Is Consciousness involved in wave function collapse?

[G01]

Yes. Shuffle a deck and pick out a card. Look at the card. I predict you will see a particular card. That prediction is not made by any interpretation of physics that does not include consciousness, because a mixed-state statistical treatment of the outcome of that process is always completely adequate until you actually have a consciousness there. Again, note that I never need to understand consciousness, or model it, or include it in my theory, I merely recognize the observational fact that physics needs no concept of a particular outcome, rather than a statistical distribution, until I have a consciousness involved.

We may just have to agree to disagree on this topic.

I see how, working in your interpretation, consciousness would be related to the nonunitary portion of the measurement, but I don't think you've shown that this interpretation is better than many worlds or the CI. Perhaps you have some philosophical arguments, but the above does not count as a new testable prediction.

If instead you said, "Look pick a card. The probability that you pick that card is X, the probability that you pick this other card is Y."

That's the type of prediction that right now, QM has trouble with. We want to be able to predict which classical state in which our system ends up, and the probability that it ends up in that state over any other. We need a model for this portion of the measurement interaction.

However, you are not supplying a model. You are taking the known fact that the system ends up in a certain state, and saying, "Hey, look. The system has to end up in a particular state when we look at it. Physics describes how systems end up in particular states, but is developed by our minds. Therefore physics is inseparable from our conscious minds. Our conscious minds must have something to do with this measurement."

That is not a scientific argument. It is an interpretation of a not completely understood area of QM that is supported only by the fact that it is consistent with your philosophy of science and language.

You don't provide any mechanism by which consciousness collapses the WF. You can't provide any quantitative prediction about which state the system will collapse into. You only offer the trivial statement that the system will end up in a particular state. That's not a useful scientific prediction. At best, it is a check that your are still consistent with nature.

Until those last two conditions are met, it's not true to say that consciousness must be involved in the hypothetical, predictive, scientific theory of quantum measurement.

I'm not saying I have the answers. I don't. Most of the time I tend to be a "shut up and calculate" kind of guy. When I do talk about this stuff, I tend to focus on the unitary part of the process that we understand. I'm as confused as the next guy. You philosophy of science and language even has some appeal to me.


However, it's not correct to say that something must scientifically be the case if that something provides no mechanism, no real predictive power, and whose support is derived from a philosophy of science that not everyone must agree on.

1) but what is this 'interaction' between the quantum system and the measuring device?

2) how does the quantum system know the measuring device is there?

Hi bugatti79. Sorry, we are talking in very abstract terms here. The actual interaction will depend on what your actually measuring. For example, if we are measurement spin moments of nuclei, the interaction would be magnetic in nature.

As far as how does the system "know" about the measuring device: Well it "knows" because of the interaction. Interacting with the measuring device is what we mean when we say, the system "sees" the measuring device or "knows about" the measuring device.

 

[Ken G]

We may just have to agree to disagree on this topic.

I see how, working in your interpretation, consciousness would be related to the nonunitary portion of the measurement, but I don't think you've shown that this interpretation is better than many worlds or the CI.

What I've tried to stress is that what I'm talking about is not an alternative interpretation than CI or many worlds, it is an inherent component to CI and many worlds. Indeed, without the consciousness of the physicist to register an experimental outcome, many worlds is not even an interpretation any more, it is just quantum mechanics, and there simply is no CI interpretation. This is because there is no CI without physicists, CI is all about what the physicist is doing, and what CI means by a physicist is someone conscious (whether it's the consciousness, or intelligence, or ability to register perceptions, is all the same in what I'm saying, I've made no effort to distinguish those concepts because we don't know how to).

If instead you said, "Look pick a card. The probability that you pick that card is X, the probability that you pick this other card is Y."

That's the type of prediction that right now, QM has trouble with.

No, that is exactly the kind of prediction that quantum mechanics has no trouble with, so much so that you don't even need an interpretation of quantum mechanics to answer that. What QM has trouble with, and why you need interpretations, is the next step, when you don't say the probability is X, but you say the card was actually X. That's where the problem appears, and that's also the step adjudicated by the consciousness of the experimenter.

However, you are not supplying a model.

The model to do that is already here, that's what quantum mechanics does, that's what the theory of decoherence does. All of that is before you even get to an interpretation, all the interpretations include that part, as does the things one learns in a textbook on quantum mechanical calculations.

You don't provide any mechanism by which consciousness collapses the WF.

