Is there a fundamental flaw in our understanding of space and conservation laws?

[vanesch]

I agree entirely with what JesseM writes here

MWI is *a* view on how *a formalism* works. That is, one had added a kind of "reality-hypothesis" to the formalism, and MWI does this in a minimalist way. Whether one finds this plausible or not, whether one finds it in agreement with one's philosophical convictions or not is a different matter. But one cannot deny that this view exists.

There is in fact not much use in attacking such a view. One can find it useful or less useful.

It seems to me that this is really a philosophical argument. I could reply that each copy of Alice and Bob *does* see a definite outcome, and likewise, once they have time to communicate, they also see a definite outcome for whatever copy they ending up sharing the same "world" with, so the correlations they see are perfectly "real" (if the universe had a different rule for 'mapping' copies of Alice to copies of Bob, the correlations seen by a typical copy could be different, so the correlations are a consequence of objective physical laws rather than something subjective). It's true that there are multiple versions of Alice and Bob in each experiment, but offhand I don't see why this is any more of an objection to the notion of "definite outcomes" than the fact that, in a spatially infinite universe, there is sure to be another region of space somewhere where there is are exact physical duplicates of Alice and Bob who share identical past light cones up until the moment of measurement, at which point they get different results than "our" Alice and Bob. You could say "yes, but that's not really the same experiment, it happened in a different region of space", but can't a many-worlds advocate say that the other "copies" of Alice and Bob are in a different region of Hilbert space? Of course, this is also a philosophical argument about what we mean by the words "real" and "definite outcome", but my point is that your denial that these words can be applied to the MWI is equally philosophical. But the brain-in-the-vat scenario involves dismissing the possibility that our sensory experiences tell us anything about the laws of nature, or that they are genuine "empirical data". In the MWI interpretation your experience isn't a "delusion" in the same way, it's just limited to a subset of everything that is "really going on" (much the same is true of the Bohm interpretation, where you will never have access to the full information about all the hidden variables). Scientists in the MWI can discover the mathematical form of the laws of quantum mechanics through experiment, just as they can in single-universe interpretations.

And when you say the MWI involves dismissing empirical data, what is that data exactly? Do you think there's any empirical data we see that we would not expect to see if the MWI were in fact true? I suppose this question is a little ill-defined because of problems relating the MWI to our observed empirical probabilities, like the preferred basis problem...my point is just that if we have some general notion of physical systems splitting into multiple copies all the time in some lawlike way, there's no reason to expect such a universe to necessarily look any different 'from the inside' than a non-splitting universe with probabilistic laws. So, if some sort of splitting-universe solution can in principle explain the correlations in the Aspect/EPR experiments in a lawlike way (as opposed to a 'conspiratorial' way like the brain-in-a-vat scenario) without violating locality, there is no good reason for dismissing this as a possible "loophole", even if one's personal philosophical views incline one to consider it very implausible.

 

[ttn]

The following quotes are probably a correct analysis (I accept your authority as a Bell connoisseur) that Bell (as many other people) had a serious emotional disliking for MWI. It is - as I often said - the main (and often only) objection people have against MWI. Bell was amongst them.

I think you misunderstood. Bell's (and my) disliking of MWI is not "emotional", or at least not fundamentally emotional. It's true, I have negative emotions about MWI -- but that's the *effect* of my thinking it is a bad theory by professional physics standards, not the other way round. I am quite certain the same is true for Bell. He thought that MWI couldn't be taken seriously as a scientific theory, not because he arbitrarily "hated" it, but because he thought carefully about what scientific theories are supposed to do. He hated it then because it failed to do those things and instead tried to play some pseudo-scientific mindgame.

Well, MWI goes as well with relativity as quantum theory in general. That means, with SR, things are more or less fine ; with GR, in as much as the background is fixed, it is fine, and in as much gravity must be considered as a quantum phenomenon, nobody knows.

But at least with SR, we can cope, and the proof is that EVERYTHING in the formalism of quantum theory remains Lorentz-invariant.

I believe there is more to SR than Lorentz invariance. For example, I can trivially write down a fully Lorentz invariant version of Bohmian mechanics, simply by putting in an "ether" (preferred frame) but then giving some made-up Lorentz invariant dynamical law which that ether is supposed to satisfy. It's exactly the same as having air for sound waves to propagate in: the fact that there is a "preferred frame" for sound waves (in which, e.g., they propagate isotropically) doesn't violate Lorentz invariance, because the preferred frame is made real by positing some *matter* (the air) which defines that frame, and that is fully consistent with Lorentz invariance so long as one can give some Lorentz invariant laws for that new matter. So, do the same thing for the ether -- instead of regarding the "preferred frame" as somehow fundamental to the structure of spacetime, you just say there's this new kind of matter which obeys Lorentz invariant laws, but there's some one frame in which the matter is "at rest".

You see that's possible, right? So would you therefore regard such a theory as fully consistent with SR? I think most people (including myself) wouldn't. It's cheating, somehow. And the "somehow" is what you need to flesh out to understand why "consistency with SR" and "Lorentz invariance" are not precisely the same thing.

But that wasn't even really the point I was making. As we've already covered, in MWI, there is no such thing as matter moving and interacting in 4-D spacetime. That exists only as a delusion in the minds of the "subjective conscious observers"
which also play such an important role in MWI. So it's very odd to say that the theory is fully special relativistic, when what SR is all about is the structure of that 4D spacetime. Or the same point a slightly different way: Lorentz invariance is *fundamentally* about the transformation properties of 4-D spacetime coordinates, i.e., the coordinates of events in 4D spacetime. But according to MWI there *are* no events in 4D spacetime.

So the point is that MWI is "consistent with SR" in about the same way it's "consistent with the data acquired in the Aspect experiment." In both cases, the theory provides a kind of fairy tale which helps you understand how you could come to be "validly deluded" into thinking the things in question... but that is not exactly the same as a theory *explaining* the things in question... at least not in the normal, historically practical sense of those terms.

But please take this as merely a recap of the same things I've been saying over and over again, which I know you don't accept. So don't feel the need to argue against them yet again, or accuse me of failing to understand your arguments.

Well, that is because your contributions are only statements where you want to argue about the silliness of MWI - arguments which have been analyzed, and which nevertheless come back each time when you post about it.

I of course would say the same thing in reverse: despite my having made quite clear why it's insane to take MWI seriously, you nevertheless come back to it each time I poke my head in this forum to see what's going on. (And no doubt you're doing so *between* those times as well, if you believe in such a thing as "what's happening when I'm not looking")...

Observations don't need to correspond to "reality". In fact, the concept of reality has not much to do with observations: observations are just that: observations. The role of a reality-hypothesis is to explain-describe-predict what kind of observations one can make (and to give a kind of "mechanism-behind-the-screens" that is responsible for us making observations).

You equate "observation" with "subjective inner-theater impression" or some such. I take observation to mean that something (read: some*thing*) was actually *observed*. If you see a pointer pointing left, you saw *a pointer pointing left*. (This is indeed a philosophical question, as JesseM has said. I'm in no way ashamed about that -- and anyways, it's not like *my* position on that philosophical question is any more philosophical than *your* position on that same philosophical question. So it's silly to think that you could argue against my position by labeling it "philosophical". It's just like the stupidity of people who say that Bohmian Mechanics is pointless/unscientific because it is "philosophical", which I guess is based on the idea that Bohm has the "preposterous, philosophical, unscientific" idea that there is actually an external physical world that it is the job of physics to describe... Anyway...)

This is exactly the difference between our views here, and it is a philosophical one. I think it is settled -- long before we get to any advanced topics like the sub-structure of atoms or how to interpret Schroedinger's equation -- that when we perceive we are actually perceiving a real physical world of physical objects external to our bodies. Those external physical objects (and/or the "matter" they're made of) are real. That is just completely *settled* and non-negotiable. The job of physics is then to understand in further, microscopic detail, what the properties of these entities/matter are, how they interact, what smaller pieces they are made of and how those interact, etc.

For you, as I understand it and by contrast, all of that stuff is left as completely negotiable. For you physics, isn't basically about explaining the hidden underlying structure of directly perceivable entities (or even directly perceivable attributes of those entities, such as their positions), but rather it is about explaining how we might come to have certain "subjective inner theater impressions". That is, you want physics to simultaneously address questions such as "why are atoms about an angstrom across?" and "is there really an external world that my experience is experience *of*?" As you know, I find this bizarre. But, for whatever it's worth, that's at least what our basic philosophical difference is.

Now, your argument is exactly this:
that MWI suffers from the following defects:

1) although you recognize that MWI can explain all of our observations, you consider it not to be an acceptable reality-hypothesis because these observations are not "real" (or not all that is real or whatever).

No, that's not right. I don't recognize that MWI can explain all of our observations. I think it fails to explain them. Instead of explaining them, it attempts to explain them *away* -- in the sense of telling a story about why I shouldn't have taken those apparent observations to be genuine observations. But as I explained above, this is because we disagree fundamentally about what "observation" means in physics. What I do recognize is that MWI can account for (I can't quite bring myself to use the word "explain" here) all of our (or, rather, my) "subjective experience". The problem is, so can solipsism or brain-in-vat theory. And they do it in essentially similar ways. And yet those latter are properly rejected by reasonable people as not being scientific, or at least as not able to be taken seriously in a scientific way. And that is also true of MWI, in my opinion, and for exactly the same reasons.

Because there is a difference between disliking a certain reality-view (such as MWI, or Bohmian mechanics or whatever), and claiming that it is self-contradictory.

That's not the relevant set of taxa here. Solipsism isn't self-contradictory. And "disliking" in a purely emotional sense has nothing to do with it. Professional judgments determine (or should determine) one's emotions, not vice versa. The relevant criteria are how well various theories live up to the professional standards for serious scientific theories. For example, evolution is a good scientific theory, creation science is not. The atomic theory of matter is good, solipsism is not. And the point isn't just that "good = true". There are all sorts of theories that turned out to be false, but which were nevertheless entirely reasonable as candidate theories for a time -- e.g., Ptolemy's epicycles theory is wrong but wasn't crazy or unscientific like "creation science" or solipsism or astrology.

But I fear there's no point getting into a discussion of what, exactly, these standards are, since we can't even agree on very fundamental philosophical points (which are hierarchically prior even to a discussion of proper standards for assessing candidate theories in science) such as whether the table I see in front of me is really there.

The argument you put forward does not show MWI to be self-contradictory. My argument against Bohmian mechanics (that it is not lorentz-invariant in its inner workings), doesn't render it self-contradictory either.

I agree. My argument wasn't attempting to show that MWI is self-contradictory. Rather, it's that any *case* for MWI (which is a scientific case in the sense of citing the results of various crucial experiments) will have to be self-refuting, since the theory the case is supposed to be a case for, says that those cited experiments never actually had the results we "subjectively experienced" them to have.

Now, if I may add one comment: I think that Bohmians are such avid MWI-dislikers, because MWI has taken out a major argument they thought they had: they thought that there was a loophole-free argument to put relativity aside. They thought that it was shown, once and for all, that the main criticism of Bohmian mechanics (namely it not being Lorentz-invariant), was a dead argument, as Bell's theorem showed that quantum predictions were incompatible with relativity. Quantum predictions without Bohmian mechanics. If quantum mechanics ITSELF killed relativity, then, of course, the way was cleared for Bohmian mechanics, and its main criticism fell on the floor. Relativity was untenable if quantum theory was to be right. And hence, no more requirement for Lorentz invariance.

This definitely has an element of truth, though the way you say it makes it sound like it's a deliberately dishonest thing -- like we (Bohmians) see that MWI is standing in the way of what would otherwise be a really good argument for Bohm, and so we viciously attack it because we don't want to face the truth that our argument is actually no good. You could put the same point more fairly if you stated it this way: Bohmians seem to understand Bell's theorem better than most (and btw Bell counts as a Bohmian here!). They recognize that there's no "hidden variables" assumption, no "realism" assumption, no "determinism" assumption, etc. If you grant that experiments actually have definite outcomes, and then if you require relativistic local causality, you get the inequality. And since we know, with more certainty than we know practically anything else in science, that experiments *do* have definite outcomes, it follows that we have to *give up* relativistic local causality. And that is nice, because it shows why the earlier-perceived barrier to accepting Bohm's version of QM, is actually no barrier at all. So it allows you to have all the *virtues* of Bohm's version (and anyone who has studied it honestly knows that there are *many virtues*), with no cost at all. But then these weirdo MWI people come along and say -- oh, but you can save relativistic local causality if you drop the requirement that experiments have definite outcomes (and drop also the whole relativistic worldview of physical events in 4D spacetime). And that's true, but come on!? Who could take that seriously?! It's about the equivalent of those annoying weirdos who come along and say "you don't have to believe that your grandparents were monkeys, if you just accept that jesus christ is your savior and he created the world 6,279.8 years ago on a tuesday." It's of course true in some sense that you can avoid the relevant conclusion you don't like if you accept that nonsense, but who could possibly accept that and think that what they were doing was *science*?

