Exploring Uncertainty: Schrodinger's Cat and the Nature of Observation

[BernieM]

A cat in a box with a poison that may or may not be released, that hinges on a random event. Until the box is opened, the cat is both dead and alive, until a single state is forced by an observation.

Would it be just as valid to pose that the cat and the poison existing in the box is also uncertain? That when you open the box, there may be a dead cat in the box, a living cat in the box or no cat at all.

 

[DaveC426913]

For starter's it was just a thought experiment. In reality, the state inside the box (i.e. a cat as well as mechanisms) would very quickly decohere.

But, to answer your question as stated:

No. There is nothing in QM that would cause anything inside the box to simply disappear.

While there are multiple paths possible (atom decays, poison released, cat dies)(atom does not decay, cat lives), those paths still have a logical cause-and-effect from beginning to end. There is no cause-and-effect leading to the result of (no cat in the box).

 

[Nugatory]

A cat in a box with a poison that may or may not be released, that hinges on a random event. Until the box is opened, the cat is both dead and alive, until a single state is forced by an observation.

No one has ever seriously suggested that the cat would end up "both dead and alive".

Schrodinger created this thought experiment about ninety years ago to point out a problem with the then-current understanding of quantum mechanics: the math seemed to predict that the cat could be in a superposition of dead and alive until the box was opened, even though that couldn't possibly be right and no one thought it might be (the problem becomes even more clear when you consider the "Wigner's Friend" version of Schrodinger's thought experiment).

It took a few more decades to find the answer to this problem. Google for "quantum decoherence" or try David Lindlay's layman-friendly book "Where did the weirdness go?" to see the current understanding.

 

[BernieM]

Thanks for the answers. I never thought Schrodingers Cat was a real proposition, or that the cat could actually be dead and alive, a particle in both states at the same time, etc. I was considering things like virtual particles or quantum tunneling and whether the variation I suggested of the Schrodinger Cat thought experiment might fit there, allowing for a state that is neither dead or alive (ghost state? still existent but in a form we can't see?) Perhaps I should have used that instead.

 

[Nugatory]

Thanks for the answers. I never thought Schrodingers Cat was a real proposition, or that the cat could actually be dead and alive, a particle in both states at the same time, etc. I was considering things like virtual particles or quantum tunneling and whether the variation I suggested of the Schrodinger Cat thought experiment might fit there, allowing for a state that is neither dead or alive (ghost state? still existent but in a form we can't see?) Perhaps I should have used that instead.

For any macroscopic object anywhere near as complicated as a cat, the quantum mechanical prediction (once we allow for decoherence) is that the cat is unambiguously dead or alive whether we open the box and look or not; we don't know which it is until we look, but that's just because we haven't looked yet. The situation is no different than if we flip a coin and then look to see whether it landed heads-up or tails-up - as the title of Lindley's book suggests, there's no quantum weirdness involved.

The situation is different for smaller and less complex systems; for example, it is possible to prepare a subatomic particle in such a way that its state can be written as a superposition of other states and then to maintain it in that state for an extended time. However, if that's what you have in the box, you don't have any sort of variant of Schrodinger's thought experiment - that thought experiment is specifically about understanding what happens when you apply QM to macroscopic objects like cats.

Now, if you're trying to understand what it means to say that the state of a subatomic particle is "a superposition of other states"... That's a whole different question, and any really satisfactory answer will require looking at the mathematical formalism of the theory. English-language descriptions like "It is both A and B" or "it is neither A nor B" are not exactly wrong, but they are so open to interpretation as to be seriously misleading (that you will hear both of these mutually exclusive explanations is a very strong hint that neither is especially good). Giancarlo Ghirardi's book "Sneaking a look at God's cards" is a pretty good layman-friendly introduction.

 

[BernieM]

Well that was a really decent answer. Funny that I never made the connection for Schrodinger's Cat to only be applicable to the macroscopic world. I took it to apply to particle and wave states of matter in the subatomic world. Funny that I never made the proper connection of the experiment.

