Transcript discussion Bub and Pitowsky

July 19 16.00
Two dogmas about quantum mechanics
Speakers: Jeff Bub & Itamar Pitowsky
Commentators: Chris Timpson
Floor speakers (in order of appearance):
Ladyman
Valentini
Wallace
Maudlin
Lehner
Rae

31


Comment by Timpson

1. Some quick comments. The comments I’m afraid are going to be somewhat polemical so forgive me in advance. There are a bunch of quibbles I could raise, particularly with that last claim that we were hearing, particularly that one can de-clone in Everett, that’s a suprising kind of claim given that it’s a theorem of quantum mechanics that one can’t clone in Everett which is just straight-out quantum mechanics so clearly there’s some jiggery-pokery about what one means by whether or not one can clone. Whether de facto it turns out in advance that people don’t clone … whether it’s ruled out as a matter of law. I think it’s that latter one which people understandably take the claim to be. That’s a quibble; I want to focus on the big picture. So, the big picture here is that Jeff and Itamar .. are interested in this question of .. locating in quantum mechanics in the space of possible theories that allow … information theories of various kinds, and they’re interested in what kinds of information-theoretic constraints can narrow down … quantum mechanics uniquely as the one correct theory. Now, I think that’s and extremely interesting project to be engaged in but I don’t see it as a project which actually tells us anything about the ontology of the world, or which is a candidate for telling us anything about the ontology of the world ….. It is not an approach which is going to tell us which interpretation of quantum mechanics is .. a candidate which we should adopt, be it Bhorian instrumentalist, Bohmian or what not. Why not? Because it’s about .. pictures of the theory as a theory, not about pictures of what the theory says about the world. So, in providing their argument that Everett and Bohm are ruled out by focussing on the no-cloning theorem in this kind of approach I think really there isn’t – the argument here isn’t a logical one … logical principles, it’s really a kind of rhetorical positioning. Okay, that’s my claim. And what I want to do is to redress the rhetorical balance here. So, basically the rhetorical position is that we line up the Bohmian and Everettian .. with a bunch of well-known loosers, so we have Carnap versus Quine and .. the Everettians and Bohmians with Carnap and the other loosers, poor old Lorenz and .. Einstein is on the side of the angels. Lorenz was a looser, oh dear, Everett and Bohm are like Lorenz. This is what the rhetoric is. I just ... both sections.

2. Regarding the two dogmas, my immediate response is that, well, as Carnap might well have said to Quine, one man’s dogma is another man’s axiom. The dogmas here are supposed to be: no dynamical
.. theory?of measurement is possible and that quantum states aren’t ontologically significant. So .. want to line themselves up with bold, bright young Quine. Who’s the Carnap figure here? Well, I think the natural candidate is Bell. Now, what would Bell have made of the proposal that we have been presented with today? I think he would think it’s rather peculiar to be told that it follows as a theorem from the theoretical structure that we’re commited to (that we subscribe to) that we can’t have what indeed we demonstrably do have. I think he’d think that’s a peculiar kind of argument. We demonstrably do have adequate dynamical analyses of measurement in quantum mechanics; they’re called Everett and Bohm. Okay, granted, measurement when we use the phrase ‘quantum measurement’ doesn’t mean measurement when we’re talking about classical measurement. Okay, fine, intuitively we know that. There are Kosher?-Speker?kinds of results of course which tell us that, and Jeff and Itamar’s use of the no-cloning theorem is just another example of that. Measurement means something different in quantum mechanics; that’s not a good reason to reject the Everett and Bohm .. it’s not a good reason to say that what they do when they look at interactions of a particular kind doesn’t make any sense. Of course it makes sense and it is extremely valuable to go through those processes. So, to close off this section, basically no difference in focussing on the no-cloning theorem from the kind of conclusion which can be drawn from the Bell-Kosher-Speker results. Those things don’t tell us that we can’t have a complete dynamical account of measurement.