That is correct, nor does the logic of my argument require any such mechanism. And if someone says "if there's no mechanism, it isn't science," then they just don't understand science. Newton gave no mechanism for why F = ma, after all. Science works from the other direction-- we first decide what we need to explain, and then we try to explain it with models, and sometimes the models involve enough other elements to be considered a mechanism. But the real point is, first we have what we must explain, then we try to explain it. What I've demonstrated is that what we must explain is how a consciousness comes up with a single outcome, when every theory of physics we have only tells us statistical distributions over an ensemble of outcomes. This is just plain true, all physics theories have that property, whether quantum mechanical or classical. You are welcome to suggest one that does not if you doubt it.

You can't provide any quantitative prediction about which state the system will collapse into.

Obviously, something like that would not only be a different theory from quantum mechanics, it would contradict quantum mechanics.

However, it's not correct to say that something must scientifically be the case if that something provides no mechanism, no real predictive power, and whose support is derived from a philosophy of science that not everyone must agree on.

That is just not true. There are a whole host of things that scientifically must be the case, yet provide no mechanism and no predictive power. That is indeed the very building block of science-- I would call it a "raw observation." And I'm saying that the role consciousness plays in obtaining a particular outcome, when all the theories of physics produce only statistical distributions (when you include the necessity of noise and uncertainty), is a raw observation. You are asking me to explain it before you will recognize it as true, and I'm saying that you are asking science to work backwards. We don't start with explanations, we start by recognizing what it is we need to explain.

 

[Fredrik]

I just want to say that I agree with Ken's comments about consciousness. The key point is that an interaction that doesn't produce a persistent record that a human can interpret as a specific result of the interaction wouldn't be considered a measurement. This isn't a magical property of consciousness, or some highly speculative pseudo-theory that tries to explain something. It's just a statement about what sort of interaction we would consider a "measurement".

 

[Ken G]

That's an excellent way to put it Fredrik, thank you. I think a lot of the problem was with all the magical properties attributed to consciousness, and I am referring only to the central issue that you also focus on: the ability for a consciousness to register an outcome. This is a very fundamental problem for physics, because physics is only good at predicting probability distributions when you call on it to achieve high precision, yet when we test those predictions, we do it with ensembles of fairly precise individual outcomes. There's a kind of disconnect there which exists even in purely classical contexts, yet is very reminiscent of the famed "Heisenberg gap" in quantum/classical interfaces.

 

[bugatti79]

We may just have to agree to disagree on this topic.

I see how, working in your interpretation, consciousness would be related to the nonunitary portion of the measurement, but I don't think you've shown that this interpretation is better than many worlds or the CI. Perhaps you have some philosophical arguments, but the above does not count as a new testable prediction.

If instead you said, "Look pick a card. The probability that you pick that card is X, the probability that you pick this other card is Y."

That's the type of prediction that right now, QM has trouble with. We want to be able to predict which classical state in which our system ends up, and the probability that it ends up in that state over any other. We need a model for this portion of the measurement interaction.

However, you are not supplying a model. You are taking the known fact that the system ends up in a certain state, and saying, "Hey, look. The system has to end up in a particular state when we look at it. Physics describes how systems end up in particular states, but is developed by our minds. Therefore physics is inseparable from our conscious minds. Our conscious minds must have something to do with this measurement."

That is not a scientific argument. It is an interpretation of a not completely understood area of QM that is supported only by the fact that it is consistent with your philosophy of science and language.

You don't provide any mechanism by which consciousness collapses the WF. You can't provide any quantitative prediction about which state the system will collapse into. You only offer the trivial statement that the system will end up in a particular state. That's not a useful scientific prediction. At best, it is a check that your are still consistent with nature.

Until those last two conditions are met, it's not true to say that consciousness must be involved in the hypothetical, predictive, scientific theory of quantum measurement.

I'm not saying I have the answers. I don't. Most of the time I tend to be a "shut up and calculate" kind of guy. When I do talk about this stuff, I tend to focus on the unitary part of the process that we understand. I'm as confused as the next guy. You philosophy of science and language even has some appeal to me.


However, it's not correct to say that something must scientifically be the case if that something provides no mechanism, no real predictive power, and whose support is derived from a philosophy of science that not everyone must agree on.




Hi bugatti79. Sorry, we are talking in very abstract terms here. The actual interaction will depend on what your actually measuring. For example, if we are measurement spin moments of nuclei, the interaction would be magnetic in nature.

As far as how does the system "know" about the measuring device: Well it "knows" because of the interaction. Interacting with the measuring device is what we mean when we say, the system "sees" the measuring device or "knows about" the measuring device.

