Talk:Interpretations of quantum mechanics/Archive 2

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Semiotic Interpretation of Quantum Physics

I'd like to see the semiotic interpretation of Quantum mechanics added to the list.

In the semiotic interpretation a quantum particle is more like an "event" (in both space and time) rather than a "thing" travelling in space over time. To ask where this event is, other than where it takes place, is like asking where London is other than where it is.

But a quantum particle is also more than just an event. It is also a "signifier", like the letter "X" or the letter "R". On their own letters doesn't mean much - but in combination with other signifiers (particles) they start to signify something. For example, the letters X, R, A, and Y, when combined, form a word ("xray") which starts to mean something - if only to us humans. In semiotics this meaning is called the "signified".

By applying semiotics to physical processes we can ask what the combination of particle-like events signify. Although this is somewhat peculiar it is also strangely effective.

The particle-like events that occur in an interference experiment can be regarded as signfying an interference pattern.

Note how this is a reversal of the "normal" questions. Instead of asking what the interference pattern means we ask what the particle-like events mean? And instead of looking for their meaning in the "cause" of such we look for what they mean in the effect they produce.

In the semiotic interpretation the "cause" of A is anything which signifys A. The effect of B is anything signified by B.

How does the wave function fit into this?

The wave function is a generalisation of interference patterns. It is that which is specifically signifed (or rendered) by particle like events in an interference experiment. For any given experiment the wave function can also be signified (or rendered) by appropriate transforms of classical measurements - of slit widths and screen distances etc.

A characteristic of the wave function - or an interference pattern (which is much the same thing) is that the wave exists over space and time. This contrasts with a particle which exists in space and time.

This is why one can not simultaneously measure both a particle and a wave at the "same time". Because the meaning of "at the same time" is inconsistent with the measurement of waves which are measured over time (and space).

By analogy we do not measure the distance between A and B at point A, or at point B. We measure the distance between A and B.

Anyway, there is a lot of stuff in quantum physics that makes sense when interpreted in terms of semiotics. This is but an introduction.

Carl Looper (220.101.184.56) 05:10, 11 January 2006 (UTC)

Participatory Anthropic Principle (PAP)

Seperate comment begins here: I don't know much about all of this. But currently the paragraph for PAP is illogical in its treatment of consciousness. PAP claims that consciousness causes wave function collapse. But the paragraph also states that an experiment could be done which would both test PAP and define consciousness. But this is circular reasoning; we are then defining consciousness as something which engenders observation, which causes wave function collapse. Thus such an experiment would NOT say anything about consciousness as it is generally understood. SOMEONE PLEASE MAKE THE NECESSARY CHANGES! I did, but it was undone. David Seperate comment ends here —Preceding unsigned comment added by 78.131.31.31 (talk) 22:40, 15 October 2009 (UTC)

I have read the source given at the end of the PAP paragraph, and I cannot find a basis for the statement "However, the experiment would need to be carefully designed as, in Wheeler's view, it would need to ensure for an unobserved event that it remained unobserved for all time" contained within. Wheeler does not appear to express any such view on any such experiment, according to the source.--68.46.187.78 (talk) 02:23, 26 October 2008 (UTC)

The point about PAP is that the quantum superposition is only resolved when it is observed - which fixes reality. After reality is fixed everything else in reality - past, present and future - must accommodate that new fact. Something may be free at the moment, but if observed later it will be fixed now (i.e. at that moment we were just talking about) if that is necessary to reach the observed outcome later. However, I must admit I can no longer find the relevant quote from Wheeler.

Another description of Wheeler's take on reality from

http://www.sciam.com/article.cfm?id=pioneering-physicist-john-wheeler-dies

"Elaborating on this idea, Wheeler evokes what he calls the "surprise" version of the old game of 20 questions. In the normal version of the game, person A thinks of an object—animal, vegetable or mineral—and person B tries to guess it with a series of yes-or-no questions. In surprise 20 questions, A only decides what the object is after B asks the first question. A can then keep choosing a new object, as long as it is compatible with his previous answers. In the same way, Wheeler suggests, reality is defined by the questions we put to it."

Aarghdvaark (talk) 00:00, 28 February 2009 (UTC)

Landé's interpretation

Is it a good idea to add the interpretation of Alfred Landé to the set of interpretations? I guess it would need a page of its own too where his interpretation is explained. INic 23:20, 16 January 2006 (UTC)

Umm, good question. I think the goal of this article was to list the "major" interpretations; I know that there are dozens of variants and "minor" interpretations (i.e. not widely accepted/discussed). I am not sure how we should go about treating these variants. Can you provide a summary of the Lande interpretation here? Is your intent to describe this as a piece of physics history, or to present this as something that has modern adherents? linas 00:26, 17 January 2006 (UTC)

Neither, more as showing the logical possibility of a different approach to the interpretations problem altogether. In the same way as Bohms interpretation always is mentioned, not because it has (or ever have had) many adherents, but because it shows an interesting possibility. Landé's interpretation never did a big impression, as far as I know, so I'm afraid it's historical import on physics history is very limited too. It has been a while since I read his books, but I'll re-read them and write a short summary here. (His opinion is closest to my own opinion in these matters, so I remember that I liked what I read.) INic 01:30, 24 January 2006 (UTC)

Prowave interpretation

I was recently very favourably impressed by the "Prowave Intepretation" by Dan R. Provenzano.

From: http://www.quantummatter.com/wave.html

Abstract:

It is widely accepted in Quantum Mechanics that measurements reveal the particle nature of elementary quanta, but there are many interpretations on how these ``particles move from the emitter to the point of measurement. This paper introduces in the ProWave (for ``Propagating Wave ) Interpretation of Quantum Mechanics, The basic idea is that elementary quanta always exist in the form of a wave, and always travel in the form of a wave, described by Schrödinger evolution, but are always measured each at a single location. This concept replaces all interpretations based on quanta traversing a particle path with the notion of a propagating wave coupled with a new concept of ``Quantum Energy Localization. It is argued in this paper that the ProWave Interpretation explains all known experimental results in a ``realistic way that would have pleased Einstein, Schrödinger, deBroglie and all those who are currently looking for a sensible way to understand the implications of Quantum Theory. As examples, the 2-slit experiment, and EPR experiment, and a quantum eraser are interpreted in the ProWave picture.

Peter Morris Adelaide 220.238.50.102 13:11, 12 February 2006 (UTC)

This is not appropriate for this article. linas 01:08, 14 February 2006 (UTC)

About decoherence

The table contains "decoherence" under the title "interpretations" but decoherence (as you can read in the article's page) is not an interpretation it is "a process". So what is it doing there on that table?!?--Pokipsy76 21:15, 28 May 2006 (UTC)

Quite right. The decoherence entry should be merged with the consistent histories entry -- despite that they disagree about many of their properties! I'll merge them if no-one else wishes -- let me know. PS I've updated the MWI entry, which had some misleading values. --Michael C Price 20:59, 12 June 2006 (UTC)

Couldn't wait, I've merged the decoherence tag into both the Many Worlds and Consistent Histories comparison. Note: I've made some properties indeterminate - which seems appropriate for a quantum table!! --Michael C Price 21:22, 12 June 2006 (UTC)

I think that yellow color has too much visive impact...--Pokipsy76 14:07, 13 June 2006 (UTC)

Pick any new colour except black :-) --Michael C Price 14:14, 13 June 2006 (UTC)

Are these two really the same? I was very surprised to read this article, and find that there was no decoherence picture (which I would ascribe to Zurek, and personally I think that a large number of physicists subscribe to). I would say that the consistent histories interpretation is a different interpretation (which I would ascribe to, among others, Omnes and Griffith)... Do people agree or not? --138.253.77.57 19:16, 6 October 2006 (UTC)

An interpretation is something people argue over; no researcher disputes the utility of decoherence as a mechanism: hence decoherence is not an interpretation itself. Zurek says he subscribes to the existential interpretation. --Michael C. Price ^talk 19:38, 6 October 2006 (UTC)

Price and the others who have objected to this are correct. Decoherence is not an interpretation, it's part of the theory itself. It should not appear here as its own section: this creates the impression that it is an interpretation of quantum mechanics when this is not the case. Certain interpretations are of course very interested in accounting for decoherence, but this is another matter. The section for decoherence must simply be removed.--Auspex1729 (talk) 16:39, 24 August 2008 (UTC)

Many worlds and consistent histories

As a footnote to the consistent histories entry in the table at the bottom of the article there is the comment

If wavefunction is real then this becomes the many-worlds interpretation.

I don't think this is true. The mathematical formalism of MWI and consistent histories are different.--CSTAR 14:17, 13 June 2006 (UTC)

In what way? I'm fairly sure they are the same. Whether you deal with density matrices (Consistent Histories) or universal wavefunctions (MWI) is irrelevant, the maths is isomorphic and they both evolve in a unitary fashion. Which is why Gell-man and Zurek are sometimes cited as Consistent Histories advocates, some times as Many Worlders. Everett used what we now call decoherence (building on Bohm, who is largely unrecognised in this context, I believe) to explain the loss of interference between worlds. --Michael C Price 14:25, 13 June 2006 (UTC)

Indeed both formalisms have "histories". In MWI however, a history is a sequence of measurement outcomes and the universal state is a superposition of these measurement histories; in consistent histories a history is a sequence of projections (each projection corresponding to a projective measurement at some time). A family of histories which satisfies a "consistency property" (it's defined in the consistent histories article but not very clearly -- that article needs work). As the article state "The consistent histories approach can be interpreted as a way of understanding which sets of classical questions can be consistently asked of a single quantum system". But I don't know that the state can be obtained as a superposition of "histories". There might be some way of obtaining formal equivalence of the two approaches, but as far as I know, this hasn't been done.--CSTAR 15:56, 13 June 2006 (UTC)

The theories -- MW & consistent histories with real wavefnctn -- have to be the same since they agree about the state of the wavefunction (uncollapsed) and its reality. Every thing can be deduced from the wf. Classical, decohered probabilities are additive (=consistent) in both approaches. Zurek describes the consistent histories approach as post-Everett for this reason -- it's just that the role of the various density operators is the focus with Zurek, the u'wavefunction with Everett; switching between them is trvial. --Michael C Price 16:40, 13 June 2006 (UTC)

"Many worlds"

I read in Penrose that "many worlds" is (about) the same as what I have been calling a direct interpretation, but I did not catch that from this article. Suppose we substitute "Many classical worlds" or "Superposition of classical worlds" and spell out "multiverse" as quantum universe or quantum reality? That doesn't really describe the point of view, but it may be clearer than "Many worlds", which sounds mystical.

This whole disagreement seems unreal to me, because I have never had any problem with, or heard any serious objection to, the idea that matter is a wave since I first read it when I was about ten years old. David R. Ingham 03:14, 28 August 2006 (UTC)

Your proposed use of "many worlds" would be WP:OR. "Many worlds" is what it is called in the literature and is in wide usage. The "direct interpretation", by contrast, is a minority term used by who? --Michael C. Price ^talk 03:58, 28 August 2006 (UTC)

How about "Quantum reality" for the name of this subsection? There is only one world, but it is most accurately described by a state vector, not by classical coordinates. David R. Ingham 04:20, 28 August 2006 (UTC)

No, there is NOT only one "world" according to MWI. If you think this you shouldn't be editing the article. Please stop inserting your own original terminology everywhere. That's not what Wikipedia is about. Use standard terms. Read WP:OR. --Michael C. Price ^talk 05:19, 28 August 2006 (UTC)

What I am saying is that the article is not clear as it stands, even to an expert. I should be able to get some idea of whether or not I am interested in "Many Worlds" before reading about it in other places also.

