# Schrödinger's Cat: Could someone explain what this is supposed to mean? **URL:** \ **Category:** Factual Questions\ **Created:** [May 8, 2005, 10:43pm UTC](https://boards.straightdope.com/t/schrodingers-cat-could-someone-explain-what-this-is-supposed-to-mean/302976 "2005-05-08T22:43:09Z")\ **Posts on this page:** 14\ **Page:** 3
**Author:** ![Meeko](https://avatars.discourse-cdn.com/v4/letter/m/e495f1/32.png) [@Meeko](https://boards.straightdope.com/u/Meeko)\ **Post date:** [May 11, 2005, 5:54am UTC](https://boards.straightdope.com/t/schrodingers-cat-could-someone-explain-what-this-is-supposed-to-mean/302976/41 "2005-05-11T05:54:18Z")
Is this similar to that one guy claiming that all odds in poker are 50/50 … either you get your card or you dont? (Not my particular take on poker mind you) ---
**Author:** ![Alan\_Smithee](https://avatars.discourse-cdn.com/v4/letter/a/2acd7d/32.png) [@Alan\_Smithee](https://boards.straightdope.com/u/Alan_Smithee)\ **Post date:** [May 11, 2005, 6:03am UTC](https://boards.straightdope.com/t/schrodingers-cat-could-someone-explain-what-this-is-supposed-to-mean/302976/42 "2005-05-11T06:03:17Z")
I appreciate **Frankenstein Monster** ’s input (and I wouldn’t mind hearing a “rebuttal” from the “traditional” side at about the same level of accessability.) But mostly I’d like to know the answer to my and **tim314** ’s question above. If different observers are part of different but overlapping systems, can the waveform collapse in such a way as to give two different results to the two observers? If so, does this require multiple universes, or what? If not, aren’t the results determinate? ---
**Author:** ![antechinus](https://avatars.discourse-cdn.com/v4/letter/a/fbc32d/32.png) [@antechinus](https://boards.straightdope.com/u/antechinus)\ **Post date:** [May 11, 2005, 7:03am UTC](https://boards.straightdope.com/t/schrodingers-cat-could-someone-explain-what-this-is-supposed-to-mean/302976/43 "2005-05-11T07:03:48Z")
Hit me if i’m wrong, but doesnt this come down to something like this. Small things Very small (quantum) things like atoms, because they are so small, are less likely to interact with their surroundings and thus transfer information\*. So these things exist as a wave function, which only collapses when they do interact with other things. Big things Containing lots of little things, are interacting with themselves, air, light, lots of stuff, and in the process giving up information about themselves. Thus, big things do not exhibit the quantum sillyness of unobserved little things. \*Since observation is the transfer of information (beyond the system in question). Or is that a bit too simplistic. ---
**Author:** ![David\_Simmons](https://avatars.discourse-cdn.com/v4/letter/d/9de053/32.png) [@David\_Simmons](https://boards.straightdope.com/u/David_Simmons)\ **Post date:** [May 11, 2005, 7:17am UTC](https://boards.straightdope.com/t/schrodingers-cat-could-someone-explain-what-this-is-supposed-to-mean/302976/44 "2005-05-11T07:17:37Z")
> [@antechinus](#): > > Hit me if i’m wrong, but doesnt this come down to something like this. > > Small things > Very small (quantum) things like atoms, because they are so small, are less likely to interact with their surroundings and thus transfer information\*. So these things exist as a wave function, which only collapses when they do interact with other things. > Big things > Containing lots of little things, are interacting with themselves, air, light, lots of stuff, and in the process giving up information about themselves. Thus, big things do not exhibit the quantum sillyness of unobserved little things. > \*Since observation is the transfer of information (beyond the system in question). > > Or is that a bit too simplistic. That’s about what my source, Dr. Wilcox, wrote. I’ll go check out the book tomorrow and get his exact words. ---
**Author:** ![Frankenstein\_Monster](https://avatars.discourse-cdn.com/v4/letter/f/ccd318/32.png) [@Frankenstein\_Monster](https://boards.straightdope.com/u/Frankenstein_Monster)\ **Post date:** [May 11, 2005, 7:32am UTC](https://boards.straightdope.com/t/schrodingers-cat-could-someone-explain-what-this-is-supposed-to-mean/302976/45 "2005-05-11T07:32:45Z")
> [@Alan Smithee](#): > > If different observers are part of different but overlapping systems, can the waveform collapse in such a way as to give two different results to the two observers? If so, does this require multiple universes, or what? If not, aren’t the results determinate? IANAQuantum Mechanic and I hope that a “traditional” QM expert could clarify this, but meanwhile, from [here](http://plato.stanford.edu/entries/qt-measurement/) and [here](http://plato.stanford.edu/entries/qm-copenhagen/) > [@](#): > > The paradoxes and questions raised by the measurement problem have spawned a host of interpretations of QM, including (…) the Everett-Wheeler “many worlds interpretation” … Most physicists bypass these philosophical resolutions of the interpretative difficulties of QM, and revert instead to some version of the Bohr interpretation. So, what is the Bohr Interpretation (= Copenhagen Interpretation)? > [@](#): > > Bohr maintained that the physical properties of quantum systems depend in a fundamental way upon experimental conditions, including conditions of measurement… > > But, instead of taking the dependence of properties upon experimental conditions to be causal in nature, he proposed an analogy with the dependence of relations of simultaneity upon frames of reference postulated by special relativity theory: … My understanding is that QM doesn’t “require” multiple universes. They are just one of the many attempts to “understand” QM. Call it a matter of taste. As for the traditional (Copenhagen, non-multiple-universes) interpretation of your scenario: my understanding of the Copenhagen Interpretation is that you have to describe (= write down the formula that predicts the result) the whole experiment you are doing. The result of a measurement depends not just on the system you are measuring but also on the system that does the measurement and the way you do the measurement. With your two experimenters, each would have his own formula for the experiment he/she is doing. A separate formula for each of the experimenters. The experimenter in the room “collapses” his own formula and the experimenter outside the room “collapses” a different formula. They could see the same outcome or even, in some strange cases, a different outcome. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [May 11, 2005, 7:51am UTC](https://boards.straightdope.com/t/schrodingers-cat-could-someone-explain-what-this-is-supposed-to-mean/302976/46 "2005-05-11T07:51:42Z")
> [@](#): > > They could see the same outcome or even, in some strange cases, a different outcome. Once everyone involved sees the outcome, they’ll agree on what that outcome was. Unless you’re using the Many Worlds interpretation, in which case observers in different “worlds” could make conflicting observations, but that doesn’t really count, since those observers who disagree can never communicate their disagreement with each other. As I said in the spinoff thread, the simplest interpretation is to use no interpretation at all. You can accurately predict the probability that, when you open the box, the cat is alive, and this prediction is independant of your interpretation of quantum mechanics. What the status of the cat is before you make your observation depends on your interpretation, but that’s not properly a question of science, since science deals only in observables, and you can’t observe the cat before you observe it. ---
**Author:** ![EarthStone777](https://avatars.discourse-cdn.com/v4/letter/e/7ab992/32.png) [@EarthStone777](https://boards.straightdope.com/u/EarthStone777)\ **Post date:** [May 11, 2005, 3:48pm UTC](https://boards.straightdope.com/t/schrodingers-cat-could-someone-explain-what-this-is-supposed-to-mean/302976/47 "2005-05-11T15:48:20Z")
> [@Chronos](#): > > As I said in the spinoff thread, the simplest interpretation is to use no interpretation at all. You can accurately predict the probability that, when you open the box, the cat is alive, and this prediction is independant of your interpretation of quantum mechanics. What the status of the cat is before you make your observation depends on your interpretation, but that’s not properly a question of science, since science deals only in observables, and you can’t observe the cat before you observe it. So you’re cutting to the chase using Occams Razor? Hey, I could have said **Chronos** is skinning the cat using Occams Razor… FWIW, I agree with you **Chronos** , all the other ideas seem overly complex and unwieldy. Nature (imho) likes a simple solution. ---
