Will the Real Schrodinger’s Cat Please Stand Up

The story of Schrodinger’s cat is well known, and one of quantum theory’s most popular phrases on the world stage. You can find his cat on t-shirts, bumper stickers, internet memes and the like. However, few know the origins of the cat, and how it came into being. I suspect many do not understand it beyond the “dead and alive at the same time” catchphrase as well. Not surprisingly, it was Einstein who was at the center of the idea behind Schrodinger’s cat. In a vibrant discussion between the two via letters across the Atlantic, Schrodinger echoed Einstein’s concerns with the following:

Contained in a steel chamber is a Geiger counter prepared with a tiny amount of uranium, so small that in the next hour it is just as probable to expect one atomic decay as none. An amplified relay provides that the first atomic decay shatters a small bottle of prussic acid. This and -cruelly- a cat is also trapped in the steel chamber. According to the wave function for the total system, after an hour, sit venia verbo [pardon my language], the living and dead cat are smeared out in equal measure.

This was the first mention of Schrodinger’s cat, and one would not be incorrect in stating that this paragraph from a letter was where the cat was born. However, the original idea behind the thought experiment was from Einstein and his loathing of the wording of the Einstein-Podolsky-Rosen (EPR) paper. He expressed his frustrations with Schrodinger with a few simple examples, who then catapulted it into his famous paradox . In this article we’re going to explore not so much the cat, but the meaning behind the thought experiment and what it is meant to convey, while keeping it simple enough for anyone to understand. So next time you see it on a t-shirt, you will be able to articulate the true meaning and know the real Schrodinger’s cat.

Reality or Statistical Probability?

Cloud Comics: Schrodinger vs Heisenberg

Einstein was not an opponent of quantum theory in its totality. He believed the theory was useful for predicting statistical outcomes. His problem was the idea that it was a complete description of reality. He did not believe nature was statistical, but rather that it was completely deterministic, just as classical physics is. Quantum theory was useful in predicting experimental outcomes, but that’s all it was… just a tool for physicists. Not a complete theory of nature.

He threw what he hoped to be a knockout punch with a paper known as the Einstein-Podolsky-Rosen (EPR) paper that would show how silly all this quantum business really was. We’ve covered EPR on a few occasions now, so I’ll not bore you with rehashing the details. Instead, let us focus on why Einstein was not happy with it, and how it birthed Schrodinger’s cat. He said to Schrodinger in one of the many letters between them that it was “smothered by the formalism“. He was frustrated that his main argument was buried by complexity, even though it was relatively simple to understand. He explained with an analogy.

Imagine you have in front of you two red Sparkfun boxes of the same size and shape, and an Arduino. You place the Arduino in one of the boxes and then shuffle them to the point that you are unable to know which box contains the Arduino. There is a 50% chance you will find the Arduino in the first box you open. The question is – is this a complete description? Remember that quantum theory links statistical outcomes with reality, and Einstein is trying to hammer a wedge between the two. Now, back to the question: if there is a 50/50 chance of finding the Arduino in the first box, is this a complete description? We can only have two possibilities:

  • A) No. The Arduino is in one box or the other. There is no more information that can be gained from the system that will tell us which box the Arduino is in, and we are forced to resort to probabilities — there is a 50/50 chance of finding the Arduino when we open the first box.
  • B) Yes. The Arduino is in both boxes at the same time, and is localized into a single box at the instant we open the lid of one of the boxes and make an observation. Done repeatedly, we find the Arduino in each of the boxes 50% of the time. The system is probabilistic in nature, and no other description is required.

Einstein was trying to show the absurdness of quantum theory as a description of reality. Yes, it can accurately predict probabilistic outcomes, as in scenario B. But scenario A is a much more sensible description of reality. This was the key point that Einstein wanted to address in the EPR paper. A simple point, perhaps too simple.

The Birth of Schrodinger’s Cat

Image via Live Science

Schrodinger had developed wave mechanics some years back, and still believed that wave functions of quantum particles played a role in their behavior.  He and Einstein butted heads on this. Einstein believed wave-functions described statistical probabilities of systems (in scenario B, the Arduino would have a wave function allowing it to be in both boxes at the same time. Opening a box would “collapse” it into one box or the other), but DID NOT describe a physical reality in any way. He would dream up yet another thought experiment:

Apply a wave function to a charge of gun powder. Over the course of time, the equation blurs the lines between an exploded and non-exploded state. There can be no reasonable description of reality that can place the gun power in both an exploded and non-exploded state at the same time.