Well, OK, maybe I slid over into slanting it just ever so slightly the other way there...

 

[Thrice]

You guys know any good resources on this? Wouldn't mind contributing, but I realize I don't know this anywhere as well as you do.

 

[vanesch]

He thought that MWI couldn't be taken seriously as a scientific theory, not because he arbitrarily "hated" it, but because he thought carefully about what scientific theories are supposed to do. He hated it then because it failed to do those things and instead tried to play some pseudo-scientific mindgame.

However, for coming to that conclusion, he ADDED extra requirements on what a scientific theory is supposed to be like (and had to discard others).

The one, only and single non-disputable criterion for a scientific theory is:
it should correctly predict observations.

All the rest is open to matters of personal taste up to a point, and the case can be made for each of them. But, no theory can claim to be a scientific theory if it doesn't correctly predict observations. This is necessary.

The requirement, however, that elements of observation should be elements of ontology in a specific way, is a totally arbitrary extra requirement, but the fundamental requirement which made Bell (and Bohmians) conclude about their distaste for MWI. There is NO ABSOLUTE REQUIREMENT for observations to be corresponding in a simple and evident way with an element of ontology. Now, of course, it would render things simple. So I can very well understand a DESIRE for this requirement, but it is nothing more than this: a desire.

In the same way, absolute simultaneity could be an extra requirement of the same kind, it would be nice to have it, but there is no reason to take this as an absolute sine-qua-non requirement for a scientific theory.

There is a requirement that *I* put forward as an extra requirement, and that is: one should aim for a MINIMUM of universal principles from which a maximum of formalism can be derived. Now, again, this is not an absolute requirement, however, it is a very pragmatical one. Indeed, without such a requirement, there is no limit upon the possible alternatives, with as an extreme, just a list of all events in the universe, without any "law". So amongst all possible variations on scientific theories which are empirically equivalent, I think this requirement (which is not absolute) is a good guide. It is essentially Occam's rasor.

I believe there is more to SR than Lorentz invariance. For example, I can trivially write down a fully Lorentz invariant version of Bohmian mechanics, simply by putting in an "ether" (preferred frame) but then giving some made-up Lorentz invariant dynamical law which that ether is supposed to satisfy.

Indeed, as you outline, if you introduce some physical stuff, the ether, as living on spacetime, you could formulate the theory in a lorentz-invariant way, or in the SR way (which, to me, is the same: SR says that all physical entities should be functions over the spacetime manifold, and that means that their coordinate representations should satisfy Lorentz-invariant mappings. Lorentz transformations are nothing else but the transition maps between orthogonal coordinates on the spacetime manifold).

It's exactly the same as having air for sound waves to propagate in: the fact that there is a "preferred frame" for sound waves (in which, e.g., they propagate isotropically) doesn't violate Lorentz invariance, because the preferred frame is made real by positing some *matter* (the air) which defines that frame, and that is fully consistent with Lorentz invariance so long as one can give some Lorentz invariant laws for that new matter.

The problem is that Lorentz-invariance should then require that when the air is set in motion, things change. You hence have to introduce a genuine beable which is that "ether". But nothing observable has ever been related to that "ether". In order for that beable to make sense (and to use your OWN requirements of having observations to correspond to reality), there should be something physically observable to that ether. The air is observable. We can put it in motion, and study the sound waves in moving air.

But the ether is just an invention to make the theory fit a required symmetry which it doesn't have. You are inventing beables in order to simulate effects which would otherwise be derivable from a simple principle. One should propose experiments which demonstrate the physical reality of that "ether", and you should also demonstrate how it comes that other stuff interacts in such a way with the ether, as to make everything appear as if it didn't, but underwent some specific transformation in such a way as NOT to respect Lorentz invariance in its inner workings, but nevertheless in observable phenomena.

Because this is the problem with introducing an ether. If it is physically present (such as the air in your example), then there should be (unless very strange conspiracy) observable effects which are not Lorentz invariant without that ether. In other words, one should be able to MEASURE that specific reference frame in which the ether resides. In the same way as sound experiments can give you the rest frame of the air.

So, do the same thing for the ether -- instead of regarding the "preferred frame" as somehow fundamental to the structure of spacetime, you just say there's this new kind of matter which obeys Lorentz invariant laws, but there's some one frame in which the matter is "at rest".

Yes, yes. See, you are doing worse than MWI-ers. You are inventing totally unobserved physical things of a totally different kind, with totally unknown interactions in a conspirational way (the ether) in order for your stuff to comply to some principle you've actually killed, but which you need in order to be able to require compatibility with phenomena which ARE observed (SR effects).

You see that's possible, right? So would you therefore regard such a theory as fully consistent with SR? I think most people (including myself) wouldn't. It's cheating, somehow. And the "somehow" is what you need to flesh out to understand why "consistency with SR" and "Lorentz invariance" are not precisely the same thing.

Consistency with SR means that the ontological elements of the theory should be defined over spacetime. Strictly speaking, making the hypothesis of an ether, as some physical stuff, which fills spacetime, is consistent with SR. The above proposal of an ether is not a violation of SR, it is a violation of Occam's rasor. One needs to introduce a lot of extra postulates, of which the only aim is, to restore some broken symmetry.

But that wasn't even really the point I was making. As we've already covered, in MWI, there is no such thing as matter moving and interacting in 4-D spacetime. That exists only as a delusion in the minds of the "subjective conscious observers"
which also play such an important role in MWI. So it's very odd to say that the theory is fully special relativistic, when what SR is all about is the structure of that 4D spacetime. Or the same point a slightly different way: Lorentz invariance is *fundamentally* about the transformation properties of 4-D spacetime coordinates, i.e., the coordinates of events in 4D spacetime. But according to MWI there *are* no events in 4D spacetime.

Well, this is a misunderstanding. In MWI, there is a 4D spacetime all right. However, it is not the place where MATTER lives. It is the place where the operators live, which determine the interactions (which determine the unitary evolution). The Schroedinger picture makes this less evident, but in the Heisenberg picture, this is quite clear.
On one hand, you have Hilbert space, and on the other hand, you have spacetime over which operators over that Hilbert space live. All operators over hilbert space are indexed over spacetime, in such a way, that they transform under a (projective) representation of the Lorentz group.
So the spacetime manifold is there all right. Only, we thought that we had maps from R into that spacetime (which we call particles), or that we had tensor fields over spacetime (classical field theory), and it turns out that we have field operators over spacetime. In other words, spacetime exists as the basis space for a vector bundle in which the fibres are sets of operators over Hilbert space. So the structure is more complicated than originally anticipated, but spacetime is there all right.

Now, in specific cases, all this machinery looks a lot like a set of mappings from R into spacetime (in which case we restore classical relativitic particle dynamics) ; and in some cases it looks like tensor fields over spacetime (classical relativistic fields). But the structure is much richer.

What makes this "relativistically correct", is that the basis space is still 4D spacetime.

The funny thing is that people don't invent this *just for the sake of saving relativity with no observational consequences*. On the contrary, this kind of structure is full of predictions which can be verified. Gauge invariance is closely related to the above vision, and has also been a powerful principle from which a lot of OBSERVATIONALLY CORRECT STUFF has been derived.

In other words, the unitary part of quantum theory, as it shouldn't surprise anyone, is ENTIRELY relativistically compatible.

I of course would say the same thing in reverse: despite my having made quite clear why it's insane to take MWI seriously, you nevertheless come back to it each time I poke my head in this forum to see what's going on.

You classify as "insane", incompatible with some extra, and to a point arbitrary, extra requirements you've put upon physical theories. This is exactly what I call "emotional".

If I say that Bohmian mechanics is not, in its spirit, compatible with SR, and that that is one of the reasons why I do not prefer it, despite quite some merits, this is not an emotional statement. If I would say that Bohmians study some insane idea, because it is not compatible with my preference to keep relativity, I'm being emotional.

You dislike MWI for its lack of 1-1 relationship between ontology and observations. That's your good right. But qualifying MWI as "insane" because it is not compatible with this extra requirement of yours, is an emotional statement.

You equate "observation" with "subjective inner-theater impression" or some such.

It will be hard to convince me of anything else ! Can YOU tell me what is an observation, totally independent of any subjective inner-theatre impression ?

I take observation to mean that something (read: some*thing*) was actually *observed*. If you see a pointer pointing left, you saw *a pointer pointing left*. (This is indeed a philosophical question, as JesseM has said. I'm in no way ashamed about that -- and anyways, it's not like *my* position on that philosophical question is any more philosophical than *your* position on that same philosophical question. So it's silly to think that you could argue against my position by labeling it "philosophical". It's just like the stupidity of people who say that Bohmian Mechanics is pointless/unscientific because it is "philosophical", which I guess is based on the idea that Bohm has the "preposterous, philosophical, unscientific" idea that there is actually an external physical world that it is the job of physics to describe... Anyway...)

It is a philosophical act to make an ontology-hypothesis.
Choosing to equate observation with ontology is ONE possible answer one can prefer. It is called "naive realism". Refusing to make an ontology-hypothesis is called solipsism. Doing something else, is, well, making yet another ontology-hypothesis.

No answer to this question is "more" or "less" philosophical than another.

This is exactly the difference between our views here, and it is a philosophical one. I think it is settled -- long before we get to any advanced topics like the sub-structure of atoms or how to interpret Schroedinger's equation -- that when we perceive we are actually perceiving a real physical world of physical objects external to our bodies. Those external physical objects (and/or the "matter" they're made of) are real. That is just completely *settled* and non-negotiable.

Saying that is adhering strongly to a specific philosophical vision, which is called "naive realism". In as much as philosophy goes, nothing is settled.

It is the refusal to re-consider that position, which is, I repeat, an entirely philosophical position, which can lead one astray.

My position is that philosophical questions should be answered in such a way as to interfere minimally with what we have formally, and which allow us to build theories on a small set of principles. I think also that "naive realism" is a nice thing to have, if we can, but if another answer to the question seems more appropriate in order to set up a theoretical framework, then so be it.

The job of physics is then to understand in further, microscopic detail, what the properties of these entities/matter are, how they interact, what smaller pieces they are made of and how those interact, etc.

You see, you are IMPOSING a philosophical viewpoint upon the workings of a scientific theory.

For you, as I understand it and by contrast, all of that stuff is left as completely negotiable. For you physics, isn't basically about explaining the hidden underlying structure of directly perceivable entities (or even directly perceivable attributes of those entities, such as their positions), but rather it is about explaining how we might come to have certain "subjective inner theater impressions". That is, you want physics to simultaneously address questions such as "why are atoms about an angstrom across?" and "is there really an external world that my experience is experience *of*?" As you know, I find this bizarre. But, for whatever it's worth, that's at least what our basic philosophical difference is.

Yes, that is exactly true. I base this onto two points:
1) the only thing we really know, observe etc... are our "inner theatre impressions". All the rest is hypothesis.
2) we should minimise the number of principles, on which to derive a maximum of powerful formal machinery in order to allow one to predict what are going to be these inner theatre impressions.

Now, ONE possible solution could be that we make the hypothesis that there is a 1-1 relationship between our impressions, and actual ontology. In other words, that we take, as a working hypothesis, "naive realism". It is amazing up to what point this works ! It is amazing up to what point one can actually make simply 1-1 hypotheses, and use this as quite a consistent way of organizing one's sensations. In other words, it is quite amazing how we can simply assume that there is an object such as a chair, and that our sensory impressions (auditive, visual, sensory...) are compatible with that one single hypothesis. So this seems indeed a good idea. Classical mechanics is for a large part based upon that.
However, certain observations have shown the limit of that working hypothesis. As such, I have no difficulty taking on another working hypothesis. After all, it was almost too nice to be true, that there were simple things which were actually there, and which could simply explain these impressions!

No, that's not right. I don't recognize that MWI can explain all of our observations. I think it fails to explain them. Instead of explaining them, it attempts to explain them *away* -- in the sense of telling a story about why I shouldn't have taken those apparent observations to be genuine observations.

