Isn't the cat experiment bullshit outside of illustrating a macro version of this? Like, there's no superposition in reality or the cat is too big to be subject to quantum mechanics where these shenanigans happen or something?
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The problem is to isolate the box enough. If you can heard the cat meow, then it counts as a "measurement" and the superposition disappears. If you can hear there is no heartbeat of the cat, then it counts as a "measurement" and the superposition disappears. If the cats move and the box wags, then it counts as a "measurement" and the superposition disappears...
So in practice the experiment only works with very small systems with a few atoms. Each year there are experiments with bigger systems, but they are usually frozen and very small. Perhaps one day we will be able to do the experiment with tardigrades.
Hearing at all implies sound waves. If there are soundwaves there is air inside the box. If there is air inside the box the system can't be in a superposition because it has long since decohered.
In other words it's not the act of "listening" that collapses the wave function but the interaction of air molecules that creates the sound in the first place, even if no one is there to listen.
A "measurement" or an "observation" doesn't actually require a human, or a living thing. It's just a physical interaction.
This does not help at all, quantum systems - even messy complex ones, boxes containing cats and air - evolve unitarily and do not destroy a superposition. The box and the air and the cat can interact all they want, you will still have a superposition of cat dead - and box and air in some state - and cat alive - and box and air in some other state. And no point are we addressing how we get rid of one half of the entangled state. And even if I see the box wiggle a bit, we only declare that me watching the box somehow was an measurement and that collapsed the wave function without answering the question why I did not simply get entangled with the box, too.
> quantum systems - even messy complex ones, boxes containing cats and air - evolve unitarily and do not destroy a superposition
This is wrong, they do collapse, you can test this using the double slit experiment. No human has to be there for the wavefunction to collapse and thus break entanglement.
If what you said was true then this wouldn't be a part of physics, physics is only about stuff we can measure not nonsense that has no meaning.
It's not at all clear how, when, and why wave function collapse happens. The only known quantity there, decoherence, is related to interaction between a coherent quantum system and a noisy decohered environment, which causes the original system to decohere itself.
Theoretically, as far as it is known today, it is possible for a quantum system of any size, and at any temperature, to remain in a coherent state, if it could be perfectly isolated from the rest of the world.
Any limits to this process, where a perfectly isolated system would undergo wave function collapse without any outside interference, would be a Nobel-prize worthy discovery and would constitute a new theory of physics: quantum mechanics and QFT allow no such process.
Edit: and the double slit experiment is unrelated - the screen and the slit are noisy classical systems that are not in any way isolated from the rest of the classical world. In principle, if you performed the experiment using a screen and slits made entirely of atoms entangled with the emitter, and isolated perfectly from the rest of the world and the CMB and everything, you'd get a different result: this is what QM predicts.
> quantum mechanics and QFT allow no such process
Double slit experiments shows collapse happens without any outside interference, nothing outside of the room has to do anything for the resulting measurements to show collapse happened. You can then afterwards enter the room, look at the recorded data and see that collapses did happen.
So you must have misunderstood what those theories says about collapses. They don't say exactly when collapses happens or what causes them, that is the unsolved problem, but we can know that they do happen.
In the normal experiments we perform, the screen is already a classical system, collapsed / entangled with the rest of the "classical world" (depending on the interpretation). When the particles interact with the screen, of course their wave function collapses as well.
But what if instead you had a screen that was itself a quantum system? You'd then use the Schrodinger equation without any collapse (without the Born rule) to describe the results. You'd have to perform other measurements on the quantum screen to obtain a classical result that can be interpreted, of course. But nothing in QM says that there is some size limit, so in principle it should be possible to prepare a quantum system of any size, including the size of a typical double slit experiment screen, that doesn't collapse.
Quantum mechanics, i.e. the Schrödinger equation, and wave function collapse are incompatible. While not measured a quantum system evolves unitarily, when measured the quantum system ends up in the measured Eigenstate which is a non-unitary change. But any measurement device is also a quantum system and therefore the combined system should also evolve unitarily. And now we have a serious problem to which nobody has a generally accepted answer.
> evolve unitarily and do not destroy a superposition
Unitary evolution can destroy superposition of the small subsystem by introducing negligible amount of superposition into large system.
Not sure if that is true, but assuming it is, does this really count as destruction instead of moving it around and spreading it? After all unitary evolution is reversible and I could therefore reconcentrate the superposition into the small system. It would however seemingly be a good explanation how to get from the decayed non-decayed superposition of a few atoms to an almost alive or almost dead cat. This is however also what makes me doubt your statement, if true, why are we still puzzled about Schrödinger's cat?