I see, so measuring quantum systems involves some form of electromagnetic fields and hence its a 'physical' interaction.

Ok, thank you for the clarity.

 

[G01]

The key point is that an interaction that doesn't produce a persistent record that a human can interpret as a specific result of the interaction wouldn't be considered a measurement.

That's an excellent way to put it Fredrik, thank you.

OK. I agree with this. But I think I'm getting stuck on the use of the word consciousness. Let me give an example:

We build a robot that sets up a two state system in some superposition. The robot then measures the system, finding it in some eigenstate. It records the eigenstate. The wavefunction collapses when we read the data sheet the robot prints out then? Or the second, non-unitary portion of the measurement process happens when we read the data sheet?

The way I would have looked at this scenario before this conversation is as follows:

The whole measurement process happens when the robot interacts with the system. The system collapsed into a random eigenstate, though the robot is not conscious. It is in a definite eigenstate, though I can't know what one it ended up in.

 

[G01]

it is in a definite eigenstate, though i can't know what one it ended up in.

Yes. Shuffle a deck and pick out a card. Look at the card. I predict you will see a particular card. That prediction is not made by any interpretation of physics that does not include consciousness, because a mixed-state statistical treatment of the outcome of that process is always completely adequate until you actually have a consciousness there. Again, note that I never need to understand consciousness, or model it, or include it in my theory, I merely recognize the observational fact that physics needs no concept of a particular outcome, rather than a statistical distribution, until I have a consciousness involved.

ahhhhhhh...OK. I see your point. You are talking about a much broader problem in physics than I originally grasped.

 

[Ken G]

We build a robot that sets up a two state system in some superposition. The robot then measures the system, finding it in some eigenstate. It records the eigenstate. The wavefunction collapses when we read the data sheet the robot prints out then? Or the second, non-unitary portion of the measurement process happens when we read the data sheet?

Yes, the robot participates in the decoherence, but that just puts the robot into a state that physics would need to describe as a probability distribution, or mixed state. We don't have a robot, just like the original system, in a particular state until the consciousness shows up to register that.

The way I would have looked at this scenario before this conversation is as follows:

The whole measurement process happens when the robot interacts with the system. The system collapsed into a random eigenstate, though the robot is not conscious. It is in a definite eigenstate, though I can't know what one it ended up in.

Yes, I agree with all that with one caveat: where does the "definite eigenstate" come from? The equations of physics would never want to give you a definite eigenstate, they'd always want to give you a probability distribution of all of them. But we can't just leave it at that, even though it's what our equations give us, because of our consciousness, and our consciousness doesn't perceive that distribution. This is the only reason we're not all "many worlds" quantum mechanicians. I think you are seeing what I mean now.

 

[G01]

I think you are seeing what I mean now.

Yes, we were having completely separate conversations for most of page 2. Special thanks to Fredrick for his post. I had to reread it several times. Then, I think I finally realized what you guys were saying.

So, the part of the measurement process represented by the nonunitary evolution in quantum mechanics exists throughout all of physics, I see what you mean. We get specific results, even though our process of modeling nature only gives us probability distributions.

So, what your saying is that our method of modeling nature is incomplete or limited, and the measurement problem is a symptom of that?

 

[BruceW]

Ken G - you seem to have been confusing the term "mixed state" with the term "pure state".
Pure state means each particle is in a superposition of eigenstates. Mixed state is when each particle is in one eigenstate, but there is a certain percentage of particles in each state, which gives a probability.

 

[BruceW]

For example, when you said that decoherence puts the robot into mixed state, you mean pure state.
Decoherence doesn't put something from a pure state into a mixed state.
In fact, it is collapse that puts something from a pure state into a mixed state.

 

[Fredrik]

Pure state means each particle is in a superposition of eigenstates. Mixed state is when each particle is in one eigenstate, but there is a certain percentage of particles in each state, which gives a probability.

It's also when you're dealing with a single object that you know is in an eigenstate, but you don't know which one. This is the situation when your measuring device has just measured something, but you haven't yet examined the result. Since you don't know which of its eigenstates it's in, you would have to describe it as a mixed state.

 

[Ken G]

So, the part of the measurement process represented by the nonunitary evolution in quantum mechanics exists throughout all of physics, I see what you mean. We get specific results, even though our process of modeling nature only gives us probability distributions.

Exactly.

So, what your saying is that our method of modeling nature is incomplete or limited, and the measurement problem is a symptom of that?

