Is Polarisation Entanglement Possible in Photon Detection?

[stevendaryl]

What I meant to say with my model that even if you are allowed to include back whatever information you want you can't make the model consistent.

I'm sorry, I don't understand what you mean. What's not consistent?

 

[A. Neumaier]

It's not though. If Bob has a proper mixed state due to ignorance of the true state, then even though he can't tell the difference, someone else who knows the true state, can. If Alice flips a coin, and with probability 1/2 sends a spin-up particle to Bob, and with probability 1/2 sends a spin-down particle to Bob, then Alice knows ahead of time what Bob's spin measurement result will be. So for Alice, that's different from the case of an improper mixed state, where nobody knows ahead of time what Bob's result will be.

The underlying assumption here is that the single system has a true state, and that this state is pure. Both are unprovable assumptions.

In general, if someone has a wrong state due to ignorance he will make wrong predictions of the full observable statistics. Ignorance therefore has no place in physics - Nature behaves independent of what we choose to know or ignore.

 

[vanhees71]

This requires having a proper pure state of the big system! But if all you have and measure is the state of the small system, you cannot distinguish it. That's why it is called a state!

Similarly with a glass of water. If you consider the bigger system that includes a camera that had observed the process of warming or cooling, you can recover from its state additional information about the history of the glass of water.

I agree, of course, with that. Maybe, I've misunderstood your previous posting.

 

[stevendaryl]

The underlying assumption here is that the single system has a true state, and that this state is pure. Both are unprovable assumptions.

Nothing in science is provable, but the assumption that it is possible for Alice to produce a pure spin-up state for Bob is empirically testable, in the sense that Alice can repeat the same experiment over and over and note how often it is that when she prepares a state that is spin-up in the z-direction, Bob measures spin-up in the z-direction. The hypothesis that it is a pure spin-up state can be tested, and all tests are consistent with that assumption.

 

[stevendaryl]

In general, if someone has a wrong state due to ignorance he will make wrong predictions of the full observable statistics. Ignorance therefore has no place in physics - Nature behaves independent of what we choose to know or ignore.

You're contradicting yourself here. By definition, an improper mixture does reflect ignorance about the true state.

And you're wrong that ignorance has no place in physics. The use of probability gives us a way to reason in the presence of uncertainty/ignorance.

Anyway, you're venturing into philosophy here, and I don't find your philosophy of science very compelling. Let's stick to the physics. The situation in which Alice flips a coin and sends a spin-up state to Bob if her coin is "heads" and sends a spin-down state if her coin is "tails" is certainly a different situation than the case where Bob measures the spin of one member of an entangled two-particle system. In the first case, Alice knows ahead of time what result Bob will get, and in the second case, she doesn't. Those are clearly different situations. But from Bob's point of view, they are both described by the mixed state with equal weights of spin-up and spin-down. In the first case, the mixture reflects Bob's ignorance and in the second it does not. You say "ignorance has no place in physics", but I think that's silly.

 

[stevendaryl]

My pet peeve is that often when people say "Philosophy has no place in physics", what they really mean is "No philosophy that is in disagreement with mine has a place in physics".

 

[zonde]

I'm sorry, I don't understand what you mean. What's not consistent?

Let's say Alice receives photon, she argues that it is either H polarized or V polarized. Now she measures it at some angle $\alpha$ and gets click in either channel#1 behind PBS or the channel#2. Let's say it was channel#1. Now she says, well it was either H photon that with probability $p_1$ went into channel#1 or it was V photon that with probability $p_2$ went into channel#1. Bob does the same with angle $\beta$. Now we (who observe both Alice and Bob doing their reasoning) argue that either Alice and Bob both got H photon (say we created entangled state with correlated polarizations) or both got V photons. We try out every possibility with that condition in place and there is none that is consistent with QM predictions for pure state probabilities (expected $p_1$ and $p_2$ for Alice and Bob). We argue that: well, maybe we picked wrong pure states. They were actually not H and V photons but say +45deg./-45deg. photons. But it turns out that if Alice and Bob would use different angles $\alpha_2$ and $\beta_2$ we end up with the same conclusion that we can't get pure state probabilities right whatever we try.

