Q: Before you open the box, isn’t Schrödinger’s cat alive or dead, not alive and dead?

The original question was: If I don’t open the lid of the bunker, the cat inside is either dead OR alive.  Why does Schrödinger say that the cat is both alive AND dead if I do not perform the act of observation?  Isn’t he confusing AND for OR?

Physicist: He’s very intentionally saying “and” and not saying “or”.

When something is in more than one place, or state, or position, we say it’s in a “superposition of states”.  The classic example of this is the “double slit experiment”, where we see evidence of a single photon interfering with itself through both slits.

The patterns on the screen cannot be described by particles of light traveling in a straight line from the source.

The patterns on the screen cannot be described by particles of light traveling in a straight line from the source.  They can be described very easily in terms of waves (which go through both slits), or by saying “particles of light certainly do behave oddly, yessir” (hidden variables).

Schrödinger’s Equation describes particles (and by extension the world) in terms of “quantum wave functions”, and not in terms of “billiard balls”.  His simple model described the results of a variety of experiments very accurately, but required particles to behave like waves (like the interference pattern in the double slit) and be in multiple states.  In those experiments, when we actually make a measurement (“where does the photon hit the photographic plate?”) the results are best and most simply described by that wave.  But while a wave describes how the particles behave and where they’ll be, when we actually measure the particle we always find it to be in one state.

“Schrödinger’s Cat” was a thought experiment that he (Erwin S.) came up with to underscore how weird his own explanation was.  The thought experiment is, in a nutshell (or cat box): there’s a cat in a measurement-proof box with a vial of poison, a radioactive atom (another known example of quantum weirdness), and a bizarre caticidal geiger counter.  If the counter detects that the radioactive atom has decayed, then it’ll break the vial and kill the cat.  T0 figure out the probability of the cat being alive or dead you use Schrödinger’s wave functions to describe the radioactive atom.  Unfortunately, these describe the atom, and hence the cat, as being in a superposition of states between the times when the box is set up and when it’s opened (in between subsequent measurements).  Atoms can be in a combination of decayed and not decayed, just like the photons in the double slit can go through both slits, and that means that the cat must also be in a superposition of states.  This isn’t an experiment that has been done or could reasonably be attempted.  At least, not with a cat.

Schrödinger’s Cat wasn’t intended to be an educational tool, so much as a joke with the punchline “so… it works, but that’s way too insane to be right”.  At the time it was widely assumed that in the near future an experiment would come along that would over-turn this clearly wonky interpretation of the world and set physics back on track.

But as each new experiment (with stuff smaller than cats, but still pretty big) verified and reinforced the wave interpretation and found more and more examples of quantum superposition, Schrödinger’s Cat stopped being something to be dismissed as laughable, and turned instead into something to be understood and taken seriously (and sometimes dropped nonchalantly into hipster conversations).  Rather than ending with “but the cat obviously must be alive OR dead, so this interpretation is messed up somewhere” it more commonly ends with “but experiments support the crazy notion that the cat is both alive AND dead, so… something to think about”.

If it bothers you that the Cat doesn’t observe itself (why is opening the box so important?), then consider Schrödinger’s Graduate Student: unable to bring himself to open one more box full of bad news, Schrödinger leaves his graduate student to do the work for him and to report the results.  Up until the moment that the graduate student opens the door to Schrödinger’s Office, Schrödinger would best describe the student as being in a superposition of states.  This story was originally an addendum to Schrödinger’s ludicrous cat thing, but is now also told with a little more sobriety.

The double slit picture is from here.

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26 Responses to Q: Before you open the box, isn’t Schrödinger’s cat alive or dead, not alive and dead?

  1. satyajit says:

    The value of psi gives the probability of a photon being in a particular state at a given point of time. It never implies that a photon is in multiple states at that point of time. How do you then say that the shrodinger’s model required particles to behave like waves and be in multiple states?

