My question is quite simple. In the thought experiment of Schroedinger's cat: When the scientist measures the state of the cat, its wavefunction collapses into either the alive or dead state. But wouldn't then the scientist in turn be in a superposition of measuring dead respectively alive until someone opens the door to the laboratory and asks the scientist about the outcome of the experiment (and therefore measures the state of the system)?

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    $\begingroup$ I deleted an off-topic comment thread. EDIT: and another batch of off-topic comments. Everyone please remember that comments are meant for suggesting improvements to the question, including by requesting clarification, or for linking to related resources. $\endgroup$ – David Z Jul 6 '16 at 19:52
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    $\begingroup$ Your question is exactly equal to the Wigner's friend thought experiment. $\endgroup$ – Emilio Pisanty Jul 7 '16 at 9:27
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    $\begingroup$ You just wrote an xkcd comic! I'm seeing Wigner's friend's friend's uncle's dog simultaneously getting and not getting his dinner based on the living/dead superposition of the whole chain of observers. $\endgroup$ – Todd Wilcox Jul 8 '16 at 15:43
  • $\begingroup$ What if instead of the cat, the box contains a human subject, aware of his/her own existence and an observer of his/her own state of health, at least up until the point the decay trigger fires (if it does at all)? Presumably, there is no superposition inside the box, but the experimenter observing from outside remains in superposition until the box is opened? Is this just a variation on Wigner's friend? $\endgroup$ – Anthony X Jul 9 '16 at 17:50

This is is known as the Wigner's friend thought experiment. According to the many World's interpretation, the superpositions are not a problem. The whole universe ends up in a superposition where all experimental outcomes are realized, but such a superposition is entangled with the environment, from a macroscopic point of view it takes the form of a superposition over a set of consistent histories.


In a bubble chamber experiment, film was the detecting medium, but film was taken automatically, by the thousands of frames. These bobbins of film went to the various laboratories involved in the experiment, and were scanned for interesting events which were measured and the cross sections for the interactions recorded.

This is a clear example of an experimental measurement of a quantum mechanical level interaction . It is easy to see where the quantum mechanical level ends and the classical regime begins. There has never been a doubt whether the events recorded on the films were there or disappeared at the next scanning, (i.e. had a quantum mechanical probability to have disappeared). The film record is demonstrably permanent and therefore classical. The cat thought experiment could be carried out in a laboratory easily (fortunately no sadist has done so) . In what follows I make the analogy film - cat, pointing up where the quantum mechanical regime to be studied is and where the recording classical regime.

Here is the first Omega minus seen, the cherry at the bottom of the decuplet pyramid, a prediction of the quark model.

omega minus

The bubble chamber picture of the first omega-minus. An incoming K- meson interacts with a proton in the liquid hydrogen of the bubble chamber and produces an omega-minus, a K° and a K+ meson which all decay into other particles. Neutral particles which produce no tracks in the chamber are shown by dashed lines. The presence and properties of the neutral particles are established by analysis of the tracks of their charged decay products and application of the laws of conservation of mass and energy.

The omega minus was there from the exposure of the film. That is the detector. Any scanner will find it again and again, because the film is not a quantum mechanical state. It is a macroscopic classical detector. The quantum mechanical state was when the K- interacted with a proton in the bubble chamber, which had a probability of manifesting in the chamber. The recording of the event is detection.

In an analogous way, the cat in this (misguided in my opinion) thought experiment is the detector, the film, on which the experiment with the radioactive probabilities is recorded. It is the radioactive probabilities that are in a quantum mechanical state. The cat is the film. Until looked at, similar to the film being blank or recorded with an event, it is guessed to be dead or alive. Whatever it is, is decided by the quantum mechanical probability, and will be a stable solution of the problem.