There are no "apparent observations". Observations (inner theatre experiences) are just as "real" as anything. Hell, it is the only thing we know there to be! But observations are a RELATIONSHIP between an inner theatre, and some hypothetical real world. That relationship is real, but the hypothetical real world ITSELF doesn't need to be equal to the relationship.

But as I explained above, this is because we disagree fundamentally about what "observation" means in physics. What I do recognize is that MWI can account for (I can't quite bring myself to use the word "explain" here) all of our (or, rather, my) "subjective experience". The problem is, so can solipsism or brain-in-vat theory.

No. Solipsism has no predictive power, nor does "brain-in-vat" theory, if you don't know the intentions of the evil scientist. It is the only reason, btw, not to adhere to them: the hypothesis doesn't bring in any organizational power of our sensations. MWI does. In MWI, you get just the same predictive power as in "standard" quantum theory. It helps one make predictions.

And they do it in essentially similar ways. And yet those latter are properly rejected by reasonable people as not being scientific, or at least as not able to be taken seriously in a scientific way.

I wouldn't say that solipsism is rejected on a scientific basis ! Only, it doesn't bring in anything useful. If it would, one should consider it. But solipsism essentially says: "subjective experiences happen", period. It is not the basis of any *prediction* of those subjective experiences, and it is on that account that it isn't of any use for scientists.

The relevant criteria are how well various theories live up to the professional standards for serious scientific theories. For example, evolution is a good scientific theory, creation science is not.

Yes, but the only reason for that is that creation science cannot explain certain observations.

The atomic theory of matter is good, solipsism is not.

Again, because from solipsism, you cannot derive many predictions.

But I fear there's no point getting into a discussion of what, exactly, these standards are, since we can't even agree on very fundamental philosophical points (which are hierarchically prior even to a discussion of proper standards for assessing candidate theories in science) such as whether the table I see in front of me is really there.

Well, because you already take on philosophical answers as non-negotiable even before the question is examined.

 

[vanesch]

continued...

I agree. My argument wasn't attempting to show that MWI is self-contradictory. Rather, it's that any *case* for MWI (which is a scientific case in the sense of citing the results of various crucial experiments) will have to be self-refuting, since the theory the case is supposed to be a case for, says that those cited experiments never actually had the results we "subjectively experienced" them to have.

Again, again. MWI is not self-refuting, because it doesn't claim that cited subjective experienced results weren't subjectively experienced. There is no need for the results to "ontologically be there" in order for a theory to say how results can be subjectively experienced.

The only need there is, is that subjectively experienced results (which are seen as a relationship between a subjective internal theatre and another, objective world) must FOLLOW AS PREDICTIONS for subjectively experienced results. You couldn't, indeed, start from observations, build a theory around that, and then come back with predictions that would be different, concerning those (subjective) observations, than the ones one started with.

Observations being the result of a relationship between a "subjective theatre" and "a postulated reality", there is no need for observations to correspond to that postulated reality ON ITS OWN.
We only need to build a postulated reality, AND a relationship between a subjective theatre and that postulated reality, so that the relationship comes out all right. It is of course a handicap to only possesses one side of that relationship, but with sufficient thinking, one can come to such an overall scheme. A guiding principle in doing so, is to try to limit the number of postulates to a minimum.

This definitely has an element of truth, though the way you say it makes it sound like it's a deliberately dishonest thing -- like we (Bohmians) see that MWI is standing in the way of what would otherwise be a really good argument for Bohm, and so we viciously attack it because we don't want to face the truth that our argument is actually no good.

Yes.

If you grant that experiments actually have definite outcomes,

Bzzt. Input of a philosophical requirement, which is often labeled "the realism" requirement. Very sensible, indeed. But nevertheless, an extra requirement, namely of "naive realism".

And since we know, with more certainty than we know practically anything else in science, that experiments *do* have definite outcomes,

We don't know that AT ALL, and we could never scientifically prove this, given that it is a philosophical position. If it were scientific, it could be falsified. You can never falsify even solipsism in a scientific way.

And that's true, but come on!? Who could take that seriously?! It's about the equivalent of those annoying weirdos who come along and say "you don't have to believe that your grandparents were monkeys, if you just accept that jesus christ is your savior and he created the world 6,279.8 years ago on a tuesday." It's of course true in some sense that you can avoid the relevant conclusion you don't like if you accept that nonsense, but who could possibly accept that and think that what they were doing was *science*?

The only way for preferring the non-creationist viewpoint is that it can explain more, with less input. If there wouldn't have been the fossile record, and there wouldn't have been other illustrations of evolution, but if on the other hand, we regularly saw appearances of the angel Gabriel and so on, then it would be scientifically more sound to accept the creationist viewpoint. Because it would THEN have a higher degree of prediction than that silly theory of evolution for which no single observational element was ever presented.

However, the creationist viewpoint is not entirely insane a priori. There could have been a creation 6000 years ago. Only, they need to put in A LOT of extra hypotheses, and each new dinosaur bone that is dug up, needs a new hypothesis on their side to explain it.
In other words, to keep to their cherished philosophical/religious viewpoint, they introduce *a lot of extra hypotheses* which seem moreover to be quite conspirational (like, God put those dinosaur bones in the ground, so as to "make us think that evolution was a principle, and hence to test our faith that it isn't, even though it seems to explain observations").

As such, for people, desperately wanting to adhere to a religious/philosophical viewpoint, creationism is a good viewpoint. It gives them ease of mind. Their preferred philosophy is saved. They have sacrificed about all predictional value of their theory in order to do so, and have to put in a lot of observational results by hand, but at least, their view on how the world "should be" is saved...

And they feverishly ATTACK a competing viewpoint, which discards their favorite philosophical viewpoint, but has, with much less input of principles, a much higher yield of predictive value: evolution.

Mmmm...

 

[MaverickMenzies]

I have another (probably pointless) question concerning MWI.

In the case of a pure entangled state between our quantum system and observer, MWI provides an interpretation for

|spin-up> |a> + |spin-down>|b>,

in that each term of the superposition should be treated as a separate copy of the universe. How does this interpretation deal with mixed entangled states? For example, suppose we include an environment in our system and then unitary evolutions give

|spin-up>|a>|e1> + |spin-down>|b>|e2> ,

and if i trace out the environment then i am left with:

rho = tr(|e1><e1|) |spin-up>|a><spin-up|<a| + tr(|e2><e1|) |spin-down>|b><spin-up|<a| + tr(|e1><e2|) |spin-up>|a><spin-down|<b| + tr(|e2><e2|) |spin-down>|b><spin-down|<b|

This is the state shared between the system and the observer. How do we interpret this state within MWI?

 

[vanesch]

I have another (probably pointless) question concerning MWI.

In the case of a pure entangled state between our quantum system and observer, MWI provides an interpretation for

|spin-up> |a> + |spin-down>|b>,

in that each term of the superposition should be treated as a separate copy of the universe. How does this interpretation deal with mixed entangled states? For example, suppose we include an environment in our system and then unitary evolutions give

|spin-up>|a>|e1> + |spin-down>|b>|e2> ,

and if i trace out the environment then i am left with:

rho = tr(|e1><e1|) |spin-up>|a><spin-up|<a| + tr(|e2><e1|) |spin-down>|b><spin-up|<a| + tr(|e1><e2|) |spin-up>|a><spin-down|<b| + tr(|e2><e2|) |spin-down>|b><spin-down|<b|

This is the state shared between the system and the observer. How do we interpret this state within MWI?

There are two answers to this.

The first is, that there will always be a split, which is binary:
"observer body" versus "rest of the universe".

So if the state is:
|spin-up>|a>|e1> + |spin-down>|b>|e2> ,

and the observer body is described by |a> and |b> then we have that the split between the observer body and the rest of the universe takes on the form:

|a> (x) |spin-up>|e1> + |b> (x) |spin-down>|e2> ,

(where I put (x) to emphasize the tensor product split between "observer" and "rest-of-universe").

So to the observer body degree of freedom correspond two distinct terms, |a> and |b> which are supposed to be corresponding to two different "subjective experiences" which emerge from these states.

The "rest of the universe" is now the union of the spin system and the environment. But state |a> will be compatible with both an environment e1, and a spin up state, while state |b> will be compatible with both an environment e2 and a spin down state.

Now, the nice thing about decoherence is that in this kind of decomposition, the "classical body states", together with the "classical environment states", together with the "classical pointer states" all come together in different terms. (ok, there is a caveat to this: we shouldn't think of basis states here, but as subspaces in Hilbert space, sufficiently compatible with classical, coherent states: there can still be wild oscillations within these subspaces).

So it is a bit as with the definition of inertial frames in Newtonian physics: although the definition is, strictly speaking, somewhat circular (inertial frames are rest frames of particles on which no forces act, and particles on which no forces act are particles which move in a uniform way in inertial frames...). Classical body states are states which appear in decohered systems of the above kind, and states are decohered when the different contributions are factors of classical states... But FAPP, there is no ambiguity of what is a classical pointer state, or a classical body state.

The statistical ensemble of "subjective observer states" is then given by the different classical bodystates that occur in the decomposition, with probability measure given by the hilbert norm squared of the term in which it occurs.

The second answer is this:

The use of the partial trace already pre-supposes the application of the Born rule, in other words, when doing this, we already accept the passage to a statistical mixture *as observed* by one of its components.

So we are already talking about the statistical ensemble of "subjective observer states" in a way.

Now, for any two environment states |e1> and |e2> which respect themselves, we have that < e1 | e2 > is essentially 0. From this follows that the reduced density matrix of "observer+spin system" is given by:

rho = tr(|e1><e1|) |spin-up>|a><spin-up|<a| + tr(|e2><e1|) |spin-down>|b><spin-up|<a| + tr(|e1><e2|) |spin-up>|a><spin-down|<b| + tr(|e2><e2|) |spin-down>|b><spin-down|<b|

= tr(|e1><e1|) |spin-up>|a><spin-up|<a|
+ tr(|e2><e2|) |spin-down>|b><spin-down|<b|

= |spin-up>|a><spin-up|<a|
+ |spin-down>|b><spin-down|<b|

In a "basis of classical body states" (here, |a> and |b>), this is a diagonal density matrix (it is always the case, when tracing out the environment, under the assumption of full decoherence).

It comes down to having a statistical ensemble of "subjective experience" (in 1-1 relation with classical body states), in which we have:

50% of the |spin-up>|a> state, and
50% of the |spin-down>|b> state.

In other words, this generates (without surprise) the same statistical ensemble of "subjective observer states" as in the previous approach.

 

[MaverickMenzies]

Interesting. Thank you for your answer.

 

[ttn]

Another question in the same spirit as the above... Suppose the state is

|1>|A1>|B0> + |2>|A2>|B0>

where, in order, the three factors refer to degrees of freedom pertaining to

* some physical system (like maybe the spin of a particle being up=1 and down=2 or whatever)

* the state of one observer ("Alice", with A1 = Alice's body/brain is in the state where she observes spin up, etc.)

* the state of some other observer who isn't observing or paying attention or even nearby (Bob). Note that the Bob part of the state is the same in the two superposed terms.

So here's the question: vanesch always talks about the "counting of terms" as being based on orthogonal "body states" of the observer. But which observer? If the state of the universe is the state above, is there just one universe, or two? Or is it "two for Alice" and "one for Bob"? Or what?

PS -- this is the kind of thing Bell had in mind when he noted that MWI is "extravagantly vague". It moves all of the physics into a very weird place (some boundary between "subjective mental impressions" and a "physical world" that is nothing like the physical world we know) and is *thereby* extremely difficult to actually formulate rigorously.

 

[vanesch]

Or is it "two for Alice" and "one for Bob"?

You've got it. Two for Alice, one for Bob.

Worlds are an observer-dependent concept in MWI.

This is exactly how MWI can weasel out of Bell's theorem.

However, in order for one to maintain such a state, there can be no interaction between "alice" and "bob", and the only way to make one sure about this, is to have them at spacelike separations. Sooner or later, via a common environment, they will entangle, and then both sets of worlds (those from Alice PoV and those from Bob PoV) will coincide.

 

[ttn]

You've got it. Two for Alice, one for Bob.

Worlds are an observer-dependent concept in MWI.

This is exactly how MWI can weasel out of Bell's theorem.

Yes, good, I understand that, but it's good to get something out in the open so clearly for once!

However, in order for one to maintain such a state, there can be no interaction between "alice" and "bob", and the only way to make one sure about this, is to have them at spacelike separations. Sooner or later, via a common environment, they will entangle, and then both sets of worlds (those from Alice PoV and those from Bob PoV) will coincide.

