Talk:Many-worlds interpretation of quantum mechanics

New intro

The theory does not attempt to explain superposition of states! That's an entirely different, well understood issue. It tries to explain, among other things. measurement. CSTAR 05:19, 2 Dec 2004 (UTC)

I'm going to revert it, although we should try to incorporate suggestions and improvements of Aarchiba into the reverted version; the newer (previous to the revert) version has too many incorrect statements.CSTAR 05:26, 2 Dec 2004 (UTC)

Additional comments

We should take Aarchiba's attempt to improve the article seriously, although I objected to several facts of his newer ersion:

• Superposition as I mentiuned above
• The phrase parallel universes at the beginning of the article, gives me at least, the creeps.
• The important fact isn't so much the parallel universes themselves, but the superposition of the so-called parallel universes (interpreting the mathematics that follows)
• Stuff was moved out of the intro that should have been there, such as Everett's leaving physics.

CSTAR 05:49, 2 Dec 2004 (UTC)

Hmm. I wasn't entirely happy with the old version, but perhaps we should include it here, for easier comparison.

The reason I wrote it is because I find the article very difficult to read in spite of a reasonable physics background; for a general encyclopedia we can surely do better.

I'll try to address the points:

• Superposition: probably a bad choice of phrasing. The intent was to explain that it tries ot understand superposition of states not as a temporary phenomenon but as a basic property of the universe.
• The "parallel universes" was there not because it describes the theory but because it describes what popular culture has done with the theory (and that will give many readers some idea of what's going on).
• The history I moved to another section in the interest of having a short lead section.

Let's try to make a wishlist for the article. See below. --Andrew 07:03, Dec 2, 2004 (UTC)

I think the article needs:

• A good intro that can at least tell the non-technical reader whether they're in the right place - at most two or three paragraphs (good luck!)
  • It's one of the leading interpretations of quantum mechanics
  • It views the universe as endlessly branching: all possibilities happen
  • It led to the idea of parallel universes in fiction
  • It was invented by Everett
• A longer intro, still for the non-technical reader, that describes what problems it's intended to resolve, what problems it introduces, and maybe a simple example (no, simpler than the "simple example")
• A history section describing who invented it, who developed it, and who based which further theories on it
• A section comparing it with alternative interpretations
• The hardcore technical stuff in the article now
• Good linking to explanations sufficient to have a hope of understanding the hardcore technical explanations

Please just go ahead and edit the list; if you think something'll be controversial flag it, but otherwise it's easier to hack directly on this. --Andrew 07:03, Dec 2, 2004 (UTC)

Relative states

Is the article about the relative-states formalism or the many-worlds interpretation? The article claims they're different and that it's bad to confuse them, then seems to go off and talk about relative states (only?). Perhaps relative states formalism should have a separate article? --Andrew 07:03, Dec 2, 2004 (UTC)

They're really the same; that was the point of the mathematics that follows. Doesn't the article say they are the same right at the top? I wasn't aware the article said they're different somewhere else, although that statement may be in some legacy paragraph; who knows, I'll have a look (although later my connection is very slow now).

I also don't like the annotation on the graphic. It refers to decision; QM has nothing to do with decisions by anybody. Such language seems to suggest there is a connection between consciuousness and QM and I think I am in good company when I assert there isn't. I will make a new graphic to replace it. CSTAR 14:41, 2 Dec 2004 (UTC)

"Rejoining" universes?

I'll preface this by saying that I am in no way a physicist and am mostly interested in the philosophical implications of this theory. Having said that, I have heard or read somewhere that there is an alternate or modified many-worlds theory in which universes that have branched apart can come back together (as it were) with the collapse of certain wave functions (or some other process unknown to me). Does this sound familiar to anyone, and if so, should it be included in the article? --Anakolouthon 21:59, 6 Dec 2004 (UTC)

See Michael Price's FAQ where that possibility is briefly mentioned. I would rather not put that in. CSTAR 23:12, 6 Dec 2004 (UTC)