Now that I have some idea what is meant by it, the term "Many World" sounds to me somewhat as though it has been made up to be derogatory. On the other hand, if it does mean that there are really many worlds, then the impression I got from Penrose is wrong, and the point of view I learned in school and used to do research is missing here. David R. Ingham 16:37, 28 August 2006 (UTC)

You're being vague. If you have a specific criticism of the many worlds article take it to its talk page. That an "impression" you got from Penrose is confusing is not really newsworthy or surprising, although I see no conflict between what he said about ascribing reality to the wf and the existence of many worlds. --Michael C. Price ^talk 08:29, 29 August 2006 (UTC)

One universe

The table contains the column "One universe?" and all the interpretation have "Yes", so what is the function of this colums if there is always one possible answer?--Pokipsy76 17:44, 13 June 2006 (UTC)

The many worlds interpretation uses many universes, something which is often counted against it by its detractors (a violation of Occam's razor). The table has that column so that the comparison can be made between interpretations that use many universes and those that do not posit other universes. However, the column currently reads "Yes" for MWI, I suppose because to say that MWI has extra universes is almost misleading enough to be wrong. See the footnote attached to that entry. -lethe ^{talk +} 17:48, 13 June 2006 (UTC)

So why don't we erase the column or why don't we change the title of the column so that the difference can be expressed more clearly?--Pokipsy76 18:05, 13 June 2006 (UTC)

The fact that people use this property of MWI to compare it to other interpretations means that we ought to keep the column, in my opinion. As for changing the title, what do you have in mind? Another solution would be to change the "Yes" to a "No", which would make the purpose of the column clearer. -lethe ^{talk +} 18:11, 13 June 2006 (UTC)

I think we could change "yes" to "no" and explain better in a note in order to be not too much misleading.--Pokipsy76 18:16, 13 June 2006 (UTC)

That's one possibility, but "universe" has become such a loaded term that there would be a need for a big footnote. Here's another possibility: that we retitle the column "multiple histories" or something similar? The two decoherent approaches would say yes, and the others no, with a qualified no for Bohm, since it contains multiple wave histories and a single particle history. --Michael C Price 18:29, 13 June 2006 (UTC)

I suggest to name the column "single history" because the "No" in the other part of the table denote something strange. Moreover there should be added a note to explain what does "history" mean in the context. But what if we call the column "One world"?--Pokipsy76 08:20, 14 June 2006 (UTC)

World, like universe, is rather interpretation-specific and just enough POVish to raise hackles in some quarrers. I suggest "unique history" .....? I'll change it and see what you think. --Michael C Price 10:20, 14 June 2006 (UTC) Done, and table re-ordered into a more natural grouping with a few extra clarifications.--Michael C Price 10:56, 14 June 2006 (UTC)

That's OK for me, now we need a note to explain what is a "history"!--Pokipsy76 11:23, 14 June 2006 (UTC)

Relational Quantum Mechanics and Modal Interpretations of Quantum Theory

We need some people who are comfortable with this topic to describe these interpretations of QM. RK 19:14, 17 June 2006 (UTC)

I can do Relational QM...for a start I am rewriting the Stanford quote, and providing a link to the published paper on the subject (which is stored at the arXiv). How long should this be? I am also going to write a full article on RQM, but that'll be a while coming.--Byrgenwulf 11:29, 15 July 2006 (UTC)

Removed passage

Hi, I'm new to Wikipedia and so if I made an editing mistake or am breaking any rules please let me know. I have deleted the following section from this article -

'Though interpretation continues to be a current topic in the philosophy of science, from the viewpoint of many physicists, the question it poses is meaningless, since answers to it are untestable. Likewise many philosophers see this interpretational quest for meaning as representing a simple category error, since philosophical arguments are clearly not empirical, and thus not "testable" in a scientific sense, though they are open to the criticism of logic.'

The author has cited no evidence for his suggestion that many physicists think the question it poses (regarding the conceptual foundations of quantum theory) is meaningless. Moreover, the assertion that the reason for this is 'since answers to it are untestable' reflects a misunderstanding of the nature of the philosophical investigation of theories. What would it mean for an interpretation to be 'testable'? The philosophy of science isn't science, but whether this is a point against it is irrelevant to this article. In bringing up issues regarding the meaninglessness of non-empirical statements etc. the author has strayed from the point of the article, which is to discuss interpretations of quantum mechanics.

'Likewise many philosophers see this interpretational quest for meaning as representing a simple category error, since philosophical arguments are clearly not empirical, and thus not "testable" in a scientific sense, though they are open to the criticism of logic.'

Again the author has not cited evidence that many philosophers support such a view. Although incidental to my argument for removing the passage, I do not think he could find any evidence for this view. He is using the terms 'category error', 'empirical' and 'testable' out of context and is obviously not familiar with the terminology. In any case, the passage is merely speculation on the views of physicists and philosophers without citation.

I think I agree with your opinion, the section as it was couldn't stay. Surely the point of view it describes actually exists but we need some reference. Remember to sign your messages using the "sign" button on the edit box or the "sign your name" link below it.--Pokipsy76 08:45, 27 June 2006 (UTC)

Yes, the point of view that the author is trying to describe is roughly verificationist positivism regarding non-empirical propositions. However, the author doesn't describe it at all accurately and I would guess is not familiar with the relevant literature. Nor does the author provide support for his claims regarding the popularity of the view. I can't speak for physicists but I know that virtually no one holds to an unreformed verificationism in philosophy currently.

In any case, the point is that even if described properly it would be irrelevant to the article. If it is acceptable to make claims regarding what physicists and philosophers think about foundational epistemological issues in this article, then we would be justified in adding similar ones to every article concerning specific topics in the philosophy of science. It would be as irrelevant as commenting on what falsificationists think of scientific method at the start of every article on, say, the biology of dogs.

--Camcolit 10:32, 27 June 2006 (UTC)

The author? This is WIkipedia for heaven's sake. Are you imagining the "author" sitting doing in a room and actually writing this? Writing for WP is like trying to doing scholarly work crouched in a foxhole in the middle of a battlefield - between four armies.

You should look at the history list, to see how this evolves.

In particular, look at these edits to the article

• [1]
• [2]

--CSTAR 15:39, 27 June 2006 (UTC)

Direct interpretation

Penrose

Here is some source material, from The Road to Reality" by Roger Penrose, 2004, section 21.6, (top of p. 508 in my copy):

If we are to believe that anyone thing in the quantum formalism is 'actually' real, for a quantum system, then I think that it has to be the wavefunction (or state vector) that describes quantum reality. (I shall be addressing some other possibilities later, in Chapter 29; see also the end of 22.4.) My own viewpoint is that the question of 'reality' must be addressed in quantum mechanics—especially if one takes the view (as many physicists appear to) that the quantum formalism applies universally to the whole of physics—for then, if there is no quantum reality, there can be no reality at any level (all levels being quantum levels, on this view). To me, it makes no sense to deny reality altogether in this way. We need a notion of physical reality, even if only a provisional or approximate one, for without it our objective universe, and thence the whole of science, simply evaporates before our contemplative gaze!

So his interpretation is direct: it is reality. (I assume that the other possibilities he considers later would be no more pleasing to the classical mind.)

Weinberg

I put this into two articles' discussion some time back, but it is also relevant here.

Physics Today, April 2006, "Weinberg replies", p. 16,

... but the apparatus that we use to measure these variables—and we ourselves—are described by a wave function that evolves deterministically. So there is a missing element in quantum mechanics: a demonstration that the deterministic evolution of the wave function of the apparatus and observer leads to the usual probabilistic rules [Copenhagen interpretation].

So the "Copenhagen interpretation" and its competitors are not philosophy nor basic physics. They are empirical rules, that have not yet been fully justified theoretically, for using the classical approximation in quantum experiments. David R. Ingham 05:01, 16 August 2006 (UTC)

Hi, I hope someone at interpretations of quantum mechanics (where I see you have already posted your comments) can answer. But briefly, MWI is deterministic (Weinberg) and classically realist (Penrose). --Michael C. Price ^talk 07:31, 16 August 2006 (UTC)

"Hidden variables"

In section 23.2, while introducing entanglement, Penrose (above) points out that there being one wave function for all the particles in a quantum system makes the number of dimensions depend on the number of particles as an exponent, instead of as a multiplier. This vastly increases the number of variables needed to specify the state of a system. Since the number of particles always greatly increases during an experiment, the instruments, etc. becoming part of the system, it is hard to imagine why anyone would want to introduce more "hidden" variables not to know either. David R. Ingham 00:05, 23 August 2006 (UTC)

Introduction

Hi,

I've rewritten the introduction section which did not seem adequate to me, although I admit that what I put down does seem a little clunky. - Rafaelgr 16:15, 7 September 2006 (UTC)

I've taken the liberty of adding a paragraph or so to the material introducing the measurement concept in the "Obstructions to direct interpretation" section. (I also modified some of the existing material there, mainly to help blend with my expansion.) My goal was to make this material more accessible to non-physicists. Randallbsmith (talk) 05:48, 30 December 2008 (UTC)

consistent histories - vandalism? i'm no physician...

the sentence "According to this interpretation, the purpose of a quantum-mechanical theory is to predict probabilities of various alternative histories." is a joke/bullshit/vandalism/callitwhatyouwant, or is it real?-- ExpImp^talkcon 22:56, 19 November 2006 (UTC)

No, I think that is for real. Admittedly it might be a bit clearer, although how, I'm not sure! --Michael C. Price ^talk 01:07, 20 November 2006 (UTC)

Other voices? Perhaps a quote of an actual Ph. D. in Physics saying that? Sorry, but i cant imagine hard-nosed physics buffs suddenly wandering off to lala-land to think about alternative histories.... can you?-- ExpImp^talkcon 23:17, 20 November 2006 (UTC)

Murray Gell-Mann hard-nosed enough for you? --Michael C. Price ^talk 02:02, 21 November 2006 (UTC)

Theory of Incomplete Measurements

The "incomplete measurements" approach derives quantum mechanics axioms from properties of the physical processes we are willing to call measurements. It does not require a conscious observer, but rather a selection among physical processes to isolate those that can legitimately be used as measurements. An noteworthy consequence is the suggestion that the normalization condition for the wave function needs to be restricted to whatever the measurement process can legitimately return as a result. It also gives a physical interpretation not just to the amplitude, but also to the phase of the wave function.

The original paper is at http://cc3d.free.fr/tim.pdf. It's very recent, has not been published in print, and it did not receive much scrutiny. If the case of Afshar's experiment is any indication, Wikipedia accepts references to very recent work. —The preceding unsigned comment was added by Descubes (talk • contribs) 09:08, 4 December 2006 (UTC).

I would say that this section, and the Theory of incomplete measurements article, are likely not to be notable enough for Wikipedia, and are essentially original research. Comments? -- Spireguy (talk) 02:56, 9 June 2009 (UTC)

Objective/Spontaneous Collapse Theories

This article makes no mention of a class of approaches to QM, the Objective (or spontaneous) collapse theories, of which the most famous example is the Ghirardi, Rimini and Weber theory).

As the Sanford Encyclopedia of Philosophy [3] article states..

