# Schrödinger's cat

Discussion in 'Physics & Math' started by fess, Jan 30, 2020.

1. ### DaveC426913Valued Senior Member

Messages:
15,208
A superposed object need not be recorded to have its wave function collapse.

Modern understanding is that a superposed object merely needs to interact (say, with other atoms) to collapse the wave function.

3. ### arfa branecall me arfValued Senior Member

Messages:
7,150
When the human observer opens the box, the cat is dead or alive; the glass vial of poison gas is broken or whole; the mechanism that breaks the vial is activated or not. In each case, classical information about the states of classical objects, is copied from one classical object to another.

But we know that quantum information can't be copied. The information in the potential decay of a radioactive sample cannot be copied. Only one of the classical measurement devices can store this hidden information (even when the box itself hides information about states). After this, the information can be copied precisely because it's classical.

All opening the box does, then, is copy some information about classical states (dead or alive, broken or not). If the cat is alive, then the vial isn't broken and the mechanism that breaks the vial isn't activated; the non-decayed sample is also in a classical 'state' because of what you, the cat, the vial and the mechanism also know, type of thing.

5. ### phytiRegistered Senior Member

Messages:
472
How would you know that, without an observation?
My point is, quantum physics has not introduced anything new, just different. It still reduces to ignorance, not understanding how/why the universe works at some level of detail. If someone knocks on your door, you have to look to see who it is (or check your security system).

7. ### QuarkHeadRemedial Math StudentValued Senior Member

Messages:
1,715
objects are not superposed, states are.

8. ### DaveC426913Valued Senior Member

Messages:
15,208
It's not a simple explanation. There are books written on the subject.
But you can't really argue against it without studying up on it.

9. ### CptBorkValued Senior Member

Messages:
6,355
According to the Quantum Decoherence interpretation of wave mechanics, all observers will see both a living and dead cat when the box is opened. However, the portion of the observer wavefunctions seeing a living cat will be almost completely disjoint from the portion of their wavefunctions observing a dead cat, effectively resulting in the establishment of two separate universes, each displaying a classical result.

10. ### Q-reeusValued Senior Member

Messages:
4,083
Decoherence per se is a mechanism/framework applied to various interpretations, not itself an interpretation of QM.
Decoherent Histories aka Consistent Histories is an interpretation - with various sub-interpretations within that fold.
https://plato.stanford.edu/entries/qm-decoherence/

I'll repeat from earlier posts - the notion an exceedingly complex and continuously internally evolving (on multiple hierarchical levels) entity 'cat' could ever be thought of as existing in a well defined QM 'state' is crazy thinking. The cat is intrinsically classical. Dead or alive.

11. ### CptBorkValued Senior Member

Messages:
6,355
Actually to put that to a real test you'd need to isolate the cat from all external interactions, including gravity, but I'll agree that even most physicists subscribing to the Copenhagen interpretation would assert that the cat's atoms would constantly "observe" each other and it doesn't end up getting spooked by its own corpse.

12. ### James RJust this guy, you know?Staff Member

Messages:
35,554
Sure, but I doubt that you truly believe that the Eiffel tower is both standing and collapsed at the same time. You may not know its current state, but I'm fairly sure you believe it has one - more specifically, that you'll always find the Eiffel tower in a regular, "classical" state, and never in a quantum superposition.

I don't think you can just brush the problem under the carpet by pretending that quantum superpositions are just like classical states that we have limited information about. They demonstrably don't behave like classical states.

13. ### phytiRegistered Senior Member

Messages:
472
That's my point, exactly. Our knowledge of a system may be expressed as probabilities, quantum physics, weather, gaming, etc. What we think does not influence the material world, no more than a fast moving spacecraft alters the rate of a distant clock, or the dimensions of the universe.
In the period about 2000, the missing solar neutrinos problem was solved. It offered an opportunity for multiple superposition of states, but was explained with transformations.
The classical view of, you have to look/measure to know still works because that's how the mind works. Every new scientific discovery only shows the degree of ignorance for human thinking. Another option is to go down the 'yellow brick road'!

