Quantum Interpretation Poll (2011)

[Rap]

But what's weird is that in Many World, the cat can be in pure state even without measuring it. Maybe you are trying to imagine a cat in Many world-like setting.

I wonder why a cat can be in pure state in Many worlds while impossible in Copenhagen. Rap or anyone?

If the wave function is "real", having existence independent of an observer, then the very existence of the cat implies a pure state. In the CI, the wave function is the most complete encoding of our knowledge of the cat possible, and is not "real" in the same sense. In classical physics, you cannot know the position and momentum of every particle of the cat without destroying the cat. Nevertheless, you can carry out thought experiments, saying "if I did know the position and momentum of every particle in an isolated system, then I can draw the following conclusions". In QM, I don't see any problem in saying "if I did know the wave function of an isolated many particle system, then I can draw the following conclusions". So in that sense, the cat can be in a pure state in the CI.

 

[rodsika]

If the wave function is "real", having existence independent of an observer, then the very existence of the cat implies a pure state. In the CI, the wave function is the most complete encoding of our knowledge of the cat possible, and is not "real" in the same sense. In classical physics, you cannot know the position and momentum of every particle of the cat without destroying the cat. Nevertheless, you can carry out thought experiments, saying "if I did know the position and momentum of every particle in an isolated system, then I can draw the following conclusions". In QM, I don't see any problem in saying "if I did know the wave function of an isolated many particle system, then I can draw the following conclusions". So in that sense, the cat can be in a pure state in the CI.

Why, is knowing the position and momentum enough to define the pure state in the cat? Or must one know every history of every particle in the body like knowing how it got there since the birth of the cat? Maybe this is why it is difficult even in essence to think a cat can be in pure state because you don't know all the history of every particle. To know it you have to measure, and measuring it entangled the cat to all your particles making the problem worse. I think it's similar to Bohr and Einstein debate where Einstein gave thought experiment about HUP being violated. But if you have to take the complete information, it can't be violated as reasoned by Bohr.

Anyway. Why not treat the wave function as real. That way, the cat is in pure state in a close system and it can be dead or alive. For those who don't consider it real. What do you think they think is the buckyball doing in between, how can it interfere with itself? Don't you think there is a disconnect that physicists only care about measurement and not what occurs before it. In General Relativity, this is also true. But in such basis stuff as double slit experiment where you are dealing with a particle, I think one must really be able to picture what is going on in between. The only reason it can't be picture is if we are living on a computer simulation where only measurements make sense and we can't bridge the veil the separate them and us. You think this is possible? If the wave function is not real, what else could the particle be doing in between? What is your best guess?

 

[camboy]

Anyway. Why not treat the wave function as real. That way, the cat is in pure state in a close system and it can be dead or alive. For those who don't consider it real. What do you think they think is the buckyball doing in between, how can it interfere with itself? Don't you think there is a disconnect that physicists only care about measurement and not what occurs before it. In General Relativity, this is also true. But in such basis stuff as double slit experiment where you are dealing with a particle, I think one must really be able to picture what is going on in between. The only reason it can't be picture is if we are living on a computer simulation where only measurements make sense and we can't bridge the veil the separate them and us. You think this is possible? If the wave function is not real, what else could the particle be doing in between? What is your best guess?

De Broglie-Bohm. None of these things is even an issue in that interpretation - it simply doesn't have a measurement problem. And it gives a very precise picture of what goes on in a double-slit experiment, because it is a dynamical theory of motion, rather than a theory of observation.

Wave goes through both slits, particle goes through one, wave influences particles to clump in regions of constructive interference (19 words - it's not so hard, is it?).

 

[rodsika]

De Broglie-Bohm. None of these things is even an issue in that interpretation - it simply doesn't have a measurement problem. And it gives a very precise picture of what goes on in a double-slit experiment, because it is a dynamical theory of motion, rather than a theory of observation.

Wave goes through both slits, particle goes through one, wave influences particles to clump in regions of constructive interference (19 words - it's not so hard, is it?).

　

De Broglie-Bohm is almost refuted already. We know that particle accelerators like LHC can precipitate new particles upon collision of the original particles. Since in Bohmian mechanics, a particle is always a particle, then any particle accelerator experiments refute it. We also know particles quantum tunnel. Bohmian mechanics forbid it. This means Bohmain mechanics is already dead on arrival. There is no particle acceleration when Bohm first suggested. Now everyone knows particles are just temporary. Since Bohmian is all about the permanence of particles. It's obvious it doesn't tally with reality. Can anyone here defend the Bohmians?

