Famously, Schrödinger's cat is found to be both dead and alive within a closed system - at the mercy of quantum mechanics. But why is the cat "both dead and alive"? For the Copenhagen interpretation, according to Heisenberg "the wave-function represents a probability, but not an objective reality itself in space and time."

The conceptual construct of "dead" or "alive" is a 100% non probabilistic state (at least as conceived by an individual within his frame of reference). This 100% certainty can be seen as an 'objective reality' for the individual with that information.

If I knew that someone (that I was not observing) was driving a car and had a 50% chance of death, they would not be objectively "both alive and dead" to me, rather given the probabilities they would be "neither alive nor dead". Any positive truth statement cannot be backed up by (non-existent) observational evidence, so no positive truth statement, beyond some assumed estimate of the probabilities, is valid.

Does it make more sense to say that when a quantum system is not observable (is closed), whether a wave function or a cat, non-probabilistic conceptual statements with regards to what is inside the system will be incomplete?

  • $\begingroup$ Have you looked at the recent question and answers here physics.stackexchange.com/q/266606 ? $\endgroup$ – anna v Jul 8 '16 at 15:55
  • $\begingroup$ @IlyaGrushevskiy: are you happy the answers to the question Anna linked address your question? If so I will close this as a duplicate. $\endgroup$ – John Rennie Jul 8 '16 at 16:03
  • $\begingroup$ It's not both alive and dead but it is rather alive or dead. $\endgroup$ – user36790 Jul 8 '16 at 16:37
  • $\begingroup$ @MAFIA36790 the OP is referencing before the cat is viewed, I believe, so it would be both dead and alive. $\endgroup$ – heather Jul 8 '16 at 17:05
  • $\begingroup$ @MAFIA36790, I see, thanks for clarifying. $\endgroup$ – heather Jul 8 '16 at 17:07

Well, first of all, Schrödinger's cat is just a thought experiment. What the thought experiment's main point is that the radioactive substance is both decayed and not decayed when not observed. What observed means here means an act that can be used to get information about the system, with or without a person or any other conscious thing. It could be like a electron interacting with a photon. In the thought experiment, the box has to be assumed to be able to shield anything from going into the box and the radioactive substance have a half life of the time of the experiment. It is meant to show that an object or system can be in a linear combination of states. The box as a system will contain a radioactive substance that will kill the cat should it decay, but any sound or photons or even neutrinos are not able to enter or leave the box.

If the half life of the substance is 30min but you leave it for an hour, the probability of the radioactive substance decaying is 75%, thus the cat is '75% dead and 25% alive'

Now in your case, there isn't anything that will kill you that is probabilistic, unless you actually decide to gamble your life(tip:Normally a bad idea). Photons and also other particles from outside the car are also observing and observe(able to deduce) if you are dead or alive, as it would be easy for the particles, if they have their own mind, to know if you are alive or not and also exit your car.

The only way the person could be 50% dead is if the car is shielded from any observations by anything which is practically not feasible.

The wave function of any system is completely deterministic if you know the initial conditions. But upon observation or measurement, it becomes completely random. The common term that represents observation is wave function collapse

Hopefully this clears any question you have in mind

Some of the answers also touched on the many worlds theory, well, the theory stems from the fact that observation is completely random. It says that all the possible states that the system or object being observed will be happening simultaneously.

  • $\begingroup$ That the radioactive substance is in a state of superposition is not true. Only the radioactive substance plus the surrounding vacuum are in such a state. The nucleus itself, if we factor out the unknown vacuum quantum field wave function, is in a well defined state within a very small amount of time after the alpha, beta or gamma have left. That the cat can be in a superposition is also not true because it is being constantly measured by the classical gravitational field that it creates. Schroedinger pretends that none of these effects exist. $\endgroup$ – CuriousOne Jul 8 '16 at 19:35
  • $\begingroup$ @CuriousOne Yes, the point you made about the decay product not having to peeks is correct, I'm taking what an outside observer will calculate. But for Gravity, the box I'm assuming is a hypothetical that only exist in the quantum realm that shields everything, that includes possibly the graviton. But no one is certain about it's existence, and if it do you can not shield from it as the gravitational field is just described by GR. As for quantum gravity, it depends on the amount of gravity there actually is, should there be less than h, ... $\endgroup$ – Ariana Jul 8 '16 at 19:51
  • $\begingroup$ ...it will not be considered an observation as you can't observe affects below h. $\endgroup$ – Ariana Jul 8 '16 at 19:51
  • $\begingroup$ The point is that the physical vacuum is the outside observer. It doesn't matter if we peak, or not, the nucleus has been decohered by nature trough the mechanism of special relativity, especially when a gamma is involved, which can not be caught by any local observer, i.e. the vacuum state is not knowable for the local observer! The same argument can be made for the cat and its gravitational field. Gravitational waves, no matter how weak, will always decohere a macroscopic body. $\endgroup$ – CuriousOne Jul 8 '16 at 20:00