3. Okay, so on to Lorenz as a looser. As Harvey Brown and I have argued in print, we think of drawing this analogy with what went on with relativity in order to motivate .. the doing-down of Everett and Bohm kind of backfires in a big way because Einstein himself came to realise that the original way in which he’d formulated theory in 1905, the principle.. theory approach, was the wrong way of formulating relativity. He himself came to realise that actually something more along the lines of what Lorenz was doing was the right thing to do. Because he realised and is quoted himself as saying that he’d commited a great sin in 1905 in treating measuring rods and clocks as unanalysed .. just primitives and really .. atomic configurations subject to dynamical laws .. and if we want to know why ..the Lorenz.. contract and time-dilate we’d better see what those laws tell you about the behaviour of those things. Now, people couldn’t do that in 1905, obviously, because they didn’t have the ground-level theory.. constructive theory to build up from so Einstien .. stroke genius went to .. I can sidestep for the momen pro tem? .. constructive theory in order to draw some conclusions about what any .. this kind of constructive theory should look like, but that’s not to reject the thought that … constructive theory … postulate the laws .. you see it going to follow … and see what rods and clocks .. behave isn’t actually what we want to end up doing. So, Einstein himself would want to … against the principle theory kinds of approach which he first had and which Jeff and Itamar are suggesting we ought to have towards quantum mechanics. So, when we’re looking at constructive theories underlying the principle-theory version of special relativity there are two kinds of ways of going about this as Harvey and Oliver .. have argued. On the one hand there is the kind of standard view which is that we postulate Minkowsky spacetime as an ontological structure which explains Lorenz contraction and time dilation, I think Michel likes that way of doing it, alternatively, the way that Harvey and Oliver prefer, is to say that actually it’s just a fact that the dynamical laws have certain symmetries associated with them .. entails that Lorenz contraction and time dilation will indeed occur. So those are the two kinds of constructive ways of doing special relativity. So rather than lumping Everett and Bohm with the ‘Lorenz as a looser’ kind of tag we should really be seeing putting Everett and Bohm together with what Minkowsky was doing, and I don’t think anybody thinks Minkowsky was looser.

4. So, to conclude, I don’t think there are any troubles raised for adopting enthusiastically the Everett interpretation that come from reflecting on the no-cloning theorem.

Bub
5. A quick minute reply.. If one at least entertains the possibility that it makes sense to consider a universe in which no-cloning is in principle impossible, that we really discovered in the case of quantum mechanics is that .. there are information are information sources which can’t be cloned, that in principle can’t be cloned, then, in this might be false, but let’s say that we entertain this possibility. If we entertain this possibility then there is I think .. two questions. First of all, one can consider: well, if you say that how do you make sense of quantum mechanics in the sense of is there some remaining problem and what we take to ourselves to have done is show look, you can make sense of probabilities that way and so there isn’t any further conceptual problem. Now, you might accept that and say okay (you might not accept that but if you do accept that), yea, but I still would like to understand why can’t we clone.

6. Now, that I think is an interesting question which might have an interesting answer but I don’t think the question just as posed that way, why can’t we clone, can be answered now. I mean it might be that in terms of some future physics there will be an interesting answer to that which is different from . sorry .. I don’t think that the question is just silly, or uninteresting. But I do think it’s an uninteresting answer to say that I’ll tell you why we can’t clone, let’s suppose that we can clone and now there some sort of .. it’s not that we can’t clone in principle and then give some sort of dynamical story about why as a contingent matter of fact we can’t clone right now. We could if things were a little different, if the decoherence process was different or something like that. That’s in effect denying the claim here to start off with that .. this is the import of quantum mechanics that we’re looking at a world in which cloning is in principle impossible, so

Timpson
7. It’s a corollary of quantum mechanics

Bub
8. Well the claim here is, look, there is no conceptual problem if we take that as the message of quantum mechanics, so to speak. And I guess we .. Itamar suggested this morning that the situation would be as if we have .. thermodynamics with a law like ‘no perpetual motion machines’, and someone came up with an explanation of why there can be no perpetual motion machines in terms of statistical-mechanical analysis, but that statistical-mechanical analysis was idle in terms of machinery .. I mean it isn’t today, you get new results out of it ...but if it was sort of completely idle and simply gave you a kind of comfortable story about why one couldn’t have a perpetual motion machine, I think that would be more analogous to the situation that you have with, say, Bohm’s theory or many-worlds. They don’t do anything more than provide a dynamical explanation of no-cloning within a framework in which cloning is possible.