 

[Fyzix]

At first I thought you were genuinly trying to understand the quantum world, now you look more and more arrogant.
Hopefully Demystifier will shut you down...

 

[camboy]

　
De Broglie-Bohm is almost refuted already. .. It's obvious it doesn't tally with reality.

You sound so serious, yet I'm afraid it's quite obvious that you simply don't understand anything about it.

We also know particles quantum tunnel. Bohmian mechanics forbid it.

On the contrary, unlike any other interpretation, it explains tunnelling.

Since in Bohmian mechanics, a particle is always a particle, then any particle accelerator experiments refute it.

You're talking about the non-relativistic Schroedinger level of theory, I take it? Well, the orthodox interpretation of non-relativistic QM doesn't explain particle creation and destruction either. Why require that of de Broglie-Bohm and not of orthodox QM? (incidentally, 'Bohmian mechanics' is an outdated term used by a single research group which Bohm himself strongly objected to - the current usage is de Broglie-Bohm theory or pilot-wave theory - see e.g. Wikipedia).

If you're talking about quantum-field theoretic generalizations of de Broglie-Bohm, they make the same predictions as the regular QFT.

I would give references, but since you clearly haven't read anything on the subject you should try something something simpler first. I'm sorry to be so harsh, but if you're going to violently attack other people's work then it is common politeness to at least try to understand it first.

 

[rodsika]

　

I read last week that a science advisor and mathematician extraordinaire Arnold Neumaier already declared Bohmian mechanics null and void so I thought you guys refuted it already. Neumaier said:

"I lost interest in Bohmian mechanics once I noticed that all electrons in hydrogen atoms in the ground state stand still at arbitrary positions. I wrote up the report as my farefell address."

...

"As my paper cited before shows, Bohmian mechanics is inconsistent with standard QM once time correlations are taken into account."

...

"The reality is much worse. Bohmian mechnaics achieves agreement with orthodox quantum mechnaics _only_ through measurement considerations. Thus they are able to hide any weakness in their approach by burying it in the fuzzyness of measurement problems."

...

"But I don't care anymore about BM, and therefore won't defend my statement in the paper further."

But I learned a while ago another science advisor Demystifier stated (I thought science advisors words here are final that was why when I heard Neumaier stated it, i believed him immediately):

"BM is NOT all particles. BM is particles AND wave function. It is the wave function that causes the particle to move through the barrier. It does not teleport through it, but simply gets more energy to "jump" over it."

　

So Bohmian Mechanics is still alive.

But isn't it that in orthodox QM, a prisoner has non-zero probability of quantum tunneling outside a cement wall? In Bohmian M, no matter how much energy you give the prisoner, you can't quantum tunnel him outside of the cell without accelerating his body to the cement walls shredding it to pieces. Here QM has more flexibility. Isn't it?

 

[Rap]

Why, is knowing the position and momentum enough to define the pure state in the cat? Or must one know every history of every particle in the body like knowing how it got there since the birth of the cat? Maybe this is why it is difficult even in essence to think a cat can be in pure state because you don't know all the history of every particle. To know it you have to measure, and measuring it entangled the cat to all your particles making the problem worse. I think it's similar to Bohr and Einstein debate where Einstein gave thought experiment about HUP being violated. But if you have to take the complete information, it can't be violated as reasoned by Bohr.

"Entangled" to me means, for example, two downshifted photons created from one using nonlinear optics. The spins are certainly opposite, and measuring the spin of one will give you the spin of the other, measured with respect to the same axis. Is this your meaning of entangled? I think that if you prepare a pure state, it will not matter if the particles associated with that state are previously entangled. That would mean there are many different types of photons, each behaving differently depending on their previous history of entanglement, and I don't think this is the case. The same is true of the cat. In any case, you are talking about the state preparation problem, and I agree it would be a massively difficult thing to prepare a cat in a pure state. But I don't see the reason, in principle, that it could not be done, any more than I can see the reason, in classical physics, that a gas could not be prepared in a particular microstate. I can prepare one particle in a pure state, I can prepare two with more difficulty, and I don't see that there is a particular number of particles where it suddenly becomes impossible, unless you want to bring in the size and age of the universe or something like that.