'...this approach consists in accepting that the dynamical equation of the standard theory should be modified by the addition of stochastic and nonlinear terms. The nice fact is that the resulting theory is capable, on the basis of a unique dynamics which is assumed to govern all natural processes, to account at the same time for all well-established facts about microscopic systems as described by the standard theory as well as for the so-called postulate of wave packet reduction (WPR). As is well known, such a postulate is assumed in the standard scheme just in order to guarantee that measurements have outcomes but, as we shall discuss below, it meets with insurmountable difficulties if one takes the measurement itself to be a process governed by the linear laws of the theory. Finally, the collapse theories account in a completely satisfactory way for the classical behavior of macroscopic systems'.

1Z 15:52, 11 January 2007 (UTC)

Looks like a good addition. Worth also pointing out that this approach is not really a new interpretation of the standard laws of quantum dynamics. Rather it is a new set of postulated dynamics, since they are modifying the canonical equations. --Michael C. Price ^talk 01:21, 12 January 2007 (UTC)

Merge?

The merge tag needs to be addressed. Does it make sense to split this article at all, or to address philosophy/"controversy" in another article? -Ste|vertigo 05:28, 5 February 2007 (UTC)

I would strongly suggest a Vote for Deletion. This is riddled with original research--or free-wheeling speculation, if you like--and is a distant cousin to anything encyclopedic in tone. —Preceding unsigned comment added by 136.142.21.213 (talk)

Oops, that looks unclear. I meant that I support the delete of the "QM, phil., and controversy article." Not the "Interp of Q.M." article, which I think is altogether nifty. —Preceding unsigned comment added by 136.142.21.213 (talk)

I also support the delete of the 'philosophy and controversy' article. Even the title seems out of place - I don't think it's an area unto itself, unlike 'interpretations of quantum mech'. I will start a discussion on the other page with a move to delete it. Mogudo 05:23, 13 April 2007 (UTC)

Transactional Interpretation - really indeterministic?

Hi,

in the comparison table of the various interpretations of quantum mechanics, John Cramer's Transactional Interpretation is indicated as an "INDETERMISTIC" one. Is this classification really feasible or does it have to be changed to "determistic"? After all, the transactional interpretation has to presuppose a blockworld scenario with a prexististing future to allow a handshake between past and future events. This would of course imply determinism, because past and future are already fixed.

If there is no rational argument in suport of an indetermistic transcational interpretation, I will chgange this point in the table.

Kind regards, Dr. Guenter Bechly —Preceding unsigned comment added by 193.197.147.9 (talk) 08:34, August 24, 2007 (UTC)

I think you are confusing predestination with determinism. --Michael C. Price ^talk 08:38, 24 August 2007 (UTC)

Is it possible that you confuse predestination and predictability? Can you conceive of a deterministic universe without predestinated future (or of an indeterministic universe with predestinated future)? I think both are strictly correlated like two sides of a coin. What is not correlated is predictability, thus a deterministic universe could be inpredictable even though the future is predetermined, but this is not an issue of ontology but of epistemiology (e.g. Chaos theory is based on determinism but explains unpredictability). Therefore, I still raise the initial question: Does the transactional interpretation imply indeterminism or determinism, or is it at least compatible with both indeterminism and determinism? —Preceding unsigned comment added by 193.197.147.9 (talk) 10:35, August 24, 2007 (UTC)

Can you conceive ..... of an indeterministic universe with predestinated future - Yes. Different words mean different things. --Michael C. Price ^talk 10:39, 24 August 2007 (UTC)

According to the definition of "determinism" by Van Inwagen (1983): Determinism may also be defined as the thesis that there is at any instant exactly one physically possible future. Enough said!? And if this is accepted: Is the Many World Interpretation really a deterministic interpretation in the strict sense (like Bohmian mechanics for example), or rather a stochastic causal interpretation: All outcomings are causally defined by the wavefunction, but there is no internal causal reason for a particular eigenstate in a particular branch of the multiverse (e.g. no causal reason for a specific quantum fluctuation or radioactive decay which is still stochastic in the Many Worlds Interpretation). Kind regards, Dr. Guenter Bechly (p.s. sorry if my physical understanding is only layman like, but I am biologist and geologist, no physicist) —Preceding unsigned comment added by 84.56.182.65 (talk) 18:46, August 24, 2007 (UTC)

That's a nice definition of determinism, but does it mean the same as predestination? I don't think so, simply because predestination is slippery religious concept (see predestination). So let's not talk about predestination in a physics article.

The MWI is deterministic, because it is the deterministic wavefunction that defines reality. Same with Bohmian mechanics, where the particle's course is deterministic. Copenhagen is indeterministic since we have wf collapse. Even Cramer admits that the TI is isomorphic to Copenhagen; ergo it is indeterministic. --Michael C. Price ^talk 19:03, 24 August 2007 (UTC)

Michael, it was you (not me) who introduced the notion of predestination to our discussion. I am talking about determinism in the sense of a block universe and block time view. If you compare this with a religious concept of predestination you are free to do that, but nevertheless block time and block world are purely scientific corolloraries of Minowski's spacetime interpretation of Einsteinian relativity. Myself, I am an atheist and Bright and I am not interested in metaphysical aspects of any kind of predestination. My point is: Is there really an ontological difference (whatever you might call it) in the world view of TI and MWI, and I am still not sure about this. It still looks to me that TI ist more deterministic, because it implies a single blockworld history (thus, endorsing counterfactual definiteness, contrary to MWI), while MWI does not imply a particular blockworld history but all possible ones, and therefore is rather indeterministic. There is no open future in TI, but well in MWI. Could you please elaborate on this crucial point instead of semantics? —Preceding unsigned comment added by 84.56.182.65 (talk) 09:40, August 25, 2007 (UTC)

I still think you're confusing predestination with determinism. Anyway I am not going to continue to debate flakey original research here. As I've already said -- and which you completely ignored -- Cramer says TI is mathematically isomorphic to CI (which is indeterministic). The MWI is deterministic, because it is the deterministic evolution of the wavefunction that defines reality. Same with Bohmian mechanics, where the particle's course is deterministic. End of story. --Michael C. Price ^talk 18:06, 29 August 2007 (UTC)

John Gribbin's take on TI is that future events are "already there", which is almost, but not quite, determinism. However, that is contested by some. 1Z 10:32, 29 August 2007 (UTC)

As I've said, "already there" is predestination, not determinism. --Michael C. Price ^talk 18:06, 29 August 2007 (UTC)

Saying it again does not make it correct. Predestination is a religious concept, but block universe is a physical concept which has nothing to do with predestination at all. Predestination does involve a notion of destiny which again involves a notion of teleology in the sense of destiny predetermined by a higher power, while block universe is just another term for fourdimensionalism and eternalism in the spacetime worldview of modern physics. —Preceding unsigned comment added by 84.56.203.190 (talk) 18:18, August 29, 2007 (UTC)

And denying it again doesn't make it false either. --Michael C. Price ^talk 18:28, 29 August 2007 (UTC)

Michael, can you please stay objective and rational, thus answering raised questions instead of contributing just short polemic statements without any arguments! Here is one question you should answer: What is the causal explanation for a particular outcome in a particular worldline in MWI? For example, how is the radioactive decay or non-decay is explained when it is measured? The wave function only gives a causal explanation for the complete set of possible outcomes, but not for a particular event or state. If MWI does not offer a causal explanation for a particular decay event or for a quantum fluctuation it must be considered as indeterministic. The second question you should answer is, is there an open future in MWI and/or TI? —Preceding unsigned comment added by 84.56.203.190 (talk) 18:32, August 29, 2007 (UTC)

I've already made some salient points which have remained unanswered: Cramer says TI is mathematically isomorphic to CI (which is indeterministic). The MWI is deterministic, because it is the deterministic evolution of the wavefunction that defines reality. Same with Bohmian mechanics, where the particle's course is deterministic. End of story. --Michael C. Price ^talk 18:34, 29 August 2007 (UTC)

And Deutsch says TI is isomorphic to MWI... 1Z 13:20, 30 August 2007 (UTC)

Have you a link for that? I doubt that Cramer agrees since he is quite anti-MWI. :-) (I also doubt that Deutsch regards MWI as indeterministic.) --Michael C. Price ^talk 13:46, 30 August 2007 (UTC)

As for your additional MWI questions, see this FAQ and in particular question 25 "Why am I in this world and not another?" --Michael C. Price ^talk 18:41, 29 August 2007 (UTC)

You are comparing apples with oranges: CI is not an ontological interpretation but rather the viewpoint "shut up and calculate". TI and MWI are ontological interpretations that attempt to understand and explain what QM means in the real world. Therefore it is irrelevant if TI is isomorphic to CI (by the way a viewpoint also endorsed by C.F.v. Weizäcker). All interpretations that do not modify the mathematical formalism of QM are isomorphic to eachother. You did not answer my questions. Are you aware that philosophically it is impossible that a deterministic universe produces an open future? However, MWI implies numerous possible futures (thus an open future from each subjective point of view), while TI does not. —Preceding unsigned comment added by 84.56.203.190 (talk) 18:44, August 29, 2007 (UTC)

CI can be considered as including an objective collpase process. The current article treats that as a differnt variation from shut-up-and-calculate.1Z 13:20, 30 August 2007 (UTC)

There are shades of CI according to how they interpret the collapse, ranging from objective "real" collapse to just an adjustment of subjective knowledge. --Michael C. Price ^talk 13:51, 30 August 2007 (UTC)

Responding to the earlier unsigned comments:

• You did not answer my questions. I did precisely that by my pointing you to a MWI FAQ (which I wrote, BTW).
• TI/CI and MWI are not mathematically isomorphic to each other: (i.e. (CI=TI) =/= MWI).
• Are you aware that philosophically it is impossible that a deterministic universe produces an open future? - no, and I don't believe it either -- and I don't intend to debate non-standard OR. --Michael C. Price ^talk 18:56, 29 August 2007 (UTC)

I am sorry, you did not ask any questions, thus I could not answer them. But I had some questions that you did not answer yet. The viewpoint that determinism is incompatible with free will and an open future is sufficiently mainstream to consider it anything but non-standard. How would you reconcile determinism with an open future? I absolutely accept your disagreement, but I would like to hear your arguments. I am not discussing here to impose my point a view, on the contrary, I am still in the process of building up my point of view. I would consider it a real pity, if you reject further discussion because you seem to have considerable knowledge about this subject. By the way: Of course I know the Many World FAQ written by you, and I also know the anthropic point of view concerning the question why we are thinking we are in a particular world. However, I still do not see a causal explanation for particular quantum states and events in MWI, but well in TI and Bohmian mechanics. To me MWI still looks more stochastic than deterministic. —Preceding unsigned comment added by 193.197.147.9 (talk) 06:54, August 30, 2007 (UTC)

I did not ask you to answer my questions, it was my points you ignored (repeatedly). And sorry, I do not intend to broaden the discussion by including the eternal bugbear of free-will (debates about free-will never get anywhere) -- let's stick to determinism. Determinism has a precise physical meaning: no random effects, which, in the MWI view, means a deterministic evolution of the wavefunction (i.e. without collapse), since it is he wavefunction that is taken to be the fundamental entity. It's quite simple: The MWI is deterministic, because it is the deterministic evolution of the wavefunction that defines reality. - that is a sourced position that is maintained by all the founders of MWI and (as far as I can recall), every other commentator of note, whether or not they agree with MWI. If you have specific issues with determinism re MWI then I suggest you ask on the MWI talk page, after reading the MWI article, where you are likely to get a more focussed response my myself or others. I am quite happy to improve the clarity of the MWI article by debate. --Michael C. Price ^talk 09:22, 30 August 2007 (UTC)