Right.

I've basically made the point I want to make, which is that you have to take all of the "different worlds" and "splitting" and all that (which *sounds* like it's supposed to be what the *dynamics* of MWI is fundamentally *about*) with a giant grain of salt. All of that is really just talk that can't be taken seriously/physically. And then it's not so clear what is left that *can* be taken seriously/physically. It shows nicely how subjective/internal/solipsist this gets. Incidentally, doesn't what you said above also raise some questions about Lorentz invariance? Not any real ones now that we know all the talk about "splitting worlds" and such is just empty talk and not something that is to be taken seriously by the theory. But anyway, if the physical effects that get Alice and Bob entangled (later) propagate at some slow speed (like the speed of sound, which is probably about right for many such situations) then what some MWI people might (erroneously) think of as "facts" that the theory says something about (like how many worlds there are at some time) aren't really facts at all, because they aren't Lorentz invariant. So, another nice illustration of why you can't take any of this seriously -- to do so is to spoil the (alleged) consistency of MWI with relativity.

Anyway, I didn't really want to discuss this. I just thought it would be a nice way to bring something important out into the open, which was a minor tweak of the other question that got asked. I'll hopefully find some time later to respond to your other post on the more philosophical stuff.

 

[mgelfan]

Interesting discussion ... sort of.

MWI is a most unattractive way of, er, looking at things. It certainly doesn't explain any of the stuff that it purports to explain. It seems to be saying that since the quantum processes involved in the generation of experimental observations (data) are pretty much unknown (ie., there's a measurement problem and an accompanying wave function interpretation problem), then let's just interpret the superposition of all possible outcomes as meaning that all possible outcomes actually happened (even though that interpretation doesn't actually mean anything). MWI also supposedly solves the locality/nonlocality problem.

Now, the measurement problem really is a problem in that physicists really don't know much about measurement (or emission, for that matter) processes at the quantum level. But I think that we can agree that the MWI solution doesn't provide any new knowledge of such processes.

The locality/nonlocality problem hasn't yet attained the status of being a real problem (everything seems to behave as if locality were the reigning standard ... and no superluminal anything has been produced for our consideration), so maybe its solution by way of MWI is premature.

ttn's idea (and demonstrations) that MWI is silly on several different fronts is more appealing than vanesch's adherence to MWI -- which adherence doesn't promise to help his (vanesch's ... or anybody else's for that matter ) understanding of physics or of the world. Of course, if MWI could be used to improve our knowledge and facility regarding, say, high T_c superconducting, or dark energy, etc., then I'll have to reconsider.

 

[ttn]

The one, only and single non-disputable criterion for a scientific theory is: it should correctly predict observations.

Sure, but that doesn't really help given our philosophical disagreement over the meaning of "observations". You say MWI does correctly predict our observations, and I deny this. So it doesn't really help move the discussion forward to just assert that what I mean by "observation" adds something to what "observation" really means. It's already clear that we disagree, and not clear (to anyone but you) that you are right.

The requirement, however, that elements of observation should be elements of ontology in a specific way, is a totally arbitrary extra requirement, but the fundamental requirement which made Bell (and Bohmians) conclude about their distaste for MWI. There is NO ABSOLUTE REQUIREMENT for observations to be corresponding in a simple and evident way with an element of ontology.

It's a "totally arbitrary extra requirement" to think that there are such things as pointers and that they point? Come on. That's the kind of talk that wins you friends among other freshman in a philosophy 101 class maybe, but it isn't the kind of talk that serious physicists take seriously. We know there are pointers, and that they point... and we're now onto working on more interesting questions.

Indeed, as you outline, if you introduce some physical stuff, the ether, as living on spacetime, you could formulate the theory in a lorentz-invariant way, or in the SR way (which, to me, is the same: SR says that all physical entities should be functions over the spacetime manifold, ...

So, the wf in MWI is not "physical"?

The problem is that Lorentz-invariance should then require that when the air is set in motion, things change. You hence have to introduce a genuine beable which is that "ether". But nothing observable has ever been related to that "ether". In order for that beable to make sense (and to use your OWN requirements of having observations to correspond to reality), there should be something physically observable to that ether. The air is observable. We can put it in motion, and study the sound waves in moving air.

Your parenthetical comment shows that you completely misunderstand what I think about "observation". It's not that reality = observation (meaning, whatever you observe is real *and* everything that's real is directly observable). I take the word "observation" quite literally. To observe something is to *see* it, literally. I'm observing a table right now. And now my cat. I've never directly observed an atom. Nor an ether. Some things (like tables and cats) can be known to exist because we observe them. Other things we know exist only through a chain of inference. So don't think that just because some hypothetical entity (like say an ether, an atom, dark matter, whatever) is unobservable, therefore my philosophy requires me to disbelieve its existence.

But the ether is just an invention to make the theory fit a required symmetry which it doesn't have.

I agree. Don't forget I said from the beginning that I didn't think this scheme was really a way to "relativize" Bohm's theory. It's obviously cheating, in roughly the way you describe. We agree there. I only raised it to point out that there is more to "consistency with relativity" than mere Lorentz invariance of the defining equations.

Consistency with SR means that the ontological elements of the theory should be defined over spacetime.

So, pray tell, which are the ontological elements (defined over spacetime) in MWI?

Strictly speaking, making the hypothesis of an ether, as some physical stuff, which fills spacetime, is consistent with SR.

That's a bizarre statement since basically the whole point of SR was to try to account for certain things *without* positing an ether (which bothered Einstein because it was arbitrary and unobservable). So you're saying (for example) the Lorentz ether theory is consistent with SR?

The above proposal of an ether is not a violation of SR, it is a violation of Occam's rasor. One needs to introduce a lot of extra postulates, of which the only aim is, to restore some broken symmetry.

I agree that that's a serious problem for the "relativistic Bohm theory" I proposed. It'd be more honest to just posit the ether and say "this conflicts with relativity" and let the chips fall where they may.

Well, this is a misunderstanding. In MWI, there is a 4D spacetime all right. However, it is not the place where MATTER lives. It is the place where the operators live, which determine the interactions (which determine the unitary evolution).

But, as you say, the operators are a human mathematical tool, right? Nobody thinks they somehow are (or correspond to or describe) anything *physical*, any *matter*. So *is* there any matter in MWI? Is there anything else, anything physically real, that lives in 4D spacetime?

And if not (and given your comments above) how can you say MWI is consistent with SR?

On one hand, you have Hilbert space, and on the other hand, you have spacetime over which operators over that Hilbert space live. All operators over hilbert space are indexed over spacetime, in such a way, that they transform under a (projective) representation of the Lorentz group.
So the spacetime manifold is there all right. Only, we thought that we had maps from R into that spacetime (which we call particles), or that we had tensor fields over spacetime (classical field theory), and it turns out that we have field operators over spacetime. In other words, spacetime exists as the basis space for a vector bundle in which the fibres are sets of operators over Hilbert space. So the structure is more complicated than originally anticipated, but spacetime is there all right.

Yes, the 4D spacetime is "there" in the theory, no doubt. But the question is: what, if anything, is "there" in the 4D spacetime? Nothing, so far as I can tell. But that's hardly consistent with our precious observations. (or SR)

Now, in specific cases, all this machinery looks a lot like a set of mappings from R into spacetime (in which case we restore classical relativitic particle dynamics) ; and in some cases it looks like tensor fields over spacetime (classical relativistic fields). But the structure is much richer.

What makes this "relativistically correct", is that the basis space is still 4D spacetime.

Not what you said before!

You classify as "insane", incompatible with some extra, and to a point arbitrary, extra requirements you've put upon physical theories. This is exactly what I call "emotional".

But what you call an arbitrary extra requirement is simply that physical theories should be able to account for our direct observation of physical objects! To deny that *is* insane (he says emotionlessly).

You dislike MWI for its lack of 1-1 relationship between ontology and observations.

No, you misunderstand, as I explained above.

It is a philosophical act to make an ontology-hypothesis.

Wow, you just defined most of physics out of existence. So when Copernicus said that the Earth really goes around the sun, he was doing philosophy, not physics. When Maxwell and Boltzmann argued that there are unobserved little particles (atoms), they were doing philosophy, not physics. When Rutherford inferred from his scattering data that these atoms have a nuclear structure, he was engaging in a "philosophical act", not doing physics. And so on and so on.

Choosing to equate observation with ontology is ONE possible answer one can prefer. It is called "naive realism". Refusing to make an ontology-hypothesis is called solipsism. Doing something else, is, well, making yet another ontology-hypothesis.

I'm not a naive realist.

You see, you are IMPOSING a philosophical viewpoint upon the workings of a scientific theory.

True, I am. I'm considering it settled (by philosophy, prior to any discussions of physics) that there are cats and tables and pointers, that the things we directly observe are actually there.

What's ironic is that you are also "imposing a philosophical viewpoint" -- it's just that you (wrongly) think your philosophical viewpoint is "obviously right" or "scientific" or some such, so all you see is everybody else's philosophical viewpoint. It's like: "everybody but me speaks with an accent!"

Yes, that is exactly true. I base this onto two points:
1) the only thing we really know, observe etc... are our "inner theatre impressions". All the rest is hypothesis.

I completely disagree with that.

There are no "apparent observations". Observations (inner theatre experiences) are just as "real" as anything.

I meant by that to distinguish between your understanding of "observations" and mine. "Apparent observations" are what *I* call what you call "observations" in MWI -- they are merely "apparent" because (as I describe it) they are delusional, because there is no really existing *object* of the observation (the way there is when I veridically observe a table or cat).

 

[ttn]

continued...

Again, again. MWI is not self-refuting, because it doesn't claim that cited subjective experienced results weren't subjectively experienced. There is no need for the results to "ontologically be there" in order for a theory to say how results can be subjectively experienced.

But if somebody mentions (I dunno, say) the results of the Davisson-Germer experiment as evidence for their theory, that only goes anywhere if I understand them to mean the *actual* realized results of that experiment (as opposed to the "subjective fantasy" that I or someone else had about the results of those experiments). It's only evidence if that's what actually *happened*. Dreams aren't evidence for scientific theories. And so, given that, such evidence can never be evidence for MWI since MWI asks us to reject the assumption that all the experienced stuff actually happened. Accepting MWI renders what might otherwise have been evidence (for MWI or anything) into non-evidence (for MWI or anything). So any such argument as "this and that experiment had thus and so outcomes, so you ought to believe in MWI" must necessarily fail.

We don't know that AT ALL, and we could never scientifically prove this, given that it is a philosophical position. If it were scientific, it could be falsified. You can never falsify even solipsism in a scientific way.

We can never scientifically prove that there are tables and cats? I think you seriously misunderstand "scientific proof". Scientific proof is what we resort to when we can't just directly perceive something!

 

[vanesch]

Yes, good, I understand that, but it's good to get something out in the open so clearly for once!

I always claimed that "worlds" is an observer-dependent concept (one should say "many subjective worlds, one objective one" ; or in other words: a statistical ensemble for subjective experiences, one single objective world). That's why all those "splittings" and so on are not objective phenomena. They are subjective phenomena.

I've basically made the point I want to make, which is that you have to take all of the "different worlds" and "splitting" and all that (which *sounds* like it's supposed to be what the *dynamics* of MWI is fundamentally *about*) with a giant grain of salt. All of that is really just talk that can't be taken seriously/physically.

Why not ? That's not an argument. From the start, in MWI, we state that there is a difference between "subjective experience/world/observation/..." and objective ontology, but that there is a rule to find the statistical ensemble describing the subjective experience/world/observation.

Now, it seems that each time you rediscover that basic assumption, that you restate how impossible/not serious/... this must be. But since we took it as a starting position, you will find it scattered all over the place.

It shows nicely how subjective/internal/solipsist this gets.

Yes, but that was said from the start.

But anyway, if the physical effects that get Alice and Bob entangled (later) propagate at some slow speed

Nope, that will essentially be light speed. It is sufficient that a single photon makes "contact" between both environments of Alice and Bob, for them to get entangled. The single most interaction that can leave a "classical trace" *in principle*.

(like the speed of sound, which is probably about right for many such situations) then what some MWI people might (erroneously) think of as "facts" that the theory says something about (like how many worlds there are at some time) aren't really facts at all, because they aren't Lorentz invariant.

But that's the point. Given that most decoherence occurs via the EM field (for instance, entanglement with the cosmic microwave background), this is essentially "at light speed".