 

[Simon Phoenix]

So I don't think it should be too surprising that your altered scenario can violate Bell's inequalities

Indeed it isn't - it surprised me way back when when we first realized it and applied it to generate a QKD protocol, but that's because I had been unthinkingly applying the mantra "violation = entanglement".

I certainly think the 'easiest' way to understand all of the QKD work is in terms of collapse - and it's the way I approach the problem being confident that I'm not going to get the wrong predictions. Of course we can interpret all of the QKD stuff in whatever 'interpretation' we like and get the same answers. Speaking purely for myself I find the 'collapse' way of thinking about it to be cleaner and to leave aside all of the legitimate concerns about FTL 'changes of state' that it implies to a lovely Summer sunset with a glass or two of beer

 

[stevendaryl]

Let's say Alice receives photon, she argues that it is either H polarized or V polarized. Now she measures it at some angle $\alpha$ and gets click in either channel#1 behind PBS or the channel#2. Let's say it was channel#1. Now she says, well it was either H photon that with probability $p_1$ went into channel#1 or it was V photon that with probability $p_2$ went into channel#1. Bob does the same with angle $\beta$. Now we (who observe both Alice and Bob doing their reasoning) argue that either Alice and Bob both got H photon (say we created entangled state with correlated polarizations) or both got V photons. We try out every possibility with that condition in place and there is none that is consistent with QM predictions for pure state probabilities (expected $p_1$ and $p_2$ for Alice and Bob). We argue that: well, maybe we picked wrong pure states. They were actually not H and V photons but say +45deg./-45deg. photons. But it turns out that if Alice and Bob would use different angles $\alpha_2$ and $\beta_2$ we end up with the same conclusion that we can't get pure state probabilities right whatever we try.

Sorry, I still don't understand what you are talking about. What do you mean when you say "none that is consistent with QM predictions for pure state probabilities"?

In the entangled two-photon state, you have the state $\frac{1}{\sqrt{2}} (|H\rangle |H\rangle + |V\rangle |V\rangle)$. We do the trace business and Alice uses the mixed state $\frac{1}{2} |H\rangle \langle H| + \frac{1}{2} |V\rangle \langle V|$. Bob uses the same mixed state. The meaning is that if Alice measures the polarization, she'll get $H$ with probability 50% and $V$ with probability 50%. Actually, that density matrix says that if Alice measures the polarization at ANY angle, she will get $H$ with probability 50% and $V$ with probability 50%. Same with Bob.

Now, if you ask Alice what the probability is that she and Bob get the same polarization, she can't answer that question using her's and Bob's density matrices, because those have thrown away correlation information.

So what inconsistency are you talking about? Do you mean an inconsistency of the type: "The theory says we will find X, but in actually X is not the case."

 

[stevendaryl]

Indeed it isn't - it surprised me way back when when we first realized it and applied it to generate a QKD protocol, but that's because I had been unthinkingly applying the mantra "violation = entanglement".

I certainly think the 'easiest' way to understand all of the QKD work is in terms of collapse - and it's the way I approach the problem being confident that I'm not going to get the wrong predictions. Of course we can interpret all of the QKD stuff in whatever 'interpretation' we like and get the same answers. Speaking purely for myself I find the 'collapse' way of thinking about it to be cleaner and to leave aside all of the legitimate concerns about FTL 'changes of state' that it implies to a lovely Summer sunset with a glass or two of beer

Yes, this reminds me of philosophical discussions about the nature of mathematical objects. Platonism is the position that mathematical objects (such as numbers or sets or functions) exist independently of humans, and when we do mathematics, we are just discovering pre-existing truths. When people are seriously talking about the philosophy of mathematics, they tend to reject platonism as silly: What does it mean that these things exist? Where do they exist? But when you're actually doing mathematics, Platonism seems as good a philosophy as any, and it allows you to get on with your work without worrying too much about the deeper meaning of what it is that you are doing.

