Schrödinger’s Kitteh

December 15, 2012 • 5:05 pm

If it were Schrödinger’s Dog, imagine how much less physics the layperson would know! Schrödinger’s Gedankenexperiment mit Katze was a felicitious (and felinitious) meme.


The original suggestion, from Wikipedia:

“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 psi-function of the entire system would express this by having in it the living and dead cat (pardon the expression) mixed or smeared out in equal parts. It is typical of these cases that an indeterminacy originally restricted to the atomic domain becomes transformed into macroscopic indeterminacy, which can then be resolved by direct observation. That prevents us from so naively accepting as valid a “blurred model” for representing reality. In itself, it would not embody anything unclear or contradictory. There is a difference between a shaky or out-of-focus photograph and a snapshot of clouds and fog banks.”
—Erwin Schrödinger, Die gegenwärtige Situation in der Quantenmechanik (The present situation in quantum mechanics), Naturwissenschaften (translated by John D. Trimmer in Proceedings of the American Philosophical Society)

38 thoughts on “Schrödinger’s Kitteh

  1. What about ψ(out-of-body-hanging-with-Jesus)? Also, what with nine lives and all, who decided on the orthogonality ψ(HAI) and ψ(OH NOES!) implied by the normalized coefficients?

    1. I would expect most persons successfully laying anywhere to at least have a working grasp of the law of gravity. Or a grasp of a handle on the floor, or some such contrivance.
      Actually, most people do have some reasonable grasp of physics – laws of gravity, motion, flow etc. Where things start to fall down is over electricity (particularly AC electricity). I guess that “after” that (depending on your local examination board’s curriculum), things get worse rapidly.
      General understanding of chemistry is worse, I’d reckon.

  2. The great thing about Schrodinger’s Cat is that it anticipates a future generation of physicists who would be all “probability waves, yadda yadda, whatever”. By creating a thought experiment where some macro-scale event is tied up with a quantum event, he made sure no one would ever become complacent about QM.

    BTW, I am curious what Jerry thinks of Schroedinger’s book What Is Life?.

    1. “What has physics got to do with it”? (Or maybe that is what I would say.)

      Ironically, thermodynamics do seem to constrain life’s origins a lot.

  3. I’m so glad decoherence came along and killed that particular ca… experiment by tying it to the environment. Too much “need observer” classical Copenhagen spiritualism-in-disguise.

    1. Well that was the point of Schrodinger’s thought-experiment,surely. He didn’t agree with Bohr and wanted to show how silly the Compenhagen interpretation was (is).

      1. And yet I suspect most people who’ve heard of it still think that Schrodinger’s Cat shows how quantum theory “is”.

        I’ve lost track of the number of times I’ve had to explain that Schrodinger was actually taking the piss out of Bohr’s ideas.

      2. Trimmer’s translation tends to neutralise Schrödinger, to neuter him even. It leaves out much of Schrödinger’s mordant style, especially his irony, and softens his sideswipes at Heisenberg and Bohr.

        Schrödinger was a quite a vivid, delightful, and when he felt like it, vitriolic, scientific writer. I have attempted a new version based on his 1935 original. Significant nuances at variance with Trimmer’s version are given in italics. Emphasis added by Schrödinger himself is given in bold.

        One can construct even quite burlesque cases. A cat is confined in a steel chamber, along with the following infernal machine (which the cat must be prevented from tampering with): in a Geiger counter there is a tiny amount of a radioactive substance, so tiny that in the course of one hour one atom may decay, or just as likely may not; if it does, the counter tube registers and actuates through a relay a small hammer which shatters a small phial of prussic acid. If the system has been left to its own devices for one hour, one may infer that the cat is still alive if no atom has decayed in the meantime. The ψ-function of the whole system would represent it so that the living and the dead cat (sit venia verbo) are mixed or smeared out in equal proportion.
        Typically for such cases, an indeterminacy originally constrained to the atomic domain is translated into macroscopic indeterminacy, which can be decided by direct observation. This inhibits our naive acceptance of a “washed-out model” as a representation of reality. There is a difference between a shaky or out-of-focus photograph and one recording clouds and fog banks.

        Source of Schrödinger’s 1935 paper: (free access, despite being hosted by Springer)

        1. the following infernal machine

          That’s a reference to Maxwell’s Demon, do you think?
          I had a friend (now deceased) who could out-physics me by a large margin ; she hosted a website on the UK ISP “Demon” as “”
          Unsurprisingly, she was a cat-lover too ; see the footer link on the page linked above.

            1. The link I tried to post above was to the Wayback machine, but for some reason it didn’t work. So I’ll try again.
              The choice of the unusual word “infernal” in Occam’s translation is unusual for modern English, which is why I’m wondering if (with a translator’s license) he’s inserted a knowing pun. Or maybe the “demonic” connection is in the original German?

              1. Here.

                Well, it _is_ a classical demon, which runs into problems at the level best described by statistical physics. However, when you look at the details on that level, you may “run into the uncertainty principle” in the sense that it is (IIRC) used to assess the minimum thermal energy that is communicated at various stages.

                An overview doesn’t need to go into such details.

              2. If I remember correctly, the “demon” in Maxwell’s demon is lexically closer to “daemon”, which I think originally meant tiny elves or something that would move stuff around. But I am not a linguist, so I might be wrong.