Let us see more detail, from the wikipedia article:

One can even set up quite ridiculous cases. A cat is penned up in a steel chamber, along with the following device (which must be secured against direct interference by the cat): in a Geiger counter, there is a tiny bit of radioactive substance, so small, that perhaps in the course of the hour one of the atoms decays, but also, with equal probability, perhaps none; if it happens, the counter tube discharges and through a relay releases a hammer that shatters a small flask of hydrocyanic acid. If one has left this entire system to itself for an hour, one would say that the cat still lives if meanwhile no atom has decayed. The first atomic decay would have poisoned it.

The quantum mechanical probability in this version, is the probability that the radioactive substance will decay within the hour. This probability is controlled by the quantum mechanical wave function of the nucleus, (the analogue of the K- proton interaction) multiplied by the number of nuclei in the radioactive sample. The further steps in the experimental setup are macroscopic: the trigger of discharging the tube, the breaking of the poison and the cat are the detector setup (analogue of hydrogen bubble chamber, camera, film). The cat is the film because it is the permanent record of what happened within that hour (analogue of a film frame being a permanent record of whether there is an interaction or not).

Please note that the cat is a superfluous sacrifice to a vague analogy. The counter would have been enough if it was recorded on an audio tape. The cat takes the place of the tape. The quantum mechanical probability function controlling the event is the original decay probability function.

Cat and film are in the classical regime, because h_bar is such a small number that the Heisenberg Uncertainty is always satisfied by macroscopic ensembles, larger than nano dimensions. True quantum mechanical behaviors, i.e. controlled by a probability density, appear macroscopically only in very special situations (as superconductivity for an example).

In general macroscopic objects are in the classical regime, the framework of the density matrix helps acquiring an intuition how the transition happens, between quantum mechanical wave functions and classical dimensions.

  • $\begingroup$ Are the dashed lines quasi particle? Or are the particles that don't interact with the enviroment? $\endgroup$ – HopefullyHelpful Jul 8 '16 at 17:22
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    $\begingroup$ @HopefullyHelpful They are neutral particles, i.e. leave no ionization trace in the chamber, but their decays into charged tracks reveals their existence. $\endgroup$ – anna v Jul 8 '16 at 17:24

It depends of what interpretation of quantum mechanics you are using. By interpretation it is meant that the mathematical predictions of the quantum mechanics formalism are the same, but the philosophical meaning of each is what differs. In the copenhagen interpretation that you seem to describe, the wave function collapses when a conscious observer makes a measurement. Before the measurement the system can be in a superposition of states. It is supposed that the cat is not conscious thus, in that interpretation the wave function does not collapse and the cat is in a superposition of states. If a human opens the box then the function collapses to either live or death. The human observer will not be in a superposition of states.

But this is an old interpretation and the are many others. In particular, in the Bohm interpretation there is no wave function collapse and the cat is in a specific state (either alive or death) regardless of he being observed or not.


This is an excellent question and stresses one of the weird features of quantum mechanics.

Indeed, the scientist would in turn be in a superposition. And we could even measure this if we'd be able to maintain coherence of such large systems.

Ultimately, your question is asking for the solution of the Measurement problem: Why don't we see any superpositions? There are no cats running around half dead and half alive. Neither do we see scientists being in a superposition of having measured this or the other.

However, the measurement problem has remained an unsolved problem. All we have are some interpretations.

  • $\begingroup$ Classical objects are really difficult to place in a super-position. Certainly very large objects such as humans or cats may very well not be possible. So far only micromechanical oscillators have been placed in a QM superposition. There's currently an experiment proposed to place a bacterium in a QM superposition $\endgroup$ – user122066 Jul 6 '16 at 21:56
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    $\begingroup$ The measurement problem is not unsolved. It only comes about because people insist on treating observers non-quantum-mechanically. This was understood a good 40 years ago now. $\endgroup$ – user10851 Jul 7 '16 at 9:13
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    $\begingroup$ This is not quite true. The measurement problem arises precisely because we do not want to treat the observer non-quantum-mechanically. $\endgroup$ – Zetaman Jul 7 '16 at 9:20
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    $\begingroup$ @ChrisWhite In fact it's even better than "not unsolved": we don't just have one solution to the problem, we have multiple! Which is the best? Different physicists have different opinions! Choose your favorite. Until we get better experimental evidence, it's in a superposition of all of them! /s $\endgroup$ – R.M. Jul 7 '16 at 17:46