The collapse interpretation is almost universally rejected by people who think deeply about physics. But if you're just doing physics, and wanting to get answers that you can compare with experiment, then the collapse interpretation works as well as any.

 

[zonde]

Sorry, I still don't understand what you are talking about. What do you mean when you say "none that is consistent with QM predictions for pure state probabilities"?

In the entangled two-photon state, you have the state $\frac{1}{\sqrt{2}} (|H\rangle |H\rangle + |V\rangle |V\rangle)$. We do the trace business and Alice uses the mixed state $\frac{1}{2} |H\rangle \langle H| + \frac{1}{2} |V\rangle \langle V|$. Bob uses the same mixed state. The meaning is that if Alice measures the polarization, she'll get $H$ with probability 50% and $V$ with probability 50%. Actually, that density matrix says that if Alice measures the polarization at ANY angle, she will get $H$ with probability 50% and $V$ with probability 50%. Same with Bob.

Now, if you ask Alice what the probability is that she and Bob get the same polarization, she can't answer that question using her's and Bob's density matrices, because those have thrown away correlation information.

So what inconsistency are you talking about? Do you mean an inconsistency of the type: "The theory says we will find X, but in actually X is not the case."

Can we speak about mixed state before Alice actually makes her measurement? I suppose so. Let's say we prepare for Alice beam of light that for first 5 seconds is H polarized and for next 5 seconds we switch polarization to V. If she measures her beam with polarizer at an angle $\alpha$ then for first 5 seconds her "click" rate will be say $p_1$ and for next 5 seconds it will be $p_2$. And $p_1+p_2$ gives probability 0.5 of the whole expected photon count.

 

[stevendaryl]

Can we speak about mixed state before Alice actually makes her measurement? I suppose so. Let's say we prepare for Alice beam of light that for first 5 seconds is H polarized and for next 5 seconds we switch polarization to V. If she measures her beam with polarizer at an angle $\alpha$ then for first 5 seconds her "click" rate will be say $p_1$ and for next 5 seconds it will be $p_2$. And $p_1+p_2$ gives probability 0.5 of the whole expected photon count.

Okay, but I don't understand what this scenario is supposed to be illustrating.

 

[zonde]

Okay, but I don't understand what this scenario is supposed to be illustrating.

These $p_1$ and $p_2$ agree (within some limits) with predicted probabilities of QM for pure states. This does not work within in my extended model of Alice and Bob i.e. you can't get similar $p_1$ and $p_2$ right.

 

[stevendaryl]

These $p_1$ and $p_2$ agree (within some limits) with predicted probabilities of QM for pure states. This does not work within in my extended model of Alice and Bob i.e. you can't get similar $p_1$ and $p_2$ right.

Something is not clicking for me. I don't know what you mean by "you can't get similar $p_1$ and $p_2$".

Maybe a little elaboration on the meaning of density matrices. Although there is something a little subjective about density matrices (they include uncertainty about the true state), if you have a way to reproduce the same situation over and over again, then it is possible by statistics to zero in on a precise density matrix. If you get statistics for measurement results at a variety of filter orientations, there will only be one density matrix consistent with those statistics (I'm pretty sure). In the scenario you're talking about, the procedure for producing photons is changing with time, so I don't think that there will be a unique density matrix that can be discovered empirically (unless the time dependence is periodic, and the same pattern repeats over and over). So in terms of repeated measurements, you can distinguish between a pure state and a mixed state. But what you can't distinguish, empirically, is proper versus improper mixed states. Statistics for measuring at a variety of filter orientations is not going to tell you whether you have a proper mixed state (a pure state that is chosen randomly, with the same probabilities, over and over) and an improper mixed state (due to looking at one component of an entangled pure state).

