Why don't we bury Schrodinger's Cat?

[bushmonk]

TL;DR Summary

Why would something think a cat, living or dead, is in a quantum state. Isn't the physical process of measurement happening all the time in a cat, living or dead, effectively starting a new quantum state?

What’s with Schrodinger’s Cat?

I seem to be missing something. I keep on hearing about Schrodinger’s cat. Why? Why would anyone think that a cat, alive or dead, is in a quantum state?

Every quantum experiment involves a quantum state, subject to the law of superposition, being physically manipulated, followed by a physical process known (problematically) as measurement. An electron wave can go through both slits to get to a detecting screen because that is what quantum states can do. Nevertheless the electron only triggers one detector and simple, classical probabilities govern that choice. Whether we look at the detector to see if it registered a hit or not does nothing to change whether or not it was triggered. It was either triggered or not. It is not in a superposition of triggered and not-triggered until such time as we have a look. The physical process of measurement takes place in the detector, not in the mind of the observer.

Surely the cells in a living cat impose a measurement on, say, an oxygen molecule in the bloodstream, compelling the molecule to make a choice to enter the cell or keep going. The oxygen cannot remain in a superposition of outside and inside the cell, at least not for long. The cat is not in suspended animation. Processes are happening.

Similarly, a dead cat is a decaying cat. Processes are happening in it. A molecule of the poisonous gas is absorbed and must make a choice as to whether to disrupt this molecule or that.

Living and dead cats are a blizzard of choices interspersed with brief periods in which things evolve by Schrodingers equation. Each choice ends a quantum experiment and begins a new one. It is not in a superposition of “before” and “after” the choice is made.

“Cat states” are a misnomer. They refer to macroscopic states that are in a quantum state and are in a state of superposition. It requires a great effort to prepare such a state. You cannot create it by sticking it in a box.

Presumably Schrodinger was protesting the idea that a measurement had to do with the conscious awareness of the observer. However, conscious awareness is not inherent in the Copenhagen interpretation. David Bohm in his very orthodox Quantum Theory, 1951 pp. 583ff explains that conscious awareness is not required to register a measurement.

That we don’t know whether the cat is dead or alive until we open the box is no more mysterious than not knowing whether the coin I flipped is heads or tails until I look at it. Nothing of importance happens to the coin when I look at it. Nothing of importance happens to the cat when I open the box and look in. Superposition, on the other hand, is physically important. It explains how interference patterns come to be. For the cat in the box, radioactive decay is the critical event that releases the poisonous gas. Classical probability even manages to describe the decay process itself, indicating that superposition plays a neglibible role in the entire experiment.

Am I missing something? And if not, why isn’t Schrodinger’s cat dead and buried.

 

[PeterDonis]

Why would anyone think that a cat, alive or dead, is in a quantum state?

Because everything is in a quantum state, according to QM. Objects like cats don't stop having quantum states just because they're macroscopic.

Surely the cells in a living cat impose a measurement

According to the modern decoherence viewpoint, yes, in any macroscopic object the atoms in it are constantly "measuring" each other. But this in itself does not "fix" a scenario like Schrodinger's Cat, it just makes it clearer where the problem lies. The decoherence viewpoint makes it clear that, once the radioactive atom decays, the cat dies--it doesn't wait to die until you open the box. But the decoherence viewpoint does not help to answer the question of why the cat should be either alive or dead, instead of being entangled with the radioactive atom in a superposition of "atom not decayed, cat alive" and "atom decayed, cat dead", even though the latter state is what you get when you apply Schrodinger's Equation.

At present, we have no definitive answer to that question; we just have different QM interpretations that make different claims about it; in the Many Worlds interpretation, for example, the cat is in exactly that superposition, even after the box is opened! But discussion of interpretations belongs in the interpretations subforum, not here. As far as "basic" QM is concerned, we do not know why the cat is observed to be either alive or dead and the atom is observed to either be not decayed or decayed, and we never observe an entangled superposition such as I described. We just know that that is the empirical fact, and we know we can model it in QM by updating the wave function when we know the result of a measurement.

 

[Dale]

Burying a cat alive is illegal. So this would put the experimenter in a state of superposition of “imprisoned” and “not imprisoned”.

Do you think granting agencies would provide bail?

 

[topsquark]

...Do you think granting agencies would provide bail?

Some of them might.

(Scene shifts to several people walking into an office with a sealed box. The door closes...)

Uh, oh! Here we go again!