  2. Frank Weyl says:

    Schrödinger’s cat was an analogy and used to give a macro idea to explain a micro idea. (read quantum). The entire cat would have to be entangled with itself, the box and its contents, the laboratory and the observer’s consciousness.
    YES CONSCIOUSNESS!
    It is an attempt to give a classical view to reader’s imagination.
    QFT has many and esoteric ‘hidden variables’ including the mechanism of consciousness as part of its hidden and holistic understanding of the substrate
    underpinning the ‘laws’ governing our particular universe which has all the correct values/constants for life AND consciousness.
    This does not give credence to a god, which to my mind is a medieval encumbrance to explain the world/universe that we inhabit.
    At this moment in our/universe evolution, doesn’t it seem strange that the very essence of reality is actually within our imagination and not its true value?
    Why is this then…
    So far the universe has created a being that can actually think about its own existence and therefore question why and interrogate the home in which it was created.
    I say that although we have only scratched the surface of our being and the (mathematical/physical) boundaries of reality, the road ahead is paved with humanities unswerving desire to KNOW more about how and why.
    I think that is why the universe requires its own understanding…via consciousness.
    At the moment all conscious beings have no intellectual ‘thumbs.
    The breakthrough will come when we find the connection between quantum mechanics and gravity.
    Someone within these questions asked what a true circle was and suggested it was an infinite polygon…
    Infinity and zero do not exist. They are confined to the limits of our reality.
    Each base of every section of the polygon can only be Planck length.
    Due to the pixelisation of space-time, this is the limit to a real curve in space-time.
    Is this true? Well I presume so. The equation is quite simple.
    Lp=sqrt (h-bar*G/c^3)= 1.616048616*10^-33 cm

  3. Emma says:

    But isn’t the reality that the cat isn’t both it’s either, and man just thinks he’s controlling existence by observing it? Isn’t the whole thing just saying that probability, or possibility, exists? Even with the split wall experiment I feel like there are many holes in what effect is actually happening to the particles. I’ve read that the observer effect occurs even with unconscious observation, and detection, so isn’t the detection method obviously just altering the results and not because of some effect of consciousness? I feel like the whole thing is really just that any state can happen, and there’s no superposition of states, it’s literally just that we can guess things, but that means nothing to the actual state of reality. For the record I do think of consciousness as a force but not necessarily one that effects reality until it motivates action.

  4. Lucas Nogueira says:

    I think that is more correct the statement that before observation we can’t talk about the life of the cat.

    We are not able to talk about things that can’t be revealed.

  5. Greg Robert says:

    I’ll see you and raise you one. It’s not and vs or, it’s neither.

    The property of being alive or dead doesn’t exist until someone or something asks the question (makes a measurement) then it takes form.

    I do not believe superposition is literally true. It’s just the way physics teachers struggle with the concept.

  6. swampyankee says:

    Please correct me if I’m wrong, but if I understand the experimental protocols properly for the “single particle produces interference pattern” experiments, the experimenters don’t look at the results of one particle, but of a large number of particles sent through the experimental apparatus one at a time, and the interference pattern is built up over time.

    This makes perfect sense without going into all the wooey-ness that some people seem to associate with QM: there is x% probability of the particle hitting the target at a given point. Given the nature of probability, there is really no difference in sending 106 particles through at one time or sending 1 particle through a second for a million seconds.

    As to the cat? I fail to understand why the presence or absence of an external observer makes any difference to the state within the box, as opposed to our knowledge of the state within the box.

  7. donjoe says:

    The cat is both dead AND alive before you open the box AND after you open the box. According to Everett’s Many Worlds Interpretation upon opening the box and making the quantum measurement (i.e. translating the quantum state to classical states via decoherence) the world splits into two orthogonal configurations: one where the cat is dead and there’s a “you” who opened the box and saw the cat dead, and one where the cat is alive and there’s a corresponding “you” who opened the box and saw the cat alive. So not only does it continue to be both dead and alive at the same time, YOU yourself will split into parallel copies that each sees the cat in one of its possible classical states. The equations of QM require this to be the case. Otherwise you have to make some terms of the equation mysteriously disappear according to the older, superstitious, variant of “wavefunction collapse”.