In Schrodinger's cat experiment, the scientist is assumed to be an "classical observer" of the state of the cat, and thus all observations made by the scientist are assumed to be classical observations adhering to physics as we knew it before quantum mechanics came along. The thought experiment focuses on what sorts of statements about reality that scientist could make which are consistent with the predictions coming from quantum mechanics despite doing some operation (i.e. waveform collapse) to get a classical measurement out of a quantum mechanical system.

If you choose not to presume that the scientist is not a classical observer, but rather an entity bound by merely quantum mechanics, it is reasonable to instead assume that the scientist and the cat (and the radioactive isotope) become entangled by their interaction. This is a completely valid interpretation of QM, it just doesn't explore the philosophical question that Schrodinger's cat was intended to explore.

The philosophical side of the question is also why the topic of consciousness comes up regarding Schrodinger's cat. If you really start digging at the philosophy of science, it all eventually boils down to the question of what does it mean to observe the world, as a conscious living being. When we think about "observations" intuitively, disregarding QM for a moment, the observations we make fit well with the "classical" model of the world. They fit rather poorly with the QM world. Thus it is this classical intuitive observer that we need to line up with the data we observe, or we must admit that we have a contradiction in our observations.

On the practical side, it is extremely difficult to get macroscopic objects to exhibit non-trivial quantum behavior because the particles in such objects are typically very decoherent. Thus, they are well modeled by using the Central Limit Theorem, ensuring the variance around the expectation is so small that we can squint and hand wave it away for all intents and purposes.

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    $\begingroup$ The thing is, treating the scientist as a classical observer is wrong. It might be a valid approximation in some cases, but taken too far you end up with ridiculous things like "wavefunction collapse" (which isn't a real thing in physics). I don't know what Schrodinger himself was trying to get across, but we know most of that generation were flat out wrong about much of quantum mechanics so it doesn't really matter. $\endgroup$ – user10851 Jul 7 '16 at 9:12
  • $\begingroup$ @ChrisWhite I think it may actually be a bit more nuanced than that. I would say treating the scientist as a classical observe may be wrong, and is inconsistent with the results from QM. There is nothing that actually proves QM, or any scientific principle. We merely have lots of evidence to suggest that QM is really how the world works. We can't disprove the dualist claim that there is some classical observer deep in the heart of every living thing. Worse, all of our evidence we've gathered to defend QM has been observed by conscious beings, and we don't really know... $\endgroup$ – Cort Ammon Jul 7 '16 at 14:23
  • $\begingroup$ ... what that fully means. However, it does mean all of the mathematical probabilities used to defend QM are based off of this concept of us making sense of the world we observe with our "senses." Those fundamental underpinnings of science could actually be wrong! (and are wrong in the case of philosophical questions such as Laplace's Daemon). I see Schrodinger's cat as an attempt to span that great divide and connect QM with the philosophy of science, and that's no small task. $\endgroup$ – Cort Ammon Jul 7 '16 at 14:26
  • $\begingroup$ Sorry, Chris, but the observer has to treat himself (or at least the parts of his brain that evaluate the information) as a classical object because that's a basic postulate of quantum mechanics, something that is absolutely essential for quantum mechanics to work at all. The brain of someone else may be treated just like any other observable external bound state of particles but unless there is some observer at the end who treats himself as a classical object, no prediction of the laws of physics may ever be verified. To fight against the need for observers means to misunderstand 100% of QM. $\endgroup$ – Luboš Motl Jul 8 '16 at 16:22
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    $\begingroup$ @LubošMotl Must a prediction of the laws of physics be verified for those predictions to be true? This is a question that digs deep at the heart of the philosophy of science. Such questions ring similarly to the question of whether mathematics is the foundation of our reality, or whether mathematics was constructed to make sense of our reality. $\endgroup$ – Cort Ammon Jul 8 '16 at 18:07

Large classical objects such as cats or humans are next to impossible to place in a superposition of states. They basically decohere immediately.