Timpson
9. .. that’s at this point the quibble I had at the beginning about exactly what one means by no-cloning, kind of does get important

Pitowsky
10. .. you know, the one thing I opened with was, I think it’s really a place to take off and I think in many of the approaches this is not ……. This is the fact that you have a structure here, you have the structure which is a realistic structure and this is what you should follow. I mean it’s not as if there’s no alternative, our point is not a polemic point and it’s probably .. this is the question that Simon had, is it a realistic interpretation .. it’s a realistic interpretation in the following sense: you have events and you have measurement results .. nothing to do with whether human beings are there or not , because measurements are not privileged in the theory, so you’re not told an event is a .. of probability theory.. there’s no analysis of what an event is in probability theory . You have a whole set of those that are actually occurring or ostensibly occurring, and you have a whole lot of very interesting structure on that and this is the structure of the Hilbert space. So this is realism about the structure of the Hilbert space. It’s realism about operators, not realism about a wave function, and in that respect it’s completely orthogonal to the. It’s realism about events and operators, projection operators.

Ladyman
11. Well, I had ... when you were talking about cloning theory because it’s obvious that the uncertainty relation implies the no-cloning theorem. If we could clone, the we could make lots of copies of the same state and measure different observables again … violations of the uncertainty relation. So then it just seems like what’s at issue between the Bohmian take on things and your take on it is just like with the uncertaintly relation, you think it’s in principle a fact about the world and they think it’s a contingent epistemic limitation on what we can practically do and so I’m not really sure how much progress we’ve made by thinking about things in terms of no-cloning rather than the uncertainty relation. But the question I had was: do you think that, granted the uncertainty relation implies no-cloning, do you think that no-cloning implies the uncertainty relation.

Pitowsky.
12. Yes, this is it, so it implies many other things so that’s why it’s stronger. You mean just uncertainty.. ?

Ladyman.
13. Well, I mean..

Pitowsky
14. But you have non-commuting operators. No-cloning implies that, in particular

Bub
15. Yea, or rather there is …. If you consider a large class of theories and just impose a no-cloning constraint, these theories would be a class of theories which are no-signalling theories and in terms of the definition of no-signalling they would also be theories where … there is a set of informationally incomplete observables with respect to which you can reconstruct the state, so there are certain features of this class of theories … this describes a very large class of theories which includes quantum mechanics and classical mechanics and the claim here is that the principle that demarcates classical from non-classical theories is the no-cloning principle or really the no-broadcasting principle and so it’s really a general result which has nothing specific to do with quantum mechanics.
I mean no-cloning basically demarcates classical from a whole class of non-classical theories

Sudbery
16. ….comment on behalf of another looser…. If non-cloning does turn out to be a fundamental principle it seems a shame that Giancarlo Ghirardi should have his name associated with it because the excuse … and Zurek to publish their paper was a paper by…..

Bub
17. Oh, that’s right, yeah

Sudbery
18. ….pointed out that there were funamental principles which showed that this couldn’t happen. Now that paper by Hewet? .. followed by Giancarlo Ghirardi ….pointed out that there was a fundamental principle which showed that therefore .. leading to rejection .. he was overruled and therefore wasn’t listened to by any of his associates … and therefore…

Bub
19. Oh yeah, I think that we did ….isn’t that right?

Timpson
20. Well actually this is no-broadcast which is Fooks, Barnham and that lot, so, and what we have here is different and was only proven in, when, 1996?