Anyway. Why not treat the wave function as real. That way, the cat is in pure state in a close system and it can be dead or alive. For those who don't consider it real. What do you think they think is the buckyball doing in between, how can it interfere with itself? Don't you think there is a disconnect that physicists only care about measurement and not what occurs before it. In General Relativity, this is also true. But in such basis stuff as double slit experiment where you are dealing with a particle, I think one must really be able to picture what is going on in between. The only reason it can't be picture is if we are living on a computer simulation where only measurements make sense and we can't bridge the veil the separate them and us. You think this is possible? If the wave function is not real, what else could the particle be doing in between? What is your best guess?

The question of what the particle is doing in between is a scientifically improper question. Its like asking what the momentum of a particle is after I have measured its position. That too is an improper question. There is no way to measure the position and momentum simultaneously, and to ask a question for which, in principle, there is no testable answer is improper.

There is no problem assuming the wave function is real until you come to the question of "Wigner's friend". It is the central problem, addressed at the very beginning in Everett's "many-world" thesis (http://www.pbs.org/wgbh/nova/manyworlds/pdf/dissertation.pdf) because he realized that this is the central problem in the discussion of wavefunction reality. (He did not call it "Wigner's friend" however). I disagree with his conclusions because his many-world resolution to the problem is untestable, but his statement of the problem is excellent.

 

[rodsika]

"Entangled" to me means, for example, two downshifted photons created from one using nonlinear optics. The spins are certainly opposite, and measuring the spin of one will give you the spin of the other, measured with respect to the same axis. Is this your meaning of entangled? I think that if you prepare a pure state, it will not matter if the particles associated with that state are previously entangled. That would mean there are many different types of photons, each behaving differently depending on their previous history of entanglement, and I don't think this is the case. The same is true of the cat. In any case, you are talking about the state preparation problem, and I agree it would be a massively difficult thing to prepare a cat in a pure state. But I don't see the reason, in principle, that it could not be done, any more than I can see the reason, in classical physics, that a gas could not be prepared in a particular microstate. I can prepare one particle in a pure state, I can prepare two with more difficulty, and I don't see that there is a particular number of particles where it suddenly becomes impossible, unless you want to bring in the size and age of the universe or something like.

Are you saying that the moment you can create a pure state cat. You can put it in any superposition you like such as dead/alive, pregnant/not pregnant, cancer/healthy, virgin/used, old/young, etc.? Without any limitations whatsoever? If there is limitation, maybe a pure state cat can only have superposition of positions of being on the left and right side at the same time and not on being dead/alive, pregnant/nonpregnant, etc.?

 

[Rap]

Are you saying that the moment you can create a pure state cat. You can put it in any superposition you like such as dead/alive, pregnant/not pregnant, cancer/healthy, virgin/used, old/young, etc.?

I think, in principle, yes.

 

[rodsika]

I think, in principle, yes.

How is that possible. Pure state in the cat body simply meant all the particles are entangled. How can you make this create both old and young cat? Hmm.. are you saying that if you can put the old Einstein in pure state, he can become young again... like the fountain youth? What in pure state makes this possible at all? Hope you can explain a bit more so it can be clear to both of us the limitations and possibilities. Thanks.

 

[tom.stoer]

You have to specify what "create a pure state" and "put it in a superposition" means. In QM this means "preparation of the state" and that's what is done in setting up an experiment with well-defined initial conditions (position, momentum, ...).

You can prepare a superposition "alive + dead" simply by following Schrödinger's Gedankenexperiment: take the cat, the box, ... - put the cat into the box - close the box and wait for some time (defined by half-life) - voila

 

[rodsika]

You have to specify what "create a pure state" and "put it in a superposition" means. In QM this means "preparation of the state" and that's what is done in setting up an experiment with well-defined initial conditions (position, momentum, ...).

You can prepare a superposition "alive + dead" simply by following Schrödinger's Gedankenexperiment: take the cat, the box, ... - put the cat into the box - close the box and wait for some time (defined by half-life) - voila

But according to a physicist, Ken, it is not even possible in principle to know the cat pure state before closing the box. So the Schroedinger Cat thought experiment is impossible in principle. Which of the following part don't you agree:

(Ken stated)

"No, that is just exactly what you could never do, not even in principle. Because the only way to know that would be to do measurements on the cat, but that would involve a measuring device, so immediately the cat becomes a subspace of the thing that is a pure state. Unlike measuring the spin of a single particle, where there is no information being ignored, if you measure an "entire cat", there is vast amounts of information you could never get a handle on, like herding cats (literally). There's no measurement like that which even in principle could result in complete information about the cat's wave function that could be treated as a closed system going forward, too much of the data (all the phase coherences) that would need to be tracked is going to be entangled with the instruments doing the measuring, not to mention the brain processing that information.