Last edited: Feb 13, 2020
14. ### CptBorkValued Senior Member

Messages:
6,355
It's not just a matter of humans not having all the information about a quantum system, it's a matter of nature itself not deciding on the outcome until it's measured or a wavefunction collapses on its own, as demonstrated by multiple different experiments (in the quantum decoherence multiverse interpretation, nature allows every outcome in its own universe, but that still makes it impossible to know which of those universes we now inhabit without measuring).

15. ### phytiRegistered Senior Member

Messages:
472
I don't believe in 'mother nature' or 'father time', nor the personification of inanimate matter. The nova occurs, not because someone was star gazing, but because the object reached an unstable state, in terms of all the physical laws that are in place, regulating its behavior.
In eating at my favorite restaurant, ordering fish instead of beef, doesn't put me in a different universe. It gives me a modified/alternate history resulting from my choice.
There are many possibilities but only one outcome.
Science has enough of a challenge to discover how a single universe works, why complicate it. I compare that to an incompetent weatherman, who predicts all types of weather, expecting one of them to occur, thus verification of his forecast.
Examining scientific theories and experimentation, over its history, the trend is echoed in this statement, "it's more complicated than we originally thought'. Theories are made and theories are revised. All the mental concepts, including the wave function, are attempts to explain the world in terms the human mind can comprehend, thus they are naive and overly simplistic.
The earth is a perfect sphere. No, it's an ellipsoid, since it bulges at the equator. No, it's pear shaped, since the northern hemisphere has a different distribution of mass then the southern hemisphere.
The (invisible) planetary orbits were modified with increasing complexity.
Forming knowledge is a continuous process of refinement.
The truth is, no one knows how the universe works, and just make it up as they go.

16. ### arfa branecall me arfValued Senior Member

Messages:
7,150
In modern information-theoretic interpretations of QM (viz quantum computers at IBM and maybe elsewhere), a multiverse isn't necessary.
In order for there to be some computation, quantum states have to be 'manipulated'; you can do this to a state and not need to know (even in principle) how it changes. You have the information that a state has been manipulated. The existence of an alternative universe where the state was not manipulated isn't relevant to the problem.

A state manipulation is an interaction, manipulating a state so it becomes classical information is therefore a computation (aka a measurement). A computation by definition transforms some input into a different output (usually). A 'computation' that doesn't transform information is a communication of same. Classically we have the freedom to encode information how we like, transmit it, then decode it; this leaves the original information (or a copy of it) unchanged--encoding and decoding is an identity operation on information.

What about quantum encoding? There isn't the same freedom; we have to accept the encoding nature hands us.

And I note this helpful hint from IBM for wannabe quantum programmers: In a QC with n qubits, the qubits' locations are all classical. Each of the n qubits 'knows' there are n-1 other qubits in a classical position (type of thing). When you turn a QC on, n qubits are in an unknown entangled state (of positions). That's why it helps to be able to tell one from the other . . . (quantum joke)

Last edited: Feb 16, 2020
17. ### arfa branecall me arfValued Senior Member

Messages:
7,150
I found some of the explanations in stuff I've read, about what's different about quantum manipulations and classical ones some help, but I can't say it's the definitive approach.

Engineers ignore a lot of theoretical physics when they build quantum (or even classical, these days) computers. They tend to whittle it down to what's useful (no shit?); in this case useful means a useful set of 'quantum' manipulations, generally between pairs of qubits, via (erm, completely esoteric) quantum gates. A quantum gate is perhaps another way to represent a pair of particles interacting at a vertex (a Feynman diagram), without most of the associated gauge fields and all that; just a dash of vector calculus does the trick, in a vector space with two complex dimensions. If you like you can kick in some Dirac notation, say.

To my opening: a big difference is how classical information can be hidden, compared to hidden quantum states. A coin hidden in a box can still be heads or tails, you manipulate the state by shaking the box, now you know something else because of how coins interact with the inside of boxes when you shake them.