After several more letters and simple thought experiments, Einstein won over Schrodinger to his side. Schrodinger had become convinced that his wave functions were indeed not a description of reality. He would go on to merge the macro state and the quantum state in one experiment by replacing the Arduino with a cat and the gun powder with poison in an attempt to create a “quite ridiculous case” as he put it.

Quantum Theory Wins Again

This created quite a problem for the Copenhagen interpretation of quantum mechanics, which insists that reality can only be defined by our measurement devices. Asking what the state of the cat is prior to measurement is without meaning. But Einsteins and Schrodinger’s view that the wave function of the cat simply represents a statistical probability of the cat being alive or dead is much more sensible.

Bohr had no response to the Schrodinger cat paradox other than the canned “The measurement instruments must be treated classically”. That just means that the cat must be treated classically and cannot enter into the superposition of alive and dead at the same time.

At this point, the physics community was growing weary of the seemingly endless philosophical debates. Quantum theory worked in the laboratory and most physicists just used it because it worked, and turned a deaf ear to its challengers. And this sums up the theory into the modern day. There are still unanswered philosophical questions out there, but the theory continues to hold up under experiment. And will likely continue to do so in the foreseeable future.

There are more advanced approaches to the paradox, which includes the consequences of Quantum Decoherence, and we did not touch on the deeper question of what exactly happens during the “collapse of the wave function”, which happens when an observation is made.  That will be for a future article, but please feel free to carry on in the comments below.


The Quantum Story, by Jim Baggott. Chapter 17  ISBN-978-0199566846

51 thoughts on “Will the Real Schrodinger’s Cat Please Stand Up

  1. Einstein loved his cat analogies. When asked to describe radio, he explained. “You see, wire telegraph is a kind of a very, very long cat. You pull his tail in New York and his head is meowing in Los Angeles. Do you understand this? And radio operates exactly the same way: you send signals here, they receive them there. The only difference is that there is no cat.”

  2. The Copenhagen interpretation of quantum mechanics is one of those examples where good enough is the enemy of better. It may well be that some other interpretation is closer to the absolute truth, but there is little gain to be released at this point in changing.

    1. The silliness that is the Copenhagen Interpretation is the main cause of 99% of the stupid things quacks spout about Quantum Mechanics all around. It’s really indefensible that people still take it in any seriousness.

      1. Furthermore, when I was in college there were 11 interpretations of quantum mechanics, only one of which was the Copenhagen interpretation. (I’m told that there are about 2 dozen now.)

        Some of the others are completely boring and contain no new-age woo, yet accurately predict the outcomes of experiments.

        Without appealing to multiple universes or simultaneously live/dead cats.

      2. While what you write is true, I strongly doubt that it would slow down the woo-mongers who anyway are now spinning Everett’s Many-Worlds Interpretation into even higher orders of nonsense. The point I was making that there is no compelling reason to change unless that change comes packaged with some major new insight that would have sufficient utility to make the change worthwhile.

        1. Of course Quantum Mechanics rules all interactions between atoms, including the ones on your DNA, and it affects how all chemical interactions work, this is hardly news (though detailed studies on exactly how it affects things is always welcome).
          In my view, the main problem with the Copenhagen Interpretation is the “magic” Observer who goes around collapsing waves and giving fuel to cranks.
          Substituting CI with any one of the other interpretations that don’t really add anything to QM, but don’t have badly defined external actors influencing things would at least cut down on that.

          1. The woo crowd gets confused because “observer” seems too similar to “person”. CI works without people, the observer can be anything at all: a measuring device (delayed choice quantum eraser) or a stray photon from cosmic rays, or even a muon caused by that photon. Anything that interacts with the system either blurs the wave function more, or causes a collapse.

          2. My point was we observe quantum mechanics because we are quantum mechanics. I think it’s circular not special to the individual but special to existence. IE the observer is everything or one of the few new theories to QM.

            This however does not rule out CI which should be obvious, and why einstein was a contrarian when he could not see that the box itself is an observer and why his arseduino analogy is circle jerking.