Calogero hypothesis

There's been a short notability discussion for this page. See its Talk page. It's non-pseudo but as with many QM theories, noone seems to have picked up on it either. Does it warrant a short mention on a page like this one, or should it go up for deletion? EverGreg 19:09, 24 August 2007 (UTC)

Thanks. My gut instinct is that if it's non-pseudo it should stay, but I'll have a look before I vote KEEP.  :-) --Michael C. Price ^talk 19:13, 24 August 2007 (UTC)

Decoherence approach

It seems to me that in the paragraph "decoherence approach" we don't actually present a philosophical interpretation of anything, we just explain a mathematical result (and a corresponding actual physical phenomenon) in common english language. Don't we? This looks very strange because the paragraph is a subparagraph of "Summary of common QM interpretations".--Pokipsy76 08:50, 16 September 2007 (UTC)

Again on decoherence

In the introduction we read:

Quantum decoherence is the prominent physical approach to the subject.

but if we follow the link we discover that quantum the coerence is not a philosophical or interpretational issue: it is just a physical phenomenon. So that phrase looks very strange to me.--Pokipsy76 09:01, 16 September 2007 (UTC)

It should be explained more clearly that decoherence is a uncontroversial tool used in various controversial interpretations (such as consistent histories, existential inter', MWI, Bohm's pilot wave).--Michael C. Price ^talk 13:09, 19 November 2007 (UTC)

There are three different sections on this talk page about how decoherence isn't a philosophical interpretation, and that seems to be the consensus. Are there any objections to simply removing it?--Auspex1729 (talk) 18:16, 24 August 2008 (UTC)

Minority QM interpretations new article

I created a new article to catalog the obscure QM theories. any QM school of thought not well known enough to be listed in the main interprations article, should be listed here vroman (talk) 05:31, 1 April 2008 (UTC)

"Consciousness causes collapse" vs. "Quantum mysticism"

This ("Interpretation of quantum mechanics") article seems "good" (at least to someone struggling to understand quantum mechanics amid the popular pseudoscience...) and seems to be maintained by some "serious" people.

"Consciousness causes collapse" is here, but, the link to the "Main article:" leads to "Quantum mysticism". However, there's lots of [unique/different] good info (I think) in the old "Consciousness causes collapse" article at

http://en.wikipedia.org/w/index.php?title=Consciousness_causes_collapse&oldid=193438546

PLEASE HELP: how do we recover/reinstate/etc. such useful info? I tried unsuccessfully to BOLDly "undo" the redirection...

My guess is that the editors *here* can't be aware of actions taken in the "Main articles" linked from here.

If someone can fix/improve the "Main article: Consciousness causes collapse", and perhaps (briefly?) explain how/why it became "Quantum mysticism" (even after I tried to "undo" the redirection) I'd really appreciate it.

Thanks! —Preceding unsigned comment added by Curious1i (talk • contribs) 03:12, 13 March 2008 (UTC)

I think "Consciousness causes collapse" is a red rag to a bull for some people. I have redone this section as the Participatory Anthropic Principle, which is a theory of Johnny Wheeler - who has just died and was the last of the titans of 20th-century physics. I hope this will mean people won't dismiss it as mysticism. To me it seems a "better" theory than many-worlds (in the sense of William of Ockham). Aarghdvaark (talk) 17:56, 3 May 2008 (UTC)

The PAP seems more a matter of cosmology. Wigner seems to be the primary source of the role of consciousness in collapse and Penrose, for example, credits him for this. I will make a rewrite of this after I have assembled the sources. Colonel Warden (talk) 18:14, 3 May 2008 (UTC)

OK. True about cosmology, but the interpretation of quantum mechanics and cosmology are intertwined, e.g. many-worlds is as much about cosmology (why this universe) as quantum mechanics.Aarghdvaark (talk) 06:49, 4 May 2008 (UTC)

Quantum Logic

Why is quantum logic included as an interpretation here? The development of quantum logic had some interpretational motivation, but this is not universally shared by authors in the field. The logic itself does not prescribe any ontology or give a reason for side-stepping the ontology, and doing one of these is a requirement of any interpretation of the formalism. Quantum logic is utilized frequently independently of interpretation, as is evidenced by its pervasive use in articles on quantum information theory. Quantum logic is, by itself, no more an interpretation than the path integral formalism. The path integral formalism is given, in certain interpretations like MWI and Consistent Histories as a consequence of the prescribed ontology, but the formalism is part of the theory of quantum mechanics itself and its use does not commit one to an interpretation which puts emphasis on accounting for it by reifying the histories.

Likewise, the empiricist and realist positions of authors such as W.M. de Muynck and B. von Frassen may happen to use quantum logic, but there is a marked distinction between the logic and an interpretation which uses it. By way of comparison, deformation quantizations of heisenberg manifolds are used by other authors as an alternative to the axiomatization of quantum logic, and these quantizations are not taken by anyone as an interpretation in their own right. Yet, if quantum logic is included as an interpretation, then so must its competitor as well.

Its inclusion here is extremely dubious, and requires a very convincing and reputable citation if it is to stay.--Auspex1729 (talk) 15:33, 24 August 2008 (UTC)

I'll go further. Even claiming that quantum mechanics presents us with the need for a non-classical logic (as some have done) and, indeed, even going so far as to claim that there is something more fundamental about this logic than the classical logic is not yet an interpretation. Putting aside the fact that someone who made such a claim would be making a very grand claim indeed, this kind of position would be akin to the dispute between frequentists and Bayesians in probability. It is, more properly, a dispute on the metaphysics of a logic, not on the foundations or metaphysics of quantum mechanics. Pressing the example, frequentist and Bayesian interpretations of probability have both found their uses in particular interpretations, but they are not unique to them (we could, for example, have a frequentist proponent of hidden variables, and a Bayesian propenent of the same interpretation). For this reason, interpretations of the probability in quantum mechanics are not included in this list as interpretations of quantum mechanics itself, and neither should quantum logic be presented as an interpretation of the theory.--Auspex1729 (talk) 15:52, 24 August 2008 (UTC)

I think the acid test for whether something is an interpretation is whether it claims to resolve the "measurement problem" and other assorted quantum paradoxes. If it makes such a claim then it is an interpretation. By this touchstone then I agree that the path integral or sum over histories approaches, for example, are not interpretations. However I do recall some people claiming that quantum logic resolves the quantum paradoxes -- whether or not the claim is true is besides the point; it's the intent that is the issue. Unfortunately the issue is clouded, it seems, in that the term "quantum logic" seems to include a range of stuff. --Michael C. Price ^talk 20:32, 24 August 2008 (UTC)

Alright. I'll leave it as it is for now. I'll try to get my hands on some copies of W.M. de Muynck and B. von Frassen's books so I'll have some citations which clarify the issue. That will allow us to retain the quantum logic in the context of the interpretations which rely heavily upon it.--Auspex1729 (talk) 23:59, 24 August 2008 (UTC)

If I'm right that the term "quantum logic" is a broad umbrella term (something which I've only come to suspect as result of our conversation), then this would definitely be worth mentioning in the QL article.--Michael C. Price ^talk 08:19, 25 August 2008 (UTC)

Fung Yau's Final Interpretation

I think there should be some discussion about whether to include this into the article. I'm new to Wikipedia so I don't know what the exact procedure is. —Preceding unsigned comment added by Randommuser (talk • contribs) 04:26, 1 November 2008 (UTC)

You're right that this doesn't belong here. This contribution falls under the category of original research. The reference material seems to be self-published.--CSTAR (talk) 04:31, 1 November 2008 (UTC)

Decoherence, again..

I read:

Thus decoherence, as a philosophical interpretation, amounts to something similar to the many-worlds approach.

this seems incorrect to me: decoherence (as you can read in its page) is not a "philosophical interpretation", it is a mechanism, a process, something which belong to science rather than philosophy.--Pokipsy76 (talk) 10:41, 23 November 2008 (UTC)

Bohm's interpretation

The complete description of a quantum system in Bohmiam Mechanics is given by its wave function $\psi$ and by the actual particle configuration $Q=(Q_{1},\ldots ,Q_{n})$ (here $Q_{i}=Q_{i}(t)$ is a point of $\mathbb {R} ^{3}$ that depends on time t). The wave function $\psi$ satisfies Schrödinger's equation and the particle configuration Q satisfies the guiding equation (which implies that the statistical distribution $|\psi |^{2}$ is conserved --- a property normally called equivariance).

When you make a position measurement, what you get is precisely the Q's. Therefore, the Q's are actually observable. Thus, saying that the Q's (the positions of the particles) "cannot be measured directly" is simply wrong.

It is true, however, that you cannot directly measure the Bohmian trajectory $t\to Q(t)$ (or, more precisely, you cannot measure what the Bohmian trajectory would have been like if you had not performed the position measurements) by making a succession of position measurements at different times, because such measurements collapse the effective wave function of the particle (although not the wave function of the universe, which never collapses), altering its trajectory.

As I said in the edit comment, the "locations" are not observable. See the Bohm interpretation FAQ section where this is explicitly dealt with (and sourced). If you have a problem with it take it to the Bohm interpretation talk page (although you'd be rehashing something that always comes us. Still the FAQ could be improved...) Either way, this is not the appropriate article (too much detail, and dubious claim). --Michael C. Price ^talk 06:47, 23 February 2009 (UTC)

Do you mean the section entitled "Frequently asked questions" on this page: Bohmian_mechanics ? It is not claimed there that the Q's (i.e., the particles positions) are not observable. On the contrary (see the second question and second answer on that section).Dvtausk (talk) 14:36, 23 February 2009 (UTC)

Last question, last answer. --Michael C. Price ^talk 14:39, 23 February 2009 (UTC)

Well, the last question asserts that Bohm's particles are observable entities (that's essentially correct, although it would be better to say that their positions are observable entities, i.e., the Q's are observable). Then the answer contains a (disputable) claim by Everett that the particles are superfluous (that's actually a common dispute between supporters of MWI and supporters of Bohmian Mechanics). The text then jumps to the conclusion that the particles are unobservable --- and you are implying that this is the same as saying that the particles positions (the Q's) are not observable.

The resolution of the dispute on wether the particles are observable depends on a clearer specification of what "observing" something means. On the other hand, here is a very basic and elementary fact (known to any student/researcher of Bohmian Mechanics): in Bohmian Mechanics, a measurement of the position of the i-th particle at time t gives you $Q_{i}(t)$, where $Q(t)=(Q_{1}(t),\ldots ,Q_{n}(t))$ is the solution of the Bohmian guiding equation.Dvtausk (talk) 15:05, 23 February 2009 (UTC)

No, it is a common assumption amongst Bohmites -- but that doesn't make it a a "very basic and elementary fact". Also I should point out that the logic of Everett's argument is not dependent on whether MWI is true. --Michael C. Price ^talk 15:22, 23 February 2009 (UTC)

First of all, I don't want to get into an argument about wether or not Everett is right about the claim that the particles are superfluous (I think that he is wrong about that, but this is not relevant to the present discussion); all I'm saying is that "particles being superfluous" is not the same thing as "positions of particles in Bohmian Mechanics not being observable" (you are probably missing the point that the judgement about what has been observed in a given experiment is not absolute, but theory dependent). Moreover, your comment above about the "assumption amongst Bohmites" doesn't even make sense: the statement that a measurement of the position of the i-th particle at time t gives $Q_{i}(t)$ is part of the very formulation of Bohmian Mechanics. It doesn't matter wether Bohmian Mechanics is true or wether belief in Bohmian Mechanics is rationally justifiable, the statement about measurements of positions and the Q's is not a statement about the world or about what we should believe about the world, it is simply a statement about what Bohmian Mechanics says about the world.