But when you switch on a QC (let's say you work for IBM), although you can maybe see n separate cavities so you know about classical positions of qubits, you can't tell one qubit state from another; that's the hidden information in one sense. You perform a sequence of manipulations on a pair of qubits; each manipulation manipulates n qubits, because quantum particles interact when they get near each other. In IBM's 5-qubit online qx machine, the manipulations are essentially signal injections into 5 cavities via a set of waveguides (check out the specs).

Apart from what you or I might learn about what quantum states are by trying to learn how to put a quantum algorithm together, notably just a 5-qubit machine takes up an entire basement, costs an awful lot of time and money and probably needs constant monitoring by a small army of . . . engineers.

18. ### SchmelzerValued Senior Member

Messages:
5,003
It says not even this. It says that the state describing the cat in the box is a superpositional state. In the minimal interpretation, it does not say anything about the cat itself.

Some interpretations say there is nothing beyond the state, thus, the state is the same as the cat. Other, realist interpretations say that the actual state of the cat exists beyond the wave function, and is either a living cat or a dead cat, but the wave function does not contain this information.

19. ### HalcRegistered Senior Member

Messages:
227
While I agree, I'm not sure of the difference between our two wordings (cat in superposition, vs. state of system (with a cat) being in a superpositional state). Yes, I'm aware that the state of the rest of the box is entangled with the cat and thus not just the cat is in some isolated state on its own.

I think there are some realist interpretations that deny a counterfactual state for the cat. Some interpretations for instance posit the wave function itself being real. Just saying not every realist interpretation say it. A counterfactual interpretation like Bohm's would definitely say the cat has a definite state.

My personal choice is RQM, which is not a realist interpretation. Under any local interpretation, it isn't difficult to put the cat into superposition.

20. ### SchmelzerValued Senior Member

Messages:
5,003
The wording .... If we have an interpretation where we have a wave function $\psi(q,t)$ which describes, essentially, the preparation procedure, and an actual configuration $q(t)$, it is not really clear what "state" means. It could indeed mean the quantum state $\psi(q,t)$ as well as the actual state $q(t)$.
There is, of course, MWI. It indeed claims to be realist. But IMHO it does not even count as an interpretation at all. It is simply ill-defined pseudo-science. Whenever I have asked for some precise definitions for the objects they talk about the answer was silence.
I see no justification for giving up realism as long as realistic interpretations exist. Moreover, one would have to give up even causality (the classical notion which contains Reichenbach's principle of common cause, not the weak replacement of signal causality) and logic as well (the logic of plausible reasoning a la objective Bayesian interpretation of probability, see Schmelzer, I. (2017). EPR-Bell realism as a part of logic. arxiv:1712.04334

21. ### iceauraValued Senior Member

Messages:
30,845
Causality has already been given up - or rather: relegated to "folk science", the category of useful but unsound heuristic notions and mental shorthands and rules of thumb - in the course of more sophisticated analysis, without any motivation in QED. There is little loss in that.

Logic, meanwhile, fits quantum theory (as it does all of mathematical perception or abstraction) just fine.

22. ### SchmelzerValued Senior Member

Messages:
5,003
That you personally may have given up causality is quite obvious, but that is your personal problem. If somebody else has given up causality, feel free to link some information about this.

23. ### HalcRegistered Senior Member

Messages:
227
Your wording implies that there is an actual state $q(t)$ which means it either has been measured, or that you're asserting counterfactuals. I'm kind of assuming the latter based on the context.
RSF (Everett, '57) has but the one postulate and realism isn't mentioned in it. MWI (DeWitt, 73) seems to add the realism (existence of the universal wavefunction, and ontologically distinct worlds), and I think it has some fatal faults, but what do I know?
OK, your biases are definitely showing. It isn't pseudo-science if it doesn't predict different results than what is observed.
You don't seem open at all, so silence is perhaps them dismissing you. Not sure what you mean by 'the objects they talk about'. What, the cat? 'The cat' is part of the state of system as you close the box. Most common language terms like 'cat' hold meaning for our everyday subjective experience that have no direct correlation to the concepts in Hilbert space.