  8. Greg Robert says:

    Please see my comment above. The “aliveness” of the cat is a non-existent property. It doesn’t exist until you measure it. This is totally consistent with the many-worlds theory (though I don’t accept that interpretation but you clearly do). This is the same as saying that an electron does not have a “position” until measured. Now that is a fair version of many-worlds, but some would maintain (as do you) that it DOES have a position, in fact it has ALL POSSIBLE positions and the act of measurement is an act of choice, not an act of “creation” (in the sense of “creating” the property position).

    Let’s face it: there is absolutely NO experiment you can propose, even in principle, to prove many-worlds in EITHER of the forms I’ve given above. It’s not possible to gain (or at least confirm) knowledge until you make observations. Observations are the very stuff of which knowledge is made. No one has ever demonstrated that ANYTHING exists except in the context of particle-like events. Events, in fact, are the ONLY thing accessible to our senses. And, as far as I can tell at this time, there is no-such thing as a wave-like event. All of our detectors are based on observations made of two particles (and only) two interacting. Wave interference, for example, is observed only by observing large numbers of particle events (such as interactions with particles on the surface of a detector) which we then aggregate to give us a mental “picture” or “metaphor” of a wave.

    Remember that both Bohr and Einstein insisted on detailed descriptions of experiments and their methodology before measurements are even defined (as in “what do we mean by the word ‘position’? “) much less made and recorded. We should only talk about physics in terms of events and predictions. Physics has nothing to say about things when they are not interacting; it can’t even say that they exist.

  9. donjoe says:

    “The “aliveness” of the cat is a non-existent property. It doesn’t exist until you measure it.”

    You’re playing with words and talking about what sounds more like philosophy than physics. I say the “aliveness” exists and is unknown until you measure it. That’s how physics should work: independently of whether anyone is looking at the experiment. Otherwise we might easily deviate into the superstitious “consciousness causes collapse” interpretation, which has already been falsified experimentally.

    “Let’s face it: there is absolutely NO experiment you can propose, even in principle, to prove many-worlds in EITHER of the forms I’ve given above.”

    You don’t seem to be very familiar with MWI yet you hold very strong opinions of it. 🙂 Yes there are experiments that can be proposed in principle that would prove MWI, since MWI makes unique predictions. See here Q36 and Q37: http://www.anthropic-principle.com/preprints/manyworlds.html

  10. Greg Robert says:

    >>>You’re playing with words and talking about what sounds more like philosophy than physics.

    No, not at all. The entire idea of “existence” is a human construct. Just because you like to believe in continuity of existence doesn’t give credence to the idea. We know ONLY observations. Get used to it. Our entire world, as is sensible (in the literal sense) to us is one of interactions. No one has ever observed anything existing without observing it. That’s a tautology of course, but a true one. Physics speaks ONLY of interactions. Everything else is metaphor and unsupported interpretations. Again, read Bohr and Einstein. And for that matter, Newton too.

    This is one of the reasons that Everet’s [sp?] many worlds theory is so badly understood when it is expressed in words rather than math.

    “Existence” has no real scientific meaning. Only momentary, and so far instantaneous observations. The quantification of time and space, of course, TBD.

  11. donjoe says:

    “No, not at all. The entire idea of “existence” is a human construct. Just because you like to believe in continuity of existence doesn’t give credence to the idea. We know ONLY observations.”

    Yep, definitely philosophy – epistemology, to be exact (answering the question of what can be known and on what basis).

    Your epistemological theory is bunk. If we only ever accepted things we had directly observed there would be no Physics (as we currently know it) because we would never use common elements between our different observations to infer any general rules of behavior of reality and consequently we would never write such things as “laws of physics”, which are basically statements that we expect some very specific outcomes if we perform some specific experiments in the future. And once we’ve tried long enough to falsify such a “law of physics” in many different ways and failed every time, we get confident enough to say “such and such is how things always work” (even when we’re not observing them) and we go on to use that belief/knowledge to build technologies that actually work.