Yes. Though I don't think I'd use the term "symptom", because that suggests there is something sick in our approach. Instead, I think there is not anything sick in our approach, I think the whole point of our approach is to be limited. We already have the real world, it is not the goal of physics to recreate it. It is the goal of physics to understand a simplified version of it, so the limitations of physics and the limitations of our minds are perfectly matched. That's a feature of the measurement problem, not a bug of it-- we need to pass reality through the filter of measurement if we are to have any hope of understanding what aspects of it we can understand. The question then is, is the wavefunction, or the statistical distribution classically, the real thing we are passing through the filter of individual measurement, or are those just the mathematical tools we use to analyze whatever survives that translation? I tend to see it the latter way, so I would say the measurement problem is a reflection of the purpose of physics, but "symptom" certainly carries much of the same connotations.

 

[Ken G]

Ken G - you seem to have been confusing the term "mixed state" with the term "pure state".

No, the issue is that a closed system evolves unitarily, so remains in a pure state even if a decohering measurement occurs within it. However, when you project this pure state of the full system onto the quantum subspace, you get a mixed state, which is a probability distribution of all the possible outcomes, with incoherent relative phases.

What you are talking about comes later, after we have witnessed the state of the pointer. Then we reject all other members of the mixed state, and only then do we get a pure state for the quantum subspace. That entire process is what we mean by "measurement", but this last phase is nonunitary, not described by any equation of physics, and heavily reliant on our consciousness, as described above.

 

[Ken G]

For example, when you said that decoherence puts the robot into mixed state, you mean pure state.

No, the robot is still a subspace of the entire system. There is no way to get nonunitary evolution of the entire non-conscious system, if quantum mechanics is a good theory. This means the robot, being a subspace of a pure state, is projected onto a mixed state. That's what a mixed state is-- a projection of a pure state onto a subspace that has nonzero components on multiple eigenstates of the subspace.

Decoherence doesn't put something from a pure state into a mixed state.

Yes, it certainly does. For example, if we start with a vertically spin-up particle, and send it through a Stern-Gerlach apparatus inclined horizontally, and don't look at the outcome, we have a mixed state of a photon with two different spins going two different ways. Usually, you hear it said that the photon goes one way or the other, and is in a pure state, but that's because they are implicitly assuming someone looked. If no one looks, one can imagine a hypothetical observer serving the purpose, and say it is a pure state and we just don't know which, but that still requires applying the conscious action of that hypothetical observer. If you simply don't do that final step, it remains in a mixed state, just like an ensemble does (the ensemble is exactly the configuation where no one is considered to have looked at the individual members).

In fact, it is collapse that puts something from a pure state into a mixed state.

If you reread the above, you will see there are two steps to "the collapse." The first step, the decoherence, destroys correlations between the eigenstates, but cannot choose an outcome for the final eigenstate because it is a unitary evolution. The second step, the choosing of the final pure state, is nonunitary, does not appear in any of the equations of quantum mechanics, and is imposed manually by our conscious need to agree with our experience. So collapse first puts a pure state into a mixed state, and then consciousness somehow necessitates that we consider the mixed state as collapsed into a registered pure state-- unless we don't imagine anyone looking, and just leave it as an ensemble, in which case it is still a mixed state of the quantum subsystem.

 

[BruceW]

I think you took the term "mixed state" to mean a quantum superposition of pure states. But this is not the case.
A mixed state and a quantum superposition of pure states are two different things.
After decoherence happens, the two things are no longer physically distinguishable.
But this doesn't mean a quantum superposition becomes a mixed state after decoherence. Collapse is required for the mixed state to actually happen.

 

[Ken G]

I think you took the term "mixed state" to mean a quantum superposition of pure states.

Certainly not, a quantum superposition has definite phase relationships, leading to interference. That's exactly what a mixed state does not have. My use of the term is standard, as confirmed by Fredrik. Is there some particular point that is relevant to what you are saying?

A mixed state and a quantum superposition of pure states are two different things.

Yes, nor did I ever suggest otherwise.

After decoherence happens, the two things are no longer physically distinguishable.

That isn't making much sense. A quantum superposition of pure states is easily distinguished from a mixed state. After decoherence, the quantum superposition is the entire system, which we cannot even write the wavefunction for except in pictorial terms a la von Neumann. The mixed state is the projected substates, such as what often gets called the state of the quantum, a statistical representation that embodies the information we have about the quantum.

But this doesn't mean a quantum superposition becomes a mixed state after decoherence. Collapse is required for the mixed state to actually happen.

