My point is that there is no superposition of dead and alive. The cat will cause the probability function to collapse long before we open the box. What am I missing here? Isn't the cat capable of collapsing the probability function? If not, why?

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    $\begingroup$ I think that was exactly Schrödinger's point when he proposed this thought experiment. The question he was interested in was: What makes the superposition collapse? Why do cats collapse the superposition and not electrons? $\endgroup$ – Peter Shor Jul 20 '19 at 20:43
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    $\begingroup$ @PeterShor Have you a reference showing that was his intent ? $\endgroup$ – StephenG Jul 20 '19 at 20:59
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    $\begingroup$ If a cat could collapse the wavefunction, then the contents of a perfectly isolated box containing a cat would not evolve according to the Schrödinger equation. So, the rules of physics would have to be such that if some configurations of atoms happens to form a cat (or some other animal), then the rules for time evolution would be different. But you can't really have different fundamental evolution rules for different states, especially given that in QM you can expand a state in terms of other states, so some generic state could have a component containing a cat. $\endgroup$ – Count Iblis Jul 20 '19 at 20:59
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    $\begingroup$ @StephenG: From Wikipedia: "Schrödinger did not wish to promote the idea of dead-and-alive cats as a serious possibility; on the contrary, he intended the example to illustrate the absurdity of the existing view of quantum mechanics." You can also read a translation of the relevant part of Schrödinger's original article. $\endgroup$ – Peter Shor Jul 20 '19 at 21:03
  • $\begingroup$ The cat will collapse the wave function of what? Until a human opens the box, wrt the human, the cat + poison system will be in a superposition of dead cat + poison activated and alive cat + poison not activated. It is really important as to with respect to whom we are talking about the wave function. See Everett's original thesis: www-tc.pbs.org/wgbh/nova/manyworlds/pdf/dissertation.pdf. You don't have to buy into the Many Worlds "Interpretation" to appreciate this point. For example, see the introductory section of the linked thesis. $\endgroup$ – Dvij D.C. Jul 22 '19 at 1:28

A theorem of von Neumann says that it doesn't make a bit of difference whether you model the cat (or anything else along the causal chain between closing the box and opening it to observe the cat) as capable of collapsing the wave function. You'll make exactly the same testable predictions no matter where along the way you place the collapse.

(The argument occupies the final chapter of this book, but recreating it is not a terribly difficult exercise.)

So feel free to posit that the cat collapses the wave function. Or to posit that only a human has that power. And if you prefer one story while your neighbor prefers another, let a thousand flowers bloom.


According to the Many Worlds view, no cat, nor even a human scientist inside the box, will "collapse the wavefunction". The scientist, the cat, and the radioactive particle are all components of the universal wavefunction, which simply branches when the radioactive particle both decays and does not decay. To an observer outside who can't see inside the box or interact in any way with what is in the box, the scientist both dies with the cat and survives with the cat, until the observer peeks inside the box. At that point, the outside observer's wavefunction branches because it has become correlated with the wavefunction of all that's inside the box. The idea that "consciousness" causes wavefunction collapse has no meaning. The scientist who subjects himself to the cat's fate is conscious (aware) in one branch that he survived; in the other branch he might briefly be conscious/aware that he is dying.

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    $\begingroup$ The idea that "consciousness" causes wavefunction collapse has no meaning I agree with that. I'm just not sure whether the Many Worlds view has enough evidence to be taken as the explanation for this. Is there a proof that, like you said, no entity collapses the wave function? $\endgroup$ – Eddie Bravo Jul 20 '19 at 21:08
  • $\begingroup$ @EddieBravo I'm just not sure whether the Many Worlds view has enough evidence to be taken as the explanation for this. There currently isn't any way to distinguish between many of the interpretations of QM. If your issue comes down to that then it won't be resolved, at least not currently. $\endgroup$ – BioPhysicist Jul 20 '19 at 21:38
  • $\begingroup$ @EddieBravo there are some testable variations of objective collapse triggered by various conditions such as enough mass. So far all observation is consistent with there being no such phenomenon, despite experiments with getting increasingly bigger objects (entire mitochondria) into superpositions. $\endgroup$ – John Dvorak Jul 21 '19 at 11:27
  • $\begingroup$ @JohnDvorak An superposition experiment with a cantilever has been successful, but a quick search doesn't reveal any results from a decade-old proposal to create superposition in viruses --- let alone a mitochondrion. Do you have a link to support that? I love being corrected. $\endgroup$ – rob Jul 21 '19 at 15:00
  • $\begingroup$ It's been quite some time since I heard about the experiment and I can't find an article about it right now... so I guess shooting fullerenes through a double slit will have to do until I do. $\endgroup$ – John Dvorak Jul 21 '19 at 15:16

This is why I generally prefer a "mostly subjective" viewpoint of quantum mechanics as it is really, despite looking at all the alternatives, the only one that fits the closest to the mathematics of the theory as given with no other adulterations (other ideas like MWI, Bohm, etc. really are "different theories" in that they play with the maths and seem to have a fixation on eliminating and explaining away the collapse concept, and they thus really are "empirically equivalent theories" [except when they're not!]. My thinking has been that we need to take it blunt, at face value, and see where that leads.).

