Talk:Schrödinger's cat

Apostrophe

Why the heck does this have an apostroph? Is it sort for "Schrödinger is cat"? — Preceding unsigned comment added by 176.198.62.38 (talk) 01:35, 16 February 2013 (UTC)

The apostrophe is applying possession. - Camyoung54 talk 20:56, 10 April 2013 (UTC)

"Contradiction with QM" section

I don't see what relevance the statements in this newly added section have to the article. No objects are being detected at "two different places at once" in the experiment. In experiments on "cat states", the "cat states" (superpositions) are detected indirectly, by their interference pattern. Also the content needs to be sourced. --Chetvorno^TALK 08:17, 3 August 2013 (UTC)

The relevance is that all claims that QM implies real cat states are meterphysical, and not part of the mathematical formulation of QM itself. A “cat state, is the claim that an object really does physically exist in more than one of the states given by the general state vector at the same time. Position is just one representative example for illustrative purposes. The specific experiments being discussed are immaterial. It could be spin, momentum or whatever. E.g for |psy> = |1> +|2>, a cat state claim is a claim that in real physicality that the state |1> and |2> actually exist simultaneously. QM theory specifically denies that such a measurement can ever be made, therefore such claims should not be implied. A superposition of states should not be interpreted as if such superposition implies a cat can be both dead and alive at the same time. QM rejects that such a concept could ever be proved by an actual measurement, and does not require such a metaphysical interpretation. I can make my self invisible, but only if your back is turned to me sort of thing. There is nothing to source, any credible reference on the postulates of QM states that the result of any measurement must be a single eigenvalue. Its what state vector collapse means! Kevin Aylward 09:06, 3 August 2013 (UTC) — Preceding unsigned comment added by Kevin aylward (talk • contribs)

I don't see that the article says that an object will "physically exist in more than one of the states". The article makes it pretty clear that if an observation is made, the result will be either |live cat> or |dead cat>. The term "cat state", which is not used in the article's explanation of the experiment, refers to the superposition itself, not the result of a measurement (hence the term "state"). The existence of superpositions ("cat states") has been established "unambiguously" and is not in doubt. --Chetvorno^TALK 10:50, 3 August 2013 (UTC)

The article cites experiments, for example, http://www.scientificamerican.com/article.cfm?id=quantum-microphone. This article specifically states, “For example, a single electron can move along multiple different paths or an atom can be placed in two different places, simultaneously.” This sort of text is pretty much the norm whenever QM is discussed. It seems appropriate to dispel this common misunderstanding of QM and make a specific note that standard QM says no such thing. The existence of an object truly physically existing as a superposition has never been established, nor is that possible, according to QM. Producing measurements that rely on the notion of a summed state does not negate this view. QM is essentially, a mathematically calculation tool. The state vector “sum” of states is not a physical sum. The plus sign is a standard logical OR operator as due to the requirement that the state vector is a probability function. It is clear that it needs to be clarified that a cat state does not mean that an object can physically have two simultaneous eigenvalues as it is clear many do. Kevin Aylward 12:06, 3 August 2013 (UTC) — Preceding unsigned comment added by Kevin aylward (talk • contribs)

Many do what? It's pretty obvious that the cat state doesn't have two eigenvalues, it has no eigenvalues at all since it's not an eigenstate of the "cat alive" operator. The paragraph you inserted is, as it stands now, grammatically wrong, physically dubious, and worst of all, completely unsourced. I'm sorry, but at the moment I don't see a good reason for not removing it. — HHHIPPO 14:19, 3 August 2013 (UTC)

Many claim that superposition means two simultaneous states. I agree, the cat state does not have two eigenvalues. The point though, is that that the popular accounts regarding cat states ARE claiming that “two things at once” is the meaning of such superpositions. I already pointed out that that is exactly what one article referenced in this article actually stated. Regarding dubiousness. Is it true or not true that QM demands a measurement must result in a single eigenvalue? Is it true or not true that articles are going around saying that objects physically exist in two states at once? So what’s exactly your problem? What exactly wrong with the grammar also?Kevin Aylward 16:04, 3 August 2013 (UTC) — Preceding unsigned comment added by Kevin aylward (talk • contribs)

Regardless of the argument, without a source the paragraph is original research and should be removed. -Jordgette [talk] 22:57, 3 August 2013 (UTC)

The example you gave above 1 was an article in Scientific American for general readers, the same target audience this article has. It's statement that "an atom can be placed in two different places" was referring to the superposition - it said so in the next sentence - there was nothing about objects being "observed" in two different places. Some such language is needed to describe the eigenstates of the state vector to nonscientists. Regardless of whether we like it or not, as you pointed out this language is the "norm" in RSs, which is what WP must follow. --Chetvorno^TALK 00:16, 4 August 2013 (UTC)

The article states "...an atom can be placed in two different places, simultaneously..." This is unprovable waffle. The norm does not mean that that it should not be pointed out that the norm is wrong. Its relatively easy to explain to non scientists exactly what multiple eigenstates mean. Hint. Ensemble interpretation description of a state vector for classical dice throwing. Hands up those that believe that measuring two eigenvalues simultaneously is NOT a contradiction to QM. So...whats the problem...oh whatever...Kevin Aylward 07:10, 4 August 2013 (UTC) — Preceding unsigned comment added by Kevin aylward (talk • contribs)