The process of branching can be regarded as "dimensionality increase" of the ambient trajectory space. (Think of quantum mechanical evolution as being simulated on n-qubit registers). This is related to (a) increased entanglement and (b) increased entropy (there is a well-known relation between entanglement and entropy). For this reason, the existence of rejoining universes would violate basic principles of thermodynamics. However, I'm at the limits of what I know here (beyond maybe?) CSTAR 23:35, 6 Dec 2004 (UTC)

No basic principles of thermodynamics would have to say that anything that seperates must come back together, seems logical to me considering finite energy (conservation of) and entropy limits °Togo

The increase of entropy more or less forbids universes interacting (by coming back into coherence) in the same way that it forbids a glass from un-breaking. But read the FAQ. --Andrew 06:54, Dec 13, 2004 (UTC)

Good Checklist

• I agree heartily with the sentiment of improving the introduction of this page. The current page, while very descriptive and informative, does really not give the reader a sense of what MWI actually *is* until somewhere around the 4th paragraph of the "Relative State" section.

Perhaps a good first step could be to add a summary sentence at the top? Perhaps something like

"Yet when the particles are observed, they appear as particles and not as non-localized waves. To explain this phenomenon, the Copenhagen interpretation of quantum mechanics proposed a process of "collapse" from wave behavior to particle-like behavior to explain this phenomenon of observation. The Many-worlds interpretation, on the other hand, proposes that each time an observation is made, each possible state the object could have been in splits off into a separate world or reality."

Maybe the information starting with "By the time John von Neumann..." could be put in a "History" section or something?

• I think there is some good information in here, but is is buried deeply. It would be nice to present it in a way that was quickly understandable. For example, the sentences

"One consequence is that every observation causes the universal wavefunction to decohere into two or more non-interacting branches, or "worlds". Since many observation-like events are constantly happening, there are an enormous number of simultaneously existing states."

and

"In the Copenhagen interpretation, the mathematics of quantum mechanics allows one to predict probabilities for the occurrence of various events. In the many-worlds interpretation, all these events occur simultaneously."

Are very nice. Maybe there is a clearer way to sum them up for a reader that is merely looking for a brief overview of MWI?

• Also, do you think it would be acceptable to mention the writings of David Deutsch? In his Fabric of Reality book he offers a variant interpretation of MWI, the Multiverse, which seems to be a variant on MWI.

Lastly, Henry Sturman proposes an experiment which could be used to distinguish between MWI and Copenhagen here (http://www.henrysturman.com/english/articles/multiverse.html) (see the "Proof of other universes" section). Anyone opposed to adding a link to this? (perhaps this should go on the copenhagen page as well?)

Sorry for the ramble. I'm pressed for time with schoolwork atm, or I would make some edits myself. Since this page has a lot of information and a decent-sized Talk page, I thought I would check before doing anything drastic. --Culix 01:30, 13 Dec 2004 (UTC)

To preface my remarks: My knowledge of QM is limited and I have not read Deutsch's book.

From the web page referenced, it sounds like Deutsch's book is a popularization of QM from a proselytizing MWI point of view. If we have an article on the book, we should link to that article, but as a reference text on the subject I don't think we can recommend it: the web page author correctly points out a major flaw in Deutsch's motivating example.

On the other hand, I don't think the web page's author is sufficiently knowledgeable about QM to warrant being cited as a reference. And in particular, I don't think his hypothetical experiment for testing MWI is worth linking to - it's pretty clear that it won't work (not that he claims to be sure it will). Now, there have been various working physicists who have published journal articles on experiments, theoretical and otherwise, that attempt to test MWI. Those, it would be worth referencing. It's worth pointing out, though, that an interpretation is not necessarily testable, in the way that a theory is. It's quite possible (even likely) that MWI will never differ from the Copenhagen interpretation in any prediction. The best we could then hope for would be to come up with ever more elaborate situations of waveform collapse/quantum decoherence and see which interpretation is most palatable. --Andrew 04:08, Dec 13, 2004 (UTC)

I added a new second sentence which explains what it is.CSTAR 04:27, 13 Dec 2004 (UTC)

P.S. I'm not sure it's any better with the new sentence, but at least it tries to solve the problem of explaining MWI as quickly as possible. It's always difficult to strike a balance between accuracy and comprehensibility and frustrating to attempt that balance. Ideally one would like to avoid the level of What the Bleep Do We Know?!. CSTAR 05:06, 13 Dec 2004 (UTC)

I think it's an improvement, but it could still maybe be clearer. It kind of obscures the really revolutionary part of the theory:

It attempts to explain the process of observation of a quantum system as a quantum superposition of states of several copies of the original system.