On a subjective account, the wave function belongs to you, the one outside of the box. It models, your information or knowledge about the state of affairs in the box. The transition from "live cat" to "live or dead cat" to "dead cat" starting from the initial state is just showing how your best knowledge, without looking into the box, that you can predict from that initial state, changes. All we can say about the "superposition" at the in-between point is that it means the predicted information about the answer to the question "is the cat alive?" is less than one bit.

That said, the theory does force us to admit that there really is something "odd" going on "in reality", otherwise it would just be doable with classical mechanics. But that "oddity" is more that the Universe seems to have an information limit that prevents the answers to all questions about a system from existing with perfect information at all times. That is, in some real cases, there really must be less than one bit of information, say, to some yes/no questions.

Problems identified with the subjectivity of the wave function in literature seem to be hung up on the idea that if you take it as subjective, you are taking it as subjective with some further assumptions on what the "real" reality should look like that often amount to sneaking classical mechanics in the back door, instead of letting the maths guide you as to what you can/can't say thereabout - which is that, if we make no such further assumptions, except perhaps relativistic causality, then you have to say that however it exists, physical parameters have "limited resolution" - limited, even fractions of a bit of, information.

In particular, the "real" information at the point in between queries ("measurements") must lie somewhere between the absolute maximum, as established by the Heisenberg uncertainty principle, and the level given by the Schroedinger equation, which may be somewhat less - the latter being, in effect, what @WillO's answer is one way of saying. There are endless scenarios we can imagine for the in-between that we would not be able to falsify.

As a subject, the cat may be assigned a wave function talking about the information it has regarding the contraption that is going to kill it. Of course, soon after that one "collapses" then there won't be any more wave function any more because this subject, the information-bearer, has been terminated.

Hence, from that point of view, it makes no sense to ask this question as it is posed because the wave function in question models your knowledge, not the cat's. The cat can't do anything to that. Well, maybe it can - it lets out one final scream as it dies, you hear that through the box, and update your knowledge accordingly :)


In my opinion such an experiment cannot actually be performed because the assumption that a box exists so that the macroscopic states of the objects inside the box is hidden from the exterior is false.

The state of the cat (dead or alive) has observable consequences outside the box, no matter what the material of the box is. For example, the gravitational field of a moving, breathing cat is different from the gravitational field of a dead cat. This will influence the way the objects outside the box move. If an external observer assumes that the cat is alive or in a superposition dead/alive but the cat inside is dead he will make wrong predictions about the motions of other objects he can observe.

So, in my opinion, the box is an impossible construct that has no consequences on what QM predicts. If the cat dies, it is dead for all observers, everywhere.

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    $\begingroup$ Gravitational field of almost any cluster of particles is different in different states of this cluster, but this doesn't break superposition by itself. See also Why doesn't gravity act as a measurement? $\endgroup$ – Ruslan Jul 22 '19 at 5:26
  • $\begingroup$ I do not think that the discussion you linked addresses my argument. I do not claim that objects "measure themselves" because of gravity. I claim that: 1. Gravity cannot be shielded by any box. 2. The gravitational field outside the box provides information about the mass distribution inside the box. 3. The gravitational field outside the box can be measured (within the limits of the uncertainty principle) using a torsion balance for example. From 1,2,3 it follows that the outside observer knows what is in the box, so he will "collapse" the state just like the observer inside. $\endgroup$ – Andrei Jul 22 '19 at 6:09
  • $\begingroup$ In fact, Motl agreed that " if you measure sufficient information about gravitational waves encoding the position, it will "collapse" but you likely won't". I do not understand why the discussion evolved around gravitational waves. A direct measurement of gravitational field with something like a torsion balance is perfectly doable for a cat-sized object, so, as long as such an instrument is located outside the box there can be no superposition inside. On the other hand any massive object can play the role of the torsion balance, so, I think, such superposition cannot actually exist. $\endgroup$ – Andrei Jul 22 '19 at 6:35
  • $\begingroup$ This objection can be redirected to the fullerenes in the fullerene diffraction experiment. Each of them has gravitational field depending on the position, yet if the superposition couldn't exist the experiment would fail. $\endgroup$ – Ruslan Jul 22 '19 at 9:58
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    $\begingroup$ You can't just dismiss a thought experiment because it's impossible to perform. Why does the state of a cat have observable consequences outside the box, when the state of an atom does not (otherwise you wouldn't be able to put electrons in superposition). Why exactly is there a difference between cats and atoms? This was what Schroedinger was really trying to get at with his thought experiment. $\endgroup$ – Peter Shor Aug 6 '19 at 22:17

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