I really don't understand what you are trying to say. What I said was correct, and I clearly separated what was meant by the various aspects of "the collapse." A quantum superposition (the treatment of the quantum subsystem) does become a mixed state after decoherence, that's more or less the definition of decoherence. Perhaps if you have a particular point that you are making in regard to what you view as corrections, it would be clearer why you believe corrections are needed.

ETA: for those who are uncertain about the issues of pure and mixed states, you are welcome to simply consult the generally reliable authority of the Wiki on the matter: http://en.wikipedia.org/wiki/Quantum_decoherence
"Specifically, decoherence does not attempt to explain the problem of measurement. Rather, decoherence provides an explanation for the transition of the system to a mixture of states that seem to correspond to those states we perceive as determinate." (my bold)

 

[BruceW]

For example, if we start with a vertically spin-up particle, and send it through a Stern-Gerlach apparatus inclined horizontally, and don't look at the outcome, we have a mixed state of a photon with two different spins going two different ways. Usually, you hear it said that the photon goes one way or the other, and is in a pure state, but that's because they are implicitly assuming someone looked. If no one looks, one can imagine a hypothetical observer serving the purpose, and say it is a pure state and we just don't know which, but that still requires applying the conscious action of that hypothetical observer. If you simply don't do that final step, it remains in a mixed state, just like an ensemble does (the ensemble is exactly the configuation where no one is considered to have looked at the individual members).

You say that the particle which comes out of the Stern-Gerlach apparatus is in a mixed state, but it is in a superposition of pure states. (which is why it looked like you got the terms confused). P.S. I think you meant electron, not photon.

To clarify the other stuff I was saying in my last post:
After decoherence happens, the quantum superposition of the outcomes is (approximately) equal to a mixed state. This is why collapse happens sometime after decoherence.

 

[BruceW]

In your interpretation, you have defined collapse to finish when a conscious observer realizes what the outcome was.
But generally, collapse could have finished at any time between decoherence and the point when the person consciously realizes the outcome.

(P.S. I would say the term 'collapse' meant the nonunitary evolution. But I've used your meaning of the word in this post, so I can get my point across better).

 

[Ken G]

You say that the particle which comes out of the Stern-Gerlach apparatus is in a mixed state, but it is in a superposition of pure states. (which is why it looked like you got the terms confused).

I forgot to add that we need a pointer to say which way the photon went, built into the system. That's what creates the decoherence that creates the mixed state, the Stern-Gerlach just separates them.

 

[Ken G]

In your interpretation, you have defined collapse to finish when a conscious observer realizes what the outcome was.

It's not an interpretation, it's a fact that we all experience. Interpretations are something different.

But generally, collapse could have finished at any time between decoherence and the point when the person consciously realizes the outcome.

As I pointed out above, collapse means two very different things that must be distinguished. Sometimes it just means decoherence, which means you still have a larger system that is still evolving unitarily. Other times it means you have a definite outcome, which basically means the unitary system includes the conscious observer (in many worlds) or the conscious observer breaks the unitarity (in Copenhagen).

(P.S. I would say the term 'collapse' meant the nonunitary evolution. But I've used your meaning of the word in this post, so I can get my point across better).

I never use the term at all, because it is so ambiguous. Instead, I speak of the two separate phases separately.

 

[BruceW]

Other times it means you have a definite outcome, which basically means the unitary system includes the conscious observer (in many worlds) or the conscious observer breaks the unitarity (in Copenhagen).

No, in Copenhagen, any classical object breaks the unitarity.
You place an importance on consciousness that is not a mainstream view.

 

[Ken G]

No, in Copenhagen, any classical object breaks the unitarity.

Are you aware of von Neumann's measurement theory? It is generally considered to be consistent with the CI, but you have to understand the two stages of what generally gets called "collapse" of the wavefunction to see that.

You place an importance on consciousness that is not a mainstream view.

There is a difference between something you have not understood, and something that is not mainstream. Already, you have contradicted not just what I've said, but what I quoted for you from the Wiki, and from what Fredrik said, and now you claim that you are the only one that is mainstream. G01 was able to see why that is not the case, but you are not getting through your own misconceptions here. You need to understand the meaning of a mixed state better before you can even understand what I am saying, and branding it non-mainstream will not accomplish that for you.

 

[San K]

In your interpretation, you have defined collapse to finish when a conscious observer realizes what the outcome was.
But generally, collapse could have finished at any time between decoherence and the point when the person consciously realizes the outcome.