Bub?
21. What we

Timpson
22. No-broadcasting was only proven …

Pitowsky
23. No-broadcasting is a bit more complicated

Timpson
24. And that’s the crucial one, no-broadcasting

Pitowsky
25. Yeah, because ….

Timpson
26. Yeah

Valentini
27. Yes, so I agree with Chris on this analogy with Lorenz is bad …..this whole issue of Einstein and Minkowski nevertheless rejected this objective account of the world, but what seems to me a better analogy in many respects is with the approach called energetics in the late nineteenth century when people, there was a whole group people, Oswald, Mach and various other people saying, look, these people with their models and generic? theory and these billiard balls bouncing in a container; we don’t need that, just focus on general principles to do with energy. Energy-theoretic principles like, conservation of energy, no perpetual motion … what we now call thermodynamic principles. Now, there was an argument back then saying, look, I can explain everything about gases in thermal equilibrium from these principles, why do I need this complicated mechanism underlying it. And at that time the kinetic theory of gasses was what Jeff called a sort of idle theory. People were saying things like, look, if you take a box of gas and you say it’s made of N molecules of a certain mass and so on, I can take another box of gas made of 2N molecules of half that mass and I still get the same picture. It’s a bit like a de Broglie-Bohm, you could say well I could have different trajectories and I’d get the same thing. Further, you could have argued … saying something like, look, de Broglie-Bohm says that in principle you could have non-equilibrium and you could clone and that violates our principles … you may have said back then, well, if there really are these molecules then there is perpetual motion. And in fact in 190? … under the microscope and you see pollen grains jiggling you see perpetual motion, and you may have said ……violate the principle…….. and finally of course on this analogy, another reason why it’s a good one, particularly from the point of view of hidden variables and de Broglie-Bohm is that you can show that in terms of .. hidden variables theory , no signalling is a specific contingency on equilibrium, simply cloning has been discussed in de Broglie-Bohm.. . So, there’s an alternative analogy which points in a very different direction, and again, too, I think there’s a question about what one might call an analogue of the big measurement problem. Someone back then who was saying just concentrate on macroscopic experiments and worry about what we can do with the energy .. would .. defend .. Boltzman who would have and did say but look if you’re just talking about this macroscopic world, what happens when you chop things up into little bits and at the very small scale what’s going to happen, how can you.? So … in a sense this is a reality problem, what are the things really made of, what happens when you probe things more precisely . This is why we’re not comfortable with an operational macroscopic approach. So that is how I your approach, that’s my analogy, which again puts you as Chris was saying ..in the looser ..

Bub
28. I have something to say quickly on that. Just on the issue of Einstein’s realisation that .. his rejection of his own approach and the thought that there should be a dynamical account which applies to rods and clocks and so on, I think the analogy of that in quantum mechanics as we see it as a no-cloning theory, is the whole dynamical analysis of decoherence of the quantum dynamical analysis of decoherence, so its not as if this position, once one starts .. no-cloning is just to say well, this is some sort of abstract principle, we don’t talk dynamics. I mean the solution to the small measurement problem is the quantum dynamical solution it’s the the whole story about decoherence which is a complicated story in itself .. which we have alluded to .. so that, I think, could be analogous to the dynamical story about rods and clocks in relativity theory because it’s not the same as Lorenz’s dynamical story which involves a presupposition about Newtonian spacetime structure, rather it’s a relativistic dynamical analysis and I think that’s just analogous to the decoherence dynamics. Anyway

Pitowsky
29. I want to say something about thermodynamics; it’s a very good analogy and I’ll tell you why. In the case of thermodynamics and statistical mechanics very quickly, with van der Waals? you’ve got results which you couldn’t get from thermodynamics or, at least, people did not get only ad hoc … so this is exactly the point. The point is the following: suppose that you’re right and people will probe the state in such a way as to get non-equilibrium states , so you’re going to get a Nobel prize, okay, now I’m going to adopt the theory, okay, but ….. this didn’t happen yet, I don’t see, after how many years, since 1925 or so, anyone who …… nothing like that happened at the mico-level, there’s no new phenomena, so what I’m saying is, okay, if the analogy goes, we stick with thermodynamics, we don’t stick with statistical mechanics because statistical mechanics doesn’t give us anything but a kind of pillow to … you know.