    2. In case anyone cares – “Consistent Histories” is a simple way of looking at the same thing but does not require an observer. It is 100% in agreement with the Copenhagen interpretation. It is easy to understand and IMHO makes sense…

    3. But perhaps there is something to be gained by a different interpretation. For example Stuff Schrodinger and his cat box in another box. Wait just over an hour, and Schrodinger opens the inner box and has either found a live cat or a dead cat. Or… maybe he has found both. Rather than becoming collapsing the cat wave function, he has become entangled with it. When we open the outer box, of course he reports one result or the other to us because we get entangled too. There is no mystical “observer” and Occam relaxes a bit.

      Why would it matter? If “Schrodinger in a box” is correct, the cat remains in superposition rather than collapsing when the inner box is opened (albeit entangled with a would-be cat murderer) . Ought to be testable with a sufficiently clever diffraction experiment.

      1. So would Schrodinger be entangled with the cat or would he himself be in a superposition of both observing the cat and not having observed the cat, and therefore resulting with the cat being in quadruple superposition? Without observing the observer we wouldn’t know if he has observed the cat and therefore interacted with it.

  3. For me, “Schrödinger’s Cat” is the killer proof that Quantum Theory gives the statistics of a scenario but not individual cases. Have the “Quantum Theory is the whole truth” side come up with an argument that says any different (other than just to insist that the theory should be used correctly).
    Thanks for this clear and informative article on a (for me) opaque topic, I look forward to the next instalment (and congrats on the “Einstein’s Arduino” story).

    1. Yeah, I recall a recent experiment that showed a particle in the process of being two places at once. It has been shown that QM isn’t just a “close enough” model at this point, it is the way things work. The why is debated now.

      Best analogy I’ve heard for “everything is a field fluctuation” is this: take a bed sheet, place it on a bed but don’t smooth it out; are the raised wrinkles “real” or are they just a property of the localized non-zero energy of the system?

      1. Thanks for that. I spent a happy few minutes reading about the single particle, double slit experiment and the Bonn University experiment moving a single caesium atom to two different places, simultaneously. So QM does what it says on the box, even for single items – cool.

  4. So, basically, to make a zombie we would replace the cat with a human. Then we would have a human that was both alive and dead at the same time. Thus, killing zombies would be easy, as merely opening the box would be sufficient to do the job half the time. A good headshot with a .45 should take care of the other half.

        1. Intriguing… What if we set up the experiment in reverse? Since zombies start out dead then get revived in a somewhat decaying yet active state, instead of a living being that is killed/transitioned in an indeterminate state, what if collapsing a waveform synthesizes a new particle with an undetermined wave-particle state? That is, the collapse of the waveform is not a collapse but rather part of one of the four fundamental interactions? This eliminates the observer effect and shifts everything towards light being what happens when two or more wave-particles interact with each other.

  5. My chem professor “solved” the cat conundrum this way: without open it the box, empty a few shells of buckshot into it. The state of the cat is solved.

    He was an old-school guy. Everything, to him, was “just the best model we have right now” since the ancient ‘atom is indivisible’ lead to the ‘plum pudding’ model and then the early circular orbits model. The ‘fact’ that we would never see these now quantum orbits, he thought at the time, was reason to just call it a model. Haven’t seen him since freshman year, wonder how he dealt with the images or the s and p orbital clouds.

    1. An atom in ancient Greece was simply the smallest indivisible particle of matter.

      Nowadays, we call those quarks.

      Them ancient Greeks would have simply called quarks atoms… And, should any other smaller indivisible particle be found… That would then be understood as the “atom.”

      Pretty nifty definition.

      1. No, nowadays we call those “fundamental particles.” Electrons aren’t made of quarks. But that’s cheating a bit: an atom is the smallest, indivisible particle of *each element*. If you break up an iron atom, it’s not iron anymore.

  6. “But scenario A is a much more sensible description of reality. ”

    But it isn’t, because there’s a gaping flaw in the analogy. If you put something in one box but not the other, there’s a difference that can be discovered from outside of the boxes – whether it’s the weight of the box, how it rattles, magnetism, electric charge, or how it looks like under x-rays, gamma rays… and so-on.

    There is -always- some way to peer inside the box without opening it. If there’s something in the box and not in the other, you can always find out some way to discover which box is different. Even if no person can discover which one it is, the universe “knows” one box is somehow different, and no amount of shuffling can hide which box contains the object. You can play the three cups game with a researcher, but not with reality itself because it always contains all the information about what there is.