There definitely is a distinction between the pure state $\frac{1}{\sqrt{2}} |H\rangle + \frac{1}{\sqrt{2}} |V\rangle$ and the mixed state $\frac{1}{2} |H\rangle \langle H| + \frac{1}{2} |V\rangle\langle V|$, and you can distinguish them through statistics. But you have to perform measurements at a variety of orientations to see the difference.

 

[zonde]

But what you can't distinguish, empirically, is proper versus improper mixed states. Statistics for measuring at a variety of filter orientations is not going to tell you whether you have a proper mixed state (a pure state that is chosen randomly, with the same probabilities, over and over) and an improper mixed state (due to looking at one component of an entangled pure state).

Yes certainly.

Something is not clicking for me.

Maybe you are trying to see in my argument more than I'm actually trying to claim. I'm trying to say that improper mixed state can't be modeled as statistical mixture of pure states (proper mixed state) even so observable statistics are the same for both cases (because model goes beyond observed statistics).

I'm thinking about your statement that tracing out one side just strips away information. Maybe you have a point.

 

[rubi]

I'm trying to say that improper mixed state can't be modeled as statistical mixture of pure states (proper mixed state) even so observable statistics are the same for both cases (because model goes beyond observed statistics).

And you're wrong, because it can be modeled that way.

However, it makes no sense to argue as long as you haven't learned the formalism. It can't be understood without knowledge about the quantum formalism.

 

[forcefield]

Maybe you are trying to see in my argument more than I'm actually trying to claim. I'm trying to say that improper mixed state can't be modeled as statistical mixture of pure states (proper mixed state) even so observable statistics are the same for both cases (because model goes beyond observed statistics).

I suggest that you add a physical collapse and you should see that it can be modeled.

 

[zonde]

I suggest that you add a physical collapse and you should see that it can be modeled.

Idea is that you have to specify particular pure states for statistical mixture before Alice and Bob has performed measurements.
Do you still think it can be modeled?

 

[forcefield]

Idea is that you have to specify particular pure states for statistical mixture before Alice and Bob has performed measurements.
Do you still think it can be modeled?

Yes. Alice always measures before Bob or vice versa.

 

[Zafa Pi]

You're contradicting yourself here. By definition, an improper mixture does reflect ignorance about the true state.

And you're wrong that ignorance has no place in physics. The use of probability gives us a way to reason in the presence of uncertainty/ignorance.

Anyway, you're venturing into philosophy here, and I don't find your philosophy of science very compelling. Let's stick to the physics. The situation in which Alice flips a coin and sends a spin-up state to Bob if her coin is "heads" and sends a spin-down state if her coin is "tails" is certainly a different situation than the case where Bob measures the spin of one member of an entangled two-particle system. In the first case, Alice knows ahead of time what result Bob will get, and in the second case, she doesn't. Those are clearly different situations. But from Bob's point of view, they are both described by the mixed state with equal weights of spin-up and spin-down. In the first case, the mixture reflects Bob's ignorance and in the second it does not. You say "ignorance has no place in physics", but I think that's silly.

Hi - I find your comment quite interesting. What if in the 1st case Alice flips the coin but doesn't see it land and instead a machine sends off the appropriate state to Bob. Are cases 1 and 2 still different situations, i.e. the first case reflects Bob's ignorance and in the second it doesn't?

 

[morrobay]

Well, your modified EPR experiment is basically the "collapse" interpretation, where the collapse is actually performed explicitly by Alice. In the collapse interpretation, after Alice measures spin-up for her particle, the state of Bob's particle "collapses" to spin-down. In your alternative scenario, Alice explicitly creates a spin-down particle and sends it to Bob.

So I don't think it should be too surprising that your altered scenario can violate Bell's inequalities--it's always been known that instantaneous wave function collapse was a way to explain EPR, but that was rejected by people who dislike the notion of an objective instantaneous collapse (since that would be an FTL effect).

