Home » History » The “dead and alive” cat myth

The “dead and alive” cat myth

1995px-schrodingers_cat-svgOne of the most iconic thought experiments used to explain quantum mechanics is called Schrödinger’s cat. And it is usually illustrated the way Wikipedia illustrates it, with a superposition of cats, one dead and one alive:

The article of Wikipedia on the topic is quite clear that the cat may be simultaneously both alive and dead (emphasis mine):

The scenario presents a cat that may be simultaneously both alive and dead,[2][3][4][5][6][7][8] a state known as a quantum superposition, as a result of being linked to a random subatomic event that may or may not occur.

In other words, in this way of presenting the experiment, the entangled state of the cat is ontological. It is reality. In that interpretation, the cat is both alive and dead before you open the box.

This is wrong. And I can prove it.

Schrödinger’s cat experiment doesn’t change if the box is made of glass

I can’t possibly be the first person to notice that Schrödinger’s cat experiment does not change a bit if the box in which the cat resides is made of glass.

Let me illustrate. Let’s say that the radioactive particle killing the cat has a half-life of one hour. In other words, in one hour, half of the particles disintegrate, the other half does not.

Let’s start by doing the original experiment, with a sealed metal box. After one hour, we don’t know if the cat is dead. It has a 50% chance of being dead, 50% chance of being alive. This is the now famous entangled state of the cat, the cat being “simultaneously both alive and dead”. When we open the box, the traditional phraseology is that the wave function “collapses” and we have a cat that is either dead or alive.

But if we instead use a glass box, we can then observe the cat along the way. We see a dead cat, or a live cat, never an entangled state. Yet the outcome of the experiment is exactly the same. After one hour, we have 50% chances of the cat being dead, and 50% of chances of the cat being alive.

If you don’t trust me, simply imagine that you have 1000 boxes with a cat inside. After one hour, you will have roughly 500 dead cats, and 500 cats that are still alive. Yet you can observe any cat at any time in this experiment, and I am pretty positive that it will never be a “cat cloud”, a bizarro superposition of a live cat and a dead one. The “simultaneously both alive and dead” cat is a myth.

Quantum mechanics is what physics becomes when you build it on statistics

What this tells us is that quantum mechanics does not describe what is. It describes what we know. Since you don’t know when individual particles will disintegrate, you cannot predict ahead of time which cats will be alive, which ones will be dead. What you can predict however is the statistical distribution.

And that’s what quantum mechanics does. It helps us rephrase all of physics with statistical distributions. It is a better way to model a world where everything is not as predictable as the trajectory of planets, but where we can still observe and count events.

The collapse of the wave function is nothing mysterious. It is simply the way our knowledge evolves, the way statistical distributions change as we perform experiments and get results. Before you open the box, you have 50% chances of a dead cat, and 50% of a live cat. That’s the “state” not of the universe, but of your knowledge. After you open the box, you have either a dead cat, or a live cat, and your knowledge of the world has “collapsed” onto one of these two statistical distributions.

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17 Comments

  1. pranjal says:

    I don’t think this is correct. In the glass box the photons that reflect off the cat and back to the observer collapse the wave function. In fact, Bell’s theorem shows that the cat does not have some hidden “true” state before we observe it – it is really, fundamentally, in superposition.

  2. No Mail says:

    You are misunderstanding the very basis of the thought experiment. When you see the cat it means that it has been “bombarded” it with photons – that’s why you see it – you see the reflected photons. These photons will collapse the entangled states into just one. The point of it is exactly that when you cannot observe it you don’t know what state it is in. So during that time the cat is both dead and alive.

    • You apparently totally missed the point of this post. You may think that you collapse the wave function of the cat by looking at it, since that’s what everyone repeats. But if that was the case, there would be no uncertainty left regarding whether the cat will be dead or alive when you open the box: a collapsed wave function has no uncertainty. The problem is that the uncertainty remains.