Also, there isn't to be any lorentz-independent notion of "how many worlds" there are, given that it is observer dependent, in the same way as a coordinate system of the rest frame of an observer is, well, observer dependent.

 

[vanesch]

MWI is a most unattractive way of, er, looking at things. It certainly doesn't explain any of the stuff that it purports to explain. It seems to be saying that since the quantum processes involved in the generation of experimental observations (data) are pretty much unknown (ie., there's a measurement problem and an accompanying wave function interpretation problem), then let's just interpret the superposition of all possible outcomes as meaning that all possible outcomes actually happened (even though that interpretation doesn't actually mean anything). MWI also supposedly solves the locality/nonlocality problem.

Yes. Given the basic principle of quantum theory, which says that a system state is given by an element of hilbert space, and that that hilbert space is found by considering the superposition of all possible "classical states", it seems normal to extend this concept to "observer states".

There is no fundamental difference between the physics happening in "observers" and in "systems". As such, their descriptions should be the same. But if that is true (which we accept, if we accept the basic postulate of quantum theory to be universal), then there is no way out of saying that observers must be in states which are superpositions of "different states of observation". It is simply the application of the basic postulate of quantum theory to observers, if we don't want to give them any different physics than other physical structures.

So if we accept the Schroedinger equation to be perfectly universally valid, then there is no escaping that at a certain point in time, we should consider that Alice's state is a superposition of "Alice saw up" and "Alice saw down". This follows from the linearity of the Schroedinger equation, and the assumption that this is applicable in principle to a physical structure such as Alice.

We also know that quantum-mechanically, there can be no equivalence between "superposition" and "statistical ignorance". So the superposition of Alice saw down and Alice saw up is not the same as a statistical mixture.

We also know the measurement axioms, which specify that "when we do a measurement" (which is after all, ultimately a subjective experience), then a superposition becomes a mixture of "observed outcomes". But if we refuse to accept that there happens some DIFFERENT PHYSICS in a measurement interaction, than in a "system interaction", then the meaning of this measurement axiom is simply giving us the relationship between the actual physical state (which we found to be a superposition), and a subjective experience. Hence, the "state -> statistical ensemble" transition seems to hide in the transition "physical state -> subjective experience".

Now, the nice thing of doing this - and the only reason in fact - is that we can keep the full unitary machinery of quantum theory. As such, we can keep all the nice properties of this machinery, and we don't need to introduce any ugly "change in physics due to observers". Also, we can do what we've always done in physics, that is, to take the essential formal elements as describing reality.
This is the reason why I adhere to MWI *as an interpretation of QM*: it allows you to take the formalism seriously (in the same way as you take the spacetime manifold for real in relativity, or you take "particles" for real in Newtonian mechanics). In other words, MWI allows you to resist the temptation to fiddle with the formalism for philosophical preferences. The formalism should speak for itself, and we shouldn't have any a priori over it.

The bonus one gets also (which is somehow also comprehensible) is that a lot of paradoxes that appear in QM when one considers QM just as a kind of thing that must ultimately transit to a classical world, disappear. The main difficulty being the EPR problem with Bell's theorem. It is no wonder that the paradoxes disappear: you take the formalism which provides you with these predictions in the first place, for real. As such, there is perfect agreement between the ontology, and the basic properties of quantum theory, and you avoid paradoxes.

Now, the measurement problem really is a problem in that physicists really don't know much about measurement (or emission, for that matter) processes at the quantum level. But I think that we can agree that the MWI solution doesn't provide any new knowledge of such processes.

It provides for a view which helps you not to consider it as a problem.
A bit like taking on a spacetime manifold view avoids you to consider the "problem" of "simultaneity". You UNDERSTAND why it is not a problem in that view.

The locality/nonlocality problem hasn't yet attained the status of being a real problem (everything seems to behave as if locality were the reigning standard ... and no superluminal anything has been produced for our consideration), so maybe its solution by way of MWI is premature.

Well, if you have no problem with locality in your view of QM, that is ok then, but I wonder how you consider Bell's theorem then.

ttn's idea (and demonstrations) that MWI is silly on several different fronts is more appealing than vanesch's adherence to MWI -- which adherence doesn't promise to help his (vanesch's ... or anybody else's for that matter ) understanding of physics or of the world. Of course, if MWI could be used to improve our knowledge and facility regarding, say, high T_c superconducting, or dark energy, etc., then I'll have to reconsider.

No, MWI gives me the "ease of mind" not to look for problems where there aren't any, such as locality or the "measurement problem". It makes me resist the temptation to throw out of the window good and powerful principles such as the principle of relativity.

 

[setAI]

There is no fundamental difference between the physics happening in "observers" and in "systems". As such, their descriptions should be the same. But if that is true (which we accept, if we accept the basic postulate of quantum theory to be universal), then there is no way out of saying that observers must be in states which are superpositions of "different states of observation". It is simply the application of the basic postulate of quantum theory to observers, if we don't want to give them any different physics than other physical structures.

extremely well said- this is the source of much frustration- opponents of MWI and multiverse theories/ontologies in general would have us accept a grand epicycle- a demon that adds some new physical principle that somehow magically allows only one 'real' observer state- simply because they find the implications of unitary quantum mechanics 'unattractive'

 

[ttn]

I always claimed that "worlds" is an observer-dependent concept (one should say "many subjective worlds, one objective one" ; or in other words: a statistical ensemble for subjective experiences, one single objective world). That's why all those "splittings" and so on are not objective phenomena. They are subjective phenomena.

Yes, fine. But what you are less clear about is what the "one objective one" is made of and where it is. (As far as I can tell, there's no physical matter in your "one objective world" and whatever it does contain, if anything, doesn't live in 3D physical space.)

Also, there isn't to be any lorentz-independent notion of "how many worlds" there are, given that it is observer dependent, in the same way as a coordinate system of the rest frame of an observer is, well, observer dependent.

Oh jeez.

 

[ttn]

There is no fundamental difference between the physics happening in "observers" and in "systems".

Huh? There is, to the contrary, no more fundamental difference than the difference between stuff happening "in observers" and stuff happening "in [physical, external] systems".

As such, their descriptions should be the same.

But in MWI they aren't.

So if we accept the Schroedinger equation to be perfectly universally valid, then there is no escaping that at a certain point in time, we should consider that Alice's state is a superposition of "Alice saw up" and "Alice saw down". This follows from the linearity of the Schroedinger equation, and the assumption that this is applicable in principle to a physical structure such as Alice.

...and the evasion of the ever-present fact that no such states have ever been observed to exist.

We also know the measurement axioms, which specify that "when we do a measurement" (which is after all, ultimately a subjective experience),

That's where we disagree. Performing an experiment and getting an outcome is NOT merely having a certain kind of subjective experience. Reality is not virtual reality.

This is the reason why I adhere to MWI *as an interpretation of QM*: it allows you to take the formalism seriously (in the same way as you take the spacetime manifold for real in relativity, or you take "particles" for real in Newtonian mechanics). In other words, MWI allows you to resist the temptation to fiddle with the formalism for philosophical preferences.

Except that the only way of coming even remotely close to making MWI possible to consider seriously (let alone accept), is to adopt a very radial anti-scientific philosophical position about the basic relationship between consciousness and its objects -- namely, that (for all we know) consciousness *has* no objects.

This is the same kind of BS argument one used to always hear about Bohmian Mechanics -- "oh, that's just philosophy". So it is. But for Bohm's theory, the philosophy is good philosophy, true philosophy. What the people who make this argument don't understand is that Copenhagen is just as philosophical, and its philosophy is complete anti-science nonsense (Kantian subjectivism, existentialism, logical positivism, ...). It's not valid to dismiss one thing because it's "philosophical" when what one advocates instead is equally philosophical -- and it's particularly invalid when the philosophy one advocates instead is an embarrassing unscientific pile of poo.

The formalism should speak for itself, and we shouldn't have any a priori over it.

The whole idea that "the formalism should [or could] speak for itself" is philosophy.

No, MWI gives me the "ease of mind" not to look for problems where there aren't any, such as locality or the "measurement problem". It makes me resist the temptation to throw out of the window good and powerful principles such as the principle of relativity.

Yes, it is very difficult to resist the temptation to follow empirical evidence where it leads.

 

[ttn]

extremely well said- this is the source of much frustration- opponents of MWI and multiverse theories/ontologies in general would have us accept a grand epicycle- a demon that adds some new physical principle that somehow magically allows only one 'real' observer state- simply because they find the implications of unitary quantum mechanics 'unattractive'

There it is again. "Everybody has an accent but me."

 

[vanesch]

Yes, fine. But what you are less clear about is what the "one objective one" is made of and where it is.

It is a mathematical object, of course.

If you talk about "space", that's also a mathematical object, right ? "particles" are mappings from R to that space.

 

[vanesch]

Huh? There is, to the contrary, no more fundamental difference than the difference between stuff happening "in observers" and stuff happening "in [physical, external] systems".

There is also no bigger difference than "subjective experiences" and "physical world". We take it that the physics of the physical observer (=body) is the same physics as anything else. However, you should admit that it is a totally different matter to say that the subjective experiences emerging from such a physical structure ought to follow the same physical laws. Subjective worlds are not objective worlds. It is the difference between epistemology and ontology.

...and the evasion of the ever-present fact that no such states have ever been observed to exist.

You mean, quantum superposition effects have not been indirectly observed ?
Interference has never been observed ?

What has never been observed (subjectively), is _by definition_ the superposition of two states of subjective observation. This is so *by very construction*!

You could just as well say that two different eigentimes have never been observed by one and the same observer and hence that never ever, any difference in simultaneity has been seen by one single observer.

That's where we disagree. Performing an experiment and getting an outcome is NOT merely having a certain kind of subjective experience. Reality is not virtual reality.

THIS is what I'm trying to make you see: it is a statement which is absolutely not evidently true, and in fact, for which not the slightest ounce of scientific proof can be advanced, by its very ontological nature.

When you "see a chair", do you:

1) observe directly a chair ?
2) observe directly electromagnetic radiation coming from the chair ? (holography!)
3) observe directly photochemical processes in your retina ?
4) observe directly nervous pulses from the ocular nerve ?
5) observe directly certain brain states ? Quantum brain states or classical brain states ?

Where is the scientific evidence which FALSIFIES that you are observing a brain state ? I try to make you see that your position is not given by any *scientific* argument, but by a philosophical argument (and so does mine).

This is not an argument I invent. It is a well-known position, which posits the separation between ontological and epistemological argumentation.

Except that the only way of coming even remotely close to making MWI possible to consider seriously (let alone accept), is to adopt a very radial anti-scientific philosophical position about the basic relationship between consciousness and its objects -- namely, that (for all we know) consciousness *has* no objects.

Yes, but that is not an anti-scientific position. You cannot do an experiment in which you FALSIFY that position (which is the only way of calling a position anti-scientific). The proposition is scientifically neutral. It is a philosophical position which is entirely compatible with all of science. It is not a scientific position, but it is not anti-scientific either. However, it forms the basis of a view which IS scientific, in the sense that it makes predictions of observations which are entirely in agreement with what is observed.

Ontological positions are philosophical positions. They are never scientific positions: scientific positions are purely epistemological. You make one, I make another one. Yours don't allow certain forms of scientific theories, while mine does. As such, you allow a purely philosophical position (because an ontology assumption) to interfere with the construction of scientific theories, while I don't.

This is the same kind of BS argument one used to always hear about Bohmian Mechanics -- "oh, that's just philosophy". So it is. But for Bohm's theory, the philosophy is good philosophy, true philosophy.

You see, that's where you go wrong. There is no "good" and "bad" philosophy. There is no "good" and "bad" ontology.

It's not valid to dismiss one thing because it's "philosophical" when what one advocates instead is equally philosophical -- and it's particularly invalid when the philosophy one advocates instead is an embarrassing unscientific pile of poo.

As always, it comes down to an emotional argument. There is no rational, scientific argument against MWI, no more as there is against Bohmian mechanics, or for that matter, any other form of interpretation. Your classification of "good philosophy" and "bad philosophy", and the argument of wanting ontological positions to be scientific or anti-scientific illustrate the issue.

But there is a scientifically based argument:

The whole idea that "the formalism should [or could] speak for itself" is philosophy.

That is, let us not IMPOSE a priori philosophical arguments! Let us see what we have formally derived from our epistemological knowledge (and that IS science), and then let us stop there and take that as the only suggestion for an ontological position.