(P.S. I would say the term 'collapse' meant the nonunitary evolution. But I've used your meaning of the word in this post, so I can get my point across better).

collapse, in my opinion (its all opinions here anyway ...;) about something we know very little about), would occur the moment quanta/photon is detected (an interaction has happened) by an/any instrument

 

[G01]

No, in Copenhagen, any classical object breaks the unitarity.
You place an importance on consciousness that is not a mainstream view.

Are you aware of von Neumann's measurement theory? It is generally considered to be consistent with the CI, but you have to understand the two stages of what generally gets called "collapse" of the wavefunction to see that.There is a difference between something you have not understood, and something that is not mainstream. Already, you have contradicted not just what I've said, but what I quoted for you from the Wiki, and from what Fredrik said, and now you claim that you are the only one that is mainstream. G01 was able to see why that is not the case, but you are not getting through your own misconceptions here. You need to understand the meaning of a mixed state better before you can even understand what I am saying, and branding it non-mainstream will not accomplish that for you.

EDIT: I do not have access to all of my old lecture notes on quantum measurement at the moment, so please check to make sure I don't have any mistakes below. This was pieced together partially from memory.

No. What Ken is saying is totally consistent and mainstream. It took me a while to figure out what we were talking about, as well. Just goes to show you how true it is that physicists don't think much about this. Perhaps if we work through an example it will help. (At the very least, it will help me make sure I'm really following Ken's reasoning.)

So, following vonNeumann's approach we have a quantum system in state |\psi> and a pointer in state |P> along with an external environment in state |e>. The entire system (quantum+pointer+environment) is in the state |P>|\psi>|e>. Now, there is a unitary evolution that takes this state:

|P>|\psi>\rightarrow\sum_n c_n|P_n>|\psi_n>|e_n>.

There are no cross terms in this new state. This is the simple version what we call decoherence. We all agree on this part.

Now, let's project out the environment states and see what the density matrix of our system now is. We'll get:

\hat{\rho}=\sum_n c_n|P_n>|\psi_n><\psi_n|<P_n|

Up until this point, everything is fine and well understood. The interaction with the environment has removed all interference terms and we are left with just states along the diagonal. We have a pointer state associated with each eigenstate \psi and a probability of finding anyone of those states (|c_n|^2).

Now here's the part Ken is focusing on: when we actually try to detect which eigenstate the system is in the system evolves in the following way:

\sum_n c_n|P_n>|\psi_n><\psi_n|<P_n|\rightarrow|\psi_i>|P_i><\psi_i|<P_i|
where i is a particular eigenstate. All other elements of the density matrix are zero. Only \rho_{ii} is nonzero.
i.e. This evolution takes the system, which had a non zero probability of being in multiple eigenstates, to having a 100% probability of being in eigenstate i.

This type of evolution cannot be represented by any unitary operator. This is the non unitary part of the measurement we are talking about. This is the WF collapse. This is the "measurement problem." Essentially the question is, "What is happening in this phase of the measurement."

We have discussed several ways of handling this issue:

1.Many Worlds- Our system is a member of an ensemble spread across multiple universes. We see one particular value, not because the WF collapsed, but because the WF describes the entire esemble, and we are only looking at one member.

2.Ken's suggestion is that there is something else happening when an experimenter(What Ken would call a conscious observer) actually takes a measurement.

3.Perhaps nature, in this case, is just random.

Ken is not talking about, as Fredrik puts it, some magical property of consciousness. It's just a statement that physics does not give us the results of individual events, yet we experience individual events.

And, when you think about it with 20-20 hindsight, we should have expected this problem to pop up. Physics only deals with ensembles. (This is why the result of one scattering event (classical or quantum) is not enough to show correspondence with theory.)

 

[Ken G]

That was 100% right on. And I'm not even sure I would say that "something else" is happening when a conscious observer takes a measurement ("else" from what, there's nothing to compare to), because I don't see the consciousness as actually doing something that we should be modeling. I don't think there's any way we can model what the consciousness is doing, we're too close to it. Instead, I think we are modeling everything else, and when we get done, we say "hmm, there's something missing here", just what you described above as the "measurement problem." But can we step back and try to include the missing piece in our model?

I don't think so, I think that's like seeing yourself when you walk into a crowded room. I think we just model what we can, and are not surprised when there are a few chinks. So I see the role of consciousness here as not being another term in the equation or another step in the calculation, I see it as the reason we have the question in the first place, the question that we cannot answer and maybe never will.

