Why is Schrodinger's Cat only a thought experiment?

[Ken G]

Yes, we do -- we can, for example, invoke decoherence to suggest, with high probability, the <measuring device, experiment> system transitions into a purely mixed state. Invoke past experience that it does so in a consistent manner. Use conditional probabilities, rather than collapses.

That's unresponsive to my challenge. I am well aware that we can conceive of superpositions, obviously we do that in quantum mechanics, my question is-- can you test your ideas without at any time confronting them with an apparatus you are relying on to behave classically? This is what you have not shown, and is all I have claimed that you cannot show. Remember, I am not attributing any mystical signifance of a "collapse", I would describe it in exactly the terms you use, and indeed have described it in those terms. My point is, we did it on purpose because that's how we do science, and that is all the CI says (as I've mentioned, Heisenberg tacked some baggage onto the idea of a "collapse", I've never adopted that, I'm using the version Bohr settled on where the goal is to predict a confrontation with a classical apparatus).

If we do this, the observed behavior of measuring devices is, as far as we know, consistent with unitary evolution.

No it isn't, it is consistent with unitary evolution subject to averaging over untracked information. That's a very big difference, as it is the source of the nonunitariness that the MWI is so bothered about and the CI isn't. Of course the equations of quantum mechanics work where they work, and in many other situations they require we do some noise averaging that ruins the unitariness of the behavior of the open subspace under investigation. When all you see is the shadow of that projection, to imagine what made that shadow is to engage in pedagogy, which is fine, if we know it for what it is.

Yes -- at least, I thought that you were arguing certain parts of reality assumed a priori to be classical, and that science cannot be used to question that assumption.

No, I am making no assumptions at all about reality, that's the point. I am talking about what science is doing, not reality. Science is how we address reality, and we introduce the classical models of our own instruments to do it. Our choice, our reasoning process, our concept of measurement-- that's where all the nonunitariness comes from, and it is a "problem" that needs no solving. There's simply no need for the MWI because its projection onto science is the CI. You may invoke it as a pedagogical tool at your whim, but until you make a prediction using it, you have nothing more. The real problem is asking people to believe it is the reality-- that is downright ascientific.

Now, it looks like it might be that you understand certain parts of reality as classical simply because you have not yet formulated any other understanding -- and you are asserting that everybody in the world currently suffers this limitation, and that it is insurmountable.

One more time: I do not understand any parts of reality as classical, I, and all scientists (I still await the counterexample), must treat their tools for probing reality as classical. If that wasn't successful, we'd all be out of work, but it has nothing to do with believing how reality works.

Our interface with reality is real, isn't it? Shouldn't it follow that the interface is also nonclassical?

I have no doubt that our measuring instruments are only approximately classical, and I have never claimed otherwise. My claim is that we treat them as classical, in the very core of how we do science. You see, "classical" and "quantum mechanical" are not words that describe reality (we haven't the vaguest idea how to make words that do that), they are words that describe our understanding of reality, how we treat reality. And part and parcel of that is how we treat our interfaces with reality. This is simply true-- science is a story of what is happening in our heads, inspired by our interactions with nature.

You only did half of the exercise. You apply your classical viewpoint in a domain where it is known to be consistent with reality -- but you have not also shown that reality is inconsistent with a quantum viewpoint!

"Reality itself" is neither "consistent with the quantum viewpoint" nor "inconsistent with the quantum viewpoint", it's just reality. We define what the "quantum viewpoint" is, and some parts of it involve unitary time evolution, and other parts of it do not (the confrontation with the classical apparatus that is always present). How can I say this, you keep mishearing me. I am saying that we construct knowledge according to classical models, like numbers, readings, intensities, lengths, and times. These are the "observables" and "parameters" of quantum mechanics. Why is the theory of quantum mechanics expressed in terms of classical observables? Why do we need a concept of "collapse" of a wavefunction onto a basis vector?

It is we who require that, not nature. It's in science. There is no aspect of reality that has to behave classically-- but all of quantum mechanics must be projected onto classical observables for us to test our understanding. That is where the "collapse" comes from, we did it on purpose, we looked for ways to do it and called them measurements. It's not odd that measurements involve collapses, that is the purpose of measurements.