Well the naming is a different question, but it is consistent with observation of what we call wavefunction collapse.
In Schroedinger cat experiment, the evolution is not thermalization but rather entangling the whole cat with atom state. I don't think we're really puzzled about it now, since we know that macroscopic objects can be in superposition. So cat, theoretically, can be in superposition of alive and dead until measured. I don't think there is any contradiction or confusion here physics-wise.
This isn't quite right, wave function collapse is relative so while within the box an observer's wave function of the box will be in a definite eigenstate, the wavefunction of the box for an observer isolated from the box will still be in a superposition of its states.
In other words, the fact that an observation takes place from within the box and collapses the wave function for an observer in the box does not mean that the wave function for every observer also collapses. A wave function collapse for one observer does not imply a wave function collapse for any other observer.
> wave function collapse is relative
No it isn't, why do you think this? If it was we wouldn't be able to see it in our experiments, as where the particle ends up would depend on which observer is looking, which would get absurd consequences.
This is actually an open problem in QM. We don't understand what constitutes a measurement, and so we don't know if two perfectly isolated observers could disagree on the "state" of a system in this way (as in, collapsed or not collapsed). And given special relativity, it is definitely possible that two different observers could disagree on the order that two independent measurements of the same system happened.
Do note that the result of the experiments is not going to differ, it is guaranteed to be consistent by the fact that both experiments are measuring the same quantum system. It's just that one observer is unsure of what result they will get for a subsequent experiment, while the other observer can know it exactly.
And in the many worlds interpretation, it is indeed believed that apparent wave function collapse is a relative phenomenon, caused by entanglement with the classical environment, which spreads out at the speed of light.
> We don't understand what constitutes a measurement, and so we don't know if two perfectly isolated observers could disagree on the "state" of a system in this way (as in, collapsed or not collapsed)
We do know that they can't disagree as collapsed particles behave differently than uncollapsed particles.
> It's just that one observer is unsure of what result they will get for a subsequent experiment, while the other observer can know it exactly.
Quantum systems aren't about knowledge or statistics, the wave function is an actual physical thing that changes how the particle behaves. If the wave function has collapsed it no longer behaves the same as before, so what you said here is wrong.
After a measurement, the wave function of a particle, and of any particles entangled with it, collapses. But this isn't itself a measurable difference.
For example, when performing measurements on two entangled particles at very far away places, the wave function collapses instantly, across any distance, even light years away. After that experiment, the result on the other side is 100% determined (if performed in the same basis). But there is no experiment whatsoever that could be done to tell if the collapse has happened or not from the other side.
This is the resolution of the famous EPR paradox: wave function collapse is non-local (instantaneous), but it is probably impossible to send classical information with it faster than the speed of light limit.
You're now arguing against claims no one has made, which is apparent from the fact that you seem to only quite very tiny snippets of what's said while ignoring the main argument.
You seem to just want to argue for the sake of arguing.
I provided my explanation for why this is the case. A measurement device only collapses the wave function for itself, it does not collapse the wave function for every other observer/measurement device.
Of course, a second measurement device that's completely isolated from the first measurement device can eventually interact with the first measurement device and the two will converge and share the same wave function. However, until the second measurement device interacts with the first one or with the original quantum system being observed, then from the point of view of the second measurement device, the first measurement device is in a superposition of all the possible states that it could have observed.
This can continue on and on, with a third measurement device which is entirely isolated from the second measurement device having a wavefunction that's a superposition of all the possible observations that the second measurement device will observer...
> A measurement device only collapses the wave function for itself, it does not collapse the wave function for every other observer/measurement device.
Wave function collapse is a real measurable phenomena, what you describe is not measurable so just a belief, don't mix in your beliefs with actual results.
Wave function collapse is one of many equivalent (so far as we know) interpretations of quantum mechanics and not all interpretations have wave function collapse whatsoever. With that said even in the Copenhagen interpretation which is the most popular interpretation of quantum mechanics, the moment in time when the wave function collapses is relative to each measurement device. It is not the case that just because some measurement device that has been entirely isolated from me happens to perform a measurement on a quantum system, that my wave function of that quantum system will also collapse. On the contrary my wave function remains exactly as is until the moment I perform a measurement on the quantum system which collapses my wave function of said system.
Catch 22: and if there is no air inside the box, then you are certain the cat is dead.