**Author:** ![tim314](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tim314/32/3468_2.png) [@tim314](https://boards.straightdope.com/u/tim314)\ **Post date:** [May 11, 2005, 3:57pm UTC](https://boards.straightdope.com/t/schrodingers-cat-could-someone-explain-what-this-is-supposed-to-mean/302976/48 "2005-05-11T15:57:18Z")
I guess I’ll let the spinoff thread die and keep responding here. Here’s what I’m not understanding: On the one hand, **Chronos** says that the only thing that physics can really talk about is observables. The state of the cat _prior_ to its observation is unobservable (by definition). So that’s a question of philosophy, not physics. That sounds sensible to me. _Except_, if that’s the case, why can’t we simply say that either the cat is alive or the cat is dead? I thought the whole point of the Schrodinger’s cat experiment was that it’s incorrect to say that the cat is just “alive” or just “dead” prior to its observation – otherwise, it’d be no different than classical physics. But it’s not like classical physics, right? Saying the cat is either alive or is dead (the classical answer) is less correct than saying he’s in a superposition of “alive” and “dead” states. Isn’t it? In fact, I thought there _were_ observational consequences of a system having been in a superposition – like interference effects, right? But if there _are_ observational consequences of a system being in a superposition of states, then it doesn’t seem like we can just bypass the question of whether an observer can be in a superposition of states relative to another observer. I guess what I’m saying is, even if there are several equally consistent explanations for what happens _prior_ to the measurement, there are still some that are _inconsistent_, right? Like _classical_ physics. And if classical physics is basically analogous to the wave function being “always collapsed”, it seems like we can _at least_ make statements about whether the wave function is at some point _not_ collapsed – i.e., whether you have quantum effects or not. In which case, how is it a non-physical question to ask “Is the wave function collapsed so long as one observer is observing the state, or is it still in a superposition relative to another observer?” In other words, does the fact that a system is observed by one observer prevent the other from seeing any further non-classical behavior in the system (in the sense of interference effects, etc. – I’m not talking about non-classical behavior that occurs even while a system is observed, like uncertainty in measurement, etc.) Or more to the point: can one observer observe quantum effects like interference _in another observer_? Whether this is due to multiple universes, ghost observers, hidden variables, whatever, I don’t care. I want to know if one observer observing something (such as himself) prevents it from behaving like an “unobserved” thing relative to someone else. Given that we can empirically distinguish “observed” from “unobserved” (like in the double slit experiment) it seems like this would be a legitimate question of physics. (Although if it isn’t, I’d appreciate further clarification as to way that is.) ---
**Author:** ![tim314](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tim314/32/3468_2.png) [@tim314](https://boards.straightdope.com/u/tim314)\ **Post date:** [May 11, 2005, 4:00pm UTC](https://boards.straightdope.com/t/schrodingers-cat-could-someone-explain-what-this-is-supposed-to-mean/302976/49 "2005-05-11T16:00:16Z")
> [@EarthStone777](#): > > FWIW, I agree with you **Chronos** , all the other ideas seem overly complex and unwieldy. Nature (imho) likes a simple solution. But Chronos isn’t commenting on _nature_. He’s saying what’s “really happening” is unknowable, and thus we can’t really talk about it (at least without venturing from physics into philosophy). That doesn’t mean there’s anything remotely simple about what’s “really happening.” It’s not a matter of nature trying to make things simple, it’s a matter of us choosing not to think about questions that are ultimately unanswerable. ---
**Author:** ![Mathochist](https://avatars.discourse-cdn.com/v4/letter/m/c89c15/32.png) [@Mathochist](https://boards.straightdope.com/u/Mathochist)\ **Post date:** [May 11, 2005, 4:13pm UTC](https://boards.straightdope.com/t/schrodingers-cat-could-someone-explain-what-this-is-supposed-to-mean/302976/50 "2005-05-11T16:13:00Z")
> [@Chronos](#): > > the simplest interpretation is to use no interpretation at all. For those of you following along at home, I believe this is technically known as the “shut up and calculate” interpretation. Seriously. It’s in the literature. ---