    And if it’s the word itself “existence” that’s tripping you up, we can always switch to talking about outcomes: Schroedinger’s experiment produces both outcomes – the cat death phenomenon and the cat continuing to live phenomenon, plus the associated classical observations by different copies of the observer (also a phenomenon because it’s a living organism). There, no more talk of things “existing”. 😛

    But what remains a mystery to me is why, after such a long history of assuming all the predictions of a newly discovered physics equation are true until proven otherwise, suddenly in the case of QM equations we should refrain from assuming all the predictions in the math will also manifest in reality, just because by their nature they also imply we’re not the kind of “observing device” that will ever be able to experience _all_ of the consequences of each QM calculation (all of the many worlds). Why should our knowledge and our math have to fall victim to our limitations as human measuring devices?

  12. Greg Robert says:

    You just don’t understand what science is.

    Start with Galileo and work your way forward paying special attention to Mach, Einstein, and Bohr.

    This isn’t philosophy. It’s the very definition of science and, as such, is PART of science.

    You’re doing a great job of arguing with me. Your writing sounds lucid, educated, and reasonable, however it’s nothing but denial of a fact you don’t like.

    That’s right FACT. This isn’t an opinion nor a hypothesis.

  13. Khosrow says:

    Thank you for beautifully answered the question have always had in my mind and been bothering me. xxx

  14. Philip P says:

    I followed the link to the article about the “bigger things” that have been shown to exhibit superposition. I just want to say that if you cool the cat down to 0.1 Kelvin, or whatever the “quantum ground state” would be for a cat, you can rest assured that the cat is dead.

    As for the commentator who wants to describe it with a “many worlds” interpretation, it doesn’t make any sense at this scale. “Dead” or “Alive” is a false dichotomy. If the world has to split for there to be a dead cat and an alive cat, then there have to be many, many other splits of the world to represent every other possible combination of quantum states of every particle in the cat’s body. So at every moment in time, the universe is splitting into so many other “worlds” that the idea is instantly ludicrous. Even worse, just think of how many quantum events occur at every moment of planck time across the entire universe. Now create a multiverse so that every possible next state can exist at the very next moment of planck time, for billions of years — the cat is not the only thing you don’t know about… This is a silly idea that can only even be described in the context of a small number of events that you think have only two possible outcomes.

  15. donjoe says:

    Yes it does make sense at this scale – quantum effects scale upward just fine: http://www.nature.com/news/2010/100317/full/news.2010.130.html
    What would not make sense would be exactly what you seem to be suggesting: that a law of physics should suddenly stop working just because the objects it’s “trying” to apply itself to have become “too large”. 🙂

    What breaks up superpositions is quantum measurement, not scale. Yes, typically large objects perform quantum measurements and split the universe, but not always, and certainly not in a thought experiment like Schroedinger’s Cat, which has been explicitly designed to have that option removed.

    “If the world has to split for there to be a dead cat and an alive cat, then there have to be many, many other splits of the world to represent every other possible combination of quantum states of every particle in the cat’s body. ”

    Yes there do, and there are, proportionally to how “probable” each of those states was. But you also have to keep in mind that only states compatible with the laws of physics are actually possible – you can’t have a branch where the cat has two legs and one eye that are alive and jumping around while the rest of it is lying dead. 😛

    “the idea is instantly ludicrous”

    I take it you’re not aware that the appeal to ridicule is a logically invalid argument.

    “just think of how many quantum events occur at every moment of planck time across the entire universe”

    Not quantum events but quantum measurements (formerly imprecisely and misleadingly called “observations”). I’d think about it but frankly I don’t know how to estimate that. Anyway, if the history of astronomy is any indication, the Universe is not shy of throwing… astronomical(!) numbers of things at us, so that shouldn’t be a problem in and of itself. 🙂

  16. Greg Robert says:

    Philip P:

    I agree. It does sound ridiculous. So did the number of stars in the universe sound to those who thought the solar system was “everything”.