In any case, it doesn't matter wether you agree or not with the statement about observability of the Q's in Bohmian Mechanics. The present version of the article contains this sentence:

"However, it does this by assuming a number of hidden variables, namely the positions of all the particles in the universe, which, like probability amplitudes in other interpretations, can never be measured directly."

and this is stated as if it were an uncontroversial fact. Now, pretty much any physicist working with Bohmian Mechanics will tell you that this claim is wrong, so even if you happen to think that somehow the physicists working with Bohmian Mechanics are wrong about what the theory they work with states about the world, a honest neutral Wikipedia article should at the very least point out that the claim is controversial.Dvtausk (talk) 21:19, 23 February 2009 (UTC)

Please note "wether" is spelt "whether". A side issue, but it's driving me crazy :).

To the substantive issue: I disagree that "particles being superfluous" is not the same thing as "positions of particles in Bohmian Mechanics not being observable". If the particles are superfluous in the theory (as Everett claims) then obviously they can be removed with affecting any empirical content. Therefore their paths are unobservable. Superfluous ==> unobservable, since observable ==> not superfluous. I agree that you can explicitly hard-wire the observability of the particles' paths into the theory by adding the assumption -- but that defeats Bohm's original motivation, which was to construct a realist theory without any observation postulates. In any realist, objective theory you would expect to be able to derive all the measurement features from other principles, rather than have to hard-wire observation in.

If you find a source where Everett's criticism was dealt with, I'd be interested to read it. I've never seen such feedback -- observation seems to be implicitly assumed in the manner we've outlined.--Michael C. Price ^talk 01:08, 24 February 2009 (UTC)

Sorry for the spelling mistake, English is not my first language.

The mistake in your reasoning "observable ==> not superfluous" is that you fail to take notice (as I said before) that the judgment of what it is that has been observed in a given experiment is theory dependent, not absolute (of course, what I mean here is not that when you see the number 3.7854 in a computer screen, then people with different theories would disagree that the number 3.7854 has appeared in the computer screen; what I mean is that the meaning of that number, i.e., what kind of information that it conveys about the world in a given experiment, is theory dependent).

Let me be more explicit: when you see a scintillation on a screeen (say, in the double slit experiment) Bohmian Mechanics explains this by stating that the scintillation corresponds to the detection of a particle at that position (such position is to be found in the Q variables of the theory, so, according to Bohmian Mechanics, this is a measurement of the Q's). Of course, Bohmian Mechanics can be wrong, so maybe that scintillation has nothing to do with the detection of a particle, and a different theory, with a different ontology, would explain the same scintillation as the observation of something else.

You are right about the fact that in any "realist objective theory" there shouldn't be postulates about results of measurements and observations and you are also right about the fact that having a "realist objective theory" is among the goals (I would say, the main goal) of the proponents of Bohmian Mechanics. On the other hand, there is one postulate about results of observations that is implied in the formulation of (essentially any) "realist objective theory", and such postulate is the one of perceptual realism, i.e., that what we perceive as what happens in three-dimensional physical space corresponds to what the theory says that happens in three-dimensional physical space.

Let me exemplify this using Classical Mechanics: Classical Mechanics describes the motion of n particles with trajectories $Q_{1}(t),\ldots ,Q_{n}(t)$ in $\mathbb {R} ^{3}$ (satisfying, say, the Hamilton equations). No one bothers to write down a postulate for Classical Mechanics saying that whenever the theory says that the i-th particle is in the position $Q_{i}(t)$ at time t, then we should actually find that particle at the position $Q_{i}(t)$ at time t, but this is obviously tacitly assumed when the theory is formulated. The same happens with Bohmian Mechanics.

I don't recall right now about any source dealing with Everett's argument (there is an article by John Bell called "Quantum Mechanics for Cosmologists" which contains some discussion of Everett's versus Bohm, but I don't recall the details right now). Part of my reply to Everett's argument is that a theory containing only the wave function is not a sharply formulated physical theory, for instance because it does not contain a sharp description of how the matter is distributed in the physical three-dimensional space. But I really don't want to get involved in an argument about that, because this would certainly lead to a long debate and I don't have the time for it, nor do I think that this is the appropriate forum for trying to settle a scientific controversy.

In fact, I'm astonished that such a trivial issue such as the observability of the Q's in Bohmian Mechanics lead to such a long discussion.Dvtausk (talk) 02:23, 24 February 2009 (UTC)

By the way, in your comment you mentioned "unobservability of the paths" and I stress the fact (as I did right on my first comment on this discussion) that "observability of the paths" is very different from observability of the instantaneous configuration.Dvtausk (talk) 02:23, 24 February 2009 (UTC)

Here is a quote from John Bell's "Quantum Mechanics for Cosmologists", section 4, "The pilot wave" (see his book "Speakable and Unspeakable in Quantum Mechanics", pg. 128):

"It is thus from the xs, rather than from $\psi$, that in this theory we suppose 'observables' to be constructed. It is in terms of the xs that we would define a 'psycho-physical parallelism' -- if we were pressed to go so far. Thus it would be appropriate to refer to the xs as 'exposed variables' and to $\psi$ as a 'hidden variable'. It is ironic that the traditional terminology is the reverse of this."

(he is talking about "the pilot wave theory", i.e., Bohmian Mechanics, his xs are my Q's).

According to Bohmian Mechanics (and I stress, it doesn't matter whether or not Bohmian Mechanics is true or even whether it is a reasonable theory at all, I'm just making a statement about what it is that Bohmian Mechanics says, i.e., about what is the theory about the world that the proponents of Bohmian Mechanics are proposing), the familiar macroscopic world of our experience, including the positions of pointers and ink on paper registering results of experiments are to be found in the Q's, the role of the $\psi$ is to guide the evolution of the Q's. Claiming that the Q's are not observable in a universe governed by Bohmian Mechanics is plain nonsense.Dvtausk (talk) 03:34, 24 February 2009 (UTC)

Ok, let me add a few more comments that might help you understand my point. There is a theory, let us call it solipsism, according to which nothing at all but my own consciousness exists in the universe. So according to such theory, it is false that I have sense organs that inform my brain and my consciousness about an external reality, but in fact all that is normally called "sense data" is nothing but lots of delusions produced by my own conciousness and they have absolutely nothing to do with any external reality. On the other hand, there is another theory, let us simply call it non-solipsism, according to which there is an external reality and I do have sense organs that inform my brain and my consciouness about some aspects of that external reality.

The existence of the theory called solipsism my lead some people to the conclusion that the external reality posited by the theory called non-solipsism is superfluous, since experience can be accounted for by the theory called solipsism, which posits no external reality (and this could lead some people to a --- misguided, I would argue --- attempt to invoke some sort of Occam's razor type of reasoning, that would lead them to the conclusion that solipsism is the epistemologically sounder theory).

Now, even if one happens to agree (based on the existence of the theory called solipsism) that external reality is superfluous, it would be wrong to conclude that, according to the theory called non-solipsism, the external reality is not observable. It is true that, according to the theory called non-solipsism, whenever I see a table, it is because there is a table and I'm observing the table (even though, according to the theory called solipsism, there are no tables to be observed, and the experience of "I see a table" is explained in terms of a delusion).Dvtausk (talk) 04:51, 24 February 2009 (UTC)

I don't why you think I don't understand the point about observation being theory dependent, since I explained exactly that in my last reply.

I am pleased to note that we both agree that the factual truth of MWI vs BM is not the issue. At least we can stay focused on the issue here.

Your solipsist analogy fails since it illustrates how two competing theories (S and non-S) differ in their handling of observation (a point, I repeat, I already understand), whereas the situation we have here is not of two competing theories (MWI vs BM) but an issue of whether the criticism of one theory is valid. That the criticism is made by the author of the other theory is irrelevant.

Thanks for the Bell quote. As you, say, let's not go into his critique of MWI here (although, of course, I believe it to be invalid). Bell, BTW, was never a very accurate critic of MWI, although he was very picturesque and graphic. I have his speakable and unspeakable book here with me. From your quote, though, it is evident that he too makes the same assumption that all Bohmites have to make about the observability of the particle paths.--Michael C. Price ^talk 06:34, 24 February 2009 (UTC)

1) If I understand your argument justifying the claim that "particle positions are not observable in Bohmian Mechanics", the structure of the argument is the following:

a) there is a theory T1 that does not posit the existence of entity X;

b) there is a theory T2 that posits the existence of entity X;

c) both theories T1 and T2 can account for experience;

therefore (so your argument goes), entity X is not observable in theory T2.

This argument is invalid, precisely because it neglects theory dependence of observation. Obviously, entity X is not observable in theory T1 (or, more precisely, it doesn't even make sense to talk about observability of X in theory T1, since there is no X in theory T1). But it doesn't follow that entity X is not observable in theory T2. Being observable in theory T2 is not the same thing as being observable in theory T1 (the "theory dependence of observation" that you supposably understand).

Maybe you are confusing "observing X" with "proving the existence of X". It is indeed true that the existence of theory T1, plus the fact that theory T1 accounts for experience, imply that one cannot prove the existence of X (but this is not the same as "X not being observable in theory T2"). But, of course, it is no surprise that one cannot prove the existence of X (since, for instance, it is impossible to logically refute solipsism).

2) It is not the case that the issue is of "whether the criticism of one theory is valid". Although I disagree with Everett's criticism of Bohmian Mechanics (about particles being superfluous) I am not here trying to defend Bohmian Mechanics from that criticism. I'm just pointing out that the claim that particle positions are not observable in Bohmian Mechanics (which does not follow from Everett's claim) is wrong.

You can disagree with the proponents of a theory about all sorts of things, but there is one thing about which it doesn't make sense to disagree with; it doesn't make sense to disagree with the proponents of a theory about what it is that they are proposing. And that's what you are doing. It is the job of the proponents of a "realist objective" theory to explain in which part of the mathematical formalism of the theory is physical reality supposed to be found. And the proponents of Bohmian Mechanics will tell you that the part of the formalism of Bohmian Mechanics in which reality is supposed to be found is in the Q's (different Bohmians might have slightly different ideas of exactly how the $\psi$ is supposed to be interpreted. It is supposed to be "real" in some sense, but not real like tables and chairs are real, but like a physical law is real; this is called the nomological interpretation of the $\psi$. In any case, if one were to simulate a Bohmian universe in a computer, the "output" of the program, i.e., that which gets to be considered as what is really hapenning in three-dimensional physical space is to be found in the Q's).

3) You are again insisting about the (non) observability of particle paths. Have you understood that I'm not talking about the observability of the paths? I highlighted the difference between observability of the instantaneous configuration and observability of the paths right in my first writing in this discussion and I have just stressed the difference in my last comment. If you are unable to see that "observability of the instantaneous configuration" and "observability of the paths" are not the same thing, then this discussion makes no sense.

Of course, you could wonder, how come, if I can observe Q(t) for an arbitrary t, doesn't it follow that I can observe the path $t\to Q(t)$ ? Again, I explained this right at the beginning. What happens is that measurement of positions (with a detector) significantly alters the particle paths (by collapsing of the effective wave function), so that you don't get to see what the path would have been like in case you had not measured the positions.