-Dan

 

[ohwilleke]

Am I missing something? And if not, why isn’t Schrodinger’s cat dead and buried.

Because it isn't the cat that is in the quantum state directly. The parable is a that there is quantum trigger for something that kills anything in the box.

 

[PeterDonis]

it isn't the cat that is in the quantum state directly

Yes, it is. In order to write down the Schrodinger Equation for this scenario, you write kets for the cat states involved: ##\ket{\text{dead}}## and ##\ket{\text{alive}}##.

 

[PeterDonis]

The parable is a that there is quantum trigger for something that kills anything in the box.

No, the parable is that the cat is in an entangled superposition with the quantum trigger. That can only be the case if the cat's state is a quantum state, just like the state of the quantum trigger.

 

[gmax137]

Now, @bushmonk , you see why the cat has not been buried, the poor thing continues to provide fodder for discussion EDIT ~90 years after.

 

[PeroK]

Schrodinger's cat is alive and kicking!

 

[topsquark]

Schrodinger's cat is alive and kicking!

You mean.... you opened the box?? NOoooooo!!!! The experiment is over!

-Dan

 

[DennisN]

TL;DR Summary: Why would something think a cat, living or dead, is in a quantum state. Isn't the physical process of measurement happening all the time in a cat, living or dead, effectively starting a new quantum state?

Am I missing something? And if not, why isn’t Schrodinger’s cat dead and buried.

You are missing the fact that cats have nine lives and actually have to be killed and measured nine times before they can be found in the "dead" state.

No, seriously, Schrödinger's cat is an excellent thought experiment, because it connects what happens at the atomic scale to things that happen on the animal/human scale (i.e. which is normal to us).

It is also one of the most famous thought experiments, and if you search on PF you will find extremely many threads about it. Schrödinger died in 1961 and we are still talking about his cat more than 60 years after his death! Just imagine how many discussions about quantum physics that cat has started, it is mind-blowing.

So, I say, Schrödinger was very successful with that thought experiment!

 

[f95toli]

TL;DR Summary: Why would something think a cat, living or dead, is in a quantum state. Isn't the physical process of measurement happening all the time in a cat, living or dead, effectively starting a new quantum state?

Every quantum experiment involves a quantum state, subject to the law of superposition, being physically manipulated, followed by a physical process known (problematically) as measurement. An electron wave can go through both slits to get to a detecting screen because that is what quantum states can do. Nevertheless the electron only triggers one detector and simple, classical probabilities govern that choice. Whether we look at the detector to see if it registered a hit or not does nothing to change whether or not it was triggered. It was either triggered or not. It is not in a superposition of triggered and not-triggered until such time as we have a look. The physical process of measurement takes place in the detector, not in the mind of the observer.

Sure, but this does not mean that we can't measure a cat-state. You obviously have to do many measurements, but there are lots of experiments where people generate and use cat-states for various applications (most recently quantum computing).
While these -obviously- do not use real cats; the physics/math is the same as in the original gedanken-experiment.

 

[PeterDonis]

While these -obviously- do not use real cats; the physics/math is the same as in the original gedanken-experiment.

Not quite, because the "cat states" in question are states of single qubits, not systems with ##10^{25}## or so qubits. That makes a huge difference.

 

[Lord Jestocost]

What’s with Schrodinger’s Cat?

“Schrödinger's cat" is a fairy tale which has nothing to do with serious physics. No need to think seriously about it.

 

[f95toli]

Not quite, because the "cat states" in question are states of single qubits, not systems with ##10^{25}## or so qubits. That makes a huge difference.

That is a good point. However, if you are using say superconducting qubits to create your cat state you are nevertheless dealing with the state of a macroscopic object (a condensate made up a large number of Cooper pairs). Due to the fact that system can be described by a single wavefunction you obviously still have a single qubit; but in the gedanken experiment that was also true for the cat. Not that this was in any way realistic, but as far as I remember the cat was described as a single two-state system.

 

[PeterDonis]

if you are using say superconducting qubits

Do you have a reference for superconducting qubits? AFAIK quantum computing experiments are done using polarizations of single photon states as qubits.

 

[PeterDonis]

Due to the fact that system can be described by a single wavefunction you obviously still have a single qubit; but in the gedanken experiment that was also true for the cat.

Any quantum system can be described by a single wave function. The question is how many degrees of freedom are included in that wave function. For a single qubit it's 1. For a cat it's something like ##10^{25}##. For a typical superconductor it might be of the same order of magnitude as the cat. The huge difference in number of degrees of freedom is important and can't just be handwaved away. Even in the case of the superconductor, while the particular degrees of freedom described by the Cooper pair wave function are homogeneous (and form a Bose-Einstein condensate in which all of the Cooper pairs are in the same state), they are not the only degrees of freedom present.