For comparison, I will point out that architects often assume the Earth is flat when they design buildings, and when the building is built, it works out exactly as the architect foresaw. Is this exercise a demonstration that the Earth really is flat? No -- because the building is small, the results are also consistent with the round Earth hypothesis, and so this exercise is of no use in distinguishing the two.

Right, and indeed all science works exactly like that. By the way, the Earth is not "round" either!

The exercises you propose are the same -- they are situations where classical and quantum physics are not known to be observably different, and so they shed no light on the issue!

No, that's not the point of the exercise, the point of the exercise if for you to tell me why you think it is that we explore quantum physics using entirely instruments that show no difference classically and quantum mechanically. Why is that? Why, if we want to understand quantum systems, don't we probe them with other quantum systems at every step of the way? From whence comes the need for that inevitable "last step", the "translation into the classical realm"?

I'm curious why you think that is, because you have given no counterexamples. And if we are planning on doing that translation, why should we be surprised when it breaks the unitariness? Quantum mechanics itself tells us that will break the unitariness, this is not a problem that needs solving. We don't need the MWI.

 

[vanesch]

Don't get me wrong here: I have nothing against saying that "although I cannot test it, I use this pedagogical picture because it works for me when I extend it to situations that are testable." We do that all the time, "you can picture it this way if you want to, or like this other way if you prefer, but the science just says this". I also have no objection to the MWI as such a pedagogical option, a kind of crutch to help you visualize the equations you are solving. Keep in mind, that is precisely how I view the wave function itself, so I'd have no issues with extending that to MWI.

But that is exactly how I also picture it !

The issue comes in when people start saying "my theory says that many worlds are spawned by quantum interactions" because "the universal wave function must evolve unitarily, so the subspace I am living in cannot be the whole reality". The latter is not a pedagogical picture, it is a nonscientific approach to knowledge about reality-- it is neither objective nor demonstrable. It is religion in the guise of science, an aberration that serves neither.

This is borderline. My point is that if *for the sake of argument, and pedagogical understanding*, we take on the viewpoint that quantum mechanics is universally and strictly valid in the way I pictured it the MWI way (which is nothing else but a toy world, I agree) THEN it is difficult to avoid the conclusion that we have to say that the subspace I'm living in cannot be the whole reality.
I wholeheartedly agree with the fact that this is an extrapolation, and that nature doesn't have to be like this at all. I would even say that it is probably pretty naive to think that our axiomatic systems as of today are somehow "real".

 

[Ken G]

But that is exactly how I also picture it !

And there's no problem in that, but something about your picture caused you to wonder if the concept of "objective observation" is simply too naive an idea, in the same way that "absolute time" was too naive. No harm in wondering that either, but it's a pretty radical idea that places the axioms ahead of the process that arrived at them. Let's face it, people tout MWI as something quite a bit more than a pedagogical picture for framing quantum mechanics. Have you heard of Max Tegmark's "quantum suicide" thought experiment for example? The problem is out there, I am not imagining it. I'm sure Max Tegmark knows more quantum mechanics in his little finger than I do in my whole head, but in way that's pretty much the problem-- one can get so deeply immersed in it that one loses track of how one got there in the first place.

My point is that if *for the sake of argument, and pedagogical understanding*, we take on the viewpoint that quantum mechanics is universally and strictly valid in the way I pictured it the MWI way (which is nothing else but a toy world, I agree) THEN it is difficult to avoid the conclusion that we have to say that the subspace I'm living in cannot be the whole reality.

I agree there, your statement is rock solid logic because you embedded it in a syllogism. Note this is also a syllogism: IF for the sake of pedagogical understanding, we imagine that some supreme being created the universe yesterday to look exactly like it does today, THEN it is difficult to avoid the conclusion that evolution did not occur. Also perfectly correct logic-- not intended to be taken seriously as science, but demonstrating that if we will leave the realm of what we can demonstrate to be true, the nuances of meaning you equip your position with are easily lost. "Apres moi le deluge".

I wholeheartedly agree with the fact that this is an extrapolation, and that nature doesn't have to be like this at all. I would even say that it is probably pretty naive to think that our axiomatic systems as of today are somehow "real".

Then we are in pretty much total agreement. Perhaps the last thing I would add is that the tendency to lose contact with that statement can lead to an undermining of how science got us to this point, which is the real issue I'm trying to keep the focus on. There is a powerful lure to want to feel like we understand the universe, people flock to churches to get a similar feeling. But science is based on avoiding any untestable shortcuts, and the MWI seems like cheating on that principle.

