Since it is uncertain on whether or not the cat could be dead or alive, it is concluded that it depends on the observer to make it either dead or alive. But lets look at a clock. It doesn't take observance of the gears to make it function. The clock ticks despite there being an observer. Could this mean that the famous thought experiment is false?

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    How do you know the clock ticks if you don't observe it? – velut luna Jun 11 '14 at 0:51
  • The clock ticks, and? My cat (called Erwin, BTW) doesn't need to be observed to be alive, just fed; and yet, it does not invalidate the argument. – Davidmh Jun 11 '14 at 0:51
  • @user139981 I'm talking about not observing the gears that make the clock tick. We can see the hands but not the fundamental gears – user50282 Jun 11 '14 at 1:14
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    Put your clock in a box and measure the observable: O = |broken><functioning| + |functioning><broken| – Count Iblis Jun 11 '14 at 2:06
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    @user50282: Notice that actually Schrodinger constructed his famous thought experiment with a cat not to prove the Copenhagen Interpretation of quantum mechanics but to disprove it. The Schrodinger's cat is an example of reductio ad absurdum. Schrodinger made important contributions to the theory, but he disagreed with the concept that measurements changes reality. See here: en.wikipedia.org/wiki/Erwin_Schr%C3%B6dinger – bright magus Jun 12 '14 at 8:37

"Since it is uncertain on whether or not the cat could be dead or alive, it is concluded that it depends on the observer to make it either dead or alive."

That would certainly be atrociously bad "reasoning", but that was not Schrodinger's argument at all. He introduced the cat thought-experiment as an attack on the idea that when a particle's wavefunction is spread out in space, the particle itself is objectively smeared across space in some sense.

In the cat thought-experiment, whether the cat lives or dies depends on the interaction of the killing device with a subatomic wavefunction, and Schrodinger's wavefunction equation implies that the smearing or superposition of the subatomic wavefunction, would be passed on to the cat's own wavefunction. So if the subatomic idea was also applied to the cat, then the cat would also be alive and dead at the same time - which he regarded as obviously absurd, and therefore a rebuttal of the idea that it made sense to talk about the subatomic world in that way.

The actual attitude of people like Bohr and Heisenberg was that wavefunctions are not physical entities at all, they are just calculating devices which give probabilities for observations. And then Heisenberg, especially, said it was unscientific to care about what happened between observations (positivism), so people should just be happy that the quantum equations gave the right results, even though they don't offer a picture of what happens when people "aren't looking".

Since then, among people dissatisfied with positivistic agnosticism about the reality beyond the equations, two approaches stand out. One is the attempt to make a theory beyond quantum mechanics - often called a "hidden variables" theory. The other is the Everett many-worlds theory, which says the cat really is alive and dead, but in different "worlds".

But there are a lot of people who have muddled views about reality and observation, like the view you summarized as "it depends on the observer to make it dead or alive". These people are sort of halfway between the pure positivism of Bohr and Heisenberg (which is not really a complete theory of reality, but at least it is a self-consistent attitude), and the return to objective physics (as in theories like hidden variables and many worlds). But it's philosophically confused, mixing up ontology (what is reality) and epistemology (how you know about reality).

Or at least, it's usually a sign of philosophical confusion when an ordinary physicist says something like this. If they actually believe that mind creates reality, like in a dream, or if they have a hidden variables theory in which the physical process of measurement drives the observed system into a new state (Bohmian mechanics is like this)... then I guess the statement that "observer creates reality" could be logically defensible. But the version you are rebutting, obviously doesn't make sense.

  • The claim that "hidden variables" and Everett many-worlds theory are "two approaches that stand out" is a bit strange (IMHO). Isn't the first theory (hidden variables) something that has been disproved long ago by Bell inequalities, and the second one just one of many "interpretations"? – Lou Apr 7 '16 at 12:08

That the clock functions implies that the wavefunction of a perfectly isolated clock would evolve almost deterministically. That's why Schrodinger chose a different setup involving a radioactive decay that can happen with a probability of 1/2.

I think you misunderstand the purpose of Schroedinger's Cat as a thought experiment. The idea was to show the ridiculousness and weakness of the Copenhagen interpretation of quantum mechanics, not to present a realistic scenario.

All the thought experiment is saying (and its not flawless obviously its just an analogy and analogies are not perfect) is that the cat is in both states of alive and dead because you haven't made the measurement yet. Schrodinger was giving a simplified version of whats going on and demonstrated the weirdness with this thought experiment. All that is demonstrated in other words is the quantum weirdness with macroscopic/everyday objects.

Since it is uncertain on whether or not the cat could be dead or alive, it is concluded that it depends on the observer to make it either dead or alive.

In this thought experiment the cat and the poison mechanism are used as a detector of the quantum mechanical state that will release the poison according to quantum mechanical probabilities. In a similar way, the detectors in high energy physics experiments register on disks the macr0scopic measurements from quantum mechanical reactions that happened during the collision of two particles. Before studying the data we do not know if the Higgs is there or not.

In the answer of @Mitchell Porter the logical confusions are cleared up as far as the cat experiment goes.

But lets look at a clock. It doesn't take observance of the gears to make it function. The clock ticks despite there being an observer.

The clock is a macroscopic object connected to the deep quantum mechanical framework, as all of us, via the density matrix formalism which, for the enormous number of quantum mechanical entities composing macroscopic objects ~10^23, is practically diagonal. The classical terminology of "observation" can be safely attributed to observing a clock. Therefore it is apples and oranges

Could this mean that the famous thought experiment is false?

and could not be used to falsify the thought experiment.

The clock has a well-defined, predictable behavior. If we assume a clock in known good working order, we don't have to observe each tick to know each has occurred. The nature of the phenomenon to be left unobserved has no randomly choosable state.

Schroedinger's cat, on the other hand, has been placed in an indeterminate, indeed superposed state because its fate will be decided by the entirely random event of an atomic decay. Its fate is unknowable without direct observation.

  • The cat is directly observing itself. – Count Iblis Jun 11 '14 at 2:30
  • not if it's dead – DavePhD Jun 11 '14 at 2:53

Whether the cat is dead or alive - from the observers point of view - does depend upon observation. Whether the cat is in fact dead or alive does not depend upon observation. As something cannot be simultaneously dead and alive this thought experiment can only be a negative critique. Observation or non-observation has no bearing on the thing. So a star may be observed but in reality may no longer be there due to travel of light. An image in a mirror shows the thing but the thing is not 'there' at the position of the mirror. 'Anything' could be happening outside the unobserved closed door. So observation itself is unreliable and unconnected with the actual state of the thing.

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