A bit confused about Schrödinger's cat

[Mentz114]

Isn't this an entanglement issue, so wouldn't you expect all of the people to see the same thing. But what they see would still be in question until they see it.
Again... I know I'm stumbling along here. I am clearly not qualified to be arguing this position. But, are there not still physicists that believe this? How would they argue this point?

This issue has been discussed in several threads in this forum. Try a search.

There is even a paper which purports to prove the 'ontic' nature of the wave function ( ie it is 'real') and not 'epistemic'. It is rubbish of course, because only experiments can tell if something is real.

One physicist who followed the ensemble interpretation was A. Einstein.

 

[stevendaryl]

Suppose we have an apparatus which produces electrons in a superposition of spin-up and spin-down ( in some basis) with equal probability.
The key to the thing is what you believe the italicised bit means. To me it means that half the electrons will be spin-up and half spin-down in a long run of identical repeats ( ensemble interpretation).

I would say that is definitely NOT what it means. The statistical content of such a set-up is that IF we measure the spin of the electrons along the z-direction, then we will find half to be spin-up, and half to be spin-down. Your statement says "will be spin-up", which suggests that the electron has a spin in the z-direction, whether or not we measure that spin. That contradicts Bell's theorem.

 

[Mentz114]

I would say that is definitely NOT what it means. The statistical content of such a set-up is that IF we measure the spin of the electrons along the z-direction, then we will find half to be spin-up, and half to be spin-down. Your statement says "will be spin-up", which suggests that the electron has a spin in the z-direction, whether or not we measure that spin. That contradicts Bell's theorem.

My apparatus produces one electron at a time. Half will be measured as spin-up and half as spin-down.

I'm at a loss as to what you mean.

 

[StevieTNZ]

[emoji21]I'm reluctant to even get into this, but...
Could you make the argument that the information encoded on the photographic plate is in superposition until such time as it is developed and visualized by a "conscious" entity?

I realize that this is a frustrating argument, but I don't see a logical flaw off the top of my head.
It seems like one could argue that IF conscious observation is required for "collapse", it is the act of that observation that is the actual "measurement". So, wouldn't all possible states of the photographic film be in superposition until it is visualized "consciously"? All the "observer" knows about "what's already there" is what is perceivable to his/her consciousness.

Again... I'm simply playing the devil's advocate here. I recognize that this is slippery ground.

Yes, you can make that argument -- and I see no flaw in it.

 

[Mentz114]

Yes, you can make that argument -- and I see no flaw in it.

There is no definition or recognition of consciousness in physics so you are encouraging superstition and misunderstanding.

It is outrageous to use 'magic' explanations in physics.

This thread should be closed.

 

[stevendaryl]

My apparatus produces one electron at a time. Half will be measured as spin-up and half as spin-down.

I am at a loss as to what you mean.

There is a distinction between "Half will be measured as spin-up", which I agree with, and "Each electron produced is either spin-up or spin-down - but we don't know which until it is measured." That is what you said, at first, and that is not true. That's the ignorance interpretation of quantum probabilities, and Bell's theorem shows that it is not true (not without nonlocal interactions, anyway).

 

[Mentz114]

There is a distinction between "Half will be measured as spin-up", which I agree with, and "Each electron produced is either spin-up or spin-down - but we don't know which until it is measured." That is what you said, at first, and that is not true. That's the ignorance interpretation of quantum probabilities, and Bell's theorem shows that it is not true (not without nonlocal interactions, anyway).

That difference is too subtle for me so go with the second attempt.

 

[stevendaryl]

That difference is too subtle for me so go with the second attempt.

That difference is really what motivated Bell's theorem.

 

[StevieTNZ]

There is no definition or recognition of consciousness in physics so you are encouraging superstition and misunderstanding.

It is outrageous to use 'magic' explanations in physics.

This thread should be closed.

I don't appreciate the reflection you have made on me, nor those investigating whether consciousness is the cause of wave function collapse.

 

[bhobba]

But I've also been told earlier in this thread that the cat in the box is truly in a superposition of both dead AND alive, so I'm left a little unclear on that issue.

You were told wrong

Once and for all the cat can never, ever, ever, be in a superposition of alive and dead. Its impossible, utterly impossible. I have given the reason why many times in this thread.