In other words, in as far as we are going to have to make a philosophical choice - which is: to make a hypothesis of ontology - let us try to be as unbiased as possible, and take as a suggestion (and nothing more than this), what we derived formally from the scientific method. In other words, because I have not much confidence in any a priori philosophical requirements, I try to take the position in which I give as few a priori input from that side as possible.

This is, by itself, indeed a philosophical position. You can choose something else (visibly you do so). However, there is not more or less merit to one philosophical position over another. So all argumentation of "this is BAD philosophy", this is RUBBISH, we all KNOW that it isn't (ontologically) true, ... are demonstrations of lack of genuine arguments (and essentially emotional statements).

As I said, I have never seen an argument that is NOT of this kind against MWI.

There is of course no scientific "proof" of MWI anymore than there could be a proof of Bohmian mechanics or anything else, given that they are all based upon ontological statements which are, by definition, philosophical positions, and not scientific ones. The only things that one can demonstrate scientifically, are epistemological concepts.

The advantage of MWI, for those who consider that an advantage, is that it doesn't require any change to the formalism. It takes the quantum formalism as is. In other words, in MWI, we don't permit weak philosophical a priori arguments to intervene in the formulation of a scientific theory. In MWI, we do not require a choice between fundamental principles on which the formalism is based, just for the sake of an extra philosophical requirement one might have.

It is the only use of MWI: to help one accept the formalism as it is, and to refrain from the desire to intervene in the formalism (and render it less "principle-based", just for the sake of some or other philosophical requirement). It helps one to accept quantum mechanics as something which might be a fundamental theory.

And this, by itself, has scientific merit. Indeed, although a position such as Bohmian mechanics can maybe run after the facts, and - especially in the domain of quantum field theory - with a lot of difficulty, ACCOMODATE what has been discovered formally before, admit that, if in the 30ies one would have gone for Bohmian mechanics (remember, no spin!), it would not have been an inspirationally fruitful ground in order to do things such as gauge theory. The modifications to the formalism required by the philosophical position which is at the basis of Bohmian mechanics, wouldn't have facilitated all the discoveries in QFT that followed. Bohmian mechanics is not "wired up" to suggest things such as gauge invariance. At most, with some difficulty, it can accommodate. This is because Bohmian mechanics has KILLED certain principles, of which one had to put the effect again in by hand.

THIS is why I consider it as not a happy idea to fiddle with the formalism just for the sake of some philosophical position, but rather, to let the formalism speak for itself.

EDIT: this is also why I don't mind the "shut up and calculate" position. In as far as one doesn't feel the urge to consider fiddling with the formalism for any other reason than observational, or for reasons of mathematical/formal consistency, that is fine.
It is when one starts to want to do things to the formalism JUST for the sake of philosophical positions, and by that, render the mathematical/formal system less "smooth", "simple" (Occam)... that one is making a potentially expensive mistake.

 

[ttn]

It is a mathematical object, of course.

But does that "mathematical object" in some way *describe* the "one objective world" that you say exists according to MWI?

 

[ttn]

You mean, quantum superposition effects have not been indirectly observed ?

If we're being careful with terminology, "indirectly observed" is a contradiction in terms. Quantum superposition effects have been *inferred* -- inferred from things that have been directly observed.

But my point was to object to the specific thing you mentioned -- a human observer being in a superposition of two belief states. *That* has never been observed, and neither has anything been observed from which one could infer it.

When you "see a chair", do you:

1) observe directly a chair ?

Yes.

2) observe directly electromagnetic radiation coming from the chair ? (holography!)

No. You observe the chair by means of the electromagnetic radiation -- but of course one doesn't observe *that*. One is completely unaware of it for a long time (until one takes some physics classes, or in terms of the development of history, until the 19th century or so).

3) observe directly photochemical processes in your retina ?

No, one doesn't directly observe those. They are discovered much later by advanced scientific inference.

4) observe directly nervous pulses from the ocular nerve ?

No, same as above.

5) observe directly certain brain states ? Quantum brain states or classical brain states ?

No, same as above.

Where is the scientific evidence which FALSIFIES that you are observing a brain state ?

Claims which are obviously false to begin with don't require such "falsification". The falsification is that the word "observation" actually means something, and we simply do not observe brain states. The vast majority of people have literally never observed a brain.

I try to make you see that your position is not given by any *scientific* argument, but by a philosophical argument (and so does mine).

This is where we differ. You equate philosophy = ontology = arbitrary. But then practically everything in legitimate science is arbitrary made up bogus "philosophy", and what is *left* of science according to your conception is some ridiculous game of trying to account for "subjective experience" but without allowing yourself to believe ever that these experiences are experiences *of* anything. And that is a fundamental, fatal, philosophical flaw in your whole way of thinking about all of this. Consciousness *means* consciousness *of* *something*. To be *aware* of something, is to be aware *of something*. Consciousness without an object is a contradiction in terms. So your whole conception of science which is based on this fundamental philosophical error falls apart.

By contrast, my conception of science is also based on a philosophy. But mine is based on a valid philosophy, not an invalid one. (See, we disagree about whether philosophy = arbitrary, and hence about whether there can be such a distinction as good vs bad philosophy. Ironically, it's bad philosophy which makes you think there's no such difference.) Mine is based on a philosophy that is "scientific" (though that is admittedly an imprecise term here) in the sense of being based on empirical observation -- for example, the observations by which we arrive at basic concepts like "consciousness".

Yes, but that is not an anti-scientific position. You cannot do an experiment in which you FALSIFY that position (which is the only way of calling a position anti-scientific).

Or at least so says the philosopher Karl Popper.

That's the problem here. It's just what I've been saying over and over again. You're convinced that (a) philosophy is just arbitrary crap and we shouldn't ever let it influence our scientific thinking, and (b) that you can prove that by appeal to certain philosophical doctrines which you regard as just obviously true beyond the shadow of any doubt.

Don't you see the problem there? In (b) you confess that it is necessary to have a philosophical base for one's whole conception of the nature and goals of science, which in turn influences how one assesses specific things in science. But because of (a) you refuse to take *seriously* the task of then working out what philosophy is right, and what the right conception of science is. So you're caught in a vicious circle.

Ontological positions are philosophical positions. They are never scientific positions: scientific positions are purely epistemological.

See, I think that is just preposterous nonsense. (No, that's not an emotional statement, just an honest, dry, factual assessment.)

"There's a table in front of me." "Matter is made of atoms." "There's another planet beyond Uranus which is perturbing its orbit through gravitational forces." "Novae in the sky are caused by the core-collapse of a supermassive progenitor star whose core comes to exceed the Chandresekhar limit." "Genetic inheritance occurs through the mechanism of DNA splitting and recombination."

To you all of these are evidently "philosophical", not science. But look at what is left of science without these things! As a simple empirical statement based on looking at the history of thought and which things we use the word "science" to distinguish from which others, it is quite clear that your conception of "science" is simply *wrong*. You are fatally mis-classifying what is and what is not science.

You make one, I make another one. Yours don't allow certain forms of scientific theories, while mine does. As such, you allow a purely philosophical position (because an ontology assumption) to interfere with the construction of scientific theories, while I don't.

But you see, you do. Even worse than I do. That we can't be sure whether there are really tables and cats, is a philosophical position (maybe not exactly "ontological", but "anti-ontological" and surely about the same *issues* that what you call "ontological" positions are about).

You see, that's where you go wrong. There is no "good" and "bad" philosophy. There is no "good" and "bad" ontology.

I don't agree. There is good and bad philosophy. I mean, even you know that there have been all sorts of conflicting philosophical positions put forward on all sorts of points. They can't all be equally right. I guess your view is that they're all equally wrong, equally empty, equally nothing, equally pointless. But I completely and totally disagree. The issues (or at least some of them) are crucial, and are fundamental to science. We need answers to them. And so we need to distinguish the right answers from the wrong answers.

As to ontology: someone who starts their philosophy by saying "I just know a priori that there is a god, and that he is omnipotent, omniscient, and has really great hair to boot" is doing bad philosophy. Someone who starts their philosophy by saying "I know there are tables and chairs because I *see* them and I know I am conscious" is doing good philosophy.

As always, it comes down to an emotional argument. There is no rational, scientific argument against MWI, no more as there is against Bohmian mechanics, or for that matter, any other form of interpretation.

So then how the heck can you justify spending all this time arguing about it? How can you justify considering yourself a proponent of MWI? To you this is all just a pointless game, in which we know going in that there is no answer, no way to actually establish what is true, what the world is really like (even if it takes hundreds of years)? Not to me.

That is, let us not IMPOSE a priori philosophical arguments!

What you refuse to see is that philosophy doesn't necessarily have to be "a priori". I reject completely the idea of "a priori". I'm an empiricist. And yet I believe philosophy is important and valid. To you that's a self contradiction I guess -- because you refuse to see that all your beliefs are influenced by the philosophy *you* (unwittingly?) accept.

Let us see what we have formally derived from our epistemological knowledge (and that IS science), and then let us stop there and take that as the only suggestion for an ontological position.

But I completely agree with this. It's just that I think we can be 100% certain, based on what you call "epistemological knowledge" (by which I assume you just mean "empirical", based in observation), that there are tables and chairs and cats (not to mention extra-solar planets, atoms, dark matter, etc.). You disagree because you define epistemology/empiricism/experience differently, which is a *philosophical issue*. But you are blind to the fact that you take this radical philosophical position and that it actually influences everything you are saying. Everybody else has an accent.

In other words, in as far as we are going to have to make a philosophical choice - which is: to make a hypothesis of ontology - let us try to be as unbiased as possible, and take as a suggestion (and nothing more than this), what we derived formally from the scientific method.

I agree, in so far as we are talking about questions of unobservables, things that have to be inferred from what is observed. There is no such debate over things that are directly observed. As an empiricist, I believe there is no possibly better warrant for believing something exists, than that I *see* it. Since any *other* warrant (some complex chain of scientific inference) has to be *based exclusively* on empirical/observational evidence, the end of such a chain could never be more certain than the things at its base. So if you don't think tables are real, if you don't think *seeing* them is enough evidence to "prove" that they are really there, then you are never going to be able to believe in *anything* unobservable (atoms, extra-solar planets, etc.). Indeed, you won't believe in anything at all. You'll have to end up a solipsist, trying to twist and turn to reconceive things like science in light of that radical (unscientific) philosophical position.

And this is precisely the argument against MWI that began this thread. You believe in MWI, which posits (kinda) a single objective world that is very very odd and unfamiliar indeed. What possible reason could you have to believe in this? Well, if we leave aside "a priori revelations" (which we both reject as invalid, unscientific) the answer *must* be: observation. But all of those observations that could possibly be relevant are observations of familiar material objects like tables, chairs, cats, pointers, computer screens, etc. And so if none of those things really exist as such, the whole alleged chain of reasoning that leads you from them to MWI falls apart, it fails to connect at the first link. So any such argument for MWI (i.e., any argument which appeals ultimately to *observation*, i.e., any *scientific* argument) is self-defeating. That is why it's impossible to take MWI seriously as a scientific theory.

In other words, because I have not much confidence in any a priori philosophical requirements, I try to take the position in which I give as few a priori input from that side as possible.

Well it's scorched Earth for me: I accept *no* "a priori input". Including such things as: some preposterous definition of "consciousness" that is *not* based on empirical observation, the idea that there is rational grounds for doubt about the existence of tables and chairs, etc...

There is of course no scientific "proof" of MWI anymore than there could be a proof of Bohmian mechanics or anything else, given that they are all based upon ontological statements which are, by definition, philosophical positions, and not scientific ones. The only things that one can demonstrate scientifically, are epistemological concepts.

By which you mean: "subjective inner-theater impressions." So you are a solipsist masquerading as a scientist.

...if in the 30ies one would have gone for Bohmian mechanics (remember, no spin!), ...

Huh? Are you saying "spin" can't be incorporated into Bohm's theory? If so, you display your ignorance of the theory. It's trivial. Make the wave function a spinor, and replace the guidance formula with

v ~ Im[ (psidagger grad psidagger) / (psidagger psi) ]

and change the hamiltonian in the standard ways.

That's it. You get the right predictions for stern-gerlach, and everything else handled by the corresponding orthodox non-relativistic theory of spinning particles.

Or maybe you meant something else.

THIS is why I consider it as not a happy idea to fiddle with the formalism just for the sake of some philosophical position, but rather, to let the formalism speak for itself.

Those voices you hear in your head are not the formalism.