parallel definition and parallel universe is one of your answers . you can imagine that you cant look inside the box because this work can decline the attention and the accuracy so if I let you to open the box there is two possibility 1. in one universe you have an alive cat and you might get happy also the other possibility is you ll open the box and run into an unhappy scene : the cat had been died ! so It depends you and in quantum mechanic every thing completely relate to the observer and the observer can change all the results for example : think you are a teacher and you want to know how much is the noise of your class if you go and take a look every thing will change just cause of you and children will become quite . every thing is just waves of possibility until observe . a photon in a far star is just a possibility ! then every when you look at a star take care that you have made a special fate for every photon that you can see! finally you can have both condition of the cats in parallel universes .

  • $\begingroup$ You can look into the box all the time and nature does look, even if you don't. Neutrinos can penetrate the box, whether you like it or not and so does gravity and both will decohere the experiment in no time, again whether we like it or not. Schroedinger wasn't thinking about these issues when he formulated this poorly conceived Gedankenexperiment, but that's no excuse for use who know better not to think about them. $\endgroup$ – CuriousOne Jul 8 '16 at 19:32
  • $\begingroup$ @CuriousOne : the time for low-interaction particles like that to decohere an electromagnetically bound system like that is actually pretty long, considering that we are able to do macroscopic Bell's theorem experiments: en.wikipedia.org/wiki/Bell_test_experiments $\endgroup$ – Jerry Schirmer Jul 8 '16 at 20:16
  • $\begingroup$ @JerrySchirmer: Indeed, and one of the failures of Schroedinger's cat is that it doesn't mention these things but leaves the student hanging with the false impression that it's an absolute statement about quantum states. Decoherence doesn't do that, it gets it right from the start. $\endgroup$ – CuriousOne Jul 8 '16 at 20:18
  • $\begingroup$ @CuriousOne: Fine, but you're kind of throwing a non-sequitir into this conversation, since this example is unaffected by decoherence from gravitons and neutrinos by a choice of a time frame less than the decoherence time. $\endgroup$ – Jerry Schirmer Jul 8 '16 at 20:33

There have been many questions and answers about this topic here on StackExchange. One type of response is to say that it's not a serious thought experiment, one then notes that it's not really possible to put a macroscopic object like a cat in a coherent superposition. But one can then argue that it's in principle possible (the laws of physics do not forbid creating such a state), and then one has to address this issue, but doing so depends on which interpretation of quantum mechanics one favors. Here one has to note that in the Many Worlds Interpretation (MWI), the superposition does not vanish due to interactions with the environment, it's just leads to a far more complicated superposition where the cat's state gets entangled with the rest of the World. So, the fact that you can't create a coherent superposition of the cat is totally irrelevant in that interpretation.

I favor my personal variant of the MWI, which is formally identical to it, except that I don't agree with the interpretation of the time dependent Schrödinger equation as specifying a time evolution. I don't believe that time really exists, I favor the block time view. Accordingly, I think it's more appropriate to consider the Schrödinger equation not as specifying a time evolution of the state vector, rather as just a change of basis of the Hilbert space. Or you could just work in the Heisenberg picture and consider the complete set of time evolving operators as specifying a one parameter family of observers.

So, in this picture one should interpret the superposition as just the initial state. All the information about the possible futures is present there and as long as you don't measure the state of the cat, you will still have access to the initial state. This is why both possible futures exist side by side, they already existed like that side by side right from the start, to see this you just need to apply the time evolution operator to the initial state.

After a measurement, you become entangled with the cat, but then that's a different "you" from the initial "you". All possible outcomes, all possible versions of "you", the cat etc. exist side by side in a timeless multiverse. The laws of quantum mechanics allow you to predict the probabilities of you finding yourself in a certain situation.