                I tried your original link, and could not find it: however, I did not like the idea of saying it because you said it belonged to a deceased friend. But, three cheers for the Internet Archive WayBack Machine!

              3. Software and sensible comment was Marijke’s way to immortality. I think it may have been her that introduced me to the “Wayback Machine”.
                I wish I’d met her in real life. And the “Fursome Four.”

  4. The problem with Schroedinger’s thought experiment is that it doesn’t actually say anything useful. The “observer” of any stochastic physical event is nothing but something that interacts with the effects of that event. In this case, the “observer” is the Geiger counter. It’s simply not the case that the cat is both dead and alive until someone opens the box. It’s alive until the atom decays, which either happens or doesn’t happen over any given time interval.

    1. There’s no “either” about it. It both happens and doesn’t happen (with various probabilities) over any given time interval. And the cat is both dead and alive as a result, regardless of whether anybody opens the box. The role of the Geiger counter is to decohere the different histories so they no longer interact.

      1. Thanny 1
        Kusnick 0

        There’s nothing funnier than someone who has “got it” being corrected by someone who hasn’t. 🙂

        1. Meh, I’d call it a tie at 0.5 each.

          (Thanny may be a naive classical realist, Gregory may take a many-worlds view, or they may both be far too sophisticated for this biologist to understand)

          1. Indeed, I’d go so far as to suggest that the continued lack of agreement on interpretations is a flashing neon sign that we’ve still got some serious digging to do. It’s exactly like what Mercury’s orbit was to Newtonian Mechanics.

            I should hasten to add: it’s clear that Quantum Mechanics is an amazingly successful theory. But so, too, is Newtonian Mechanics — and it remains so to this day despite its failure at large and small scales.

            Do I know what the solution is? Ha! But I’ll bet a cup of coffee that it’s not Many-Worlds. Instead, my money is on something as “more fundamental” in relation to QM as QM is to Newton. After all, that is the pattern, in all of science, and especially in physics.



        2. I can finally ask, who gets to declare which entity is an observer ? Specifically would not the kitty be an observer, being in the box with the cyanide & the hammer. Would not the her observations collapse the indeterminacy? If the observer needs a physics degree could a physics graduate be substituted for the cat and that cause the indeterminacy to collapse?
          If it is the opening of the door of the box by the disembodied observer that causes the collapse , could we nest the initial box within an other box with there own observer ? As the inner observer open the door to the initial experiment & collapses the indeterminacy to he/she or it, the outer observer still has the door closed and therefor to that observer nothing has changed. This could be nested ad-infinium

          1. With decoherence there is no observer, but a process where the system interacts with the environment irreversibly (loosely analogous to thermal dissipation):

            “Quantum decoherence gives the appearance of wave function collapse (the reduction of the physical possibilities into a single possibility as seen by an observer) and justifies the framework and intuition of classical physics as an acceptable approximation: decoherence is the mechanism by which the classical limit emerges …”.

            Here wavefunction decoupling from initial coherence, which occurs rapidly for macroscopic objects like a cat or a box, is separated from observation. Measurement will also engender decoherence in certain cases (“strong” measurements), but I am not sure that happens in weak measurements and I am almost certain it doesn’t happen in interaction-free measurements. (However as you can see, the weak measurements are, perhaps by consensus, described as “part of the causation of wave collapse” after a strong measurement has been performed. Whatever that means when you dig into the technicalities.)

            If you ask what happens if you insist on the gedanken experiment’s physics, I dunno. If it isn’t related to what we observe, it can go cat belly up very quickly.

    2. That’s ridiculous. It’s quite simple to create an experiment where Particle A interacts with Particle B and Particle B does not act as an “observer”. In fact, since space is filled with virtual particles, and every particle is constantly interacting with them, according to your characterization of an “observer”, wave functions should be constantly collapsing. Note that contrary to Gregory Kusnick’s post, this refutation relies on empirical facts, rather than dueling interpretations.

      Gregory Kusnick’s post is a bit less inaccurate, but is still a bit off.
      1. It doesn’t really make sense to claim that something both happens and doesn’t happen. The excluded middle is a logical, not physical, law. If the cat is both alive and dead, then “dead” and “not alive” are clearly not synonymous. Of course, our language is really not built for these sort of discussions, so “it both happens and doesn’t happen” is an understandable shorthand.
      2. The Geiger counter does not decohere the different histories. It is quite possible, from a theoretical point of view (albeit not a practical one), for a “Geiger counter detects radiation” and “Geiger counter does not detect radiation” state to be non-orthogonal. Or, in the usual (but wildly misleading) shorthand, the Geiger counter can be put in a superposition of “detects radiation” and “does not detect radiation” states, and those states can interfere with each other.
      3. Regardless of whether states are orthogonal, they don’t really interact with each other (or, more precisely, the statement “quantum mechanic state evolution is linear” logically implies that no states interact with each). Saying that states “interact” with each other is yet another shorthand for a complicated concept. (Although I suppose that to some extent this is more a statement about what I think the word “interact” conveys than about physical reality. Clearly, if you accept the standard physicist usage of the word “interact”, non-orthogonal states “interact” with each other).

  5. “Technically, a cat locked in a box may be alive or it may be dead. You never know until you look. In fact, the mere act of opening the box will determine the state of the cat, although in this case there were three determinate states the cat could be in: these being Alive, Dead, and Bloody Furious.”

    Terry Pratchett, Lords and Ladies.

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