?
30. It’s a rhetoric

Valentini
31. ….I think it took longer than you’re saying, there was a period of several decades where statistical mechanics was .. seen as this idle theory but, okay, let’s say this period is not as long as it has … but even so there’s still the question of what ….. people like Boltzman who spent years developing statistical mechanics, it wasn’t just because, ah, we’re doing this because we’re getting results, new physics … There was a concern about a real description of things which went beyond just crude sensations in the lab of a macroscopic box of gas, there’s no .. why can I not? .. let’s say I’m given a description of what I’m doing in a lab with a box of gas and a pump and so on. Now, why can’t I extend that description down to smaller and smaller scales? What happens? There’s no divide, no boundary

Pitowsky
32. We don’t have any boundaries

Valentini
33. ..so, the motivation, I think, reading Boltzman’s writings … is a sense that the world is built out of some smaller constituents and it’s just not good enough to have a picture of the world that is based just on crude approximately-defined macroscopic things, and these are exactly the same concerns that a lot of people have today, that when you’re talking about doing a measurement (this is why Bell didn’t like measurement as a primitive) because it’s just a fuzzy concept based on: you’ve got this piece of equipment which is just a fuzzily-defined thing

Pitowsky
34. If no-cloning is a valid principle then it must be fuzzy, that’s the theorem here

Valentini
35. ..so, well I would interpret that as: if it is a valid principle and if you take your point of view then your view is simply incapable, you have proven that your point is incapable, of solving the big measurement problem , which, you know, that’s interesting, but now we want to solve the big measurement problem.

36. Pitowski - ..I think ..did solve, by putting that as primitive

[new question]

Wallace
37. Okay, one person’s dogma is another person’s axiom is another person’s empirical datum. I can imagine living in a world where measurement was a primitive. In this world there are indestructible black boxes dotted over the landscape; they have a knob on and a button and a dial. The world’s not like that. The measurement devices we actually see in the world do not seem to be primitive, they seem to be built by very clever people, from principles that we basically think we can understand. One can look at them and deduce rather than postulate how they work, and I think in that context it’s worth remembering the real reason why we adopt statistical mechanics rather than thermodynamics or why we continue to use a form of modern thermocy.. as primitive I think is not really about fluctuations it’s about the equation of statae, it doesn’t require us to postulate as primitive . it allows us to derive it from the mathematics. So it’s not … I mean that objection is utterly unoriginal, but if there is something to be said about it I’m very interested to hear what that thing is.

Bub
38. Well, in principle the claim is simply not that the no-cloning principle entails that .. the are measuring instruments around which are just black boxes which copy …. The claim is that it follows from the principle that a quantum-mechanical analysis can be given at any level so if somebody builds a black box you can proceed to analyse any aspect of it down to its smallest microscopic constituents and keep going as much as you want with a quantum-mechanical analysis but at the end of the day there will always be some aspect of the analysis, there will always be some aspect of the physical system, which is treated simply as a classical information source. If you want to analyse that quantum mechanically then you can, you .. look at .some further bit of the apparatus which is functioning in the analysis purely as a classical information source. And the claim is that that’s a consequence of the no-cloning principle. But I find it a bit surprising that there’s all this charge of : well, if you say this then you’re sort of giving up on physics or something. It seems to me that, I’m sure nobody here believes, well maybe there are one or two people, but I doubt that anybody here really believes that real physics will be done using, well Anthony does, but to me it seems really bizarre to try and perform some quantum-mechanical calculation from the point of view of the Bohm theory in order to get some new insight about ….(I’m not only talking about new prediction, I just mean that there …..equilibrium distribution) but some new insights and new way of looking at things which is suggestive of something which quantum mechanics might be able to produce too. But this is really getting at the roots of the way the world is put together so if we look at it this way then we see things which the quantum-mechanical view would just clumsily try to reproduce.