 

[BruceW]

Ken G - wikipedia disagrees with you. If you read the page on the Copenhagen interpretation, near the bottom of the page under the heading Alternatives you'll see: Consciousness causes collapse is often confused with the Copenhagen interpretation.
Also, the von Neumann measurement scheme talks about a measuring apparatus, not a conscious person.
G01 has explained that there is some non unitary part to the measurement process that occurs. This non unitary part doesn't require consciousness. A measurement made by a lifeless computer could cause this non unitary process to occur.
In fact, it is because a human is a type of measuring apparatus that we humans cause a non unitary process to occur. Whether the human is conscious or not doesn't affect anything.
(When I say measuring apparatus, I am talking about something that causes wave function collapse), (a.k.a strong von Neumann projection in the words of wikipedia).

 

[Ken G]

Ken G - wikipedia disagrees with you. If you read the page on the Copenhagen interpretation, near the bottom of the page under the heading Alternatives you'll see: Consciousness causes collapse is often confused with the Copenhagen interpretation.

That's simply because the phrase "consciousness causes collapse" can mean about 100 different things, many of which have been carefully delineated on this thread. Indeed that was the initial disconnect between myself and G01, if you care to review that discussion. Nothing that I am saying is what that Wiki means by consciousness causing collapse, but what I do mean is completely consistent with both the Wiki I quoted for you, and what G01 just said above.

Also, the von Neumann measurement scheme talks about a measuring apparatus, not a conscious person.

All covered in the thread, you're not telling me anything I haven't already discussed in detail.

G01 has explained that there is some non unitary part to the measurement process that occurs. This non unitary part doesn't require consciousness. A measurement made by a lifeless computer could cause this non unitary process to occur.

Now I'm just repeating, but I will go through it yet again because this is the crux of the whole business right here.

A measurement made by a lifeless computer creates what is called a mixed state for the quantum system. That is because the quantum system is, at this point, a subspace of a larger apparatus. The larger apparatus, as G01 just explained, is still in a pure state according to the unitary evolution of quantum mechanics theory. The mixed state is the projection from the whole system onto the subspace of the quantum system. Nothing there creates any difficulties, nor requires any interpretations of quantum mechanics-- the full system is in a pure state so is still unitary, the projection onto the subspace is not supposed to be unitary, it's a projection from a joint wave function to a single-particle state, and that does not lead to a single-particle wavefunction at all (let alone an eigenstate of the measurable), it leads to a mixed state.

At this point, where all we have is the "lifeless computer", we do not have a single measurement outcome, we have a mixture of outcomes. This is also called an "ensemble" in mainstream quantum mechanics, the only difference is that to resolve certain difficulties in picturing what this is, we imagine lots and lots of copies of the system, instead of just one system. This makes it easier to imagine what a mixed state is, but there's really only one system there, it doesn't have to be an ensemble.

Enter a consciousness/intelligence/perceiver who thinks classically. Only now do we encounter the concept of an "actual outcome", and this creates a huge problem for quantum mechanics theory. Where does that actual outcome come from? No one knows, but here is where each of the interpretations step into provide an untestable answer. I've already outlined what those answers are above, and GO1 mentioned some of the possibilities as well. The key thing to recognize at this point is that none of that difficult business even comes up, and there's no need for an interpretation, until we factor in the presence of a consciousness and its resulting "actual outcome" perception. The physics is perfectly happy just leaving the quantum in a mixed state, if all we have is a lifeless computer. It's all related to how a conscious entity does science, and this involves the perception of an actual outcome, even though the theory provides no such concept and forces us to inject a layer of randomness to get agreement with our experiences. Because we are conscious.

So this role of consciousness is much more subtle, yet much more fundamental to everything we do in science, than what that Wiki is talking about. I know that without even reading the context of the rest of that Wiki.

In fact, it is because a human is a type of measuring apparatus that we humans cause a non unitary process to occur.

No. Measuring apparatuses are physical constructs, and so, should obey the laws of physics. Unitary evolution is one of the laws they should obey. Ergo, measuring devices should not "cause a non unitary process to occur." The whole measurement problem, as nicely described by G01 above, is the origin of the apparent non-unitarity, since it cannot come from the measuring device. Here are the answers of the main interpretations:

CI: it comes from how we do science, since the unitary evolution piece was just a tool we use at one stage of the calculation. (This is related to our consciousness/intelligence/classical processing in the "how we do science" part.)

Many-worlds: the non-unitary element is illusory, the full unitary result is there but we only see a tiny part of the real story. (This is related to our consciousness/intelligence/classical processing in the "what part of the whole we see".)