This is the key point-- the information that goes into determining a wave function is not in the entity being observed, it is in the environment doing the observing and processing that information. Physics is done by physicists, even if we can imagine the action of hypothetical physicists not actually present in the environment. If that environment does include a real brain, it might be able to treat the entity as having a pure-state wavefunction (as for the spin of a particle), but it could never be empowered to treat a cat in a pure-state wavefunction, there would always have to be too much overlooked information (indeed, judicious overlooking of information is more or less the foundational principle of physics). It is only ever the whole system including the observer that could be treated as a pure state, and only if it started out in a pure state, which brings in the issue of history."

 

[tom.stoer]

The key point is that Ken wants to "measure" the cat whereas I want to "prepare" it. That is something different.

When setting up an experiment with monochromatic, coherent light emitted by a LASER the light is by definition monochromatic and coherent w/o measuring it! You check the properties of the light before starting the experiment in order to ensure the correctness of the setup, but during the experiment you do no longer check them; you simply believe that these properties persists w/o measurement. The same applies to the cat.

The problem is different: The cat will never be in a pure state "alive" as the cat itself is always a mixed state b/c of the huge number of d.o.f. Therefore preparing the cat in a state "alive + dead" is possible, but this state is not a pure state (a ray) but a mixed state (a density matrix). This is not due to the superposition of "dead" and "alive" but due to the macroscopic nature of the cat.

But as far as I can see that does not affect the interpretation of the experiment b/c again the problem is "why the (mixed) state of the cat collapses to either dead or alive" where again dead and alive refer to mixed states. I guess decoherence is able to explain why the state of the cat collapses to one one these mixed states, but it does not explain why in a certain experiment it collapses to "dead" and not to "alive".

That means that "collapse of the wave function", "split according to MWI interpretation ..." does apply on the level of density matrices. This is technically more involved but causes the same fundamental problems regarding ontology.

 

[rodsika]

The key point is that Ken wants to "measure" the cat whereas I want to "prepare" it. That is something different.

When setting up an experiment with monochromatic, coherent light emitted by a LASER the light is by definition monochromatic and coherent w/o measuring it! You check the properties of the light before starting the experiment in order to ensure the correctness of the setup, but during the experiment you do no longer check them; you simply believe that these properties persists w/o measurement. The same applies to the cat.

The problem is different: The cat will never be in a pure state "alive" as the cat itself is always a mixed state b/c of the huge number of d.o.f. Therefore preparing the cat in a state "alive + dead" is possible, but this state is not a pure state (a ray) but a mixed state (a density matrix). This is not due to the superposition of "dead" and "alive" but due to the macroscopic nature of the cat.

But as far as I can see that does not affect the interpretation of the experiment b/c again the problem is "why the (mixed) state of the cat collapses to either dead or alive" where again dead and alive refer to mixed states. I guess decoherence is able to explain why the state of the cat collapses to one one these mixed states, but it does not explain why in a certain experiment it collapses to "dead" and not to "alive".

That means that "collapse of the wave function", "split according to MWI interpretation ..." does apply on the level of density matrices. This is technically more involved but causes the same fundamental problems regarding ontology.

But if it's a mixed state (a density matrix) then it's just classical probability of being alive or dead. Not a superposition (which involves pure state or ray). Since pure Quantum Mechanics only deal with pure state evolving unitarily. Then preparing it alive + dead is not quantum mechanics already. Just a classical problem. But I wonder why density matrix must fall under quantum mechanics since there is no longer a ray. Any idea?

 

[tom.stoer]

But if it's a mixed state (a density matrix) then it's just classical probability of being alive or dead. Not a superposition

That's not true. It would hold iff the density matrix could be written in terms of two pure states "alive" and "dead" with classical probabilities, but this is not true. Density matrices can describe both classical probabilities and quantum superposition.

 

[StevieTNZ]

The fundamental equation that governs any quantum system is the Schrodinger equation. At least, according to Howard Carmichael.