4) I reread Bell's "Quantum Mechanics for Cosmologists" and I realized that I don't agree with much of his criticism of Everett. But that's a different subject.Dvtausk (talk) 21:03, 24 February 2009 (UTC)

Let's dump the meta-terms and focus on the specifics: the problem is that Q has no effect on $\psi$. If I can somehow "see" Q then Q must have an effect in my $\psi$, since my future actions will changed by my observation of Q, which alters my future $\psi$. But Q does not have any effect on $\psi$, in any way, ever. Therefore Q cannot be observed. It doesn't matter whether you posit it can be observed, because it simply cannot be. It's like trying to posit that 1+1=3; it simply won't work because the resultant theory is internally inconsistent.

Another way of viewing it is to realise that Q is simply an assignment of reality to one branch of the universal wavefunction, $\psi$, to the exclusion of reality to the other branches -- but Bohm already accepts that $\psi$ in its wider totality is "objectively real" (a phrase Bohm repeatedly uses), so such a narrow reality assignment by Q is meaningless and contradictory. --Michael C. Price ^talk 01:20, 25 February 2009 (UTC)

There are two mistakes in your reasoning:

1) according to Bohmian Mechanics, a human being (just like all the matter) is made of particles whose positions are given by its Q's (the $\psi$ is only guiding the Q's). Therefore, if you want to know whether Bohmian Mechanics predicts an interaction between a given particle and a human being (an interaction that corresponds to the human being observing --- through some experimental apparatus --- the particle's position, for instance), you should worry about whether the Q of that given particle influences the Q's of that human being.

(yes, I understand that you are used to think that physical reality is to be found in the $\psi$, but this is simply not the case in Bohmian Mechanics; and no, this doesn't contradict the idea that $\psi$ is to be taken as something objectively real. Both $\psi$ and the Q's are to be taken as real, but they are not real in the same way. The Q's are real in the sense that they are supposed to contain a description of what is really happening in the three dimensional physical space of the Bohmian universe, they contain the information about how the matter moves around. The $\psi$, on the other hand, is real like a physical law is real (say, the Hamiltonian in Classical Mechanics), or, perhaps, like a physical field that guides the particles is real (say, like the eletromagnectic field in Maxwell's theory, an important difference being that $\psi$ is a field in the configuration space, not in physical space)).

2) While it is true that the $\psi$ of the entire universe is not affected by the Q's (by the way, that's part of the motivation for the nomological interpretation of the $\psi$), it is also true that the so called conditional wave function of a subsystem of the universe is affected by the Q's of the rest of the universe (such conditional wave function then influences the motion of the Q's of that subsystem, thus allowing interaction between the Q's of the subsystem and the Q's of the rest of the universe).

In case you don't know what the conditional wave function of a subsystem is, I'm happy to give you a short explanation (and also give you some good references written by Bohmian experts containing more detailed explanations; see, for instance, this article, section 4, bottom of pg. 9).

Denoting by $Q_{I}(t)$ the vector of particle positions of a certain subsystem I of the universe, by $Q_{II}(t)$ the vector of particle positions of the rest of the universe and by $\psi (t,q_{I},q_{II})$ the wave function of the universe, then the conditional wave function $\psi ^{I}$ of subsystem I is defined by:

$\psi ^{I}(t,q_{I})=\psi (t,q_{I},Q_{II}(t)),$

i.e., what you do is to replace the position variables $q_{II}$ in the wave function of the universe with the actual positions $Q_{II}(t)$ of the particles that are outside system I. It follows easily from the Bohmian guiding equation that the conditional wave function of a subsystem is the wave function that actually guides the particles of that subsystem.

So, basically, you're mistaken when you think that "the Q's do not influence the wave function of the universe" (which is true) implies that "the Bohmian particles do not interact with each other" (which is false).Dvtausk (talk) 03:14, 25 February 2009 (UTC)

Thanks for the interesting PDF, but I never claimed that "the Bohmian particles do not interact with each other". If they really are unobservable (and hence superfluous) then their mutual interactions are also unobservable and superfluous. Defining a mixture of $\psi$ and some Qs doesn't ensure that the resulting amalgam is observable. --Michael C. Price ^talk 08:38, 25 February 2009 (UTC)

Your argument (if I have not misunderstood it) was "the Q of a particle won't influence my $\psi$, thus I cannot observe the Q of that particle". In the formulation of Bohmian Mechanics there is, in principle, only the wave function $\psi$ of the universe, so it is simply meaningless to talk about "my $\psi$" or about "the $\psi$ of a subsystem of the universe"... unless, of course, you give a definition of what the $\psi$ of a subsystem of the universe means, and that's what the definition of the conditional wave function is supposed to make meaningful. In any case, what you are failing to realize is that, according to Bohmian Mechanics, I am not "made of $\psi$'s", I am made of particles with Q's, so "me observing the Q of a particle" is related to "the Q of that particle interacting with the Q's of my particles" (and the Q of that particle can interact with the Q's of my particles).Dvtausk (talk) 15:00, 25 February 2009 (UTC)

I fully realise that BM contains the assertion that we are made of both particles and $\psi$. That doesn't alter the fact that, whereas $\psi$ influences the particles, the particles have no influence on $\psi$. So the "Q part" of me can "see" the particles, but not the "$\psi$ part"? --Michael C. Price ^talk 16:17, 25 February 2009 (UTC)

Bohmians would tell you (and you can't disagree with them about what it is that they are proposing) that it is not true that, according to Bohmian Mechanics, matter (tables, chairs, cats, people, etc) is "made of Q's and $\psi$'s" but, rather, as I told you, that, according to Bohmian Mechanics, matter is made of particles whose positions are to be found in the Q's. Although the theory (obviously) has nothing to say about how consciousness emerges from matter, the stuff from which consciousness is supposed to emerge is to be found in the Q's, not in the $\psi$ (it is this that Bell means when he writes "It is in terms of the xs that we would define a 'psycho-physical parallelism' -- if we were pressed to go so far."). Bohmians would also probably tell you (you are right about that) that the $\psi$ is to be considered as something real (as opposed to, say, epistemic, like probability distributions on the phase space in Classical Statistical Mechanics), but they don't mean by that claim that "matter is made of Q's and $\psi$'s" or that consciousness is supposed to emerge from something that you will find in the $\psi$.

In any case, if you want to talk about "my $\psi$" (or about the $\psi$ of a subsystem of the Bohmian universe) then you have to define what that means, and if you define the $\psi$ of a subsystem in a way that is useful for the theory (i.e., something that guides the particles of that subsystem) you will get to the definition of conditional wave function that I explained to you; and it is true that the conditional wave function of a subsystem is influenced by the Q's of the particles that are outside that subsystem.Dvtausk (talk) 20:43, 25 February 2009 (UTC)

This is all very fascinating and I have been following it with interest. However, you do seem to be discussing whether Bohmian mechanics is true or, at least, good science, and not discussing really how to improve this article. The article should report what others have said about Bohmian mechanics both by its supporters and its critics. I suggest you take your discussion to e-mail. --Bduke (Discussion) 21:58, 25 February 2009 (UTC)

I'm not discussing whether Bohmian Mechanics is true or whether it is a good theory, nor do I intend to do so here. I believe that Michael agrees that this is not the subject of our discussion. This whole discussion started because I saw the statement:

"However, it does this by assuming a number of hidden variables, namely the positions of all the particles in the universe, which, like probability amplitudes in other interpretations, can never be measured directly."

in the "The Bohm interpretation" section of the main article and I know that this claim (about positions of the particles not being measurable directly) is wrong. Michael disagrees. This is (supposed to be) the only subject of our discussion. I was hoping that me and Michael would reach an agreement on this matter, but now it seems like this is not going to happen, so I propose that we simply write on the article something like: "source A claims X, source B claims Y". So, basically, I agree with Bduke, although I wouldn't be too interested in continuing the discussion with Michael via e-mail, since I can't spend that much time on a debate every time I find someone who makes an incorrect claim about Bohmian Mechanics.Dvtausk (talk) 23:33, 25 February 2009 (UTC).

If there are different interpretations out there, then "source A claims X, source B claims Y" is exactly what you should be doing and not trying to resolve the difference here, which would be original research.--Bduke (Discussion) 23:45, 25 February 2009 (UTC)

I agree with both Dvtausk and Bduke: I don't think we are going to reach agreement about either the substantive issue (the validity of Everett's criticism) or the question of quite how it should be presented, because we can't agree what it is exactly that Bohmian mechanics assumes. However I hope we can agree to confine our changes to the Bohmian mechanics article? Also we can agree that Everett's criticism is not accepted by Bohmites? Lastly it would great to state it all in the form of "source A claims X, source B claims Y", but I can't find a sourced response to Everett's position. Anyone? --Michael C. Price ^talk 01:30, 26 February 2009 (UTC)

Now that I have read the article about "No original research" I have a better understanding of the rules of Wikipedia (yes, I'm new around here) and I understand that most of my discussion with Michael was pointless; it wouldn't even matter if we had happenned to have reached an agreement on the issue.

For now, I'm proposing only changing the section of the article concerning Bohm's interpretation (later on, of course, after reading carefully the entire article I might decide to propose an alteration on something else).

Of course, in this article, the section about Bohm's interpretation is to be kept short and therefore a detailed presentation of criticism of Bohmian Mechanics and of the replies of its proponents would not be appropriate (things like that should go to the larger article about Bohmian Mechanics; that article would probably need a lot of editing, but I don't have time for this right now).

I think that Everett's criticism of Bohmian Mechanics (quoted in the FAQ of the Bohmian Mechanics article):

"Our main criticism of this view is on the grounds of simplicity - if one desires to hold the view that $\psi$ is a real field then the associated particle is superfluous since, as we have endeavored to illustrate, the pure wave theory is itself satisfactory."

can be presented and attributed to Everett. However, the claim that is right below that in the FAQ:

"In this view, then, the Bohm particles are unobservable entities, similar to and equally as unnecessary as, for example, the luminiferous ether was found to be unnecessary in special relativity."

is original research and should not be admitted, unless it can be sourced (specially the claim about the unobservability of Bohm's particles should not be admitted, unless it can be sourced). It doesn't matter if Michael believes that this claim follows logically from Everett's claim, I think that the "No original research" article makes it sufficiently clear that this kind of extrapolation is to be considered original research and should not be allowed (unless the extrapolation is directly supported by some source).

If Everett's criticism (without the extrapolation!) gets to be in the article, then I could do some research to see if I can come up with a source that answers this particular criticism.

If Michael can find a source that directly supports the view that the positions of Bohmian particles cannot be observed (and decides to include this on the article), then I would add Bell's remark (quoted by me above in this discussion page) supporting the opposite view.Dvtausk (talk) 03:07, 26 February 2009 (UTC)

The question is, when does an explanation shade into original research? Explanations are quite admissible. So, if we say that by "superfluous" they are as a consequence "unobservable" (in Everett's view, of course, which is acknowledged by the prefix in this view), that would seem okay in that it doesn't violate WP:OR. And perhaps a link to Ocham's razor as well would be helpful. The analogy with the ether is there as an explanation of what Everett meant by superfluous. What would be original research would be much of our recent discussion (sadly) about how BM might handle the issue. --Michael C. Price ^talk 09:20, 26 February 2009 (UTC)

From WP:OR:

"you must cite reliable sources that are directly related to the topic of the article, and that directly support the information as it is presented."