  • $\begingroup$ The Schroedinger equation is ontologically false, but it gives the correct results for a limited number of cases for which its false ontology maps neatly onto the non-relativistic single quantum case of quantum field theory. That makes for good physics, if we stay within the realm of applicability, but it makes for extremely poor philosophy when the interpretation of quantum mechanics based on the phenomenology to which the Schroedinger equation is restricted is being taken to far. $\endgroup$ – CuriousOne Jul 8 '16 at 19:29
  • $\begingroup$ @CuriousOne QFT has a natural wavefunctional formulation. You can limit QFT to apply to only asymptotically free particle states to calculate the outcome of scattering experiments, but the full theory is just ordinary QM applied to fields. What we measure in practice in experiments is just one small part of the theory. $\endgroup$ – Count Iblis Jul 8 '16 at 20:21
  • $\begingroup$ The problem occurs for massless fields. One can't catch a photon, as they say, so any ontology that presumes a local observer when mixed with a massless field suffers from decoherence because the total state of the observer's system plus the outgoing waves is unknowable. That, IMHO, is how nature creates a classical world to begin with: we don't have a choice, we are being measured, weighed and found too heavy all the time, just like the arrogant knight in "A Knight's Tale". :-) $\endgroup$ – CuriousOne Jul 8 '16 at 20:26
  • $\begingroup$ @CuriousOne Yes, but the structure of the laws of QM with its infinite dimensional Hilbert space is so different from the laws of classical mechanics, that it's unlikely that a description in terms of concepts from classical mechanics are anything more than an effective description. Just like you can describe the state of a fluid in terms of a local density field a velocity field etc., it's good enough in practice, but it's not going to give you an exact description of the physical situation. $\endgroup$ – Count Iblis Jul 8 '16 at 20:40
  • $\begingroup$ Schroedinger came up with this thing in 1935, I believe. Relativistic QM was in its infancy. He didn't know anything about decoherence, either, and, what's worse, he didn't think it trough, I am afraid. One can say the same about Newton and his corpuscular theory of light. Was it completely wrong? No. Is it useful? Not really. Same thing here. It doesn't teach us anything useful and, I am afraid, it doesn't ask the student to actually think critically about what is important in a physical scenario. IMHO it should not be taught any longer (we aren't teaching corpuscles, either, after all). $\endgroup$ – CuriousOne Jul 8 '16 at 20:45

Schrödinger's cat is a thought experiment about Heisenberg's cut.

The box where the cat and its death-via-decay mechanism live really represents Heisenberg's cut itself. The aim of the cat proposal was to show how weird it is to accept that QM can describe any physical system, not only elementary ones, by applying its concepts to a system including a living being.

Saying that the cat is both dead and alive has absolutely no physical significance, because if we want to consider what is in the box as a quantum system, then it is in a superposition of much, much more than two states. Indeed what would be the single state "dead"? Dead when? One seconds after we close the box? An hour after? A week after, when the cat is dead for sure because there is no more oxygen in the box? What about "alive"? There are many ways to be alive, and living things are known to have more degrees of freedoms than dead ones...

In no way can the cat be described as a plain two-state superposition, so any question about whether it is "alive and dead", "75% dead" or anything of the like is meaningless. The only point here is that, if it is correct that Heisenberg's cut can be put as far as we want in a Von Neumann chain, then the fact that there is no classical way to describe the state of the box contents translates in plain English as something like: "We have no idea what has become of the cat in the box, so much what we cannot even think about it as being an actual plain cat anymore. That's creepy, where is my cat, help me understand what's happening".

And of course, the whole point is that we still do not understand what is happening (Laloë, 2004); we still do not know what a quantum state really is.


We make a habit of treating classical statistical mixtures as being "ignorance" probabilities — the "reality" of a situation is that exactly one outcome holds, and we are just quantifying the information we have about the outcomes.

Schrödinger's cat is really just a classical problem involving a nondeterminstic event, and we would like to treat the nondeterminism as ignorance. However, there is overwhelming evidence that the nondeterministic source simply doesn't work that way.

So, Schrödinger's cat forces us to revise our interpretation of what probabilities mean (or postulate some unknown physics kicks in that forces the universe to align with our prior notions).

Language is an additional obstacle here, since it is awkward to distinguish, e.g., between talking about the event "$\text{dead}$ and $\text{alive}$" and talking about both of the events "$\text{dead}$" and "$\text{alive}$".


protected by Qmechanic Jul 10 '16 at 5:41

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