 

[f95toli]

Do you have a reference for superconducting qubits? AFAIK quantum computing experiments are done using polarizations of single photon states as qubits.

No, most quantum computing experiments are certainly NOT done using photonic setups.
You can of course do it, and there are groups and companies (e.g. PSI Quantum) that do; but other modalities dominate. The most common ones are superconducting (e..g, IBM, Google, Rigetti), ion traps (e.g., IonQ, Quantinuum) and neutral atoms (Pasqual, ColdQuanta); and there are also several others (Si spins, topological etc)

Note that not even photonic QC is necessarily done using polarizations of single photon, that technique is useful for demonstrating basic QM; but not very good for actual QC.

IBM's Qiskit website is a good intro to superconducting qubits and QC
https://qiskit.org/learn/course/quantum-hardware-pulses

 

[NickMDal]

I think this is drastically easier than everyone makes it. But the OP has a really good point, since every venue that teachers introductory physics goes through this same old thought experiment. The Great Courses quantum mechanics lectures describe sitting in three movie theater chairs at once. The lecturer then slogs through Schrodinger's cat. This metaphor just won't die.

I think it's because people haven't outwardly adopted what is a grossly simple explanation. I believe physicists like to retain the mystique.

David Chalmers spent much of his life trying to convince us that all quantum measurements are conscious. This suggests that anything interacting causally with a particle is a measurement device. Whether or not the consciousness part is true, he's earned the right to be taken seriously.

We only need to concern with the measurement, conscious or otherwise. Particles acting as particles exhibit causal behavior. There is never a causal interaction until the particle state is reached. So a measurement doesn't need to be anything more than a causal interaction initiated by a particle.

A geiger counter clicking is one such interaction. That's obviously the measurement that ends the wave behavior. So after the geiger counter clicks, superposition is over. Of course the geiger counter is a measurement device. It says so right on it!

 

[haushofer]

“Schrödinger's cat" is a fairy tale which has nothing to do with serious physics. No need to think seriously about it.

No need to think about the Heisenberg cut and the measurement problem. Just shut up and calculate already. That's "serious physics".

My bookkeeper always says he deserves a Nobel prize for doing "serious econometry" instead of Black and Scholes.

 

[bushmonk]

I made the original post. Oops. I just revived the cat. A special thanks for those that expounded QM in response whether for or against my post.

Here is my understanding. Correct me where I am wrong.
I think it is misleading to say that a cat is in a quantum state, for the following reasons:
1. A quantum state is a microstate that deterministically evolves according to Schrodinger's equation.
2. A cat is a series of microstates that are randomly and discontinuously related to each other by brief excursions of Schrodinger’s equation, and frequent wave function collapse.
3. A cat’s microstate can never be reproduced, but, for instance, a radioactive atom can be replaced by an identical one in a future experimental trial.
4. A so-called “cat state” is truly a quantum state of a macroscopic system. It is a technological triumph, not an illustration of a general principle that cats are in quantum states.
5. This does not mean to imply that superpositions don’t occur in a cat. They happen all the time, but they quickly collapse to form other superpositions. (The oxygen molecule may be in a superposition of outside and inside a cell, but not for long.)

I haven’t studied the various interpretations of quantum mechanics although I have incidentally come across several. However I think that every interpretation must acknowledge the 5 points I made above, although the language I used might not suit some interpretations.

If I understand it correctly, the basic theoretical problem is that the word “measurement” in the postulates of quantum mechanics is physically undefined. When an observer observes, a collapse has definitely occurred somewhere along the line. Knowledge of what happened is a sufficient condition for a collapse to have occurred. The question is whether knowledge is a necessary condition. The answer is no, it is not a necessary condition. What is the necessary condition? No one has come up with a precise condition.

Yet we have abundant “rules of thumb” that point the way.

For example, the chance of observing an electron going through one or other slit in a double slit experiment is zero if there is no measuring instrument there to detect it. If we are moderately careful we get an interference pattern that indicates the wave function went both ways. There was not wave function collapse until the screen. If we illuminate the slits with a light of the right frequency and intensity, but provide nothing to examine the response of the light, we still do not observe which slit the electron went through but we disrupt the interference pattern at the screen, as if we had measured the electron going through one or other slit. (Measurement is not a necessary condition for wave collapse!)