Valentini
39. The discovery of Bell’s theorem is a case where this actually happened … de Broglie-Bohm helped Bell discover non-locality .

Timpson
40. and non-contextuality.

Bub
41. ..well he knows that Bohm’s theory is non-local and he asked the question must any hidden-variable theory have to be like Bohm’s theory. But that seems to be something … different

Valentini
42. ..it is an example would be .. . insight … not just looking at quantum mechanics

Pitowsky
43. .. it wasn’t developed on the basis of Bohm’s theory, it was derived from Hilbert space. This is the point,

Valentini?
44. ..there was the question of whether .. lead to

Pitowsky
45. oh, lead to in the sense of inspiring , yes .. that I don’t contend? With

Maudlin
46. I don’t know what to say except that I don’t understand how anything about

Bub
47. I knew you’d begin with that

Maudlin
48. …..could become associated in anyone’s mind with the measurement problem. Decades and decades of talk about there being a problem, something that was identified as the measurement problem, people thought about it ……. Behaving or not behaving ….. Whatever problem that is it ain’t the measurement problem and people thought that …
Now, let me say something about making measurement primitive. From a normal information-theoretic point of view I would have thought that any interaction between two systems provides some information. I take system A and I throw it at system B what happens is that ….the actual state system B has to be in, and I can figure out what happened to system A then I would ….some information about the state of system B. From that point of view any interaction is something measurement-like and we heard early on that one of the dogmas that you said you got … I didn’t quite understand you can’t have a dynamical explanation of measurement since any interaction is sort of measurement-like and provides information, that seems to say you can’t have a dynamical explanation of anything … we have lots of dynamical explanations .. of lots of things, so

Bub
49. We can’t have a complete dynamical explanation of … there’s always going to have to be some system which is functioning as a classical information source that’s really what ..

Maudlin
50. Maybe I can just re-ask Chris’s question how can you claim that you can’t have something that exist? I mean, you say you can’t have it and I say well here’s a theory that has it …

Pitowsky
51. Again, a theory that has it, with certain dynamical conditions put from the outside .. such as equilibrium in Bohmian theory

Maudlin?
52. … deny the premis?

Pitowsky
53. Yes, you deny the premis

Maudlin
54. … the second law of thermodynamics really exactly .. classical explanation of the fact that we can’t extract mechanical energy from a heat reservoir … is somehow bad or insufficient because after all in principle you can, if you’re lucky, it just turns out that in equilibrium the chances are so small that in the history of the universe it probably never happened. Do you consider that to be an inadequacy?

Pitowsky
55. It’s an inadequacy, I think, just in one respect. If we didn’t have evidence that- . Suppose we stay with the second law of thermodynamics, the original formulation say, no … and then .. the only purpose of the atomic story would be to say, look, this is highly unlikely

Maudlin
56. Anything else that you get from the theory?

Pitowsky
57. No, just that this is highly unlikely . I think that this would not be an interesting physical theory, it would be an uninteresting physical that is just doing a simple thing in a very complicated form.

Maudlin
58. That … the only purpose of the atomic theory? ..

Pitowsky
59. No, I don’t think it was the only purpose of the atomic theory

[new question]

Lehner
60. I just wanted to sharpen Antony’s point about ….ther is exactly a parallel to .. .that’s supposed.. ……… tried to argue against Bolzman’s statistical mechanics … exactly that argument that the second law of thermodynamics comes out only as a statistical law, right, … and who was the looser? … but I mean what I want to substantially argue about your approach is that, just as Tim says, you can only claim that no-cloning implies dynamic description of measurement because you assume that measuremant has to be non-destructive, so let’s assume that measurement changes or destroys the state of the object and immediately your argument doesn’t go through at all any more, right. I mean, nobody says .. that we still have the measurement problem, it hasn’t changed a whit about the measurement problem if we assume that measurement changes the state of the measured object

Bub
61. Well, no, we’re talking about changing irreversibly?