    So, if there is no difference between the two boxes, they must be both empty or both hold an identical object.

    So, in order to put an object in a box in a way that makes the two boxes absolutely undiscernible from each other, you must impart the same difference to both boxes, and therefore, if you later find out that only one box contains an item then that item must have been in both boxes simultaneously.

    The whole dilemma in the thought experiment is dependent on the idea that you can’t peer inside the boxes, but no such box can exist. If such a box did exist, putting an object inside it would erase it from reality and that would violate the conservation of energy and information.

    1. well, no, not as I understand it. If such a box existed it would actually erase the item from existence AND double it at the same time. No information would be lost. Reality turns out to be just a bunch of wave forms that have collapsed. All the box does is “uncompresses” the wave forms to the superposition state.

      However it is an interesting thought to have the universe be an observer. By doing that you would have to collapse all wave forms simultaneously to end up with the “reality” we see. Of course, since the universe itself is just a giant waveform , it probably exists in multiple states at once….OH jeebus…I think my eyes just went crossed…

      1. “AND double it at the same time.”

        If you have two empty boxes to start with, and you put an item in one box, and find that no change happened to the other box, and both boxes are still identical – which you do – then the first box must be empty too. Therefore the object dissapeared. If you find that the second box changed after you put an object in the first, you’ve discovered a wormhole.

        “By doing that you would have to collapse all wave forms simultaneously to end up with the “reality” we see.”

        Why does the waveform need to ever collapse? What would collapse it? In my understanding, we’re living the sum of the waveforms that exist here and now.

      2. The main point is that reality never loses track of which box is which. You may shuffle the boxes around until you forget, but the (local) universe cannot forget which has the object, because the boxes cannot hide what’s inside.

        So it’s just as well you didn’t shuffle the boxes at all. Just put the object in one box and open the other – of course you can’t find anything in that box because you didn’t put anything in there.

        So the thought experiment of two boxes is fundamentally flawed – it doesn’t describe what’s happening with the quantum-weird event at all.

    2. As for the quantum weirdness involved with such experiments in real life, such as with entanglement, there’s two alternative explainations.

      1) The object is spread between two boxes and appears in one box while dissapearing from the other. If the boxes are separated by any distance this option will require superluminal transfer of information, which is why Einstein opposed it – it would break physics entirely.

      2) The object isn’t in the boxes at all. Either the object didn’t exist in the first place and you simply thought you put something in the box, or the object is destroyed by the attempt and the information and energy of the object disperses outside of the boxes, to be reconstituted later.

      For example, when you break up a particle into spin-up and spin-down particles and send them away two ways, there’s not enough information in the system to define which is which in the first place. If the particle you send is a box, you did not put anything in it. The spin of each particle is defined afterwards by the experimenters who observe the two particles, and then observe each other to see who got what.

      The imporant bit is that even after the researchers have observed their particles and each other, it still does not matter who got which spin – all they need to agree on is that the spins were different, which is all the information there was from the beginning, and no information is added or destroyed.

      If it should make a difference whether one got a spin-up particle and not spin-down, that information necessarily comes from elsewhere – even after the experiment was made – which makes it seem like some sort of retrocausality but it’s really not because the original event of one researcher finding a spin-up/down particle didn’t actually happen. It only seems like it did after the new information is added.

      1. That cat will live a long time, since dark energy seems to be preventing any future collapse of the universe. Imagine, that poor imortal cat, outliving the stars, the galaxies, and even getting to see the evaporation of the largest black holes. Then still living on til the protons that make it up ultimately suffer decay.

  7. To me the most simplistic way to see this theory is that in order verify the “state” of an item is to observe it first hand and take measurements. BUT can the act of observation and measurement alter the original “state” being measured?
    The cat is neither dead or alive until it is observed. Open the box and see the cat alive but by opening the box, the poison is released and the cat dies. You closed the box before the poison takes effect, so your observation will be different from the next person who “observes” the cat. Who’s observation is correct?

  8. For the viewpoint of the observer, quantum mechanics works. But there could be some way to check the boxes. Philosophy is important here as if superposition of states is inevitable, further research on improving instruments will be fruitless, and no research will be done even if it could lead to a way to check the box without opening it.
    What is sure, is that lack of observation leads to the need to use a probabilistic model.