When it was proposed by Schroedinger no one seriously considered it could be. It was to highlight an issue in QM that now has largely been fixed.

Please do not go down this path - it will simply be a waste of time and I will not participate in it except to point to this post.

hanks
Bill

 

[bhobba]

But, aren't there many physicists that believe precisely this troublesome concept?

Its very fringe now because its issues are better understood and the reason Von Neumann proposed it no longer applies. The other famous person to hold to it, the great mathematical physicist Wigner, recognised that later in life and did a complete 180% about face after reading some papers by Zeth on decoherene.

Thanks
Bill

 

[bhobba]

There is even a paper which purports to prove the 'ontic' nature of the wave function ( ie it is 'real') and not 'epistemic'. It is rubbish of course, because only experiments can tell if something is real.

Here is the paper:
http://xxx.lanl.gov/pdf/1111.3328v3

The interesting thing is people that use the paper to prove it must be 'ontic' didn't read it:
'Here we present a no-go theorem: if the quantum state merely represents information about the real physical state of a system, then experimental predictions are obtained which contradict those of quantum theory. The argument depends on few assumptions. One is that a system has a “real physical state” – not necessarily completely described by quantum theory, but objective and independent of the observer. This assumption only needs to hold for systems that are isolated, and not entangled with other systems. Nonetheless, this assumption, or some part of it, would be denied by instrumentalist approaches to quantum theory, wherein the quantum state is merely a calculational tool for making predictions concerning macroscopic measurement outcomes.'

Thanks
Bill

 

[Mentz114]

I don't appreciate the reflection you have made on me, nor those investigating whether consciousness is the cause of wave function collapse.

There is nothing personal intended. People are free to believe what they will just as I'm free to give a considered opinion on whether those activities can be classed as physics.

 

[stevendaryl]

Here is the paper:
http://xxx.lanl.gov/pdf/1111.3328v3

The interesting thing is people that use the paper to prove it must be 'ontic' didn't read it:
'Here we present a no-go theorem: if the quantum state merely represents information about the real physical state of a system, then experimental predictions are obtained which contradict those of quantum theory. The argument depends on few assumptions. One is that a system has a “real physical state” – not necessarily completely described by quantum theory, but objective and independent of the observer. This assumption only needs to hold for systems that are isolated, and not entangled with other systems. Nonetheless, this assumption, or some part of it, would be denied by instrumentalist approaches to quantum theory, wherein the quantum state is merely a calculational tool for making predictions concerning macroscopic measurement outcomes.'

Thanks
Bill

I found that paper a little confusing; it wasn't completely clear to me what it was they were claiming that their argument shows. But here's what I think they might be saying:

• Assumption: If a single experiment can distinguish between two states, then those states must be physically different.

If the state is epistemic, that is, it represents the experimenter's knowledge, then different states of knowledge need not be physically different. For example, if a generate a number by flipping a coin, with heads=1 and tails=2, but I don't look at the result, then I can characterize the state of my knowledge as:

Probability(result = 1) = 1/2, Probability(result=2) = 1/2.

Alternatively, I could roll a 6-sided die to generate a number from 1-6. That leads to a different state of knowledge. But a single test won't necessarily distinguish between those possible knowledge states, because if the result is 1 or 2, then that is consistent with either state. (On the other hand, many repeated independent tests would distinguish the two).

• Fact: For any two different quantum states $|\phi\rangle$ and $|\psi\rangle$, there is a single experiment that can distinguish the two. (Actually, what they seem to show is that a single experiment can distinguish $|\phi\rangle \otimes |\phi\rangle$ and $|\psi\rangle \otimes |\psi\rangle$)

So this fact together with the assumption about "physically different" states implies that two systems described by different wave functions must be physically different.

It's not clear how surprising this should be. It's obviously true, using the Born interpretation, that repeated independent measurements can always distinguish any two systems described by different wave functions. This paper shows that a single measurement is sufficient. I'm not sure how important that distinction is.

 

[bhobba]

I found that paper a little confusing;

Its been a while since I have gone through that paper so can't really help.

However it turns out to not be quite what the authors thought it was with models existing where it applies and ones where its evaded:
http://arxiv.org/abs/1203.4779#

And again its been a while since I went through it.