"Even with well-sourced material, however, if you use it out of context or to advance a position that is not directly and explicitly supported by the source used, you as an editor are engaging in original research;" (emphasis added)

Sorry, "unobservable" is not an explanation of "superfluous", "superfluous ==> unobservable" is an argument (as I have shown to you, it is an invalid argument, but it doesn't even matter whether I'm right about that). Don't try to cheat, the "unobservable" part is not directly supported by your source, it is your opinion that "unobservable" follows from "superfluous", so either find a source that diretly supports it or give it up.

The analogy with the ether is trickier... while I do believe that the author of your source (Everett) would agree that this is a good analogy for explaining his point, one could argue that you can't show that what Everett meant by "superfluous" is faithfully reflected by your analogy.

From WP:OR:

"Editors should not make the mistake of thinking that if A is published by a reliable source, and B is published by a reliable source, then A and B can be joined together to reach conclusion C. This would be a synthesis of published material that advances a new position, which constitutes original research."

As for the link to Ocham's razor, you would be joining multiple sources to make a point ("X is superfluous" + "Ocham's razor" ==> "a theory containing X is not a good theory"). Whether or not Ocham's razor applies in a given situation is not a perfectly objective and undisputable issue (recall my point, for instance, that applying Ocham's razor to support the theory of solipsism is a bad argument). So, if your source does not explicitly appeal to Ocham's razor to make the point that you are trying to make, you are doing original research.

About our "recent discussion": my arguments were presented to help you understand the issue. I will not (as you are doing) propose the inclusion of any of my arguments in a Wikipedia article, unless they are explicitly supported by a source.Dvtausk (talk) 01:09, 27 February 2009 (UTC)

Comment: many of the points that I made in our "recent discussion" are made in this article (published in the British Journal for the Philosophy of Science 59, 353-389).01:52, 27 February 2009 (UTC) —Preceding unsigned comment added by Dvtausk (talk • contribs)

Another observation: John Bell says that the Q's are observable in Bohmian Mechanics and all the people professionally working with Bohmian Mechanics will say that the Q's are observable in Bohmian Mechanics. To say that John Bell and all the people working with a theory are wrong about such a basic aspect of the theory (it is the kind of thing that one would learn in the very first lesson in a course in Bohmian Mechanics) is a very powerful claim. Can you find one single published source that directly claims that the Q's in Bohmian Mechanics are not observable? So you decided to distort Everett's claim to advance this point? Find a source that directly supports your unobservability claim or give it up.Dvtausk (talk) 13:53, 27 February 2009 (UTC)

When a scientist says that an element of a theory is "superfluous" all scientists understand this to be a reference to Ocham's razor. I forget the exact bit of wikipolicy that says this, but if a universal truth is accepted by all experts we are allowed to state this. The ether comparison could be regarded as a concrete example of an application of Ocham's razor (again, an example that all experts would accept, and not inserted by me, BTW). As I said, though, this is all contingent on "in Everett's view". And please don't say I "decided to distort Everett's claim" -- you accept that he would accept them, we are just debating about whether they are admissible here. --Michael C. Price ^talk 14:20, 27 February 2009 (UTC)

I accept that Everett would probably agree with your ether analogy and I even accept that Everett probably would agree that "superflous" is an attempt to invoke Ocham's razor (though I think that your claim that "all scientists understand this to be a reference to Ocham's razor" is too strong and certainly disputable). But pay attention to what I'm writing: I accused you of trying to "distort Everett's claim" in order to advance your argument of unobservability of positions of particles (the Q's) in Bohmian Mechanics. I insist that this is really what your are doing in that situation.Dvtausk (talk) 17:03, 27 February 2009 (UTC)

But if you accept that Everett understood/believed that the Bohmian particles were superfluous in the Ochamite sense (i.e. non-existent) then surely he also thought they were unobservable (we can't observe non-existent entities)? We can argue about whether that is admissible here, but I don't see how you can accuse me of distorting Everett's views to push my own beliefs. Perhaps I am pushing Everett's beliefs inappropriately, but not distorting them.--Michael C. Price ^talk 19:25, 27 February 2009 (UTC)

Ok, so I think I should withdraw my accusation, because apparently you are not deliberately trying to distort Everett's statement, you are honestly misunderstanding the issue. The problem is that your understanding of the concept of "observation" is too naive, you have claimed that you understood my explanations about theory dependence of observation, but from your last comment it is clear that you haven't. A claim about what it is that has been observed in a given situation is always to be understood relatively to a theory about how things are, otherwise it is meaningless (except, perhaps, when a claim about what it is that has been observed is just a crude description of raw sense data, like "I saw the number 3.7453 appear in the screen of the computer"). It is perfectly possible that something that does not even exist according to a given (empirically viable) theory is at the same time existent and observable according to another (empirically viable) competing theory.

Look, if there is one thing that is not hard to find in the Physics literature is criticism of Bohmian Mechanics. There are two types of criticisms of Bohmian Mechanics out there:

(a) intelligent educated criticism that leads to interesting debates and to the consideration of subtle points;

(b) criticism that is based on claims that are trivially false, profoundly ignorant or based on gross misunderstandings of what is the theory that Bohmians are proposing.

Everett's "superfluousness of particles" criticism is of type (a). Dealing with that criticism leads, for instance, to the consideration of the concept of Primitive Ontology (introduced by Dürr et al.) or to the related concept of Local Beable (introduced by John Bell). Some discussion about that appears in the "On the Common Structure..." article that I cited above. This would lead to a subtle interesting debate about when a physical theory is sharply formulated, about matters of Epistemology and Metaphysics.

Your criticism about "unobservability of particle positions in Bohmian Mechanics" (which appears in the main "Interpretation of quantum mechanics" Wikipedia article) is, on the other hand, of type (b) and I really do not believe that it would be supported by Everett (of course, I might be wrong about that, but I'm sure that such criticism is at least not being explicitly supported by that quote from Everett).

Now, it is not that hard to find respectable scientists writing criticism of type (b) about Bohmian Mechanics in respectable journals/books. Therefore, I wouldn't be that surprised if you could come up with a more or less decent source that directly supports your unobservability claim; if you can produce that source then, according to Wikipedia rules, it would be perfectly fine for you to include that claim in the main article and attribute it to the source that you have found (in that case, of course, I would include the claim that your source's claim is wrong and would cite a source for that --- there are many).

In summary: the unobservability claim is completely inadmissible, unless directly supported by a source. Everett's criticism is obviously admissible, the ether analogy and the link to Ocham's razor can be included, but my agreement on that would depend on the exact wording that you choose.Dvtausk (talk) 20:43, 27 February 2009 (UTC)

Okay, well, I will make a general point which you can ignore or respond to as you please (since it probably won't affect the article) and then offer a specific suggestion about what we do.

General point: The observation theory point you keep raising and insisting I misunderstand is irrelevant. MWIers generally believe that the MWI follows from the mathematical formalism itself, along with a belief in the reality of the non-collapsible wavefunction. (Everett and de Witt certainly believed that, and so do I.) Therefore -- in the MWI view -- any interpretation that accepts a non-collapse view of a real wavefunction -- such as de Broglie-Bohm's -- is necessarily forced to accept the MWI. It doesn't matter what extra assumptions or entities are postulated in an attempt to evade the MWI, the only way out of the MWI is to

1. change the existing mathematical formalism or
2. reject the reality of the wavefunction or
3. accept that it collapses

None of these options does Bohm choose. Tacking on some new entities (the particles) and an implicit assumption relating to the observability of these particles won't help. Now of course Bohmites don't accept this conclusion; if they did I guess they'd be Everettites. So in the MWI view the Bohm particles have to be unobservable because there isn't the observationable freedom left in any interpretation that accepts the three assumptions above. Okay, I'm sure you won't accept this explanation as factual, but please try to understand where MWIers come from, and why statements about observation being interpretation-dependent have no impact on MWIers.

Specific point: What form of the disputed sentence is jointly acceptable? We have at present:

However, it does this by assuming a number of hidden variables, namely the positions of all the particles in the universe, which, like probability amplitudes in other interpretations, can never be measured directly.

I suggest making the addition "and momenta" to read:

However, it does this by assuming a number of hidden variables, namely the positions and momenta of all the particles in the universe, which, like probability amplitudes in other interpretations, can never be measured directly.

As a source I cite Bohm's words from his 2nd foundational paper, section 4 ... the precise values of the particle position and momentum must, in general, be regarded as "hidden", since we cannot at present measure them.

I suggest we leave discussion of the "Everett text" in the Bohmian mechanics article to that article's talk page.

--Michael C. Price ^talk 00:21, 28 February 2009 (UTC)

I guess it is not appropriate for us to continue the scientific discussion here in the Wikipedia discussion pages for too long, because (as Bduke pointed out) it is not really relevant for the article. But, since you made an effort to explain your position in detail, I will give you a short reply.

My explanations about theory dependence of observation show that the argument "superfluous ==> not observable" (or, better, "non existent in theory T1 ==> unobservable in theory T2") is invalid. Of course, my explanations about theory dependence of observation are not supposed to show that all the arguments that lead to the conclusion of unobservability of the Q's in Bohmian Mechanics are invalid (I know, however, that any such argument must be invalid because it can be shown that the alleged conclusion is false, by an analysis of the Bohmian theory). So, if you present a different argument that leads to the conclusion of unobservability of Q's in Bohmian Mechanics, it might well be the case that the explanation of why this new argument is invalid has nothing to do with theory dependence of observation. For instance, in our recent discussion, you have presented another argument that lead you to the conclusion that the Q's are unobservable in Bohmian Mechanics (the argument involving the fact that the Q's do not influence $\psi$) and I explained to you why that argument is invalid too.

Now, you have presented a new argument... and I can easily show to you why this new argument is also invalid. The problem is that you fail to see that "being real" can have more than one meaning (I have already sketched this explanation before, but I don't mind repeating it). When Bohmians say that the wave function is real, they don't mean what you think that they mean. When a Bohmian says that the wave function is real, he/she is merely emphasizing that the wave function is not subjective or epistemic (like, say, a probability distribution in phase space in Classical Statistical Mechanics). For instance, if the wave function contains something that resembles a human being looking at a dead cat then, in Bohmian Mechanics, this doesn't mean that there really exists a human being looking at a dead cat (the theory says that you should look at the Q's in order to know whether there really exists a human being looking at a dead cat); on the other hand, in MWI, one is supposed to understand the wave function in a way that when it contains something that resembles a human being looking at a dead cat then there really exists a human being looking at a dead cat. So, what happens is that, while both MWIers and Bohmians would agree to the validity of "the wave function is real", they do not mean the same thing by "real". Bohmians do take your way out number 2... because they don't mean what you meant (and MWIers normally mean) when you wrote the word "real". Different people sometimes use the same word with different meanings. It is as simple as that.

In case you have a genuine interest in this issue, I strongly suggest that you eventually take some time to read the article "On the common structure..." that I cited above.

Getting back to the discussion that concerns the Wikipedia article: adding "and momenta" only makes the sentence worse. I suspect that the sentence that you took from Bohm's article has been taken out of context, so I have to look at the article to check what it is that Bohm meant when he wrote that. Unfortunately, it is possible that I will have to wait until monday in order to have access to Bohm's article. But I will get back to you on that really soon.

Another comment: "Bohmian Mechanics" isn't supposed to mean "the theory exactly as it is presented in David Bohm's original articles", just like, say, "Galois theory" does not mean "the theory exactly as it is presented in the original writings of Evarist Galois", but rather, the Galois theory that is presented in modern Algebra textbooks.Dvtausk (talk) 06:50, 28 February 2009 (UTC)

I'm pleased that the debate comes down explicitly to what we mean about the word "real". It means no more progress is to be made on the subject, since MWIers have very binary view of reality: either something is real or it isn't, without any shades of grey in between. Bohm in his original articles is similarly clear, since he often makes the comparison of $\psi$ with the electromagnetic field. If Bohmian mechanics has since moved on to embraces the ghostly in-between shades of reality then it would seem to inherit all the problems that go with the Copenhagen interpretation.