 

[vanesch]

If we're being careful with terminology, "indirectly observed" is a contradiction in terms. Quantum superposition effects have been *inferred* -- inferred from things that have been directly observed.

Correct.

But my point was to object to the specific thing you mentioned -- a human observer being in a superposition of two belief states. *That* has never been observed, and neither has anything been observed from which one could infer it.

Well, personally, I consider an EPR-kind of experiment as exactly such a situation. The only problem is that one had to ressort to a trick to get two human beings in a "different basis" wrt each other (which is necessary to get interference experiments): namely by having them entangle with two different basis expansions of the singlet state (by using spin analysers at different angles).

Typically, how does one determine quantum interference ? One looks at whether a statistical mixture gives the same predictions as the proposed interference. That is: we make a setup in which, say, two possible classical states are present (in one basis), and then one analyses a result. If that result can be obtained by a statistical mixture of the two classical basis states, then there is no proof of interference ; if, on the other hand, there is a difference, then there is interference.

In the double-slit experiment, that's what happens:
there are two slits, and the two classical states are: went through left slit, and went through right slit. Now, any classical statistical mixture of particles going through the left slit and going through the right slit, no matter what is their subsequent dynamics, will give you a whole possibility of pictures on the screen, but NONE OF THEM corresponds to an interference pattern. Hence, the observation of the interference pattern in the double-slit experiment is a proof of quantum interference.

Now, humans are too big objects to send then through a pair of slits, and the problem is that they quickly entangle with their environment and hence amongst themselves. As such, it is not easy to find two "classical states" of a human which we will analyse according to another basis.
But there is a trick! We can entangle a human with a spin measurement along one axis, and look ourselves at a spin measurement along another axis. If we start out with an entangled singlet state, then OUR "resulting state" will be a combination of classical states of that other human (the classical states of that other human are: saw up OR saw down, according to his axis).
Given the spacelike separation, we are also sure that no premature decoherence will take place.
Now, we have to make sure that *no statistical mixture of his states* is ever going to explain the entire "pattern of interference" (that is, the correlation tables with our outcomes!) observed. If that is the case, then we know that the human being underwent an interference effect, which is a pure quantum state effect. Well, Bell's theorem is exactly what is needed: Bell's theorem asserts that we cannot obtain a statistical mixture which will give us the entire "interference pattern" (the correlations).
As such, I claim that in an EPR experiment, we have in fact done an interference experiment on humans.

Ok, this has never been done, because the EPR effect is done by electronic means, and the temporal separation is too small for a genuine human to take notes of his observations, and "go and interfere with himself" when we observe him - but we take it that the extrapolation of an EPR experiment over, say, a few lightminutes with human observers would not essentially change the result.

This is about only interference experiment that is readily executable with humans with reasonable technology.

Yes.

You directly observe a chair ? Even if it is dark and so on ? Even if you don't have eyes, nerves or anything ? (because if the observation is direct, and is the proof of the ontology of the chair, it shouldn't depend upon observational conditions ! It is direct!)

Come on.

You observe the chair by means of the electromagnetic radiation -- but of course one doesn't observe *that*. One is completely unaware of it for a long time (until one takes some physics classes, or in terms of the development of history, until the 19th century or so).

Exactly. So it took quite some analysis to determine that what we subjectively experienced as a "direct observation" was a whole chain of physical processes. Illustrating that what one intuitively *thinks* is "direct observation" is a very complicated process.

Also, if you "directly observe" a chair in a very good hologram, is that chair there then really ? According to you, there REALLY ARE chairs inside a hologram.

Do you say hello to the little man in the mirror ?

No, one doesn't directly observe those. They are discovered much later by advanced scientific inference.

Exactly. Scientific discovery can show that what was long thought to be "direct observation" is in fact a totally different physical process, giving us the *illusion* of something real (and our brains are wired up to jump all these intermediate steps, and give us such an impression).

Claims which are obviously false to begin with don't require such "falsification".

Such certainties is where science ends and dogma begins.

The falsification is that the word "observation" actually means something, and we simply do not observe brain states. The vast majority of people have literally never observed a brain.

You mean that our sensations do not find their origin in brain activity ?

This is where we differ. You equate philosophy = ontology = arbitrary. But then practically everything in legitimate science is arbitrary made up bogus "philosophy", and what is *left* of science according to your conception is some ridiculous game of trying to account for "subjective experience" but without allowing yourself to believe ever that these experiences are experiences *of* anything. And that is a fundamental, fatal, philosophical flaw in your whole way of thinking about all of this. Consciousness *means* consciousness *of* *something*.

Ah ? What's that kind of an argument ? Where do you get that from ?
So you mean that the ontological question is evident ? That's a very remarkable philosophical statement ! As I said, it is ONE specific view, and it is called "naive realism". You are picking ONE SINGLE philosophical position upon the ontological question, and you claim that it is the one and only, and that everything else is *necessarily* bogus, evidently false, ...
I find that personally a very close-minded position.

To be *aware* of something, is to be aware *of something*. Consciousness without an object is a contradiction in terms. So your whole conception of science which is based on this fundamental philosophical error falls apart.

Now, THAT's an irrefutable argument !

Come on: you simply STATE your conclusion, and consider that as a proof that my position falls apart.

By contrast, my conception of science is also based on a philosophy. But mine is based on a valid philosophy, not an invalid one. (See, we disagree about whether philosophy = arbitrary, and hence about whether there can be such a distinction as good vs bad philosophy. Ironically, it's bad philosophy which makes you think there's no such difference.) Mine is based on a philosophy that is "scientific" (though that is admittedly an imprecise term here) in the sense of being based on empirical observation -- for example, the observations by which we arrive at basic concepts like "consciousness".

So in short, the argument is: I say something different than you, and because what I say is right, yours must be wrong. QED.

 

[vanesch]

Continued...

That's the problem here. It's just what I've been saying over and over again. You're convinced that (a) philosophy is just arbitrary crap and we shouldn't ever let it influence our scientific thinking, and (b) that you can prove that by appeal to certain philosophical doctrines which you regard as just obviously true beyond the shadow of any doubt.

I never say that MWI is true beyond any shadow of doubt!
I say that if what we know is quantum mechanics, then a NATURAL ontological view that goes with it is MWI, and moreover, if you take on that view, a lot of "difficulties" that one would otherways have, disappear.

As such, MWI is as good a view as any other. You want to demonstrate that MWI is necessarily a crap view. I think you cannot make such a statement.
However, you NEED to say that it is a crap view, because otherwise some concessions YOU make about science are much less necessary than they seem to be.

However, yes, ontology is essentially arbitrary. That is because solipsism (absence of ontology) is unfalsifiable. Understand this clearly: solipsism is entirely possible. You cannot have ANY scientific argument against it.
So any postulate of an ontology (which ADDS stuff to the empty-ontology solipsism view) is ARBITRARY. Indeed. But some are MORE USEFUL than others. Some hypotheses of reality are more helpful in organizing our subjective sensations. And most of our sensations are in agreement with the "naive reality" hypothesis. So that is, in daily life, a good working hypothesis. But understand this very well, because it is a philosophically well-established fact: ontology is never more than a hypothesis.

Don't you see the problem there? In (b) you confess that it is necessary to have a philosophical base for one's whole conception of the nature and goals of science, which in turn influences how one assesses specific things in science. But because of (a) you refuse to take *seriously* the task of then working out what philosophy is right, and what the right conception of science is. So you're caught in a vicious circle.

That is as erroneous an argument as the following:
"you confess that it is necessary to use a mathematical structure to build a scientific theory. But because you refuse to take seriously the task of working out (a priori) what must be the RIGHT mathematical structure, you are caught in a vicious circle"

But of course one doesn't know a priori what is the right mathematical structure ! Of course one doesn't know what is the right set of fundamental principles ! And of course one doesn't know what is the right philosophical ontology hypothesis ! One has to find out by doing observations, and to set up a whole which explains those observations ! THAT is science.
In as much as it is entirely open WHAT are the fundamental principles, and WHAT are the correct mathematical structures (build upon those principles), it is also entirely open WHAT must be the right ontology hypothesis.
Also, it is very well possible that different alternatives for each of these things are possible. In that case, it is a matter of taste and esthetical judgement to make a choice.

Again, ontology is a CHOICE. Given that it is unknowable (contrary to what you claim !), we can do with it what we want, in the same way as we can take the mathematical structure we want for a physical theory. Only, at the end of the day, the entire machinery must simply spit out the right subjective experiences (epistemology).

"There's a table in front of me." "Matter is made of atoms." "There's another planet beyond Uranus which is perturbing its orbit through gravitational forces." "Novae in the sky are caused by the core-collapse of a supermassive progenitor star whose core comes to exceed the Chandresekhar limit." "Genetic inheritance occurs through the mechanism of DNA splitting and recombination."

To you all of these are evidently "philosophical", not science.

On the contrary. But they are shortcuts for the actual, epistemological statements:

"my observations are mainly consistent with "a table in front of me" "
"my observations are mainly consistent with "matter is made of atoms" "
"..."

My observations include my "direct" sensory observations, sensory impressions of things I read/saw/... (books, TV, ...) and sensory impressions of contacts with other beings (stuff Alice said etc...).

As I said (that's why you have this strong desire to make it an "obviously true statement"), many many many of our sensations are in agreement with a "naive reality" hypothesis. But not all.

But look at what is left of science without these things! As a simple empirical statement based on looking at the history of thought and which things we use the word "science" to distinguish from which others, it is quite clear that your conception of "science" is simply *wrong*. You are fatally mis-classifying what is and what is not science.

No. Science is NOT about what things ARE. Science is about "observation". Science is the activity which helps us organize observations (relationships between observations). And observations are ultimately "subjective impressions". For a lot of science, we CAN make the hypothesis of naive realism. For daily life too. But, as I said, such an hypothesis is entirely arbitrary. For daily life, and a lot of science, the naive realism hypothesis is by far the simplest and most helpful. But not for quantum theory. There, it is better to change your hypothesis to another one.

But you see, you do. Even worse than I do. That we can't be sure whether there are really tables and cats, is a philosophical position

No, THAT is a philosophical FACT.

I don't agree. There is good and bad philosophy. I mean, even you know that there have been all sorts of conflicting philosophical positions put forward on all sorts of points. They can't all be equally right. I guess your view is that they're all equally wrong, equally empty, equally nothing, equally pointless.

There is no "correct" and "false" philosophy. There is "useful" and "less useful" philosophy. The only false philosophy is the refusal to do philosophy, that is, the refusal to re-consider certain "evident" positions, and the claim that certain answers are beyond doubt true.

Of course the Earth is flat, Zeus lives on the Olympus, the Earth is 6000 years old, and there exist chairs and tables. Of course space is 3-dimensional, Euclidean, and time is absolute. Of course.

But I completely and totally disagree. The issues (or at least some of them) are crucial, and are fundamental to science. We need answers to them. And so we need to distinguish the right answers from the wrong answers.

Yes, but guessing them a priori, based upon intuition, is maybe not the only way - or the best way - of getting the "right answers".

As to ontology: someone who starts their philosophy by saying "I just know a priori that there is a god, and that he is omnipotent, omniscient, and has really great hair to boot" is doing bad philosophy. Someone who starts their philosophy by saying "I know there are tables and chairs because I *see* them and I know I am conscious" is doing good philosophy.

If you say so

So then how the heck can you justify spending all this time arguing about it? How can you justify considering yourself a proponent of MWI? To you this is all just a pointless game, in which we know going in that there is no answer, no way to actually establish what is true, what the world is really like (even if it takes hundreds of years)? Not to me.

Well, nevertheless, THAT is a genuinly true statement. We don't know what the world is like, and we will never know it. There's nothing we know as absolutely true. We can only make working hypotheses, which have only that validity: they are working hypotheses, which help us to organize what we experience. If you would have studied the most basic elements of philosophy, you would know how much is fundamentally unknowable.

What you refuse to see is that philosophy doesn't necessarily have to be "a priori". I reject completely the idea of "a priori". I'm an empiricist. And yet I believe philosophy is important and valid. To you that's a self contradiction I guess -- because you refuse to see that all your beliefs are influenced by the philosophy *you* (unwittingly?) accept.

If you are an empiricist, then you should not have to have any ontological position at all. Empiricists work entirely within an epistemological framework.

But I completely agree with this. It's just that I think we can be 100% certain, based on what you call "epistemological knowledge" (by which I assume you just mean "empirical", based in observation), that there are tables and chairs and cats (not to mention extra-solar planets, atoms, dark matter, etc.). You disagree because you define epistemology/empiricism/experience differently, which is a *philosophical issue*. But you are blind to the fact that you take this radical philosophical position and that it actually influences everything you are saying. Everybody else has an accent.

