of consciousness in quantum mechanics, writ large, we must go beyond the simple issue of whether there is a human looking at the detector or not.
One can discuss that. But I doubt that was really the question asked in this thread. I think the question was indeed simply whether a human looking at a detector makes a difference. Nothing else.

That is a valid objection, but I can answer it. I do feel there are analogs in other areas of science, like entropy in thermodynamics. But the problem is never as central as it is to quantum mechanics, because quantum mechanics has a formal evolution that is unitary, which leads to things like interfering wavefunctions, but experiments show nonunitary outcomes, like individual photon counts and decoherence in general, any time one particular outcome is perceived out of all the possible ones.
If you take your position seriously then it will apply to any field of science. There are not even simple things like tennis balls, but just our perception of it. It is just more puzzling in qm.

I'm pointing out that the fundamental weirdnesses associated with two-slit experiments are fundamentally about the role of the consciousness, for the simple reason that only a conscious being can perceive a nonunitary outcome. Without the need to explain that perception, quantum mechanics works just fine treating everything as a superposition-- it's only a question of how large the closed system is.
I still doubt that the role is fundamental in physics. It can be fundamental in philosophy, but physics is indeed "shut up and calculate". Of course many people are interested in areas beyond physics, but imho things are much clearer if you keep the dividing line clear.

I agree that philosophy had influence on the development of physics and science in general like developing falsifyability (is that a word? hmm) as a criterion to distinguish between scientific and other theories. But apart from that I really vote for keeping the physics part "shut up and calculate" and taking all other issues to philosophy.
 
  • #77
Ken G
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Understanding in the meaning you use is not the task of science. It is the task of mapping (intersubjective) experimental results to a predictive model of how stuff works and minimizing the number of false predictions. Of course many want to "understand" the meaning at a deeper level. This is, however, a philosophical question, not a scientific one and it is good practice to keep this difference in mind.
I agree completely, except I view "the task of science" as having a broader mission. Indeed, if you go into any classroom where science is being taught, you will find the philosophical version there far more often than the strict predictive model version! It's a lot to ask a high school science teacher to say "now we will leave the formal realm of what science is and begin to address the philosophical ramifications" every time they want to say "forces cause acceleration" or some such. So I think we do better by tracking the difference you speak of, but not imagining that science comes equipped with a firewall between them. Science, in practice, is more like an amalgamation of those two very different goals.
Your tornado example is a classical one. Entropy is clearly more important in a scenario where a system having one state is linked to a system having many degenerate states of the same energy (like an excited atom and the vacuum).
Yes, I am using a classical theory there.
Does it? To be honest I do not really care.
A self proclaimed "shut up and calculate" type. More power to you, but as I said, many times I have seen that claim but few times have I seen it held to, the lure is just too great.
One can discuss that. But I doubt that was really the question asked in this thread. I think the question was indeed simply whether a human looking at a detector makes a difference. Nothing else.
I suspect you're right. But sometimes the questioner does not know what question to ask to get to the answer they need. We have to guess a little.
If you take your position seriously then it will apply to any field of science. There are not even simple things like tennis balls, but just our perception of it. It is just more puzzling in qm.
Yes, that is true. Indeed I'd say it's pretty clear that "tennis ball" is not a strict ontological concept, it is an effective ontological concept, like all in science. Effective ontology is all you need, so that should be fine with you (and me also), my issue is with those who demand absolute ontology.
I still doubt that the role is fundamental in physics. It can be fundamental in philosophy, but physics is indeed "shut up and calculate".
Not in those high school science classes-- it just isn't.
Of course many people are interested in areas beyond physics, but imho things are much clearer if you keep the dividing line clear.
I don't disagree, it's important to maintain that division, even though both end up being part of the mission of science.
I agree that philosophy had influence on the development of physics and science in general like developing falsifyability (is that a word? hmm) as a criterion to distinguish between scientific and other theories. But apart from that I really vote for keeping the physics part "shut up and calculate" and taking all other issues to philosophy.
That is certainly a logically sound proposition. It does make philosophy more important to scientists though!
 
  • #78
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Isn't the Quantum Eraser a Fourier Transform in action?
 