The revolutionary parts of the theory are:

• It does away with the privileged status of "observation".
• It makes copies of the whole universe.

On another level, we should be able to give a vague notion what theory this is to people to whom "quantum superposition of states" is incomprehensible.

How about:

It avoids the privileged status of observation in the Copenhagen interpretation by viewing the entire universe as a quantum superposition of many states which do not measurably interact on macroscopic scales.

--Andrew 06:50, Dec 13, 2004 (UTC)

Yes that sounds good and is accurate. Please go ahead and put it in. I believe you are suggesting it as the 3rd sentence?

I would however, keep the von Neumann part later in the intro, despite Culix suggestions (which I think are also very good). BTW, please pardon my being somehat obsessive about this. CSTAR 15:00, 13 Dec 2004 (UTC)

Not putting up with inaccurate, clumsy writing (such as my earlier attempt) is the best way to get a good article - keep it up. --Andrew 18:01, Dec 13, 2004 (UTC)

Those changes look very nice, good work to you both! And no worries about being obsessive - I wholeheartedly agree with improving and tweaking this article ;)

I agree that Deutsch's book has a somewhat religious fervor to it; though this may be because he believes it "works so well!". Perhaps you are right about not using it as a reference. Hm. I would be interested to know what you don't like about Sturman's experiment. I am not very knowledgable about the field of QM, so maybe your arguments against his experiment would help illuminate something.

I like the new sentence. I also agree that it would be nice to have something for "people to whom "quantum superposition of states" is incomprehensible". What do you think about adding on to the end of the sentence, something like " - in layman's terms, there are many overlapping universes that only affect each other in small ways." ? --Culix 21:25, 13 Dec 2004 (UTC)

Except that only affect each other in small ways is not quite right. "quantum superposition of states" is a particular way of combining states.

Think of a coin, it is a system characterized by two states heads or tails. When you toss the coin, it has one of two outcomes i.e. lands up in one of the two states. The most know-nothing approach to describing this outcome is to say it is nondeterministic. We can actually say a little more. Indeed empirically we know the outcome is probabilistic which means that with a certain probability p it comes out heads and with probability q=1 - p it comes out tails. This probabilistic formulation has an operational interpretation based on repeated trials (we can formulate this better mathematically but I won't do it). If the coin is fair p=q =1/2.

A quantum mechanical coin has a whole family of superposition of states which can be viewed geometrically as the Bloch sphere. Tails |0> is the south pole and heads |1> is the north pole. CSTAR 22:17, 13 Dec 2004 (UTC)

Not to be too picky, but I just got back from a math conference (the Canadian Mathematical Society meeting) at which one of the plenary talks was titled "A Mathematician Flips a Coin": they did some studies with high-speed video cameras and calculations and showed that a normally flipped coin is biased to come up the way it started...[1] (http://www.cms.math.ca/Events/winter04/abs/Plen.html) [2] (http://stat.stanford.edu/~cgates/PERSI/by_year.html) PDF (http://stat.stanford.edu/~cgates/PERSI/papers/headswithJ.pdf) --Andrew 23:52, Dec 13, 2004 (UTC)

Yeah, that's possible, but does that change the probabilistic interpretation? CSTAR 00:03, 14 Dec 2004 (UTC)

Fiction

I think the section on fictional works could be a little less of an attack to science fiction. Calling fictional stories "misleading" and "deficient" because they don't conform to the details of quantum theory is asking too much from fiction. The stories are scientifically inaccurate or too fanciful, but you will be misled or deficiently informed only if you confuse fiction with fact. I was the one who added Borges, BTW. I've never implied that Borges got the idea of the MWI before the scientists, only that the literary concept and the scientific theory were similar, on the surface. --Pablo D. Flores 01:10, 16 Jan 2005 (UTC)

I'm aware of that and the point I was trying to make was that Borges' story (as do others) are not illustrations of MWI because they don't involve superposition of states. I ceratinly didn't intend it to be an attack on Borges!CSTAR 03:52, 16 Jan 2005 (UTC)