(http://www.physics.auckland.ac.nz/uoa/home/about/our-staff/professor-howard-carmichael/ )

 

[StevieTNZ]

Are you saying that the moment you can create a pure state cat. You can put it in any superposition you like such as dead/alive, pregnant/not pregnant, cancer/healthy, virgin/used, old/young, etc.? Without any limitations whatsoever? If there is limitation, maybe a pure state cat can only have superposition of positions of being on the left and right side at the same time and not on being dead/alive, pregnant/nonpregnant, etc.?

A macroscopic quantum system has an infinite amount of physical configurations as a basis state.

virgin/used

lol

 

[camboy]

　

I read last week that a science advisor and mathematician extraordinaire Arnold Neumaier already declared Bohmian mechanics null and void so I thought you guys refuted it already. Neumaier said:

<snip>　

So Bohmian Mechanics is still alive.

Look, why - in your case of having zero knowledge of this subject - do you think you have the right to behave in a sarcastic manner to people who try to educate you? Someone else already accused you of being arrogant - I can see why.

Arnold is unfortunately guilty of spreading a great deal of misinformation about de Broglie-Bohm theory in these threads. There's nothing one can do about this - even if we point out correct objections he'll just proclaim his superior intellect at us (and say that he's far too clever to bother publishing an objection in the peer-reviewed literature). Doesn't mean he's right though, as has been pointed out many times - see my https://www.physicsforums.com/showpost.php?p=3269901&postcount=27" to him in another thread.

But isn't it that in orthodox QM, a prisoner has non-zero probability of quantum tunneling outside a cement wall? In Bohmian M, no matter how much energy you give the prisoner, you can't quantum tunnel him outside of the cell without accelerating his body to the cement walls shredding it to pieces. Here QM has more flexibility. Isn't it?

No - look, just read some reference, will you. Start with the http://www.tcm.phy.cam.ac.uk/~mdt26/pilot_waves.html" - if I remember rightly Lecture 3 has something about tunnelling..

 

[rodsika]

That's not true. It would hold iff the density matrix could be written in terms of two pure states "alive" and "dead" with classical probabilities, but this is not true. Density matrices can describe both classical probabilities and quantum superposition.

Uhm, how do you prepare the cat in a state "alive + dead" that is not a pure state but a mixed state? an actual example? I only know about superposition of alive and dead where both exist in pure state. But I'm not familiar with mixed state "alive" and "dead". About the outcome, maybe it's is chosen by whether the radioactive source decays or not?

 

[Rap]

How is that possible. Pure state in the cat body simply meant all the particles are entangled. How can you make this create both old and young cat? Hmm.. are you saying that if you can put the old Einstein in pure state, he can become young again... like the fountain youth? What in pure state makes this possible at all? Hope you can explain a bit more so it can be clear to both of us the limitations and possibilities. Thanks.

tom.stoer #102 said it - if you have a box and you arrange it so that, after a certain amount of time, one outcome happens with 50% probability, another with 50% probability, and you know the pure state going in, then after that time, it will be in a superposition of the two outcomes. If you put a male cat and a female cat in a box with a closed door, knowing the pure state, and after a minute the door has a 50% chance of opening and staying open, 50% chance of never opening, then after a few months, the contents of the box will be in a superposed female cat pregnant/not pregnant state. I guess if you had the cat travel around in circles at near the speed of light with 50% probability, then after a year it would be in a superposition of (a little bit older)/(a year older), but you couldn't make it younger.

This is all too simplistic, but it is the general idea. Once you understand this, you can start to get into the details.

I think Ken G. eventually allowed that you could, in principle, close the box knowing a pure state.

The problem is different: The cat will never be in a pure state "alive" as the cat itself is always a mixed state b/c of the huge number of d.o.f. Therefore preparing the cat in a state "alive + dead" is possible, but this state is not a pure state (a ray) but a mixed state (a density matrix). This is not due to the superposition of "dead" and "alive" but due to the macroscopic nature of the cat.

I think that the fact that the cat is macroscopic does not prohibit you, in principle, from describing it by a pure state. I can describe one particle by a pure state, two with more difficulty, etc. There is no particle number at which it suddenly becomes impossible to describe them as a pure state.

But as far as I can see that does not affect the interpretation of the experiment b/c again the problem is "why the (mixed) state of the cat collapses to either dead or alive" where again dead and alive refer to mixed states. I guess decoherence is able to explain why the state of the cat collapses to one one these mixed states, but it does not explain why in a certain experiment it collapses to "dead" and not to "alive".