So this single particle inequality violation is just constructed by Alice to demonstrate FTL effect.
I don't understand why more attention is not given to CFD to account for inequality violations : https://arxiv.org/pdf/1605.04889.pdf
Instead of relying on a bizarre feature that has no known physical mechanism.

 

[Simon Phoenix]

Instead of relying on a bizarre feature that has no known physical mechanism.

If we assume that a 'state' is some objective property of a system, and we accept the 'axiom' of quantum mechanics that says if we make a von Neumann measurement of type I (basically what has been called a 'filter' type measurement) then the result of the measurement will be an eigenvalue of the measurement operator and the measured system will be in an eigenstate of the measurement operator after the measurement, then there is also no known physical mechanism that will achieve this.

The only way out of this (that I can see) is to assume that there is no meaning to a system being 'in' a state and the word 'state' means a mathematical quantity that is merely descriptive of our knowledge and not descriptive of some objective physical property of an entity. Measurement is then simply an 'updating' of our knowledge, a bit like (but not exactly like) when we update a probability distribution based on new data (measurements). In the quantum case we're not updating a probability but something from which we can derive probabilities. This doesn't really tell us what a measurement 'is' in physical terms but just describes its effect on our knowledge (whatever that rather vague term actually means). It also doesn't really explain (to my mind, at least) why our 'knowledge' has to be encoded in a mathematical object that evolves according the Schrodinger equation (involving physical things like energy and interactions), lives in an abstract complex space, has such close connections at a deeper level to classical mechanics, and yet is not supposed to model 'reality' in any objective way. I would (grudgingly) agree that this 'knowledge' viewpoint makes more coherent logical sense, but as a physicist it leaves me very unsatisfied because I no longer have any real physical 'picture' of what's happening but must deal with things in a very operational way using vague terms like 'knowledge' or 'what can be known' in order to interpret things.

Decoherence will give us the correct density operator if we remain ignorant of the actual result, but it does not explain why we get a particular pure state after measurement (if we know the result decoherence predicts an incorrect density operator). Of course in order for these comments to make sense we must believe that the formalism of QM applies to single measurements on single systems - and not just to ensembles.

 

[vanhees71]

If you prepare the value of only one observable (e.g., by filtering) then in general you don't know in which eigenstate the system is. Suppose you determine observable $A$ to have the value $a$ and if $|a,b \rangle$ is a complete orthonormal set of eigenvectors, and if we assume for simplicity that $b \in \{1,2,\ldots,n\}$ ("$n$-fold degeneracy"), then you'd rather associate the state
$$\hat{\rho}=\frac{1}{n} \sum_{b=1}^n |a,b \rangle \langle a,b|.$$
That choice is due to the maximum-entropy principle (or the principle of least prejudice) in the sense of information theory. You must not assume something you don't know. You have a pure state if and only if $n=1$.

 

[A. Neumaier]

in the sense that Alice can repeat the same experiment over and over and note how often it is that when she prepares a state that is spin-up in the z-direction, Bob measures spin-up in the z-direction.

This is a test of the preparation of an ensemble, not of a single state.

By definition, an improper mixture does reflect ignorance about the true state.

This assumes that ignorance affects the outcome of physical experiments, and hence the state of a system.

Now I am ignorant about most experiments done in the word, but my ignorance obviously doesn't affect their outcome.

In fact, nobody's ignorance can make a difference since ignorance is a property of the state of a brain while measured is some information about the state of a tiny quantum object usually completely unclupled to any brain. Most measurements are done automatically without anyone observing the details, so there is no distinguished knower whose ignorance might be relevant.

Ignorance about a (pure or mixed) state simply means that when one assigns an arbitrary state for it it is likely to give wrong predictions - unless this arbitrary state happens to be the physical state.