Thanks
Bill

 

[AlexCaledin]

I am reading Heisenberg, "Physics and Beyond", - there is a remarkable discussion with Werner's friend Robert, a philosopher; it happened in 1920.

Robert: "I should expect that atoms would, in any case, behave quite differently from the objects of everyday experience. I could imagine that attempts to divide matter even further might lead us to fluctuations and discontinuities from which it would be quite possible to conclude that matter has a grainy structure. But I also believe that the new structures will elude all our attempts to construct tangible images, that they will prove to be abstract expressions of natural laws rather than things."
Werner: "But what if we could see them?"
Robert: "We shall never be able to see atoms themselves, only their effects."
Werner: "That's a poor excuse of an answer. For the same remark applies to things in general. In the case of a cat, too, all you can see is the reflection of light rays, i.e., the effects of the cat, and not the cat itself. And when you stroke its fur, the situation is much the same!"
Robert: "I'm afraid I can't agree with you. I can see a cat directly, for when I look at it, I can—indeed, I must—transform my sense impressions into a coherent idea. In the case of the cat we come face to face with two aspects: an objective and a subjective one— the cat as a thing and as a notion. But atoms are quite a different matter. Here notion and thing can no longer be separated, simply because the atom is neither the one nor the other."

So, a cat is a real part(icipant) of Nature while the atom/particle and QM are merely Nature's answers to Man's questions "what's it made of" and "how to calculate it"...
The Copenhagen QM seems to be the most honest approach because it simply stops where it encounters something not very physical, like that atom-cat difference.

 

[bhobba]

So, a cat is a real part(icipant) of Nature while the atom/particle and QM are merely Nature's answers to Man's questions "what's it made of" and "how to calculate it"...

Since the cat is just as much described by QM as an atom it's as real or not real as an atom.

Be very careful what these early pioneers say - things have moved on a LOT since then.

As Wienberg says - both Bohr an Einstein were wrong (not that I 100% agree with Weinberg - but that is another story - I think if you query him about it its likely more along the lines of the issue I mention later):
http://scitation.aip.org/content/aip/magazine/physicstoday/article/58/11/10.1063/1.2155755

Likely the same with the other pioneers as well - with the probable exception of Dirac.

The real issue with QM, the rock bottom problem is its a theory about observations that appear in a classical world. How does a theory that assumes such a world from the start explain that world.

Thanks
Bill

 

[Flyx]

If you subscribe to the multiverse theory, the cat is dead and alive at the same time, but in different universes. By opening the box, you just see which universe you are in.

 

[Feeble Wonk]

If you subscribe to the multiverse theory, the cat is dead and alive at the same time, but in different universes. By opening the box, you just see which universe you are in.

I understand what you're trying to say here, but you've got to be careful with your semantics.
When considering the cat in the box from the perspective of the "Many Worlds" interpretation, there is only one cat "per world" ( in different universes, to use your words). It's either dead OR alive, because the Geiger counter either triggered the cyanide gas release OR it didn't... though, as you said, the external observer won't know which until the box is opened.

There are other QT interpretations (mostly "collapse" theories) that might describe this situation less definitively, but with MW the cat is never "dead AND alive".

 

[Feeble Wonk]

Since the cat is just as much described by QM as an atom it's as real or not real as an atom.

...as is the box, the person that opens the box and the universe in which the event takes place. (The system being observed, the physicist doing the observing, the apparatus AND the environment in which the observation is made).

 

[Paul Colby]

I guess I don't understand the cat issue either. The state vector applies to a (potentially infinite) ensemble of similarly prepared cats. It's not about a single cat, never was.

 

[Feeble Wonk]

I guess I don't understand the cat issue either. The state vector applies to a (potentially infinite) ensemble of similarly prepared cats. It's not about a single cat, never was.

If your talking about the mathematical formalism, then I suppose that's true... in that the state vector describes the "potential" states of the cat within the isolated system. But, if you're considering what is "really" happening with the cat in the box before an observation is made by opening the box, then that is an interpretational question. As Bill is fond of saying, the formalism is silent with respect to what is happening "between observations".

 

[Paul Colby]

But, if you're considering what is "really" happening with the cat in the box before an observation is made by opening the box, then that is an interpretational question.