Schrodinger’s cat was hypothesized to show the ridiculousness of the idea that knowledge of what happened is a necessary condition for wave function collapse. We know that it isn’t necessary. So why do we continue to confuse subsequent generations of people by saying that quantum mechanics says that cats can be in a superposition of alive and dead. Why keep a comatose cat on life support? Let the poor thing die.

 

[PeterDonis]

A quantum state is a microstate

No, it's a state. It doesn't have to be a microstate. A cat has a quantum state, but it's not a microstate.

A cat is a series of microstates states

See correction per the above.

that are randomly and discontinuously related to each other

Not at all. If this were the case, cats, and objects generally, would not be describable to a very good approximation using classical physics.

by brief excursions of Schrodinger’s equation, and frequent wave function collapse.

This is interpretation dependent and is off topic here; discussion of interpretations belongs in the interpretations subforum. Some interpretations, like the MWI, do not include wave function collapse as an actual physical process; it's just Schrodinger's Equation all the time.

In terms of standard QM, without adopting any particular interpretation, we can't do much to describe a cat, or any other macroscopic object, because we can't run meaningful quantum experiments on such things. We can't put them through double slits or beam splitters or otherwise test for quantum interference effects. The best we can do is to say that, according to decoherence theory, we expect, say, a cat's "alive" and "dead" states (or more correctly "alive" and "dead" subspaces of the cat Hilbert space) to be orthogonal to each other, with no appreciable quantum interference between them; and hence we expect cats to be describable to a very good approximation using classical physics.

A so-called “cat state” is truly a quantum state of a macroscopic system.

Any state in QM is a quantum state. The key property of a "cat" state is that it is an entangled state of a macroscopic system with a microscopic system, such that we cannot assign a definite state to the macroscopic system alone. We can't say that the cat is alive or dead in such a state; we can only say it's in an entangled superposition of "alive" and "dead".

This does not mean to imply that superpositions don’t occur in a cat. They happen all the time, but they quickly collapse to form other superpositions.

"Superposition" is the wrong word here, because it's basis dependent. Entanglement is basis independent and is the right thing to focus on here. Yes, in an entangled state we might view one subsystem, such as the cat, as being in a superposition, of "alive" and "dead", say, but the key property we are making use of is entanglement, not superposition.

If I understand it correctly, the basic theoretical problem is that the word “measurement” in the postulates of quantum mechanics is physically undefined.

That was a problem with QM prior to decoherence theory being developed, yes. But decoherence clarified that a "measurement" requires decoherence, which greatly helps to pin down what a measurement is in QM, since decoherence is not physically undefined; it's a definite physical process that can be and has been studied, for a number of decades now.

The measurement problem that is left over once we take into account decoherence is the problem I described in post #2 of this thread.

When an observer observes, a collapse has definitely occurred somewhere along the line.

Again, this is interpretation dependent and is off topic here.

Schrodinger’s cat was hypothesized to show the ridiculousness of the idea that knowledge of what happened is a necessary condition for wave function collapse.

Not quite. It was originally hypothesized to show the ridiculousness of the idea that cats, or macroscopic objects in general, could ever be in such entangled superposition states at all. However, we now know it's not as simple as Schrodinger was making it out to be.

So why do we continue to confuse subsequent generations of people by saying that quantum mechanics says that cats can be in a superposition of alive and dead.

We're not confusing people, we're recognizing that there is still a measurement problem even once we understand decoherence and the role it plays. That problem, as above, is described in post #2 of this thread. And a big part of the problem is that we still do not have a single agreed upon account of why we only observe cats to be alive or dead and never observe entangled superpositions such as QM predicts would be created by a Schrodinger's cat experiment. Every QM interpretation gives a different account, and many of the accounts are mutually inconsistent.

 

[sophiecentaur]

Schrodinger did a great job with his cat model. If he'd just said that the isotope is in both states until you measure them then this thread wouldn't exist and nor would all the earlier discussions throughout history. He triggered some serious thought by bringing in this surreal scenario. And he still manages to sort out the men from the boys (other genders are available, of course).

 

[Meir Achuz]

I didn't read all the responses, but the cat is like the twin paradox of relativity. They are both kept alive by people who don't like (even if they believe it) the theory.
It's interesting that the first responders, Planck, Einstein, deBroglie (I'm not sure about him.), Schrödinger, didn't like quantum mechanics.

 

[NickMDal]

The key property of a "cat" state is that it is an entangled state of a macroscopic system with a microscopic system, such that we cannot assign a definite state to the macroscopic system alone.