      If the cat is still alive, looking through the box does not impact the probability of said cat to be dead or alive when you open the box at all, since that probability only depends on the (still entangled) state of the particle. Even if you see the cat being alive right now, there is still a chance it will be dead by the time you open the glass box, because the particle disintegrated in the meantime. If you have 1000 boxes, after one hour, there will be 500 dead cats and 500 live cats whether you look at them or not.

      Ergo, looking at the cat does not collapse their wave function, and the cats are never in an entangled state. Only our knowledge about their potential death is in said entangled state. Only probabilities are entangled, not the cat itself.

      • No Mail says:

        Looks like you misunderstand the thought experiment itself. Of course that there is no uncertainty once you open the box! In what sense are you uncertain that the cat is dead or alive when the box is opened? There is no uncertainty there at all.

        When you open the box you will KNOW that cat is dead or alive. Right before you opened the box you did not know that – the cat could have been both. And there is no way to determine that. That is the paradox.

        The whole point of the thought experiment is that at quantum level there is no way to “know” something about a system without perturbing it – and that perturbation of finding out what it is – collapses the state into one of the possibilities. The “deadness” of the cat is just a joke.

        • What I’m saying is that even if the box is made of glass, the uncertainty is exactly the same. In one hour, there will be 50% of chances of the cat being dead, 50% of chances of the cat being alive. Being able to see it won’t change anything about the outcome. Seeing the cat does not collapse the wave function of the particle.

          • No Mail says:

            You are misunderstanding the purpose of the box. It is not to keep the gas in – it is to provide isolation from the external world and its perturbations.

            If you say: “being able to see won’t change the outcome” only means that you don’t understand the very gist of the problem that it is being discussed here.

            That is exactly what the paradox is about.

            Seeing and observing something pushes it into one of the states. Before the observation itself is made the thing exists in both states.

        • Are you arguing that if the box is transparent, then the particle that kills the cat will no longer decay at the same random time, with the same random distribution as when the box is opaque?

        • The whole point of the thought experiment is to convince you that the cat *is* itself in a superimposed state. Schroedinger used that thought experiment as an argument against the Copenhagen interpretation.

          I’m only making it even more obvious by building the alternate experiment where everything is exactly the same, except that you can see in the box. My alternate experiment makes it patently obvious that the cat superposition is a figment of the imagination. It is only a description of what we know about the cat, not a description of the cat.

      • Samuel Kitazume says:

        “The problem is that the uncertainty *remains*.”

        Where did you find that statement?

        • Where did you find that statement?

          The uncertainty about the death of the cat derives from the uncertainty about particle disintegration, which itself does not depend on whether the box is opaque or not.

  3. Teilo says:

    All you have done is proven that you do not understand the role of the observer in superposition. Observation is observation. It makes no difference if the means of observation is by opening the box, or looking at the cat through a pane of glass. Either collapses the wave function. You have not changed the experiment in the least. You have only caused the wave function to collapse the moment you look at the box.

    • You have only caused the wave function to collapse the moment you look at the box.

      Which collapse are you talking about? The wave function for the cat, or for the particle?

      • If it’s the particle, I don’t see how making the box transparent would change the way the particle disintegrates. So I think your statement is wrong in that case. The wave function for the particle does not collapse because I look at the cat.
      • If it’s the cat, then the particle wave function is not changed, and the probability for the cat to die has not changed either, since it only depends on the particle disintegrating. Therefore, you now say that I collapsed the wave function of the cat, but that we still have uncertainty about whether the cat will die? So I have a collapsed, yet entangled state for the cat? Explain to me how that works, mathematically… So I think that this interpretation is wrong too.
  4. Marcos says:

    I agree with some of your arguments, but not with your conclusion.

    First of all, the cat experiment is a thought experiment to help address the weirdness of quantum mechanics. No one would tell you that if you conducted this experiment meticulously, you could have a cat in a state that is both dead and alive. That is simply not possible because of decoherence (perturbation and interactions with the medium). It’s just not possible to isolate a “cat system” enough; if you could do it, you could also build a quantum computer with enough power to collapse every present working security system.