 

[Lord Jestocost]

...the quantum mechanical prediction (once we allow for decoherence) is that the cat is unambiguously dead or alive whether we open the box and look or not;

The assumption that one of the answers (dead/alive) is "objectively" realized in a classical sense in between observations or measurements is simply impossible when one restricts oneself to the quantum mechanical formalism and is thus unphysical.

 

[.Scott]

A cat in a box with a poison that may or may not be released, that hinges on a random event. Until the box is opened, the cat is both dead and alive, until a single state is forced by an observation.

If the cat did act as a quantum particle, not only would it be dead and alive, but the two versions of it would be able to interact.

 

[AlexCaledin]

If the cat did act as a quantum particle, not only would it be dead and alive, but the two versions of it would be able to interact.

- and virtual cats would be absolutely everywhere!

 

[MichPod]

It took a few more decades to find the answer to this problem. Google for "quantum decoherence" or try David Lindlay's layman-friendly book "Where did the weirdness go?" to see the current understanding.

Is this problem really considered answered already? My understanding is that the decoherence theory does not resolve the problem completely (since all components of the wave function still exist in a global superposition and there is no explanation how the one particular "classical" state may be selected/observed even after the decoherence happens), i.e. the problem of "measurement" remains unexplained.

 

[Nugatory]

Is this problem really considered answered already? My understanding is that the decoherence theory does not resolve the problem completely (since all components of the wave function still exist in a global superposition and there is no explanation how the one "classical" state may be selected/observed even after the decoherence happens), i.e. the problem of "measurement" remains unexplained.

You're right, decoherence does not "solve the measurement problem", in that it doesn't explain why we get this outcome instead of that outcome.

It does provide a physically reasonable and non-arbitrary place for the von Neumann cut, so goes a long ways towards explaining why macroscopic classical systems can be understood classically. That's enough to address the 1930-vintage difficulties posed by Schrodinger's cat and Wigner's friend - and those are the problems that most non-specialists here are starting with.

With tongue slightly in cheek, I'll suggest that the most common trajectory for a layman approaching QM is:
1) Hear from various pop-sci sources that "Until the box is opened, the cat is both dead and alive, until a single state is forced by an observation" (quoted from the first post in this thread). Between the cat and "seeing the interference pattern" you'll find this is one of the more common thread starts, and the question is basically about unobserved macroscopic superpositions of classically inconsistent (dead/alive, pattern/no pattern) outcomes. Schrodinger's point was that this problem appeared at the time to be baked into the mathematical formalism.
2) Learn about decoherence, which makes the weirdness go away for macroscopic systems like the cat. Reassured that the physics community has not really spent the entire last century in the thrall of an astounding mass delusion about endangered cats, we relax...
3) ... Until we learn that there is still a basic foundational problem. Decoherence says that the possible outcomes are all sane, but doesn't explain why there's an outcome at all.

This thread started at #1. You're coming in at #3, and there's nothing wrong with that as long as everyone is clear on the distinction.

 

[Lord Jestocost]

Is this problem really considered answered already?

The answer is "No". Decoherence is a dynamical effect that is never perfectly exact. The probability for observing a macroscopic interference effect between a dead and a live cat is never exactly zero; it’s extremely small and becomes exponentially smaller in course of time. There are good reasons to believe that the cat always exists in a definite state of health in such experiments; however, quantum theory permits in principle other experiments in which the outcome might not be so clear-cut. Perhaps we've never seen a live/dead "Schrödinger cat" only because we don't know how to look for one.

 

[MichPod]

With tongue slightly in cheek, I'll suggest that the most common trajectory for a layman approaching QM is:
[...]
3) ... Until we learn that there is still a basic foundational problem.

Could you share for one layman what is the item #4 then?

 

[Nugatory]

Could you share for one layman what is the item #4 then?

With tongue even further in cheek...
4) Unable to resolve this foundational problem, you adopt your favorite interpretation of quantum mechanics and defend it with unthinking ferocity. The resulting discussions invite comparison with the siege of Verdun.
5) Finally recognizing the futility of #4, you settle for il faut cultiver notre jardin. Not everyone makes it this far.

 

[MichPod]

il faut cultiver notre jardin

LOL.