      1. Yes, and the principle that observation influences the observer is valid also in the macro environment.
        At particle level, if you need to send a photon to find where a particle is, the photon, hitting the particle, will change it’s velocity or position, and when you get the measurement, the state has changed.

  9. Why mess with mere hours and simple cats though. In 5 Billion years the Sun will turn into a Red Giant envoloping the inner planets of the Solar system, so in 6 Billion years there’s a 50:50 shot that the human race is completely extinct unless observed otherwise. Therefore we’re all existing in a state of quantum indetermination, a wave function yet to be collapsed.

    Unless of course you like to take the statistical point of view that out of the billions of humans thought to have been born, only 93% have been observed to die, hence there’s a 7% chance you’re immortal.

  10. So let’s blow the Copenhagen Interpretation to smithereens with this….Suppose you have an observer that does not interact with the system in any way, thus avoiding all interference with the system. Does the observer “see” both states at the same time, or does the observer see only one state?

    The answer is simple, they only see a single state. Problem solved.

  11. The issue here is really not physics but in how the human brain sees the world. For example, common sense tells us that dead and alive are easy to determine mutually exclusive states of carbon based lifeforms that use DNA for replication. That may be our overwhelming experience but it does not exclude other possibilities (Princess Bride : “mostly dead” :-)). That probably doesn’t matter for the Shroedinger example which is not really about life or death but about whether the human experience of a series of identifiable events following a direction it time (no unbreaking a broken cup) is a sufficiently good model of “reality” to be broadly applicable at the subatomic level. There is a lot of evidence that it is not and the concept of wave function collapse seems to be just a convenience to allow two different models to co-exist.

    More importantly, we assume that there is such a thing as “truth”, defined as a models of our environment that provide accurate and consistent predictions everywhere and which we can be certain will not be replaced by other models into the indefinite future. The reality is that Classic Physics and Quantum Physics are just the best mathematical models we have now. They produce good predictions for certain environments but have some areas of inconsistency between them. I am sure that theories, such as multiverse theories, will be developed to reconcile them but at some point, even these theories will fall short somewhere and on it goes.

    The question I find interesting as a (former) mathematician and computer scientist is whether the amount of information in the universe is bounded. The answer would have a huge impact on future models of the “universe”. For example, an information unbounded universe can prpbably never be modeled accurately and so our models will probably have to end up being statistical in nature. On the other hand in an information bounded universe, space would have to be inherently discrete and so maybe calculus based models are just convenient simplifications. As an aside, I have yet to see a good proof of Pythagoras’s theorem in discrete space and if space is discrete, this must be possible.

  12. Quantum wave theory would suggest the cat was in the box, and outside the box and everywhere at once at the same time. Not only was the cat in the box, there were millions of cats in millions of boxes. When one cat was poisoned, an infinite number of cats were poisoned. Both inside and outside the box.
    The killing joke is that what happens to one wave form, happens to all equal wave form signatures equally.
    The solution to the dilemma, remove probability, remove the state value, replace with equal change value, solution resolves itself.
    The problem with waveforms is that its not a cat in a box, but a waveform that no box can hold.
    Dr. Rife proved this to be true. Dr. Rife also solved the issue of the viewing without disturbing the molecular structure results.
    The Rife microscope is real and functional and still exists.

    1. Rife’s claims could not be independently replicated, and were discredited by independent researchers during the 1950s. Rife’s microscope, his techniques and results he claimed have been consistently discredited by the broader scientific community, who’ve concluded that they were simply not possible given the laws of physics.

      Several marketers of other ‘Rife devices’ have been convicted for health fraud, and in some cases cancer patients who used these devices as a replacement for medical therapy have died.

  13. Schoedingers cat, quantum physics, time dilation, light as a wave theories are all probably inaccurate- in particular if you study the history of science. Science entered a massive dark age with the rise in the early 1800s of Thomas Young’s undulatory theory for light- the idea of light made of material particles collapsed- and if that was not bad enough- then when Michelson with Bell’s support tried to revive it, FitzGerald put forward the “space contraction” theory which Einstein adopted and is still with us today. What exists now, is a massive fraud or honest mistaken set of beliefs. All through the 1800s and beyond people are talking about energy (which started as vis-visa) and sometimes momentum, but yet, it seems logical that mass never changes into motion and motion never changes into mass. Just sayin’

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