 

[stevendaryl]

This is a test of the preparation of an ensemble, not of a single state.

You're making a philosophical point that I disagree with. To me, if someone flips a coin and hides the result, then I use probabilities to reflect my ignorance about the fine details of the coin-flipping process. In my opinion, bringing up ensembles is unnecessary and unhelpful.

 

[A. Neumaier]

You're making a philosophical point that I disagree with.

Most of the discussion here is philosophical; labeling a particular statement as such does not help.

To me, if someone flips a coin and hides the result, then I use probabilities to reflect my ignorance about the fine details of the coin-flipping process. In my opinion, bringing up ensembles is unnecessary and unhelpful.

Thus you and the someone will assign different states to the same physical situation. This means that in the situation you describe, the assigned state is purely subjective and contains no physics. It is a property of your mind and not of the coin. You cannot check the validity of your probability assignment; so any probability is as good as any other. Applying probabilities to single coin flips is simply meaningless.

 

[zonde]

Yes. Alice always measures before Bob or vice versa.

With physical collapse you mean that measurement of Alice's photon changes Bob's photon polarization? Meaning that if initially we model Bob's mixed state as statistical mixture of orthogonal pure states H/V then after Alice's measurement in H'/V' basis Bob's mixed state components change to H'/V' basis, right?

 

[zonde]

The only way out of this (that I can see) is to assume that there is no meaning to a system being 'in' a state and the word 'state' means a mathematical quantity that is merely descriptive of our knowledge and not descriptive of some objective physical property of an entity.

I would (grudgingly) agree that this 'knowledge' viewpoint makes more coherent logical sense, but as a physicist it leaves me very unsatisfied because I no longer have any real physical 'picture' of what's happening but must deal with things in a very operational way using vague terms like 'knowledge' or 'what can be known' in order to interpret things.

As I see this is similar to my own disappointment with defining "state" in a "what can be known" way. To me it seems that intuitive meaning of concept "state" is a model for real physical situation i.e. a model that explains our observations rather than observations themselves. And because that concept is stolen for something else it's harder to talk about model for real physical situation.

It also doesn't really explain (to my mind, at least) why our 'knowledge' has to be encoded in a mathematical object that evolves according the Schrodinger equation (involving physical things like energy and interactions), lives in an abstract complex space, has such close connections at a deeper level to classical mechanics, and yet is not supposed to model 'reality' in any objective way.

Let me oppose you here. Mathematical object that evolves according the Schrodinger equation is a bit closer to real physical 'picture' and is not quite identical to state (in "what can be known" sense).
I found this jtbell post https://www.physicsforums.com/threa...-of-schrodinger-equation.889605/#post-5596330 quite interesting and sort of confirming my sentiments. As I understand Schrödinger's intuition that helped him to arrive at his equation was this:
"is one not greatly tempted to investigate whether the non-applicability of ordinary mechanics to micro-mechanical problems is perhaps of exactly the same kind as the non-applicability of geometrical optics to the phenonema of diffraction or interference and may, perhaps, be overcome in an exactly similar way?"
So it's interference phenomena for massive particles that was starting point for him.
And it's interesting that Feynman too had some very special attitude toward interference phenomena:
"We choose to examine a phenomenon which is impossible, absolutely impossible, to explain in any classical way, and which has in it the heart of quantum mechanics. In reality, it contains the only mystery."

As I see the "the heart of quantum mechanics" is represented by phase factor. So to me it seems not very wise to hide it somewhere away or to try to drop it entirely.

 

[stevendaryl]

As I see the "the heart of quantum mechanics" is represented by phase factor. So to me it seems not very wise to hide it somewhere away or to try to drop it entirely.

I don't think anyone is saying to get rid of phase information. As you say, it's absolutely at the heart of quantum phenomena. The density matrix formulation does not get rid of relative phase information, only overall phase, which plays no role in interference.