(somewhat related) There are two infinities operating in QM when systems get big.
https://youtu.be/FrTq_m1pLz8?t=2188
Quantum mechanics forces two infinities upon us when attempting to make precise observations. The first is the need for infinitely many measurements, stemming from the probabilistic nature of quantum predictions. To obtain a sharp notion of probability, we must perform an experiment infinitely often, converging to the true probability in the limit. The second infinity involves an infinitely large measuring apparatus. This arises because any finite measuring device is itself subject to quantum fluctuations, introducing an intrinsic imprecision to measurements. The degree of imprecision scales as e^(-n), where n is the number of particles composing the measuring device. While the first infinity is often discussed and practically relevant in experimental settings, the second is less commonly addressed but conceptually significant. These infinities highlight fundamental limitations in our ability to make precise quantum measurements and become particularly problematic when gravity is introduced into the picture.
This seems to predicate what does or doesnt happen on the awareness of sentient observers. What is the evidence for that and why would the cats fate depend on whether anyone (or itself) is monitoring it in any sense of that word?
The cat isn't in some quantum state. Cat is locked in a box, with some poison and a Geiger counter. Poison is released when Geiger counter registers a particle.
Now, the process of atom decay that would be registered by a Geiger counter has some interesting properties and that's where the "quantum magic" happens.
(at least, that's my mental model of it; ask an expert, I'm not an expert)
the idea is that everything is in some quantum state, because quantum mechanics attempts to describe everything. The thought experiment is basically going from the well-observed (and explained in theory) superposition of a nuclear undergoing decay to the never-observed (but still valid in theory) superposition of a cat being both alive and dead.
Why should it be valid in theory tho? What is the purported bottleneck in physical reality that is supposed to exist whose presence makes the Schrodinger cat thought experiment realistic? How does sentient observation get the ball rolling and why couldn't that happen in the absence of such observation at this scale?
The quantum magic properties are thought to transfer to the poison and the cat.
Schrödinger's thought experiment was supposed to demonstrate that superposition itself (and/or the whole of the Copenhagen interpretation) was a nonsense concept… but every test so far says it's real, that he was wrong, and that the cat would be in both states "until observed".
But nobody knows what "observed" even means.
As far as known physics goes, nothing is too big to be subject to quantum mechanics.
It is very hard to demonstrate that fact though, the obvious way of doing it is to build a large quantum computer.
Schrödinger never believed it; he gave that as an example of something that could never happen in real life.
Cat observes itself.
That defeats the point of the exercise. The whole point was about the state of some isolated system. Otherwise we could say that the detector observes the state of the system without even needing the poison or the car
It doesn't defeat the point of the exercise, it just shows that the exercise is nonsense. The cat is an observer therefore collapses the quantum wave function and is either alive or dead.
In what sense the cat is an observer? Having eyes and a brain doesn't really mean anything in QM.
If the particle interacts, that collapses the superposition. Cats aren't any more or less blessed as observers than physicists. If either of them can observe the outcome, then both can.
Hmm. Maybe what "counts as an observer" is relative to your reference frame, and so is the concept of "superposition". And thus, though the cat is making observations within its environment, since the environment is sealed off, the cat is not "an observer" from our reference frame because there is no interaction between us and the cat. From the cat's reference frame it is probably not in a superposition - If it is observing anything, it is alive, and if it is dead, it is not observing anything. But if we observe the cat, the wave function is collapsed from our reference frame and the cat is in one or the other state.
My understanding of quantum mechanics is pretty shallow so take this with an ocean of salt.
That would mean we are all alive and dead.
If the particle interacts, that collapses the superposition.
Not at all, the superposition spreads to the interacting particles. What destroys superpositions are measurements and we do not know what measurements really are.
We do know what measurements are. "Superposition" and "collapse" are only really valid concepts in a particular reference frame. A "measurement" is an interaction in your preferred reference frame. The cat cannot "collapse" anything from your perpective - only its own. Indeed the cat itself does not experience superposition.
You can't measure something without interacting with it. So either it's the interaction that collapses it, or it's some sort of soul mumbo jumbo. If it's somehow the latter, there's no justification beyond shear anthropocentrism to suppose that physicists can do it but cats can't.
Therefore, the cat observes itself.
Doesn't necessarily tell you anything about what you would observe. The cat's observations of itself would still be in superposition in theory.
> The cat's observations of itself would still be in superposition in theory.
Not anymore, not according to this theory.
No, the results of this paper are still 100% compatible with the interpretation that the cat is in a superposition: all it says is that the two states (cat alive and cat dead) can't interact with each other at all (or more specifically, can't interact in a way that's distinguishable from random noise).