Bohm: the unitary evolution is the illusion-- it's just a cloak placed on top of the pilot-wave evolution, which is deterministic and non-unitary. The unitarity is "filled in" by physically irrelevant aspects of the wave function, and it is stripped away by the measurement. (This is the only interpretation that does not involve consciousness in a direct way, because it treats unique experimental outcomes as purely deterministic, but it cannot be tested.)

 

[yoda jedi]

The point is that consciousness has nothing to do with measurement. By no means do all measurements have to include a conscious observer. Wavefunction collapse will happen because of an interaction with an external environment

environment induced superselection.

i agree.
and there are objective collapse models and no collapse models..

 

[Varon]

That's simply because the phrase "consciousness causes collapse" can mean about 100 different things, many of which have been carefully delineated on this thread. Indeed that was the initial disconnect between myself and G01, if you care to review that discussion. Nothing that I am saying is what that Wiki means by consciousness causing collapse, but what I do mean is completely consistent with both the Wiki I quoted for you, and what G01 just said above.

All covered in the thread, you're not telling me anything I haven't already discussed in detail.
Now I'm just repeating, but I will go through it yet again because this is the crux of the whole business right here.

A measurement made by a lifeless computer creates what is called a mixed state for the quantum system. That is because the quantum system is, at this point, a subspace of a larger apparatus. The larger apparatus, as G01 just explained, is still in a pure state according to the unitary evolution of quantum mechanics theory. The mixed state is the projection from the whole system onto the subspace of the quantum system. Nothing there creates any difficulties, nor requires any interpretations of quantum mechanics-- the full system is in a pure state so is still unitary, the projection onto the subspace is not supposed to be unitary, it's a projection from a joint wave function to a single-particle state, and that does not lead to a single-particle wavefunction at all (let alone an eigenstate of the measurable), it leads to a mixed state.

At this point, where all we have is the "lifeless computer", we do not have a single measurement outcome, we have a mixture of outcomes. This is also called an "ensemble" in mainstream quantum mechanics, the only difference is that to resolve certain difficulties in picturing what this is, we imagine lots and lots of copies of the system, instead of just one system. This makes it easier to imagine what a mixed state is, but there's really only one system there, it doesn't have to be an ensemble.

Enter a consciousness/intelligence/perceiver who thinks classically. Only now do we encounter the concept of an "actual outcome", and this creates a huge problem for quantum mechanics theory. Where does that actual outcome come from? No one knows, but here is where each of the interpretations step into provide an untestable answer. I've already outlined what those answers are above, and GO1 mentioned some of the possibilities as well. The key thing to recognize at this point is that none of that difficult business even comes up, and there's no need for an interpretation, until we factor in the presence of a consciousness and its resulting "actual outcome" perception. The physics is perfectly happy just leaving the quantum in a mixed state, if all we have is a lifeless computer. It's all related to how a conscious entity does science, and this involves the perception of an actual outcome, even though the theory provides no such concept and forces us to inject a layer of randomness to get agreement with our experiences. Because we are conscious.

I can't let this go. If one puts a camera in planet pluto. It can still record the surrounding. If what you said was true. Pluto should be in mixed state, and the camera being non-conscious, should be entangled to the mixed state. So this ensemble of mixed state are still not collapsed to definite outcome (because there was no conscious observer present there). Therefore the camera shouldn't even be recording any collapsed state but mixed state. But we know the camera recorded collapsed state. Hence this simple example refutes what you said above.

So this role of consciousness is much more subtle, yet much more fundamental to everything we do in science, than what that Wiki is talking about. I know that without even reading the context of the rest of that Wiki.

No. Measuring apparatuses are physical constructs, and so, should obey the laws of physics. Unitary evolution is one of the laws they should obey. Ergo, measuring devices should not "cause a non unitary process to occur." The whole measurement problem, as nicely described by G01 above, is the origin of the apparent non-unitarity, since it cannot come from the measuring device. Here are the answers of the main interpretations:

CI: it comes from how we do science, since the unitary evolution piece was just a tool we use at one stage of the calculation. (This is related to our consciousness/intelligence/classical processing in the "how we do science" part.)

Many-worlds: the non-unitary element is illusory, the full unitary result is there but we only see a tiny part of the real story. (This is related to our consciousness/intelligence/classical processing in the "what part of the whole we see".)

Bohm: the unitary evolution is the illusion-- it's just a cloak placed on top of the pilot-wave evolution, which is deterministic and non-unitary. The unitarity is "filled in" by physically irrelevant aspects of the wave function, and it is stripped away by the measurement. (This is the only interpretation that does not involve consciousness in a direct way, because it treats unique experimental outcomes as purely deterministic, but it cannot be tested.)