**Author:** ![Mathochist](https://avatars.discourse-cdn.com/v4/letter/m/c89c15/32.png) [@Mathochist](https://boards.straightdope.com/u/Mathochist)\ **Post date:** [May 11, 2005, 4:25pm UTC](https://boards.straightdope.com/t/schrodingers-cat-could-someone-explain-what-this-is-supposed-to-mean/302976/51 "2005-05-11T16:25:52Z")
> [@tim314](#): > > But it’s not like classical physics, right? Saying the cat is either alive or is dead (the classical answer) is less correct than saying he’s in a superposition of “alive” and “dead” states. Isn’t it? In fact, I thought there _were_ observational consequences of a system having been in a superposition – like interference effects, right? Yes, there are observational effects, but in the “classical limit”, all the waves that differ by much from the classical observed behavior come paired off with another wave exactly out of phase. If I throw a ball across a room, it follows a parabolic arc. There’s a probability wave associated to it following any other path, but anything differing significantly from the classical parabolic arc cancels against another such “weird” path. This sort of thing is very clearly dealt with in one of Omnes’ books, though I forget which offhand. Try _Quantum Philosophy_ first, since it’s a worthwhile read anyway. Incidentally, one of the things people like Penrose are trying to do with their solutions to the measurement problem (which _are_ in principle testable) is to investigate the “mesoscale” and figure out how quantum mechanics becomes classical mechanics in the limit. As a side effect, all this ad hoc language about “observers” (which are never really well defined) goes away. If you have only one evolution of quantum systems and no quantum leaps, you have no need to grant some fuzzy class of systems the special status of observers. ---
**Author:** ![Omphaloskeptic](https://avatars.discourse-cdn.com/v4/letter/o/bcef8e/32.png) [@Omphaloskeptic](https://boards.straightdope.com/u/Omphaloskeptic)\ **Post date:** [May 12, 2005, 12:56am UTC](https://boards.straightdope.com/t/schrodingers-cat-could-someone-explain-what-this-is-supposed-to-mean/302976/52 "2005-05-12T00:56:30Z")
> [@tim314](#): > > But it’s not like classical physics, right? Saying the cat is either alive or is dead (the classical answer) is less correct than saying he’s in a superposition of “alive” and “dead” states. Isn’t it? In fact, I thought there were observational consequences of a system having been in a superposition – like interference effects, right? Correct; there is a measurable difference between a superposition of states and a “collapsed” state (or, in another terminology, between coherent and incoherent superpositions of states). > [@](#): > > But if there are observational consequences of a system being in a superposition of states, then it doesn’t seem like we can just bypass the question of whether an observer can be in a superposition of states relative to another observer. > > I guess what I’m saying is, even if there are several equally consistent explanations for what happens prior to the measurement, there are still some that are inconsistent, right? Like classical physics. And if classical physics is basically analogous to the wave function being “always collapsed”, it seems like we can at least make statements about whether the wave function is at some point not collapsed – i.e., whether you have quantum effects or not. In which case, how is it a non-physical question to ask “Is the wave function collapsed so long as one observer is observing the state, or is it still in a superposition relative to another observer?” In other words, does the fact that a system is observed by one observer prevent the other from seeing any further non-classical behavior in the system (in the sense of interference effects, etc. – I’m not talking about non-classical behavior that occurs even while a system is observed, like uncertainty in measurement, etc.) > > Or more to the point: can one observer observe quantum effects like interference in another observer? Whether this is due to multiple universes, ghost observers, hidden variables, whatever, I don’t care. I want to know if one observer observing something (such as himself) prevents it from behaving like an “unobserved” thing relative to someone else. Given that we can empirically distinguish “observed” from “unobserved” (like in the double slit experiment) it seems like this would be a legitimate question of physics. (Although if it isn’t, I’d appreciate further clarification as to way that is.) This is a legitimate question; and the short answer is, we don’t know. Some theories (like Penrose’s, mentioned in some earlier posts) predict that wavefunction collapse, even for a “closed” system, is _inevitable_ once different states in the superposition have sufficiently-different values of some macroscopic variables: for Penrose, four-momentum density, which couples to gravity. (Penrose’s theory would probably better be described as saying that because it’s impossible to isolate a system from gravitational effects, the entire formalism of closed quantum systems is inapplicable to most macroscopic quantum systems.) Under this kind of theory, the