I feel like this is where things go off the rails. In this experiment, exactly why is the cat in an entangled state? As I understand, entanglement has been observed in matter up to a millimeter in size under very controlled conditions. How does the cat, which is on the other side of the detector, hammer and vat of poison become entangled?

 

[PeterDonis]

exactly why is the cat in an entangled state?

Because that's how the experiment is set up. The radioactive atom's state is entangled with the cat's state; the joint state looks like this (omitting normalization):

$$
\ket{\text{atom decayed}} \ket{\text{cat dead}} + \ket{\text{atom not decayed}} \ket{\text{cat alive}}
$$

What we would actually observe, of course, is just one of those two outcomes: but QM does not explain how the transition is made from the above entangled state, which is what you get when you apply the Schrodinger Equation to this experimental setup, to one or the other of its terms, depending on whether the cat is observed to be dead or alive. That is the essence of the measurement problem in QM. Different QM interpretations give different (and mutually inconsistent) solutions to the problem.

As I understand, entanglement has been observed in matter up to a millimeter in size under very controlled conditions.

More precisely, entanglement has been tested for explicitly in controlled experiments in this range. But that doesn't mean entanglement doesn't exist outside that range. Entanglement is the normal result of any interaction between quantum systems.

 

[hutchphd]

https://www.physicsforums.com/threa...don't we bury Schrodinger's Cat?,-(2 Viewers)
Because we cannot agree as to the proper place for the burial....

 

[gmax137]

https://www.physicsforums.com/threa...don't we bury Schrodinger's Cat?,-(2 Viewers)
Because we cannot agree as to the proper place for the burial....

Whoa, this thread is about QM, not self referential loops, lol.

 

[sillyputty]

but QM does not explain how the transition is made from the above entangled state,

Isn't it also true that QM does not say that there is a transition? If true, in which interpretation does 'transition' belong?

 

[PeterDonis]

Isn't it also true that QM does not say that there is a transition?

Basic QM, without adopting any interpretation, takes no position on what "really happens". It just says that, in order to make future predictions, you need to update the wave function you use in the math once you know the result of a measurement.

 

[PeterDonis]

In which interpretation does 'transition' belong?

Any interpretation that says that collapse of the wave function is a real, physical process.

 

[NickMDal]

More precisely, entanglement has been tested for explicitly in controlled experiments in this range. But that doesn't mean entanglement doesn't exist outside that range. Entanglement is the normal result of any interaction between quantum systems.

I believe the merit of the Schrodinger's cat metaphor hinges entirely on this. What evidence is there that entanglement exists between the ejected photon and the cat itself? Saying that entanglement exists between "any" interactions is way to inclusive and is without evidence.

 

[PeterDonis]

What evidence is there that entanglement exists between the ejected photon and the cat itself?

Nobody has actually done the experiment so how could there be any actual evidence?

QM predicts that there will be entanglement, based on the Schrodinger Equation, and since such predictions have turned out to be correct in every experiment that has actually been done, it seems reasonable to explore their implications in this case.

 

[NickMDal]

Nobody has actually done the experiment so how could there be any actual evidence?

QM predicts that there will be entanglement, based on the Schrodinger Equation, and since such predictions have turned out to be correct in every experiment that has actually been done, it seems reasonable to explore their implications in this case.

QM predicts that there will be entanglement, based on the Schrodinger Equation, and since such predictions have turned out to be correct in every experiment that has actually been done, it seems reasonable to explore their implications in this case.

There are no experiments that prove entanglement continuity between consecutive macro objects to a single particle. Whether or not this is eventually proven to be possible, you can't make it the basis for your argument that the cat is in an entangled state. We have no evidence at all saying this is possible. So you might hypothesize but it does not give any new credence to the cat in a superposition.

 

[PeterDonis]

you can't make it the basis for your argument that the cat is in an entangled state

I have already said that I am not basing the argument on evidence, since there is none. I am basing it on standard QM, using the same framework whose predictions have already been verified in countless other experiments. This kind of extrapolation of a theory is commonplace in science. You are of course free to stick your fingers in your ears and refuse to pay attention, but scientists are going to continue to do it whether you like it or not. As long as the distinction is clear between a theoretical prediction, studied in order to explore its implications, and a claim of actual evidence, which is not the case here as has already been said, there is no problem with such theoretical extrapolations. If you refuse to accept this, then please stop posting in this thread since you are adding no value to this particular discussion.