I guess you have expectations of what the theory is or "should be" that are not supported by observation. (I've figured out how to do the attribution in the Quote interface, yeah!)​

 

[Feeble Wonk]

I guess you have expectations of what the theory is or "should be" that are not supported by observation.

I'm not quite sure what you mean here. The theory can only tell us what it can tell us, and I have no "expectations" beyond that. The struggle is understanding what it means at the fundamental level. That's where interpretation comes into play.

 

[Paul Colby]

The struggle is understanding what it means at the fundamental level.

Unless your struggle ends in yet a more refined theory which includes thing not qualitatively described by the current theory I have little to say. Interpretation is a pretty wide open endeavor. "God made it that way" is an interpretation, granted not one I'm particularly enamored with. On the other hand, decoherence theory is a theory aimed at addressing the measurement problem using QM. My problem is I don't think there is a problem.

 

[Feeble Wonk]

Unless your struggle ends in yet a more refined theory which includes thing not qualitatively described by the current theory I have little to say.

That is, of course, the hope.

On the other hand, decoherence theory is a theory aimed at addressing the measurement problem using QM. My problem is I don't think there is a problem.

My understanding is that it's not generally accepted within the physics community that decoherence alone resolves the "measurement problem".

 

[Mentz114]

That is, of course, the hope.

My understanding is that it's not generally accepted within the physics community that decoherence alone resolves the "measurement problem".

There is no measurement 'problem' unless you choose an 'interpretation' that carries that baggage. You've succeeded in keeping this post going much longer than is justified.

I suspect you have a bet to see how long you can keep the dead/alive nonsense going

 

[Feeble Wonk]

There is no measurement 'problem'...

Would you say that this is the consensus opinion of physicists in general?

 

[houlahound]

Give my dog a minute with the cat and problem disappears.

 

[Mentz114]

Would you say that this is the consensus opinion of physicists in general?

This is your standard reply. I'm not qualified to comment.

 

[Feeble Wonk]

This is your standard reply. I'm not qualified to comment.

That is my reply. It's a simple question. Would you say that "There is no measurement problem" is the consensus opinion within the physicist community?
If not, then why?

 

[Mentz114]

That is my reply. It's a simple question. Would you say that "There is no measurement problem" is the consensus opinion within the physicist community?
If not, then why?

To find if there is a consensus one needs a poll and some numbers. I'm unable to answer this.

My view of the measurement problem comes from studying textbooks by Ballentine, Peres, Neumaier~Westra, Holland and numerous learned articles.
If you want an argument - take it up with them.

 

[stevendaryl]

Would you say that this is the consensus opinion of physicists in general?

There is no consensus as to whether there is a consensus.

 

[Ken G]

My understanding is that it's not generally accepted within the physics community that decoherence alone resolves the "measurement problem".

The problem is that there is a lot of misunderstanding about what the measurement problem is. One problem is certainly resolved by decoherence-- the problem of how the coherences in the wave function disappear. But all that gives you is what is formally known as a "mixed state." There remains a problem-- so if the problem that remains is what one means by the "measurement problem," then there is still a measurement problem in quantum mechanics.

The problem is this. A mixed state comes up all the time in classical physics-- it's like what happens when you flip a coin and you haven't looked yet. The difference in quantum mechanics is that in classical physics that we are well aware that whatever forces cause the coin to come up heads or tails are simply being overlooked, we are choosing not to include them but we expect they are there-- the mixed state is merely a statistical representation of the actual situation. However, in quantum mechanics, people tend to want to take the formalism more seriously-- the mixed state is supposed to be the projection of the actual state onto the measurement subspace. That's not just a statistical representation of the true state. This is the measurement problem-- how seriously do we take the mixed state that appear after decoherence is all over with. This is what the different interpretations disagree on-- they all agree just fine with the process of decoherence.

 

[StevieTNZ]

However, in quantum mechanics, people tend to want to take the formalism more seriously-- the mixed state is supposed to be the projection of the actual state onto the measurement subspace. That's not just a statistical representation of the true state. This is the measurement problem-- how seriously do we take the mixed state that appear after decoherence is all over with. This is what the different interpretations disagree on-- they all agree just fine with the process of decoherence.

Yes! See pg 209 of 'Quantum Enigma', pg 241 of 'Modern Physics and Ancient Faith'