    That being said. In the presently most accepted interpretation of quantum mechanics, an electron can be in two different places at the same time (until the position of the electron is observed). This means that electrons behave AS IF they are at two different places (or take two different paths). For more about this, you can review the double slit experiment (or watch this amazing talk given by Richard Feynman). In that experiment, you cannot explain the interference observations by assuming that the electron takes only one path; it has to interfere with itself, and thus take two paths or more.

    In conclusion, yes, you are right, in the case of the cat experiment there would be 50% of the cats dead, and 50% alive (and it does not matter for practical purposes if you have either a black or a glass box). This is because the cat state inevitably interacts with the medium and will instantly collapse into one of the two classical states. BUT, an electron (if not perturbed) can and will remain in a superposition state, and the observations made by experiments can be not explained by saying “QM is statistical”. No, it’s not only that we do not know what path the electron takes, the problem is that each electron behaves as if it had taken two or more paths (and, the second weird thing is that, when you want to measure where the fuck did actually the electron go through, you no longer obtain the superposition (or interference) result, but you obtain the classical result (no interference)). That is, measurement changes fundamentally the behaviour of electrons.

    • That is simply not possible because of decoherence

      True enough, but for the sake of the argument, Schrödinger asked to do as if this limit case could be achieved. I ask that you do the same in my experiment. In any case, the argument does not depend on this at all, see my answer to Teilo’s comment.

      That being said. In the presently most accepted interpretation of quantum mechanics, an electron can be in two different places at the same time

      If I cannot convince you that a macroscopic cat is not both dead and alive, I can’t even begin to address the case of something much more exotic like an electron 🙂 . But that’s really the core of the argument. The most common interpretation of quantum mechanics states that the wave function describes what is. I’m arguing that it describes what we know. For small enough systems such as the electron, what we know may actually be all there is, but that’s a different and more complicated discussion that I did not intend to address right away. I will address it soon, though (probably around next Monday if I keep my posting schedule).

      In conclusion, yes, you are right, in the case of the cat experiment there would be 50% of the cats dead, and 50% alive (and it does not matter for practical purposes if you have either a black or a glass box). This is because the cat state inevitably interacts with the medium and will instantly collapse into one of the two classical states.

      Please note that this outcome, 50% dead and 50% alive, is the same whether the boxes are opaque or not. It depends solely on the statistics of particle disintegration, not on the transparency on the box. So, as I answered in your other comment, what I’m proving is the hypothetical decoherence of the cat is totally immaterial to the interpretation of the experiment. In physics, something that is immaterial and unobserved should, in my opinion, be disposed with.

      BUT, an electron (if not perturbed) can and will remain in a superposition state

      Yes. But this does not address the question of what you imply by “state”. If you call “state” all I know and can know about the electron, I will agree with you. If you imply that there is some magic here/there state similar to the alive/dead state of the cat, I will prove you wrong, but in a later post 😉

      I and the observations made by experiments can be not explained by saying “QM is statistical”

      Could you elaborate about what you mean with this?

      • Marcos says:

        I don’t know how to tag your answers, but I will answer them in order.

        Schrödinger did not know anything about decoherence at the time he proposed this experiment. Decoherence IS the answer for: why can’t we have a “big object” superposition? In particular, the cat is a “big object”, its “state” isn’t described by a simple wave function, but a wave function that is a combination of all the wavefunctions of the atoms and particles that compose “the cat”, and their interactions with the environment.

        You said: “If I cannot convince you that a macroscopic cat is not both dead and alive, I can’t even begin to address the case of something much more exotic like an electron.”

        I was already convinced of that. A macroscopic cat cannot be in a superposition state, because of decoherence (and its the same thing if the box is either black or made from glass). And an electron, at least in quantum mechanics, is not an exotic thing, its simply described by a wave function.

        You said: “Please note that this outcome, 50% dead and 50% alive, is the same whether the boxes are opaque or not.”