  • #79
Cthugha
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I agree completely, except I view "the task of science" as having a broader mission. Indeed, if you go into any classroom where science is being taught, you will find the philosophical version there far more often than the strict predictive model version! It's a lot to ask a high school science teacher to say "now we will leave the formal realm of what science is and begin to address the philosophical ramifications" every time they want to say "forces cause acceleration" or some such. So I think we do better by tracking the difference you speak of, but not imagining that science comes equipped with a firewall between them. Science, in practice, is more like an amalgamation of those two very different goals.
Why should one say such? Saying "forces cause acceleration" is perfectly within the realm of science. Every student should be taught a lesson about scientific theory and method once during school time, maybe adressing questions of intersubjectivity, a bit of Popper, qualia problems and such, but that is enough. As soon as that framework is established, forces and accelaration are perfect scientific concepts. There is no need address any philosophiocal implications.

Yes, I am using a classical theory there.A self proclaimed "shut up and calculate" type. More power to you, but as I said, many times I have seen that claim but few times have I seen it held to, the lure is just too great.
Taken by your above quote you have a very strange concept of what shut up and calculate should be. Forces and accelerations are "shut up and calculate". The question, what a force is microscopically and how it creates accelerations might not be, but is usually not considered in classrooms.

Not in those high school science classes-- it just isn't.
Taking the very rough models that are usable in classrooms into account, high school physics is very much shut up and calculate. What the guys in school expect, however, is often something different.

RalkoCzez said:
Isn't the Quantum Eraser a Fourier Transform in action?
That short comment is way too short to judge whether you understood the concept or not. All in all, it is not that easy.
 
  • #80
Ken G
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Why should one say such? Saying "forces cause acceleration" is perfectly within the realm of science.
The reason I view that as a philosophical statement (and one on rather shaky ground, actually), rather than a scientific statement, is that it certainly does not fall within the narrow realm of empirical demonstrability that you referred to earlier. It is a statement that certainly has its purposes, and in fact I use it all the time, but I also recognize when I do that it does not stand up to the standard of what is scientifically correct. The reason I object to saying that statement is scientifically justified in any absolute sense is twofold:

1) It asserts a particular type of causation, which actually stems from how we think about the phenomenon rather than anything that is demonstrably happening there. Instead of saying what is accurate, that we can understand acceleration better by imagining that it is caused by forces (similar to how we gain conceptual understanding of everyday life by imagining cause/effect relationships rather than simple temporal correlations which would suffice to get power over our environment), we just say that acceleration is actually caused by forces. By removing our responsibility from drawing that conclusion, we can imagine we have stated some absolute truth, around the true existence of forces, and the true presence of a causal relationship. Yet neither of those claims are scientifically demonstrable, a force is not an ontological entity (it is even defined by its effects, not by what it is), and is not even needed in some versions of classical physics. And causation is also not present in any theory-- when we say F=ma, we can imagine the causal relationship a comes from F/m, but we can just as easily imagine the causal relationship that F comes from ma. (The latter would be how forces are derived from Lagrangian mechanics, for example.)

2) The statement sounds highly ontological, yet does not identify the true sources from which the ontological elements are borrowed. As I said already, forces are scientifically defined by their effects, not by what they are, so it is already a bit scientifically imprecise to say that these things we call forces can cause anything (they are defined by what we imagine them causing, so that's quite circular). A more accurate statement is that accelerations can be organized in very useful ways by attributing them to forces, where forces are nothing more than the patterns by which the accelerations can be organized. This core circularity causes no problems when no ontological claims are made surrounding it. Such claims are just a convenience, that we all use but can create misconceptions very easily-- especially if we ourselves lose track of what semantic conveniences we have invoked.
Taken by your above quote you have a very strange concept of what shut up and calculate should be. Forces and accelerations are "shut up and calculate".
No-- not when forces are said to cause acceleration. That is not shut up and calculate-- it is an ontological claim on reality that is not scientifically justified. It's OK to say it, we're not going to make sure everything that we say is fundamentally scientifically correct, but we should be aware when we are leaving the realm of what can be scientifically demonstrated (the shut up and calculate realm), and have entered the philosophical realm of using language to understand our calculations. If I can do the calculation without believing that forces cause acceleration (which I can), then how can that statement be part of the calculation?
The question, what a force is microscopically and how it creates accelerations might not be, but is usually not considered in classrooms.
It's not an issue of detail. There is no such thing as a force, not at any scale or in any level of detail, that is not simply a non-unique, contextual, and goal-oriented concept borrowed from some formal mathematical structure (here a structure along the lines of classes of potential energy functions). We are fundamentally talking about patterns of accelerations, and how to group and quantify those patterns, and we generate the force concept to unify and simplify that task. There is never any reason to imagine that forces actually exist, or that they cause anything, but it is certainly a useful fantasy when we go to picture what our calculations are saying. Some might find the concept useful in actually carrying out the calculation, some might prefer to use a different approach that never references forces at all. Yet how many students are going to recognize these facts when they are told "the cause of an acceleration is a force"?
 