Your addition has a worse waffle. It implies that superpositions (cat states), the subject of this article, violate QM. "...if an experiment unambiguously, directly shows a true Shrödinger (sic) cat state... then QM would be falsified." Hands up those who believe with Kevin that there are no such things as superpositions in QM. --Chetvorno^TALK 08:58, 4 August 2013 (UTC)

"This poses the question of when exactly quantum superposition ends and reality collapses into one possibility or the other"

Why would "reality" collapse into one possibility or another? This just seems preposterous. I do not understand this part of the article at all. Perhaps it should be redone. 98.81.2.13 (talk) 09:21, 29 October 2013 (UTC)

It's hard to visualize, but the uncertainty principle of quantum mechanics says each radioactive atom is in a combination or "superposition" of two "states" simultaneously: one in which it has decayed and emitted a radioactive particle, and one in which it hasn't. Therefore, if the superposition doesn't collapse into one possibility or the other, the radioactivity detector will also be in two states: one in which it has detected the radiation particle and one in which it hasn't. So the flask of poison will also be in two states: broken and unbroken. And the cat will be in two states: dead and alive. Since we never see macroscopic objects like cats in two "states" simultaneously, somewhere in this chain of events the "superposition" must have "collapsed" and the world settled into the result we see: either the radioactive atom "did" decay, and the cat is dead, or the radioactive atom "didn't" decay and the cat is alive. The problem is scientists don't know where in the chain of events from atom to cat the collapse occurs. The Nobel prize in physics this year was given for progress on "cat states". --Chetvorno^TALK 16:19, 29 October 2013 (UTC)

If the cat is alive, it will know which state it is in. There is no superposed dead state in that case. The confusion is between (a) the QM uncertainty which produces a mixed state at the microscopic level, and (b) the superimposed uncertainty of those outside who are juggling ordinary probabilities of the cat being dead or alive. Imagine a repeating gun inside the box to see the latter. No QM, just a probability that the cat walks in front of the gun. Anyway, interactions are what, if anything, causes collapse, be they with humans or apparatus or cats or walls of boxes or air molecules. The wave collapse, if it happens, happens well before the box is opened. There is no live-dead cat in there, any more than there would be if the cat were replaced by a human. 51kwad (talk) 14:54, 7 November 2013 (UTC)

Relational quantum mechanics has a very nice treatment of such issues, for example Wigner's friend. -Jordgette [talk] 18:53, 7 November 2013 (UTC)

51kwad, the idea that collapse happens immediately, on the atomic scale, is one interpretation, called Objective collapse theory, but not the only one compatible with observations. The cat (or any observer inside the box) cannot resolve the question, because in those theories in which superpositions do not collapse reality "splits" and there are multiple cats, some alive, some dead. Due to "decoherence" the different realities do not interact, so each cat knows only its own history. We end up in only one branch of a branching reality. --Chetvorno^TALK 05:10, 30 May 2014 (UTC)

Exactly when does the world fork according to many-worlds?

This paragraph, intended to clarify the many-worlds interpretation, confuses me:

In 1957, Hugh Everett formulated the many-worlds interpretation of quantum mechanics, which does not single out observation as a special process. In the many-worlds interpretation, both alive and dead states of the cat persist after the box is opened, but are decoherent from each other. In other words, when the box is opened, the observer and the possibly-dead cat split into an observer looking at a box with a dead cat, and an observer looking at a box with a live cat. But since the dead and alive states are decoherent, there is no effective communication or interaction between them.

I thought that the worlds split much earlier, at the instant when the particle decayed. Thereafter there would be two cats (one staying alive and one about to be killed in a few seconds), and also two persons that would some time later observe either the alive or dead cat.

Is this interpretation wrong? Mumiemonstret (talk) 10:24, 5 December 2013 (UTC)

The wavefunction inside the box splits when the particle decays (actually, because it is the time of decay that differs between states, and the particle can decay at any time, the split begins when the box is closed, and as time passes the amplitude - probability - of the "particle decayed|cat dead" eigenstates increases). When the box is opened, the internal wavefunction interacts with the wavefunction of the external world, and the external world splits. I believe the split propagates causally, at the speed of light, to more remote regions. --Chetvorno^TALK 11:01, 5 December 2013 (UTC)

But the wave function is constantly interacting with other things inside the box. Particles, cats, the walls of the box, maybe even a human observer. Why single out the opening of the box as the interaction that causes the collapse? The wave function inside the box will be extremely coherent, as either a dead cat or a live cat, before the box is opened. We just don't know from outside which one it is, any more than we would know the throw of a dice inside the box. There is no need for a split to propagate causally at the speed of light, any more than conventional probability would require that in the dice case. There are two probabilities going on here: (1) the quantum one, which immediately macroscopically resolves itself, and (2) the conditional probability one outside the box, whether the cat will leap out alive or will be dead. Opening the box resolves the latter probability but in no way affects the former. 51kwad (talk) 10:28, 5 June 2014 (UTC)