mere act of observation by a (sufficiently macroscopic) observer is enough to effectively collapse the waveform (or at least to entangle it with the rest of the universe in a practically irreversible way). The simplest extensions of standard quantum theories to macroscopic systems, though, predict that any closed quantum system, initially in a pure state, will evolve into another pure state. Even if the closed system contains an “observer” (“Alice”) along with the “observed” system S, the combined state of S+Alice will not collapse until it is (externally) measured. The coherence of this state is in principle measurable; so these theories differ from the Penrose-like forced-decoherence theories above, and these differences are in principle measurable. In practice, though, it is extremely difficult to actually keep a system isolated from its surroundings. For all practical purposes, given the current state of technology, we may as well assume immediate wavefunction collapse whenever a system is observed by a human or any other complicated object. The coherence of mesoscopic quantum systems is an important research topic, though, so if you’re willing to broaden your notion of “observer” to things like large Bose-Einstein condensates or optical lattices, then maybe this question will start to get answered in the next few years. ---
**Author:** ![antechinus](https://avatars.discourse-cdn.com/v4/letter/a/fbc32d/32.png) [@antechinus](https://boards.straightdope.com/u/antechinus)\ **Post date:** [May 12, 2005, 3:35am UTC](https://boards.straightdope.com/t/schrodingers-cat-could-someone-explain-what-this-is-supposed-to-mean/302976/53 "2005-05-12T03:35:51Z")
> [@Omphaloskeptic](#): > > In practice, though, it is extremely difficult to actually keep a system isolated from its surroundings. For all practical purposes, given the current state of technology, we may as well assume immediate wavefunction collapse whenever a system is observed by a human or any other complicated object. The coherence of mesoscopic quantum systems is an important research topic, though, so if you’re willing to broaden your notion of “observer” to things like large Bose-Einstein condensates or optical lattices, then maybe this question will start to get answered in the next few years. What is a complicated object? Why cant any interaction of one object with another object entail an observation? i.e. can an observation be simply the entanglement of quantum states? So, when two objects interact they observe each other. At a macroscopic level this would blow out to the whole universe becoming entangled because macroscopic objects are quickly communicating information with one another at the speed of light. So you cannot have a closed system at a macroscopic level. Does the separation of macroscopic from quantum levels come down to the rate at which interactions occur? Or am I off the mark? ---
**Author:** ![Omphaloskeptic](https://avatars.discourse-cdn.com/v4/letter/o/bcef8e/32.png) [@Omphaloskeptic](https://boards.straightdope.com/u/Omphaloskeptic)\ **Post date:** [May 12, 2005, 5:11am UTC](https://boards.straightdope.com/t/schrodingers-cat-could-someone-explain-what-this-is-supposed-to-mean/302976/54 "2005-05-12T05:11:58Z")
> [@antechinus](#): > > What is a complicated object? > > Why cant any interaction of one object with another object entail an observation? i.e. can an observation be simply the entanglement of quantum states? So, when two objects interact they observe each other. Sure; you can call it an observation if you like. Standard quantum mechanics doesn’t particularly care whether the measuring device is a low-dimensional Hilbert space or an enormous one. But for small “measuring” devices (entanglement of one photon with another, etc.) experimental results exist (and are consistent with standard QM). I was just addressing the questions of the macroscopic observer (Schrodinger’s cat, Wigner’s friend, etc.), which have not yet been resolved experimentally and for which new physics may therefore exist. > [@](#): > > Does the separation of macroscopic from quantum levels come down to the rate at which interactions occur? Or am I off the mark? This is the idea of many “decoherence” theories of measurement, in both standard QM and some nonstandard theories (like Penrose’s). These are fairly elegant solutions to a lot of the problems with observers and measurements, but (again) experiments to confirm or refute them are pretty tough to do and haven’t been done for human-scale systems. [Previous page](https://boards.straightdope.com/t/schrodingers-cat-could-someone-explain-what-this-is-supposed-to-mean/302976.md?page=2)