  • #81
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If we place detectors behind the slits in Scully et al delayed choice quantum eraser experiment, could we open the shutters at the same time and 'erase' the which-way information? If we describe the whole situation using the Scrodinger equation, everything is in superposition.

Also, with Bohm Mechanics, wouldn't we expect clicks EVERY TIME we open both shutters at the same time? That would be because there'd be a definite particle in either cavity each time.
 
  • #82
Cthugha
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1) It asserts a particular type of causation, which actually stems from how we think about the phenomenon rather than anything that is demonstrably happening there. Instead of saying what is accurate, that we can understand acceleration better by imagining that it is caused by forces (similar to how we gain conceptual understanding of everyday life by imagining cause/effect relationships rather than simple temporal correlations which would suffice to get power over our environment), we just say that acceleration is actually caused by forces. By removing our responsibility from drawing that conclusion, we can imagine we have stated some absolute truth, around the true existence of forces, and the true presence of a causal relationship. Yet neither of those claims are scientifically demonstrable, a force is not an ontological entity (it is even defined by its effects, not by what it is), and is not even needed in some versions of classical physics. And causation is also not present in any theory-- when we say F=ma, we can imagine the causal relationship a comes from F/m, but we can just as easily imagine the causal relationship that F comes from ma. (The latter would be how forces are derived from Lagrangian mechanics, for example.)
I disagree with most of that. I agree that forces are not ontological entities. However, the same is true for accelerations. The point I really disagree with is "By removing our responsibility from drawing that conclusion, we can imagine we have stated some absolute truth". Any kind of absolute truth is not the job of science Determining what is really happening is also not the job of science. The job of science is to create models that correlate with experimental outcomes and have predictive power. And any theoretical scientific statement should be understood as an explanation of the model, whether "forces cause accelerations", "our universe started from the big bang" or "global warming is related to the decrease of pirate activity". These models are to be tested against our perception of reality via experiments. That is not the same as comparing it to reality, but anyway the closest we can get.

2) The statement sounds highly ontological, yet does not identify the true sources from which the ontological elements are borrowed. As I said already, forces are scientifically defined by their effects, not by what they are, so it is already a bit scientifically imprecise to say that these things we call forces can cause anything (they are defined by what we imagine them causing, so that's quite circular).
No scientific term is defined by what it is, but by its effects.

If I can do the calculation without believing that forces cause acceleration (which I can), then how can that statement be part of the calculation?
Sure you can believe that forces do not cause acceleration. That is a different model. There is no need to find a single model. And that is (also) the meaning of "shut up and calculate": Stopping at the level where one may have different models of the same situation which are equally good and predictive without having one better or more real than the other. All scientific statements similar to "forces cause accelerations" should always be understood in the framework of the model used.

Some might find the concept useful in actually carrying out the calculation, some might prefer to use a different approach that never references forces at all. Yet how many students are going to recognize these facts when they are told "the cause of an acceleration is a force"?
I do not know. Maybe many people are not aware of that. Anyway, they should be. On the other hand many students encounter for example both Newtonian and Lagrange/Hamilton mechanics and often the role of science is clarified while these topics are discussed. However, I obviously cannot say how usual this is around the globe.

Do you mind if we stop the discussion here or move it to a different topic? While the discussion was somewhat fitting in the beginning, I somewhat feel like we are hijacking the topic.