As far as i know the largest object placed in a superposition is a micromechanical resonator and there is currently work being done on placing a bacterium in a QM superposition ( quantum superposition of a classical object,Schrodinger's bacterium).

My understanding is that it is not the cat that is in a superposition but rather only the radioactive decay. The state of the cat is merely unknown.

At any rate, it comes down to interpretation. Some people may argue S-cat is ridiculous and worthless but the fact is that it holds a special place in physics as well as our culture and thinking on S-cat as one learns and grows can be invaluable to many.

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    $\begingroup$ Saying that the cat decoheres quickly in practice is correct, but that's like saying "there's no such thing as a massless, frictionless string" in response to a freshman mechanics question. $\endgroup$ – user10851 Jul 7 '16 at 9:20
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    $\begingroup$ Totally....but i think S-cat and coherence can be much more confusing than abstracting massless strings. There's also the misconception in QM that the process of looking at something is unique in that QM depends on being a human or some kind of intelligent entity. These distinctions are important to keep in mind just like it's inportant to keep in mind that strings cant really be massless $\endgroup$ – user122066 Jul 7 '16 at 13:31
  • $\begingroup$ The only problem is that both final answers are wrong, @user122066 - Strings can be massless in some theories - scholar.google.com/… - Your other conclusion is even more incorrect: every object may be brought to an arbitrary supersposition state, it's one of the most universal basic postulates of QM. It's true even in practice - a truck with a bacterium in it (and every truck has one) may be brought in practice into the superposition of two macroscopically distinct pure states. $\endgroup$ – Luboš Motl Jul 8 '16 at 16:19
  • $\begingroup$ @LubošMotl has an experiment verified that an object like a truck is in a quantum superposition? Im only familiar with a micromechanical resonator being experimentally realized in a quantum superposition of states. An object the size of a truck being in a superposition would be absolutely amazing amd really really bizarre! $\endgroup$ – user122066 Jul 8 '16 at 16:25
  • $\begingroup$ Yes, I meant a virus, not a bacterium, see nature.com/news/2009/090910/full/news.2009.903.html - the virus was located in a truck (or at least on Earth), so just like the virus, the truck (or Earth) was clearly in a superposition of macroscopically distinct states (because the properties of the virus make a distinguishable impact on the states of the truck, too, since the virus is macroscopic). Every object or system in the Universe may be in any superposition - that's a postulate of QM (absolutely basic law of Nature). $\endgroup$ – Luboš Motl Jul 8 '16 at 16:27

Schrodinger, I take it, was dramatising the epistemological and ontological paradoxes implicit in QM by considering a cat as opposed to a particle contained in a box; and hugely succesfull too, as we're still discussing it almost a century on, and its probably more widely known that his eponymous equation.

Cats, compared to particles, are vastly macroscopic objects; so being classical objects, QM doesn't apply.

If we 'undramatise' the picture, by replacing Schrodingers cat, Wigner and Wigners friend with particles and consider interactions as measurements, then we get an interpretation of QM defended by Rovelli as relational QM; he quotes Simon Kochen in '79 saying:

The basic change in the classical framework which we advocate lies in dropping the assumption of the absoluteness of physical properties of interacting systems… Thus quantum mechanical properties acquire an interactive or relational character.

He does suggest that it has a family resemblence with Everetts many-worlds ontology, but its different in kind as this is relative.