 

[vanesch]

I agree there, your statement is rock solid logic because you embedded it in a syllogism. Note this is also a syllogism: IF for the sake of pedagogical understanding, we imagine that some supreme being created the universe yesterday to look exactly like it does today, THEN it is difficult to avoid the conclusion that evolution did not occur.

I already made this point a few times
This is why it is a bad idea to try to PROVE creationists wrong. As you point out, creationism is simply not "scientific", but cannot be proven to be "wrong" in the ontological sense. The only problem is that it has *less* predictive power than evolutionary biology. *This* is why it is bad science.

However, we draw different conclusions from this. My final conclusion is one of agnosticism (that in the very end, we don't really know anything for absolutely sure about "reality", which gives us, up to a point, some liberty in choosing our preferred ontology), while you seem to draw a red line not to cross, and take as "absolutely sure" what is observed.

 

[Lester]

The explanation I've heard is that quantum superposition is an atomic-scale phenomenon, that it makes no sense to apply it to a macro-scale object such as a cat.

But that's not what the experiment is doing. The radioactive isotope is subject to quantum effects, and it seems to follow that the result can be detected (and thus acted upon) at a macro-level.

So, why would the cat not exist in two states?

My view about this matter is somewhat different from common wisdom but makes things easier to understand.

The main point is that quantum mechanics and statistical mechanics can be related each other by a Wick rotation of time variable, t to -it, in the unitary evolution operator. Then, recovering classical limit means just to take the limit of a number of particles going to infinity. This is just thermodynamic limit the can be supplemented with the condition of constant density.

So, the cat is classical and no entanglement is expected. For all practical purposes you can never observe such a superposition of macroscopic states as presumed by the Schroedinger experiment. Anyhow, you can consider objects being somewhat in the limit at this boundary and do some studies about. This has been done in different cases as Zeilinger's group with interference using macromolecules. This studies today seem somewhat blocked and some years passed by since the last significant results appeared in literature (Phys. Rev. Lett. 91, 090408 (2003)). This happened when they started to see some effect at increasing number of particles and I do not know why this group gave up producing new results. The other significant results have been seen by Haroche's group with photons recovering a result due to Gea-Banacloche with increasing number of photons (Phys. Rev. Lett. 91, 230405 (2003)). An effect due to thermodynamic limit has also been seen by Pastawski's group with a very beatiful NMR experiment (J. Chem. Phys. 108, 2718 (1998)) recently confirmed by Krojanski and Suter, Phys. Rev. Lett. 93, 090501(2004).

A recent theoretical overview has been given in M. Frasca, Journal of Physics: Conference Series 67 (2007) 012026. This effect is very easily seen in radiation-matter interaction (Dicke model) and you should consider that any measurement apparatus uses electromagnetic interaction to realize a measure. A number of refs. are

M. Frasca, Annals Phys. 313 (2004) 26-36.
Emary & Brandes, Phys. Rev. Lett. 90, 044101(2003); Phys. Rev. E 67, 066203 (2003).
Soi-Chan Lei and Ray-Kuang Lee, Phys. Rev. A 77, 033827 (2008).

Indeed, the Copenaghen interpretation does properly work when a boundary between quantum and classical worlds is given. This boundary is at infinity.

Jon

 

[Ken G]

As you point out, creationism is simply not "scientific", but cannot be proven to be "wrong" in the ontological sense. The only problem is that it has *less* predictive power than evolutionary biology. *This* is why it is bad science.

I agree with your assessment, and I think we are using a more stable and reliable understanding of what science is than the popularized version that imagines that truths are definable independently from any means of establishing them.

However, we draw different conclusions from this. My final conclusion is one of agnosticism (that in the very end, we don't really know anything for absolutely sure about "reality", which gives us, up to a point, some liberty in choosing our preferred ontology), while you seem to draw a red line not to cross, and take as "absolutely sure" what is observed.

I only do that when I step into "science", because I feel that is its crucial distinguishing feature. If I am not thinking scientifically, which I find more appropriate for appreciation of art, music, religion, and much of what we might call the human experience, then I am more agnostic about truth. The main point is, there is no unique concept of truth, but scientific truth needs to know how it differs from other forms or we get lost in our own advances and begin to serve the servant. The latter is how I view both MWI and the "landscape".