@StevieTNZ: I have to reread the original Scully paper again before I can comment on that.
 
  • #83
Ken G
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I disagree with most of that. I agree that forces are not ontological entities. However, the same is true for accelerations.
And for every single concept used in physics, yes. But accelerations have a mathematical definition as the rate of change of the rate of change of position-- the problem is more generally with the ontological baggage attached to the concept of position.
The point I really disagree with is "By removing our responsibility from drawing that conclusion, we can imagine we have stated some absolute truth". Any kind of absolute truth is not the job of science Determining what is really happening is also not the job of science.
Then you don't disagree at all-- that's what I'm saying. I'm merely noting that the sentence "forces cause acceleration" has an ontological character, which means that it is very often interpreted as a statement of what is really happening-- exactly what you say science is not trying to do.
The job of science is to create models that correlate with experimental outcomes and have predictive power.
As I said before, that is not the sole job of science, and what happens in any science classroom demonstates that very clearly. Science is not just taught "here is the model and here is its predictive power," it is taught "this is what science tells us is the truth of our reality." There is no question that science is taught that way, more often than the way you describe. If you doubt that, sit in on any astronomy lecture the day they say why the Earth isn't the center of the solar system but the Sun is, or that the Sun doesn't go around the Earth but the Earth does go around the Sun.

And any theoretical scientific statement should be understood as an explanation of the model, whether "forces cause accelerations", "our universe started from the big bang" or "global warming is related to the decrease of pirate activity". These models are to be tested against our perception of reality via experiments. That is not the same as comparing it to reality, but anyway the closest we can get.
I couldn't have said it better myself. All that isn't clear to me is why you think that disagrees with what I just said above!
No scientific term is defined by what it is, but by its effects.
You are saying that all science is epistemic rather than ontological. That is what I keep saying! Yet there are many threads going on right now about the PBR theorem and how it proves the ontological character of quantum mechanics, and another thread where Jaynes is quoted as saying that people are entering into logical fallacies if they won't admit that atoms are real. Is an atom defined by what it is, or what the effects of the concept are? If the latter, how can anyone hold that good science must assert that atoms are real?
Sure you can believe that forces do not cause acceleration. That is a different model. There is no need to find a single model. And that is (also) the meaning of "shut up and calculate": Stopping at the level where one may have different models of the same situation which are equally good and predictive without having one better or more real than the other.
I would say that shut up and calculate involves stopping even short of that-- it stops at saying that all models are just concepts we use for their effectiveness, with no ontological character except what we bring to them-- provisionally, conceptually, and in a goal-oriented way. That's also what I have been saying.

All scientific statements similar to "forces cause accelerations" should always be understood in the framework of the model used.
I agree, yet the same cannot be said generally. Have you seen the threads where the existence or non-existence of virtual particles is being hotly disputed? Why can't those on both sides of that debate just allow that the existence of virtual particles should always be understood in the framework of the model used? The problem is, some models are better than others, and so people tend to say that one particular model is the "right one" to talk about virtual particles, but someone else talks about them by modeling them in a different way. Different subdisciplines of physics even have their own particular slant, yet still the argument rages, no one is saying that none of these concepts actually exist outside of the mathematical structure that they are borrowed from.
I do not know. Maybe many people are not aware of that. Anyway, they should be.
There are probably a dozen active threads right now in which it is clear that many people are not aware of this. Indeed, on one thread I find myself being ridiculed for suggesting that even the ontological elements of quantum mechanics are provisional, contextual, and dependent on the goals of the physicist-- not just the ontological elements of ancient discarded theories. I'm branded a non-realist for noticing that human intelligence plays a role in choosing the mathematical structures from which we borrow our ontological elements in our theories!

Do you mind if we stop the discussion here or move it to a different topic? While the discussion was somewhat fitting in the beginning, I somewhat feel like we are hijacking the topic.
Yes, I don't want to be accused of hijacking. However, I feel that the basic issue we are discussing is at the heart of a very large number of threads on here-- including this one. Now I'll step back and let the delayed-choice ontological haggling go on without the recognition of how unnecessary it is to the way science actually works!
 

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