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    $\begingroup$ Careful with statements like "... QM doesn't apply". Do not confuse the model with reality - in reality (to the best of our knowledge), quantum physics always applies. It's just that from certain point (depending on your requirements), its differences from other, more classical models, can be ignored, and you "default to classical physics". But the reality is still quantum, and there's no reason to believe otherwise. Whether you're a cat or a collection of a few atoms, it's the same thing that makes you "stop being (measurably) quantum-like". $\endgroup$ – Luaan Jul 8 '16 at 15:08
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    $\begingroup$ @Luann: Isn't QM already a model, and aren't we applying this model to reality? I'm expecting people to understand what I mean by '...QM doesn't apply', otherwise one ends up being pedantic, and long-winded with lots of qualifiers. $\endgroup$ – Mozibur Ullah Jul 8 '16 at 15:30

The cat has no quantum state, it is a classical object, which, along with everything else in the experiment, is just "overhead" to make it accessible to the general public.

There is the decaying lump of radioactives, and the Geiger counter (detector). This is the quantum level stuff; the rest is just classical embellishment. It makes no sense whatsoever to consider the cat or the whole system to be part of the quantum state, much less so the experimenter who opens the door to find out whether the cat is alive or dead. The cat is certainly not a "detector" in this experiment, it is just the extended "display" of the Geiger detector. That's what Schrödinger wanted to point out.

Schrödinger himself (according to his original, if translated, quote, on Wikipedia) posed his paradoxon quite tongue-in-cheek; take it for what it is, don't try to interpret too much.

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    $\begingroup$ Everything is quantum-mechanical. If you choose to ignore this and pretend there exist magical classical objects in the universe, you will run into contradictions. And whatever Schrodinger thought is irrelevant -- with the knowledge that generation had of quantum mechanics they wouldn't pass an introductory course in the subject today. $\endgroup$ – user10851 Jul 7 '16 at 9:18
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    $\begingroup$ Fair enough, @Chris. That's the point. You cannot take Schrödinger's Paradoxon out of the context of the time and thought processes of Schrödinger, especially his intended meaning, which clearly was to humourously provoke thought, not as a literal experiment procedure. It's highly relevant what Schrödinger thought, the paradoxon carries his name! $\endgroup$ – AnoE Jul 7 '16 at 15:32

For the cat (which is the subject of the experiment), it is irrelevant if you observe it or not. Its state will be the same at a given 't'. Also, that state is not dependent on the observation happening or not (although, opening the door when the cat is alive will influence the cat's behavior, but that is another matter).

Universally, it is the observer that is irrelevant, not that state of the cat that is dual.

Not knowing something does not mean it is both ways simultaneously, but it means it can be one or the other way.

"It's not Schrödinger ... it's the cat !" --> to quote myself.

  • $\begingroup$ Except that quantum superposition has measurable effects on everyday life (bonding energy of benzene or ammonia, for example), so... no. Please, do not take pop-quantum-physics into this. Quantum physics is hard enough to understand as is. $\endgroup$ – Luaan Jul 8 '16 at 15:12
  • $\begingroup$ What is called quantum superposition is miss-interpreted observation. This is a whole different topic, though. $\endgroup$ – Overmind Jul 11 '16 at 7:56
  • $\begingroup$ Okay, good. So what's the alternate explanation that happens to correlate with the above mentioned facts? $\endgroup$ – Luaan Jul 11 '16 at 8:44
  • $\begingroup$ I cannot explain such a matter in just a few words. The practical fact is that today instead of basic physical explanations we have elaborate models based on nothing that practically apply for nothing. I find the tendency of searching for a complex explanation when a simple one is available most illogical. $\endgroup$ – Overmind Jul 11 '16 at 8:56
  • $\begingroup$ Ah, okay. So you're still searching for the classical reality underneath all this quantumy-mumbo-jumbo. It must be there if you just look closely enough! I think you severely underestimate how simple your explanation is, and overestimate how complex is the alternate explanation. Superposition is extremely simple - simpler than any other explanations of the same facts I've ever seen at least. $\endgroup$ – Luaan Jul 11 '16 at 9:20

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