    I don’t care for the many worlds interpretation but I can’t rule it out just because it uses big numbers.

    – g

  17. Philip P says:

    Hi donjoe, the article you linked to is about the same research that was pointed to in the OP, which states:

    “Scientists have long wanted to demonstrate superposition in larger objects but a significant challenge here is to eliminate all thermal vibrations in the object, which mask or destroy quantum effects. To achieve this, the object needs to be cooled down to its quantum ground state – at which point the amplitude of vibrations reduces to close to zero.”

    Are there real world objects (at least on earth) that are naturally in this state? What part of the cat could possibly be in this state? As I said in my first comment, if the cat is at near absolute zero, then it is certainly a dead cat. There is no such thing as a live cat at that temperature. The OP says that the cat is in superposition because the radioactive atom is in superposition, meaning that it is both decayed and not decayed. But if quantum effects are eliminated when large objects are at their normal temperature, then how can the cat exhibit superposition? And why does the decay state of the atom imply that the deadness or aliveness of the cat is also a binary quantum state of cats?

    “What would not make sense would be exactly what you seem to be suggesting: that a law of physics should suddenly stop working just because the objects it’s “trying” to apply itself to have become “too large”. ”

    If the law in question allows that quantum effects are destroyed at the least introduction of any temperature, then it certainly makes sense that the same law implies different outcomes at different scales if at some of those scales objects just don’t naturally exist in a state where quantum effects occur. I wasn’t trying to imply that the LAW stopped working.

    “I take it you’re not aware that the appeal to ridicule is a logically invalid argument.”

    Admittedly I took a short cut. I think it would take a lot of space to say what I really meant. But the appeal to ridicule is an informal fallacy, not a formal one. So no, it’s not “logically invalid.” It only shows that I didn’t include enough of an argument to convince someone else to conclude my implied outcome that the theory is wrong in some way. It would be fair to call it bad rhetorical form or not a persuasive argument of itself. So let me elaborate a little more…

    Suffice it to say that I think the many worlds interpretation represents the quasi-equivalent of a contradiction in a reductio ad absurdum argument. I’m not ridiculing the theory but rather suggesting that the implications of MWI lead to what most people would consider to be contradictory or impossible outcomes. So maybe “absurd” would have been a better word choice than “ludicrous.”

    Thought experiments can be misleading. In this particular case we have the introduction of a “measurement proof box.” Does such a thing exist? If not, then does introducing impossible items lead to impossible outcomes? It also introduces a leap of logic from the quantum weirdness of the decay state of an atom to the supposed quantum weirdness of the life state of a cat. Is that valid? If not, could it lead to invalid conclusions?

    Since you seem to know quite a bit about MWI, I’d love to ask some questions. Maybe then I’ll see how it isn’t mathematically and physically impossible. If quantum measurements in our world lead to new worlds being created, then do quantum measurements in the other worlds spawn new worlds as well? Can the math of QM describe the actual process of the world slitting in two? e.g. where does the energy and matter come from to make two worlds out of one? What happens if a new split begins before a previous one is complete? (I’m assuming it takes some measurable amount of time.) Are they in some way spatially close to each other? Are the many worlds somehow existing in a way that they cannot interact with each other? No gravitational effects between? If the next split requires that the new world be sufficiently far from all the currently existing ones so that there is no interaction, then how does QM explain that the matter of the new world traveled such a great distance in only a moment? Is it some sort of inter-dimensional travel? Or is reality able to just instantly communicate the right information to some far away location and cause an entire new world to pop into existence at once?

    The number of worlds that would have to be created over the course of time is not merely an astronomically large number, it represents an exponential growth of matter and energy so steep as to be essentially vertical after only a few iterations. I don’t think we have any physics theories that can adequately describe how such a thing can happen in reality.

    Just because something can be described mathematically, doesn’t mean it requires the existence of said thing in the real world.