Decoherence in a closed system is a mathematical approximation, and does not explain collapse. It says that a superposed state can be approximated by a mixed state.

 

[tom.stoer]

Decoherence in a closed system is a mathematical approximation, and does not explain collapse. It says that a superposed state can be approximated by a mixed state.

Sorry for being sloppy; of course it's not "collapse" but simply the fact that we observe classical pointer states instead of coherent superpositions.

 

[rodsika]

The key point is that Ken wants to "measure" the cat whereas I want to "prepare" it. That is something different.

When setting up an experiment with monochromatic, coherent light emitted by a LASER the light is by definition monochromatic and coherent w/o measuring it! You check the properties of the light before starting the experiment in order to ensure the correctness of the setup, but during the experiment you do no longer check them; you simply believe that these properties persists w/o measurement. The same applies to the cat.

The problem is different: The cat will never be in a pure state "alive" as the cat itself is always a mixed state b/c of the huge number of d.o.f. Therefore preparing the cat in a state "alive + dead" is possible, but this state is not a pure state (a ray) but a mixed state (a density matrix). This is not due to the superposition of "dead" and "alive" but due to the macroscopic nature of the cat.

But as far as I can see that does not affect the interpretation of the experiment b/c again the problem is "why the (mixed) state of the cat collapses to either dead or alive" where again dead and alive refer to mixed states. I guess decoherence is able to explain why the state of the cat collapses to one one these mixed states, but it does not explain why in a certain experiment it collapses to "dead" and not to "alive".

That means that "collapse of the wave function", "split according to MWI interpretation ..." does apply on the level of density matrices. This is technically more involved but causes the same fundamental problems regarding ontology.

After hours of reading many threads and articles. I still can't fully understand everything you said above. First I need to know. When you say mixed state, do you mean classical collapsed state or not? If not. Are you saying that if we don't know the pure state of the cat and we totally isolate it in the box. It won't necessarily go into classical state but mixed state, meaning some parts still in superposition, is this what you meant? Also you were asking why the mixed state of the cat collapsed into either dead or alive. So you meant here the cat is still in partial superposition? Do you consider the body of a person like Obama walking on street in mixed state or no state? Or is the word mixed state only referred to isolated object? Pls clarify. Sorry if I am so dumb. Thanks.

 

[rodsika]

I think he means that, if we agree on a wave function for a system, it encapsulates our knowledge of the system (subjective), but the probabilities that we calculate as we propagate the wave function forward in time, (using e.g. the Schroedinger equation) are objective.



The thing I objected to was the assumption that the cat and the device were separate systems. This seems arbitrary. I think, in principle, you can have the device and the cat as one pure state, evolving according to the Schroedinger equation. In the case of Wigner's friend, the same applies.



No. QM is about measurements and the only reality is that which is revealed by those measurements. If I said the buckyball took a classical trajectory, that would mean I could measure its position and momentum to a high degree of accuracy without appreciably disturbing it, all along its trajectory. Superposition would not be an issue, the wave function would collapse at each measurement. If you say that the position and momentum is unmeasured during its travel, then QM and superposition will apply. But then, the question of whether it was following a classical trajectory is untestable, and is therefore not a proper scientific question.



Decoherence in this case is a mathematical approximation, not a physical occurrence. It says that you can approximately replace a pure wave function by a mixed state: an ensemble of macroscopic observations which do not yield as much information as a wavefunction collapse, and whose probabilities are additive. These observations do not collapse the wave function to an eigenstate, but rather selects one macroscopic observation out of the ensemble of observations. This is what happens when you open the box. You don't collapse the wave function, measuring the position and momenta of every particle of the cat (to within Heisenberg), such a measurement would destroy the cat. You make a much less informative measurement, noting only whether the cat is alive or dead.

Rap, what do you think of this comment by eaglelake?

"4. A quantum experiment requires a measurement result. (Bohr) If we assume that the particle exists prior to measurement, as was done in the EPR experiment, then we get erroneous results. Taking the results of quantum experiments at face value, quantum particles do not have trajectories and they exist only at the instant they are detected. For this reason, Wheeler calls particle detection, "an elementary act of creation.""

You believe the wave function is not real but only a tool by the physicist. But in the EPR thing, quantum particle do not have trajectories and they exist only at the instant they are detected. So you mean the Copenhagen thought or idea the wave function is not real yet quantum particles are not classical before measurement is compatible? Or more accurately, wave function being not real and local realism false is compatible? But if it is. The wave function has to exist somewhere or else if can't tract quantum objects which are not in classical world.