I hope you can see how the two arguments I've presented against the Q-particles are just flip-sides of each other; since the particles don't exist (in the MWI view) they can obviously not have any influence on $\psi$. Obversely, since the particles have no influence on $\psi$ they have no observational status (since $\psi$ is "real enough" to contain conscious observers) and hence are superfluous, by Ocham's razor.

I shall read the "On the common structure.." article. It looks very interesting. Speak to you after you've checked Bohm's quote. --Michael C. Price ^talk 10:13, 28 February 2009 (UTC)

It is not the case that Bohmians are considering "shades of gray" between "real" and "not real". Wouldn't you say that a physical law is, in some sense, "real"? It is not real like, say, a table is real, but one could argue that it is real in the sense that it is not just "something in my head", it must be "known to Nature", since Nature follows that law and "something that is just in my head" wouldn't influence Nature's behaviour. So, it is not about being "half real" or "one third real", but about "being real in different ways". The issue is not of whether it is the MWIers or the Bohmians that are right about what the word "real" means, nobody is right about what a word means, it is just a matter of convention, different people use the same word with different meanings and it is not the case that some of them are "right" and the others are "wrong" about what it is that the word means.

In any case, the best way of presenting Bohmian Mechanics is to say (in order to avoid the unnecessary trouble with the word "real"): the theory contains the $\psi$ and the Q's, the Q's describe the positions of the particles that matter is made of (and that constitute tables, chairs and conscious observers). The $\psi$ is merely something that makes the particles move around (and one is not obliged to discuss the exact ontological status of the $\psi$, just like, in a presentation of Classical Mechanics, one does not discuss the exact ontological status of the Hamiltonian).

For instance, I think many Bohmians would agree to the following statement (I would): if you find a mathematical formalism that does not contain the $\psi$ that evolves through Schrödinger's equation (might contain other things, satisfying other differential equations) but that contains the Q's and if those Q's follow the same trajectories as they do in ordinary Bohmian Mechanics, then this new mathematical formalism should be considered as being essentially (or even exactly) the same physical theory as ordinary Bohmian Mechanics. The theory is not about the $\psi$, it is about the Q's, the $\psi$ is just a tool to describe the motion of the Q's.Dvtausk (talk) 14:14, 28 February 2009 (UTC)

One more comment: the E's and B's in Maxwell's eletromagnetism and the g's (metric tensor) in General Relativity are normally considered to be real, but it is also well understood that it is not in the E's, B's and g's that one should look for tables, chairs and conscious observers.Dvtausk (talk) 14:21, 28 February 2009 (UTC)

One of the things that comes across again and again in Bohm's two foundational papers is that $\psi$ is as real as the classical electromagnetic field, the only difference being in the domain and range of the fields, not in their reality. The reality of physical law is unclear, I'll grant you, but that is a side issue, albeit an interesting one.

The point you make about a $\psi$-less theory is interesting. Even given the Qs at some time you also need $\psi$ (or, equivalently, the quantum potential) to completely specify the initial conditions. Otherwise you can't compute the trajectories. So the Qs on their own is not sufficient.

I don't what the basis for the final statement is: it is also well understood that it is not in the E's, B's and g's that one should look for tables, chairs and conscious observers. Wasn't that precisely what Einstein's quest for a unified field theory was all about? And recent string theory?

--Michael C. Price ^talk 23:47, 28 February 2009 (UTC)

As I already mentioned, "Bohmian Mechanics" doesn't mean "the exact thing that David Bohm had in mind", but rather the theory as presented by modern Bohmians (David Bohm's articles are mostly important to historians of Science); so, if you want to become a serious critic of Bohmian Mechanics, do not worry so much about what David Bohm wrote but about what modern Bohmians write. Many Bohmians (including me) prefer to think about the $\psi$ as a law rather than a physical field (in the article "Bohmian Mechanics and Quantum Information" that I cited above, this view is defended in section 6, pg. 16).

In any case, even if one insists that $\psi$ is to be understood as a physical field (similar to an eletromagnetic field), it doesn't follow from that, for instance, that you should find in the $\psi$ information about how the matter is distributed in physical space or that it is from stuff described by the $\psi$ that the consciousness of human observers emerge. You are allowed to formulate a theory (an MWI-like theory) in which the $\psi$ describes the distribution of matter in physical space and the stuff from which the consciousness of human observers emerge, but you are also allowed to formulate a theory (like Bohmian Mechanics) in which $\psi$ does no such thing.

I think that the main reason why we do not understand each other is the following: the theory that you call "Bohmian Mechanics" is not the theory that Bohmians call Bohmian Mechanics, but rather some amalgam of MWI with the actual Bohmian Mechanics, in which the $\psi$ is to be understood as it is understood in MWI, but there are also particles following Bohmian trajectories (Q's) appearing in the theory (and, in this theory, consciousness of human observers emerges from something that is to be found in the $\psi$, not in the Q's). In this chimeric theory that is neither MWI nor Bohmian Mechanics, it is really true that the Q's are not observable and this is in fact a very stupid theory.

Have in mind that, by criticizing the chimeric theory that you call "Bohmian Mechanics" you are (perhaps unintentionally) just attacking the straw man, leaving the actual Bohmian Mechanics proposed by the Bohmians untouched.

About my comment concerning E's, B's and g's: what I mean is this; in Maxwell's theory (for instance), if I give you just the E's and B's, you will not be able to figure out from that what it is that is happening in the universe.

About the point that you called "interesting": sorry, the point wasn't to try to advance an argument (as you apparently understood) that it would be possible to have a theory with just the Q's. The point was to illustrate the (sociological) fact that Bohmians normally don't take the reality of the $\psi$ very seriously (see section 4, specially footnote number 6 on pg. 11 of the "On the common structure..." article and you will see what I mean).Dvtausk (talk) 02:13, 1 March 2009 (UTC)

I already understood your explanation that Bohmian mechanics has moved on from Bohm, and I already gave my response on why I thought that was a retrograde step.

Most of our differences come down to the reality of $\psi$ and what that means. The analogy of $\psi$ with physical law is invalid, as I tried to indicate previously; physical law is universal and system-independent, whereas $\psi$ is system-dependent and therefore forms part of the initial conditions (which everybody accepts are real).

The explanation you give for tables and consciousness not being found in the E, B and g's if I give you just the E's and B's, you will not be able to figure out from that what it is that is happening in the universe is precisely why we cannot look for them in the Q-particles with either :-) -- the Q-particles are not a complete specification of reality. In specifying the initial conditions of a system you have to include $\psi$. We are not free to redefine terms as we like.

--Michael C. Price ^talk 13:42, 1 March 2009 (UTC)

Look, Michael, I'm getting tired of debating you, it seems like you simply don't want to understand and in that case no one can help you.

How can you say that what the new Bohmians present is worse (a "retrograde step", as you say) than what Bohm presents if it is obvious from our discussion that you barely know what it is that the new Bohmians are presenting? You're obviously not familiarized with the modern Bohmian literature (I am). Your response of "why you think it is a retrograde step" was based on a gross misunderstanding of my explanation about the ontological status of the $\psi$, in which you somehow got the idea that Bohmians are considering "ghosts" or "shades of gray between real and not real".

The $\psi$ that appears in the formulation of Bohmian Mechanics is (unlike what you said) universal and system independent. The wave function of a subsystem (known as the conditional wave function of the subsystem and, under some appropriate conditions, is also called the effective wave function of the subsystem) is a concept that you define from the universal wave function that appears in the formulation of the theory and from the Q's and it is merely a convenient tool for describing the motion of the particles of that subsystem (by the way, that's a feature of Bohmian Mechanics: there is a sharp mathematical definition for the concept of wave function of a subsystem). Observe that also the Hamiltonian of Classical Mechanics (which I have been using all along in the discussion as an analogy to help you understand the nomological role of the wave function in Bohmian Mechanics) can also be defined for subsystems of the universe. And moreover, as I explained, even if we accept the original view of David Bohm that the $\psi$ is a real physical field that exerts a force on the particles that causes them to deviate from standard classical trajectories, the conclusion that you want (that the $\psi$ is something from which matter is made of and that consciousness emerges from something that is in the $\psi$) doesn't follow (observe that I'm not saying that you are forbidden to formulate a theory --- like MWI --- in which the matter distribution and the stuff from which consciousness arises is to be read from the $\psi$, I'm just saying that that's not the only option and that Bohmian Mechanics does not take that option. In order to formulate a physical theory it is not sufficient to present a mathematical formalism, you have also to present explanations of how the formalism is supposed to be connected to physical reality, things like what aspects of reality are supposed to be represented by what parts of the formalism).

The claim that you made in the last paragraph of your comment is wrong. It is true that if I give you just $Q(t_{0})$, you won't be able to predict what $Q(t)$ will be like for $t>t_{0}$. But, if I give you $Q(t)$ for all t in a given interval $[t_{0},t_{1}]$, you will know what it is that happened in the Bohmian universe during the time interval $[t_{0},t_{1}]$. As I have already mentioned, it helps to imagine a Bohmian universe being simulated in a computer: while the wave function $\psi$ would merely appear as some internal variable that the computer uses as a tool for determining how the Q's evolve, the Q's would be directly appearing in the computer screen and you would be able to watch the story of the Bohmian universe just like you watch a movie, without ever having to worry about knowing the wave function.

Soon, we will go back to the discussion about the contents of the Wikipedia articles about Bohmian Mechanics. I wonder if it will be possible to produce anything decent, considering that it has to be negotiated with someone who knows very little about the theory, but which seems to believe that he knows more than the experts that are actually doing research, publishing articles and teaching the subject at important universities. I do not consider myself an expert, I still have a lot to learn about the theory, but the types of things we have been discussing are really really elementary.Dvtausk (talk) 21:09, 1 March 2009 (UTC)

One of the things that is really really elementary is that $\psi$ models systems and so, by definition, is system-dependent. That statement is as true of the universal wavefunction as it is of the wavefunction of any subset of the universe, i.e. of any subsystem. I don't see why you decided to take umbrage over this, but it seriously calls into question the veracity of your judgment about my ignorance of basic physics, post-Bohm developments etc.

I'm sorry to see, Dvtausk, such a grumpy tone entering what has been, up to now, such a polite and intelligent debate. Time for a break, I think.

--Michael C. Price ^talk 21:58, 1 March 2009 (UTC)

Well, it is possible that I didn't understand what you meant by "system dependent" (what I understand by "system dependent" is that "there is a wave function for each subsystem of the universe"; in Bohmian Mechanics we have, in the formulation, just a universal wave function. Wave functions of subsystems are introduced, later on, through a mathematical definition, in terms of the wave function of the universe and the Q's).

I do not judge you to be ignorant of basic physics. Not at all. I just said that you are not familiarized with the modern literature on Bohmian Mechanics.

I think we should drop the scientific debate, since it seems to be taking us nowhere and it is not (as it should be) a debate about how to improve the article.

By the way, I have just looked at the main article and the present version seems fine to me. I propose to leave the present version as it is and take detailed criticism of Bohmian Mechanics and the replies of its supporters to the main article about Bohmian Mechanics.Dvtausk (talk) 22:41, 1 March 2009 (UTC)