Misleading graphic

The graphic of splitting is very misleading, not because there is an infinite branching tree, but because the origin of the tree has nothing to do (at least in any commonly accepted interpretation) with decisions being made. his graphic belongs maybe in an article decision trees. I will be happy to compose a graphic of an infinite btanching tree but without reference to "decisions".CSTAR 04:55, 5 Feb 2005 (UTC)

Non-50% probabilities

Consider a variation of the Schroedinger's Cat experiment where you only wait long enough for a 1/3 chance of the cat being dead. There are only two possible outcomes for the experiment, so when you open the box, the universe only splits two ways, and it seems to me that that makes for a 50% chance that you'll be in the branch with the dead cat. How does a probability of 1/3 come out of this? I checked the FAQ linked from the article, and quickly got bogged down in tensor calculus -- and I never had classes in anything more complicated than basic vector calc. --Carnildo 23:15, 17 Mar 2005 (UTC)

The possibility for a cat to be 1/3 dead is possible yet extremely unlikely. Chances are you will never branch off into such a universe. You have to keep in mind that i may branch of into a different world than you will. Life is just a bunch of quantum quackery. --Aero66 13:05, 18 Mar 2005 (UTC)

There is little point you trying to impose this on the article. Please try to edit in a collaborative way. If you feel the article misses some essential point, please bring it up here. Charles Matthews 20:04, 18 Mar 2005 (UTC)

"Life is just a bunch of quantum quackery."

Could you please explain this remark which you have seen fit to place in the main article? CSTAR 19:37, 18 Mar 2005 (UTC)

I will provide you with an example: Based on the many worlds interpretation of quantum mechanics, the future holds that i may never receive an answer from you although my dopplgangers (copies) will. The theory removes the certainty we previously had knowing who we would talk to tommorrow. If you are socially tied with other individuals as i am, you will understand why this is of concern. We should investigate other interpretations of our world until it is certain that many worlds theory stands. The remark in the article was not suicide note, but merely a given fact to attract attention. I will come to accept any discovered truths about our world. Aero66 20:01, 18 Mar 2005 (UTC)

In many-worlds, the different copies are correlated, so there isn't that sort of uncertainty in who you're talking to. Of course you might never receive an answer from someone, since they could disappear in your world - but the possibility of death isn't anything new.

This is not the proper forum for discussing your new ideas regarding MWI, as interesting as they may be. Original research cannot be included in Wikipedia articles, and talk pages should not be used for discussing topics irrelevant to the content of the article. -- Schaefer 17:18, 19 Mar 2005 (UTC)

I understand your concept of correlated copies, but it is important to note that each copie is a seperated being. Suppose you are replaced with Schrodingers cat in the thought expirement. Instead of having a 50/50 chance of life or death, you have a 50/50 chance of going into one door rather than the other. Door A happens to contain cake while door B contains a sandwich. If you happen to go through door A you will never taste the sandwich although your copies will. We never know which copy we are going to spend our future with. For a recap read (RE: Life is just a bunch of quantum quackery) on this discussion page. Aero66 25:01, 19 Mar 2005 (UTC)

Wrong. In MWI, you go through both doors. And since the decisions that split universes are on the quantum level, the result is about a bajillion forks of the universe, in which you go through door A, go through door B, smack face-first into the barrier between the doors, say "to heck with it" and go through door C, remain befuddled with indecision until you starve to death, and many other futures, with the "go through door A"-type and "go through door B"-type futures being the most common. --Carnildo 06:05, 20 Mar 2005 (UTC)

The point that im trying to make is that you wont experiance all these outcomes. You will split into one universe while your copies will split into the other bajillion universes. Aero66 12:00, 20 Mar 2005 (UTC)

Neutrality

There are significant sections of this article that I feel are unfairly biased towards Everett's theories. The opinion that his theory adds nothing, and is untestable (since universes are non-cummunicating) is barely mentioned (living in a high-probability universe is common to a number of interpretations, and does not provide significant evidence for one or another). Subjective polls of "acceptance", while perhaps not stated to be evidence for Everett's theory, are at least implied to be significant (section 3).

Re: Neutrality