 

[stevendaryl]

I don't think anyone is saying to get rid of phase information. As you say, it's absolutely at the heart of quantum phenomena. The density matrix formulation does not get rid of relative phase information, only overall phase, which plays no role in interference.

Quantum interference is somewhat like classical wave inference, but is subtly different: A classical wave is a wave in physical space; you have a field that extends throughout space, and propagates as a function of time according to a wave equation. A quantum wave function is a wave in configuration space. In QM, the interference effects involve interference between different possibilities.

For classical interference of say, light waves, you can understand it in terms of part of the wave goes one way (through one slit, for instance) while another part of the wave goes another way (through a different slit). The interference is not interference between possibilities, it's interference between actualities---there really is electromagnetic fields going through both slits. But when you attempt to apply that idea to quantum mechanics, it seems to me that you are forced to a many-worlds view, where different possibilities are equally real. Most people reject that interpretation, and understandably so (it seems to posit the existence of whole worlds that are unobservable), but if you reject the reality of alternative possibilities, then it's hard for me to understand what interference effects are about. (Note: The Bohmian interpretation seems more realistic than other interpretations, since it only has one world with definite positions for particles at all time. But in the Bohmian interpretation, there is still a wave function that acts as a "guide" to particle motion, and this wave function is determined by interference effects among possibilities, even though only one possibility is considered "real".)

 

[zonde]

In QM, the interference effects involve interference between different possibilities.

I would say that interference between "possibilities" is heuristic that avoids hard and currently unanswerable questions.

But in the Bohmian interpretation, there is still a wave function that acts as a "guide" to particle motion, and this wave function is determined by interference effects among possibilities, even though only one possibility is considered "real".

I don't know Bohmian interpretation very well but judging by it's key features I think that Bohmian interpretation goes in right direction. What I'm missing in that interpretation is particle effect on pilot wave. As I understand many interacting worlds interpretations is variation of Bohmian interpretation that is modeling pilot wave from many particles so it sort of fills that gap. But I have not investigated it as there is not so much to read about it at my level.

 

[stevendaryl]

I would say that interference between "possibilities" is heuristic that avoids hard and currently unanswerable questions.

In what way?

 

[Demystifier]

Thus you and the someone will assign different states to the same physical situation. This means that in the situation you describe, the assigned state is purely subjective and contains no physics. It is a property of your mind and not of the coin. You cannot check the validity of your probability assignment; so any probability is as good as any other. Applying probabilities to single coin flips is simply meaningless.

Just because the assignment of probability to a single event is subjective and cannot be checked does not mean it's meaningless. Such a Bayesian subjective assignment of probability may be useful in making decisions. This is something that people do (often intuitively and unconsciously) every day. (For instance, I have to buy shoes for my wedding (and I was never buying wedding shoes before), so have to decide which shop I will visit first. I choose the one for which I estimate a larger probability of finding shoes I will be satisfied with.)

 

[stevendaryl]

Thus you and the someone will assign different states to the same physical situation. This means that in the situation you describe, the assigned state is purely subjective and contains no physics.

That's plainly not true. You assign (subjective) probabilities to initial conditions, and then you evolve them in time using physics, to get derived probabilities for future conditions. There's plenty of physics involved.

The assumption that if there is a subjective element to your reasoning, then the entire reasoning process is nonscientific would, if taken seriously, imply that science is impossible. Whether you perform an experiment 5 times or 1 million times, there is the logical possibility that the statistics that you gather are a "fluke". To make any conclusion requires a subjective judgment that your data is sufficient to rule out some possibility. Without making such subjective judgments, you really couldn't make any conclusions in science.

Saying that the physics describes ensembles, rather than individual events does absolutely nothing to change the fundamental subjectivity of probability judgments. If the actual ensemble is finite (which it always is), then in reality, you have the same problem as single events, which is how to make judgments based on finite data.

 

[zonde]

In what way?

You don't have to speak about actualities. Meaning you don't care how to give realistic model of interference.