Lehner
62. Yes

Pitowsky
63. Right, so we didn’t talk about GRW for example. GRW is an extension of quantum mechanics. It’s an extension of quantum mechanics about which we didn’t talk. We were talking about deterministic theory

Lehner
64. Look, let me put it another way no-cloning is a consequence of unitarity right?

Pitowsky
65. We are reversing that .. Suppose that, assuming no-cloning, then you get that certain processes could not be unitary. This is the inverse theorem. The inverse theorem says if you assume no-cloning then certain processes cannot be unitary

Lehner
66. …..perfectly equivalent but that for example Everett, the core of Everett’s claim is that unitarity is universal. So how can no-cloning be in conflict with that? If the unitary evolution is all we have then of course we’ll have no-cloning in that sense

Timpson
67. I think the issue here is that Itamar and Jeff have in mind something different in no-cloning than what the standard view is. …. I think you’re dead right that given the dynamics .. subsystem …even in the most general case linear … complete positive map … the no-cloning theorem follows. But they’re interested in basically .. de facto non-happening of cloning in some abnormal branches

Maudlin
68. ..exactly the parallel with ..

Timpson
69. Yeah, because of the particular meaning they put in their …not cloning, not happening ..

Pitowsky
70. Okay, the claim is just that you need a very complicated dynamical explanation of why people don’t see cloning, a very complicated dynamical explanation for that

Lehner
71. ….was personally offended that Boltzman marred the beauty of the second law of thermodynamics ..

Pitowsky
72. But I think you all ignore the fact that there was the black body radiation and here we don’t have a black body radiation. Here you don’t have evidence

Lehner
73. This was the 1890s, right, Boltzman died without any good evidence for the atomic hypothesis … this went on for just as long as ..

Pitowsky
74. The black body radiation was there and was not explained by Boltzman’s statistics. This is something that you don’t have here. There is not a parallel

Lehner
75. This all happened long before black body radiation was even discussed

[new question]

Rae
76. ...somewhat naively and at the beginning of the talk … I understand, I think ..does not mean to claim …. At the beginning of the talk it seemed to me you were saying that information theory plus no-cloning theorm .. was all we needed to deduce the von Neuman axioms of quantum mechanics including all …if that were true that would be a huge claim. I’m sorry, it did sound like that at the beginning, so could you between you just summarise just how far down that road you actually have gone

Bub
77. Well we don’t derive quantum mechanics …. All that we did today was to produce a principle which demarcates classical from non-classical theories; it’s still an open question what other principle or principles you need to actually demarcate quantum mechanics from (which is just one of a large group of non-classical theories) from the other, let’s say ‘super quantum theories’ . Or even, for that matter, sort of sub-quantum but non-classical theories. I mean there are some theories .. at some point I mean the correlations are not as strong as the Bell correlations but they’re still not classical. You see there’s a whole range of theories … actually picking out quantum mechanics would require more than just no-cloning, and nobody knows what that is ..

Pitowsky
78. There are some counter-examples on that, but they’re perhaps, less simple than just not ..

Rae
79. ..you say that we’ve got quantum mechanics … von Neuman postulates or something like them, and then you put on, you add no-cloning to it, this sounds like an extra postulate

Pitowsky
80. No, what you’re doing is something else .. .we’re taking very general information-theoretic accounts which have no-signalling, and you add no-cloning and you see that many of the features of the measurement problem arise already. And you don’t need quantum mechanics for that.

Rae
81. Why do you need no-cloning? I mean, what’s it telling you? … ruling out that sensible people don’t believe in it anyway …

Bub
82. It gives you non-classicality. The claim is that that is the essential principle which divides classical from non-classical theories and all these non-classical theories are similar in respect to the measurement problem