Yet I must say that for the outsider it looks like physicists simply are afraid to say "we do not know it" to the public or even students. IMO, if each QM textbook had a section "we don't know" (or better such a sentence in some paragraphs), it would be much easier to learn QM for those who have some critical attitude.

I may clearly say that some paragraphs devoted to the measurement etc. looked for me (when I was a student) as a clear indoctrination rather than science and/or reason. And that dramatically hindered my ability to learn QM (which I could compensate only decades later).

 

[Mentz114]

[..]
I may clearly say that some paragraphs devoted to the measurement etc. looked for me (when I was a student) as a clear indoctrination rather than science and/or reason. And that dramatically hindered my ability to learn QM (which I could compensate only decades later).

I agree emphatically with that. The idea that a measuring apparatus has to deal with a superposition is just assumed and never justified. Without this dogmatic assumption many of the alleged problems disappear.

I found this course material very helpful.

Spin and Quantum Measurement
David H. McIntyre
Oregon State University

https://www.coursehero.com/file/7641300/SpinBook02

 

[Lord Jestocost]

Yet I must say that for the outsider it looks like physicists simply are afraid to say "we do not know it" to the public or even students.

It's not the case that all physicists simply are afraid to say "we do not know it". Listen to Richard Feynman (in ”The Feynman Lectures on Physics, Volume III”, italics in original):

“Because atomic behavior is so unlike ordinary experience, it is very difficult to get used to, and it appears peculiar and mysterious to everyone—both to the novice and to the experienced physicist. Even the experts do not understand it the way they would like to, and it is perfectly reasonable that they should not, because all of direct, human experience and of human intuition applies to large objects. We know how large objects will act, but things on a small scale just do not act that way. So we have to learn about them in a sort of abstract or imaginative fashion and not by connection with our direct experience.

In this chapter we shall tackle immediately the basic element of the mysterious behavior in its most strange form. We choose to examine a phenomenon which is impossible, absolutely impossible, to explain in any classical way, and which has in it the heart of quantum mechanics. In reality, it contains the only mystery. We cannot make the mystery go away by “explaining” how it works. We will just tell you how it works. In telling you how it works we will have told you about the basic peculiarities of all quantum mechanics.”

 

[Lord Jestocost]

Unable to resolve this foundational problem, you...

From an instrumentalist's point of view, there are no foundational problems. What "foundation"?

 

[MichPod]

From an instrumentalist's point of view, there are no foundational problems. What "foundation"?

Supposing we accept that everything is made of quantum particles, what may be the definition of the "measurement" or "measurement apparatus" from the instrumentalist point of view?

 

[Lord Jestocost]

Supposing we accept that everything is made of quantum particles...

An instrumentalist would never expose himself/herself to danger to make unprovable suppositions. "What everything is made of" is a senseless question because we have only a set of pointer readings, which exact science can study and connect with other pointer readings. So why worrying about a definition of "measurement" - related to what?

 

[MichPod]

we have only a set of pointer readings

Excuse me, do you have any definition of what is a "pointer" in your theory? And whatever "measurement apparatus" is made of, please give me its definition too.

 

[Lord Jestocost]

Excuse me, do you have any definition of what is a "pointer" in your theory?

For example, spatial or temporal patterns of "clicks" or "spots on screens". At the end, any "message" that arrives at the seat of consciousness.

 

[Lord Jestocost]

Excuse me, do you have any definition of what is a "pointer" in your theory? And whatever "measurement apparatus" is made of, please give me its definition too.

I would say, instrumentalism is not a theory, it’s an attitude towards science. An instrumentalist is not too bothered about the accuracy or reality of his/her ideas, concepts and definitions as long as they work and allow to make accurate predictions about what will happen next. An instrumentalist may even not believe that all that which we gather as a matter of convenience under headings like “that’s an electron” or “that’s an atom” or “that’s a measurement apparatus” are real, existing objects. The “physical world” is entirely abstract and without any “actuality” apart from its linkage to conscious minds. Mind is the first and most direct thing in our experience, and anything else is remote inference. In the end, we are thus talking about mental images produced by conscious mind. That‘s why "Made of what?" –questions might take one directly into the nowhere.