  18. donjoe says:

    “Thought experiments can be misleading. In this particular case we have the introduction of a “measurement proof box.” Does such a thing exist? If not, then does introducing impossible items lead to impossible outcomes? It also introduces a leap of logic from the quantum weirdness of the decay state of an atom to the supposed quantum weirdness of the life state of a cat. Is that valid? If not, could it lead to invalid conclusions?”

    If you’re going to get all strictly-technical on us, then this is the correct response indeed (rather than splitting hairs about “AND”s and “OR”s on the life-state of the cat): Schroedinger’s Cat is an outdated thought experiment that has been superseded by better information and can only serve today as an amusing curiosity or at worst as a misleading example of how QM works. Case closed. Thanks for your participation. 😛

    “Since you seem to know quite a bit about MWI, I’d love to ask some questions.”

    Sorry, not technical enough to go that far in-depth. Best I can do is point back to my earlier link to a page that answers a multitude of questions about / criticisms of MWI: http://www.anthropic-principle.com/preprints/manyworlds.html

    It’s quite unfortunate that Everett didn’t live long enough to finish writing down his theory in full and that we’ve had to rely on others to piece it together as best they could. On the bright side, MWI seems to have a promising successor now in the form of the Many Interacting Worlds Interpretation, where there is no creation of new branches – there is a fixed number of branches – and where quantum measurements rather than physically splitting things only serve to tell you more about (help you narrow down) which subset of all the branches you’re inhabiting: http://phys.org/news/2014-10-interacting-worlds-theory-scientists-interaction.html

  19. Joey says:

    I have a question about Schrodingers Equation; if you were observing the decaying radiation particle, wouldn’t that only allow one outcome based on the Turing Parodox?

  20. See Schrodinger’s Cat and the Law here:
    https://skepticalsciencereviews.wordpress.com/reviews/

    A parable about how silly people have gotten with Schrodinger’s cat and miss his original point.

  21. David Martin says:

    My question is – how do we know quantum superposition applies to an event such as the decay of a radioactive particle? There are probabilities attached to such an event, and in some situations they never change over time. But just because there are probabilities involved, doesn’t mean there’s quantum superposition involved, as there certainly is with more direct situations, for instance with waves and particles as in the double slit experiment.

    There are probabilities involved in a horse race, but no quantum superposition. So there are unavoidably situations with probabilities where superposition applies, and others with probabilities where it doesn’t. How do we know which is which? Thank you.

  22. https://skepticalsciencereviews.wordpress.com/story-land/

    See ‘Schrodinger’s Cat and the Law’ on site above. Top (TBP) is better.

    It shows the sealing the cat in the box made it easier to figure out when the cat died and that the cat is always dead or alive.

  23. David Martin says:

    This tells me nothing about the question. I’m asking a physicist, is there one around? I hope so – I’ve had some very good answers on this site earlier this year. Thank you.

  24. I am a physicist but not the one who runs this very useful site.

    The cat is either dead or alive. A cat is not in a pure quantum state that can be superimposed. It is being ‘measured’ with every breath as the box fills up with CO2.

    Even in ‘many worlds’ interpretation the cat is either dead or alive in your world whether you look in the box or not. There are a hell of a lot of worlds though. You need at least one for every possible time the cat could die.

  25. David Martin says:

    You’re just talking about what you already have in your head. That’s not how to answer a question.

    You’ve gone to an area that is mysterious to many, and seem to think that because it is, and because no-one has a complete answer, you can get away with saying some random stuff about it. Well……… you can, yes. Carry on.

  26. Schrödinger did not think the cat was alive and dead. He was poking fun at Bohr and Heisenberg. Many silly people took the joke seriously.

    A cat is not in a pure quantum state. The very act of breathing and pumping blood is a irreversible ‘measurement’

    The cat is either alive, dead or briefly dying. There some classical ambiguity as various cells die off, but that happens whether any one watching or not. Opening the box changes nothing. Sealing the cat makes it easier to estimate when it died.

    See, e.g., my parable ‘Schrödinger’s Cat and the Law’ previously posted.

    -Traruh

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