        That’s true, that’s the same that I’m saying. But you can’t extrapolate this conclusion and then say that “quantum mechanics is statistics”, because that’s not true. The cat is explained with statistics because it is not macroscopic, the electron is not. An electron can be in a state that represents to different positions at the same time, at least it behaves as if that were the case, and the (for example) double slit experiment can’t be explained by saying “the electron is either here or there, the thing is that WE don’t know where it is”, that’s simply wrong. If the electron were in one place at a time, the outcome of the double slit experiment could not be explained, even if we don’t know where the electron is. When the electron is in a superposition of two different positions, that is called a “pure state”, which makes it behave very differently from an electron that is either here or there (which would be described by a “mixed state” in quantum mechanics).

        “Could you elaborate about what you mean with this?”

        By this I mean what I wrote above. For example, the outcome of the double slit experiment cannot be explained by saying “electrons are in one place at a time, it’s just that we don’t know where they are”.
        I mentioned the Feynman talk in my last comment but didn’t give you the link, here it is: https://www.youtube.com/watch?v=aAgcqgDc-YM&feature=youtu.be

        Another thing you mentioned in your last comment is a very important questions in quantum mechanics: is the information given by the wavefunction all there is? Or is it that we are not observing the rest of it, or maybe that we can’t observe it?

        It is true that in the Copenhagen interpretation, the statement “the wavefunction is all there is” is not derived by the theory and its simply assumed. Hence the question about the so called “hidden variables”. The thing is that Bell’s theorem has already ruled out hidden variables with local interactions, which says that: even if some hidden variables exist, quantum mechanics would still be a freaking crazy theory.
        A very good account of the present state of hidden variables theories is Scott Aaronson’s book “Quantum Computing Since Democritus”, chapter 11: http://www.scottaaronson.com/democritus/lec11.html

        Quote from Aaronson: “It follows that, if we want it to agree with quantum mechanics, then any hidden-variable theory has to allow “instantaneous communication” between any two points in the universe. Once again, this doesn’t mean that quantum mechanics itself allows instantaneous communication (it doesn’t), or that we can exploit hidden variables to send messages faster than light (we can’t). It only means that, if we choose to describe quantum mechanics using hidden variables, then our description will have to involve instantaneous communication.”

        A very interesting “hidden variable theory” that is obviously non-local (because it works) is the “pilot-wave theory”, or “de Broglie-Bohm interpretation”. But, there are many problems with it that are listed in the chapter I linked from Scott Aaronson’s book.
        The problem is that no interpretation can presently be differentiated from one another by experiments, so it is meaningless to argue about one interpretation over the other (except for some interesting experiments that use weak measurements and can account for deterministic results in the double slit experiment, but more research is needed in this area to get a meaningfull conclusion). And, for all the practical purposes, the copenhagen interpretation is the most useful interpretation for doing calculations, so it will never die, at least we can say that, for now, it is the best what we can do.

        So, in conclusion, if all your argument was about the problem of the assumption that states that the wavefunction is the maximum knowledge that we can have from a particular system, you’re right. That is an assumption in the Copenhagen interpretation, but there’s nothing else to prove there, we just simply don’t know what the answer is.

        • Marcos, thanks a lot for your answer. I want to make one thing clear so that you focus your time where it matters: before you wrote it, I knew every single thing that you wrote about. You do not need to teach me quantum mechanics 🙂 But thank you for clarifying that what you meant about statistics is what I supposed you meant. I will address that soon on this blog.

          That being said, your points are quite interesting. You are just a bit ahead of my posting schedule 😉 But if you want to read ahead, you will get additional information of where I’m going here: http://cc3d.free.fr/tim.pdf (not all of it, this was written in 2007). And about your arguments on pure states, you may want to read point 4 of the comment here http://grenouille-bouillie.blogspot.fr/2007/05/another-theory-of-everything.html#c1454241459379485832. That comment was “one step ahead” of you, because the person had read the paper, and commented directly on how I address pure states in the theory. Once you have read the comment (I presume you will probably agree with it), then you can read my answer there: https://grenouillebouillie.wordpress.com/2008/02/17/an-intelligent-review-of-the-tim. Unfortunately, the discussion with that anonymous poster stopped there. Too bad, the points, like yours, were articulated and very cogent.

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