 

[stevendaryl]

I agree emphatically with that. The idea that a measuring apparatus has to deal with a superposition is just assumed and never justified.

I'm not sure what you mean by dealing with a superposition.

 

[stevendaryl]

From an instrumentalist's point of view, there are no foundational problems. What "foundation"?

You could take a macroscopic view, which is to assume that quantum mechanics is not about particles and fields, but is simply about predicting future macroscopic states (dots on photographic paper, clicks in Geiger counters, pixels on display screens, etc.) from past macroscopic states (settings of dials, which buttons you push, etc.) All the stuff about particles and fields and wave functions could be thought of as just mathematical fictions that are just used to calculate macroscopic probabilities.

I don't really think that that's a coherent approach though. For one thing, thought of as a theory of macroscopic transition probabilities, quantum mechanics is certainly incomplete: there are hidden variables (the microscopic details) that really do influence the macroscopic results.

 

[Mentz114]

I'm not sure what you mean by dealing with a superposition.

This is the assumption that a preparation results in a superposition which presents to the measuring apparatus which 'decides' which one of the components will be realized. If the apparatus is given an eigenvalue of some operator to measure, the apparatus just reports the state. It is the assumption of the superposition which causes the perceived problem.

It needs more justification than merely a rule in linear algebra.

(I think this is way off-topic)

 

[stevendaryl]

I would say, instrumentalism is not a theory, it’s an attitude towards science. An instrumentalist is not too bothered about the accuracy or reality of his/her ideas, concepts and definitions as long as they work and allow to make accurate predictions about what will happen next. An instrumentalist may even not believe that all that which we gather as a matter of convenience under headings like “that’s an electron” or “that’s an atom” or “that’s a measurement apparatus” are real, existing objects. The “physical world” is entirely abstract and without any “actuality” apart from its linkage to conscious minds. Mind is the first and most direct thing in our experience, and anything else is remote inference. In the end, we are thus talking about mental images produced by conscious mind. That‘s why "Made of what?" –questions might take one directly into the nowhere.

I can certainly see that that attitude might be fine for technology, or applied science. What you care about is building cars and cellphones, curing diseases, etc. Who cares what's really going on if we can accomplish that? But it seems to me contrary to the spirit of science. Science is driven by curiosity about what's going on behind the phenomena that is apparent to our senses. It wasn't enough to know the spectrum of hydrogen emissions (the Balmer series). People wanted to know why those frequencies. It wasn't enough to know that chemical compounds form in particular proportions of the constituent elements, people wanted to know why those combinations. Getting at explanations that are beneath the level of our senses is a principle driver of scientific research.

 

[stevendaryl]

This is the assumption that a preparation results in a superposition which presents to the measuring apparatus which 'decides' which one of the components will be realized. If the apparatus is given an eigenvalue of some operator to measure, the apparatus just reports the state. It is the assumption of the superposition which causes the perceived problem.

I don't see it as an assumption---it's part of quantum theory. If an electron is in a pure state of spin-up in the z-direction, then it will be in a superposition of spin-up and spin-down in the x-direction. I don't see an alternative to dealing with superpositions, as long as we are doing quantum mechanics.

 

[Mentz114]

I don't see it as an assumption---it's part of quantum theory. If an electron is in a pure state of spin-up in the z-direction, then it will be in a superposition of spin-up and spin-down in the x-direction. I don't see an alternative to dealing with superpositions, as long as we are doing quantum mechanics.

There is no problem with that. It is a digression. The fact that a superposition is a solution to the equations does not force us to assume that a superposition ( as opposed to a statistical mixture) is the inevitable result of every preparation.
( I have to go out for while now ... )

 

[stevendaryl]

There is no problem with that. It is a digression. The fact that a superposition is a solution to the equations does not force us to assume that a superposition ( as opposed to a statistical mixture) is the inevitable result of every prepration.
( I have to go out for while now ... )

Oh, I think that that's just an oversimplification for the sake of simplifying computations.

 

[Mentz114]

Oh, I think that that's just an oversimplification for the sake of simplifying computations.

I find that ambiguous but whichever I take it, it trivialises the issue.
I'll start a new thread sometime, maybe.