 

[tom.stoer]

@rodsika: mixed state is a technical term referring to a density matrix q http://en.wikipedia.org/wiki/Quantum_state#Mixed_states A density matrix can describe both pure and mixed states; the criterion to differentiate them is tr(q²).

 

[Rap]

Rap, what do you think of this comment by eaglelake?

"4. A quantum experiment requires a measurement result. (Bohr) If we assume that the particle exists prior to measurement, as was done in the EPR experiment, then we get erroneous results. Taking the results of quantum experiments at face value, quantum particles do not have trajectories and they exist only at the instant they are detected. For this reason, Wheeler calls particle detection, "an elementary act of creation.""

I am not sure what "if we assume that the the particle exists" means. Can you elaborate on this?

 

[rodsika]

I am not sure what "if we assume that the the particle exists" means. Can you elaborate on this?

In the EPR experiment, if the other photon pair exists on the other side, then you can determine with certainty both position and momentum by measuring both the attributes in either side. Hence EPR refutes local realism, meaning properties like position don't exist before measurements. Therefore you can't say there is still classical trajectory unless you go to the Bohmians. But our concern is pure Copenhagen. You said Copenhagen is pragmatic and doesn't care what is the deeper reality underneath.. But at least we can say that Copenhagen rejects or not compatible with classical trajectories but more of the stuff about properties don't exist before measurement variety. Refute this.

 

[Rap]

In the EPR experiment, if the other photon pair exists on the other side, then you can determine with certainty both position and momentum by measuring both the attributes in either side. Hence EPR refutes local realism, meaning properties like position don't exist before measurements. Therefore you can't say there is still classical trajectory unless you go to the Bohmians. But our concern is pure Copenhagen. You said Copenhagen is pragmatic and doesn't care what is the deeper reality underneath.. But at least we can say that Copenhagen rejects or not compatible with classical trajectories but more of the stuff about properties don't exist before measurement variety. Refute this.

To me, the statement that something does not exist means experimentally "I have searched exhaustively and it is not to be found". If something does not exist in principle, then it means "no matter how much you search, you will not find it". What can you say about something that you do not search for? Nothing.

In hard science, to search is to measure. You cannot determine if a particle exists before a measurement, because it requires measurements to make that determination. To ask if a particle exists before a measurement is an improper question. It is not a testable question. It is not a scientific question. It is an improper question and it has no answer.

If reality were God, Copenhagen is agnostic, not atheistic. An agnostic does not know if God exists, an atheist denies the existence of God. Copenhagen is like the agnostic, it does not deny or reject reality, it simply realizes that we cannot have scientific knowledge about any reality beyond what can be repeatably and quantitatively measured.

 

[rodsika]

Rap. About Wigner friend. Can you give a simpler setup to illustrate how the wave function has more to do with the observer knowledge and measurements? In the Wigner friend and the cat inside a bigger box with another scientist outside. You illustrated how the outside scientist can model Wigner in superposition of opening and not opening the box.. while Wigner would say he didn't experience any superposition, but either opening and not opening the box. Here you claimed that the wave function can't be real or there would be paradox. Is this your own argument or did you hear it elsewhere or the mainstream? But the problem is many state that the cat and Wigner can't even be in pure state in principle so no superposition at all is possible. That is why. Try to find a simpler setup using atoms or particles that can show the Wigner friend paradox. Can you think of one or any there in the literature?

 

[StevieTNZ]

Rap. About Wigner friend. Can you give a simpler setup to illustrate how the wave function has more to do with the observer knowledge and measurements? In the Wigner friend and the cat inside a bigger box with another scientist outside. You illustrated how the outside scientist can model Wigner in superposition of opening and not opening the box.. while Wigner would say he didn't experience any superposition, but either opening and not opening the box. Here you claimed that the wave function can't be real or there would be paradox. Is this your own argument or did you hear it elsewhere or the mainstream? But the problem is many state that the cat and Wigner can't even be in pure state in principle so no superposition at all is possible. That is why. Try to find a simpler setup using atoms or particles that can show the Wigner friend paradox. Can you think of one or any there in the literature?

I refer you to pages 243 and 244 of 'Modern Physics and Ancient Faith' by Stephen M. Barr. Publisher: University of Notre Dame Press