No, you stubbornly frame yourself within naive realism, without considering any ontological positions. I didn't come to quantum mechanics with an MWI view ! Children are of course naive realists, because that is a simple hypothesis which works very well for most if not all experiences children encounter in their childhood. Hence, this is deeply rooted in our intuition. They also have a "simultaneity" intuition, and they also develop a good intuition for Euclidean space. But all this, again, because that is a hypothesis which works incredibly well for daily experiences. So, naive realism works well for most of that stuff.
But when one comes to more sophisticated stuff (in fact, ONLY in physics), then this hypothesis shows its limits. Until the 19th century, naive realism even worked well within physics. But then, we grew more sophisticated, and understood that this hypothesis is not tenable. It was the birth of modern physics. Some people couldn't get over it, and they became etherists, and Bohmians.

I agree, in so far as we are talking about questions of unobservables, things that have to be inferred from what is observed. There is no such debate over things that are directly observed. As an empiricist, I believe there is no possibly better warrant for believing something exists, than that I *see* it.

That is a very very naive position. Again: do you believe that there is a little man in the mirror which ressembles you ? Nevertheless, that is what you SEE when you look in the mirror.

Since any *other* warrant (some complex chain of scientific inference) has to be *based exclusively* on empirical/observational evidence, the end of such a chain could never be more certain than the things at its base. So if you don't think tables are real, if you don't think *seeing* them is enough evidence to "prove" that they are really there, then you are never going to be able to believe in *anything* unobservable (atoms, extra-solar planets, etc.).

Exactly. You don't *believe* in anything for sure ! That's the whole viewpoint! However, you can make working hypotheses which HELP you understand what you see. If I *see* a chair in front of me, I don't have to BELIEVE that there is a chair in front of me. However, MAKING THE HYPOTHESIS that there is a chair in front of me helps me coordinate my actions and my experiences. For daily life, it is often sufficient to make the simple hypothesis that what one sees is there. It works, most of the time (not always, as with holograms).

Indeed, you won't believe in anything at all. You'll have to end up a solipsist, trying to twist and turn to reconceive things like science in light of that radical (unscientific) philosophical position.

You are trying to label the position as "unscientific", but you do this only by stating that. You don't have a single argument. I'm not a solipsist. I consider solipsism as a good exercise in "ontology hypothesis building", but I'm in fact an "ontological agnost". I am deep down, convinced that the reality of the world is fundamentally unknowable. I think that anybody who claims anything else is a seriously deluded and naive person.

However, I do think that we have learned quite a lot ABOUT the world (without even approaching what it "really" is: maybe a big computer, the mind of a deity, a mathematical structure,... who knows ?). We have learned quite a lot about relationships between experiences, observations, all that.
And, depending on what kind of relationships we are dealing with, it is often nice to set up some (evidently erroneous) mental picture of how the world MIGHT BE LIKE. But again, I don't think we have the vaguest clue what it REALLY is like, so the picture - the ontology we set up in our mind - is evidently a kleenex picture. The thing we know most, is the formal machinery that gives us more or less correct relationships between experiences, and hence it is the best source of inspiration to set up a picture.

And this is precisely the argument against MWI that began this thread. You believe in MWI

No, I do not believe in MWI. I think that MWI is a good working hypothesis as an ontology when one does quantum theory. That's all. It MIGHT eventually be "true", but we will never know. But in this respect, it isn't any worse than any OTHER ontology hypothesis, which is just as uncertain.

What possible reason could you have to believe in this? Well, if we leave aside "a priori revelations" (which we both reject as invalid, unscientific) the answer *must* be: observation. But all of those observations that could possibly be relevant are observations of familiar material objects like tables, chairs, cats, pointers, computer screens, etc. And so if none of those things really exist as such, the whole alleged chain of reasoning that leads you from them to MWI falls apart, it fails to connect at the first link.

You are still making the same argumentation error as from the start. In order for making an ontology hypothesis (such as MWI, or solipsism, or naive realism, or whatever), OBSERVATIONS DON'T NEED TO BE ONTOLOGICALLY REAL. There is strictly no need "for a table to be ontologically real" in order for there to be a "subjective experience which is consistent with what we colloquially say 'there is a table'". I only need to have a coherent set of (subjective) observations, like visual impressions of tables, pointers and all that. From that, I AM FREE TO MAKE UP WHATEVER REALITY I LIKE, as long as, at the end of the day, this reality "generates" my coherent set of observations.

See, you are taking your OWN starting position as "evidently true" (namely, naive realism, in which observations correspond to "ontologically true things"), and then argue that ANOTHER position (in which observations and ontology are distinct), must be erroneous, given that ontology is different from observations, and (here's the error) given that you take it as evident that observations are ontologically true, observations in this other system are "false", and hence the thing is self contradictory.

Your argument has the following logic:

I believe A, and A contains a statement D

You believe B and B contains the statement ~D

(D is here: observations are ontologically real)

Now, I'm going to prove that B is wrong and that A is right.
Indeed, take B. In B you have ~D.
But given A, clearly D. Hence B is wrong.

By which you mean: "subjective inner-theater impressions." So you are a solipsist masquerading as a scientist.

First of all, a solipsist can just as well do science as a non-solipsist. There is no contradiction between being a solipsist and being a scientist. The solipsist considers science as a way to explore his subjective experiences, and makes no ontology hypothesis. The statement "really exists" has no meaning for him.

As I said, I'm not a solipsist, am an agnost. I think we cannot know what really exists. We can only make hypotheses. And, given the equivalence of (unknowable) truth value of those hypotheses, we should make those which suit us. But it shouldn't have any incidence on how we try to formalize relationships between observations - which is REAL science.
In daily life (and many activities), we can get away with "naive realism", but in certain parts of physics it is easier to make another hypothesis. It's FREE!

Huh? Are you saying "spin" can't be incorporated into Bohm's theory? If so, you display your ignorance of the theory. It's trivial. Make the wave function a spinor, and replace the guidance formula with

Spin can be incorporated in Bohmian mechanics all right, namely in the quantum mechanics part of it. But spin "is not real" in Bohmian mechanics. It doesn't correspond to "a real state" of the particle. A particle with spin, or one without spin, is the same particle in Bohmian mechanics. Only, you can ADD BY HAND some stuff in the wavefunction (which is only "half-real") and there: motion AS IF there was spin. This is why I say that you can ACCOMODATE quite a lot of stuff in Bohmian mechanics, but one first has to discover it OUTSIDE of bohmian mechanics, and then it can be imported.

Spin essentially comes about because one requires the field equation solutions to be a representation of the lorentz group. From that follows that one of the few possible field equations is the Dirac equation.
But in Bohmian mechanics, there is no a priori need to have any representation of any Lorentz group, given that relativity is dead. But nothing STOPS you from putting in something like the Dirac equation. Or something else. This is how Bohmian mechanics can accommodate for spin. Now, the Lorentz group being represented by solutions, you can then go further and consider G-bundles over spacetime. Requiring fields not only to be representations of the Lorentz group, but requiring them to be sections over a G-bundle, automatically gives you gauge invariance. It is a very similar trick as the way the Dirac equation was derived (and hence spin was derived). But in Bohmian mechanics, there is no way to require naturally any G-bundle structure, given that there is just Newtonian space and time, and hence, gauge invariance is not easily *required* in Bohmian mechanics. But of course, the specific field equations that *come out of the requirement of gauge invariance* can be put into Bohmian mechanics (after the fact), and you can *accommodate* in this way, gauge invariance.
Once it is derived in another paradigm, you can easily (or less easily) PUT IT IN BY HAND in Bohmian mechanics. But the paradigm of Bohmian mechanics wouldn't have suggested it.

 

[ttn]

I have nothing to add that I haven't already said 5 times, so I'll just leave it at that. It's clear that we have fundamental philosophical differences, and that these are the cause of our disagreements over MWI/Bohm, etc. For anyone watching this, *that* is probably the most important take-home message: don't believe it when someone tells you that philosophy doesn't matter, and that you can (and should) do physics without philosophy. It just isn't true. You can *try* to do physics without any kind of philosophical input or base, but you will *fail*, and will end up being influenced by whatever philosophy you accept by accident, by osmosis. And so you will be letting all this crap which you didn't scrutinize carefully and which you probably wouldn't accept if you did, influence the way you think about and do science. And we should all be able to agree that that is not a good strategy.

Oh, one other point about spin in Bohm's theory: your answer is funny, after all that accusing me of being a naive realist (which I'm not). You say that the way Bohm incorporates spin isn't sufficiently good, because it treats spin as "merely" a property of the wave function, not of the particles, and hence not really real -- whereas, I guess, it is just obvious to you that spin really is really real. So you're a naive realist about "spin" and that's why you think Bohm's way of accounting for the relevant *observations* isn't good enough. Sigh...

 

[Schrodinger's Dog]

I have nothing to add that I haven't already said 5 times, so I'll just leave it at that. It's clear that we have fundamental philosophical differences, and that these are the cause of our disagreements over MWI/Bohm, etc. For anyone watching this, *that* is probably the most important take-home message: don't believe it when someone tells you that philosophy doesn't matter, and that you can (and should) do physics without philosophy. It just isn't true. You can *try* to do physics without any kind of philosophical input or base, but you will *fail*, and will end up being influenced by whatever philosophy you accept by accident, by osmosis. And so you will be letting all this crap which you didn't scrutinize carefully and which you probably wouldn't accept if you did, influence the way you think about and do science. And we should all be able to agree that that is not a good strategy.

Oh, one other point about spin in Bohm's theory: your answer is funny, after all that accusing me of being a naive realist (which I'm not). You say that the way Bohm incorporates spin isn't sufficiently good, because it treats spin as "merely" a property of the wave function, not of the particles, and hence not really real -- whereas, I guess, it is just obvious to you that spin really is really real. So you're a naive realist about "spin" and that's why you think Bohm's way of accounting for the relevant *observations* isn't good enough. Sigh...

Well thanks to both you and Vanesch for a really interesting exploration of two very conflicting and different interpritations anyway .

In the UK everyone learns metaphysics as part of the Physics degree, it may not be a huge part, but from what I have seen all the good universities make physics students at least conversant about philosophy, I've dabbled myself, and I'm sure some of the philosophical tomes and discussions I have, will help me to prepare better more logically consistent material for my course.

To be honest no good student should absolutely take somebodies word for it about physics, I mean you learn it, you ponder it, you discuss the implications of it, but blindly accept? no one just robotically assumes everything there told is absolutely true and there are no holes or oddities in any interpretation,hypothesis, theory, etc. At least I hope not anyway, for the future sake of physics ask questions about everything I think is the best approach, even if you don't like the answers.

 

[setAI]

For anyone watching this, *that* is probably the most important take-home message: don't believe it when someone tells you that philosophy doesn't matter, and that you can (and should) do physics without philosophy. It just isn't true.

how ironic you would say this yet argue against the MWI- Philosophy IS important- Logic is important- that is rather the point of the MWI! any philosophy that does not contain a multiverse ontology is a BAD philosophy- bereft of fundamental logic- invoking yet ignoring demons that allow only a single causal structure- which is why the vast majority of TOEs and physical theories posit or assume a multiverse-

http://arxiv.org/abs/astro-ph/0302131

 

[vanesch]

I have nothing to add that I haven't already said 5 times, so I'll just leave it at that.

I also have that impression on my side

For anyone watching this, *that* is probably the most important take-home message: don't believe it when someone tells you that philosophy doesn't matter, and that you can (and should) do physics without philosophy. It just isn't true.

Ok, but there is a difference between what you call "philosophy" and what I call "philosophy". I also consider philosophy important, but rather as a QUESTIONING. You call philosophy the *answers* (or one single "good" answer) to that questioning.
To the question, and the ponderings "what is reality ?", "what is really there ?", "what can I know about what is there ?" - which I consider important philosophical ponderings, you retain only ONE correct answer, and call THAT philosophy.

To me, philosophy is a tool to try to liberate one from one's intuitions and certainties in order to question what has always been taken for "evident". To you, it seems to be the opposite: to carve in stone what you intuitively "know" already.

You can *try* to do physics without any kind of philosophical input or base, but you will *fail*, and will end up being influenced by whatever philosophy you accept by accident, by osmosis.
And so you will be letting all this crap which you didn't scrutinize carefully and which you probably wouldn't accept if you did, influence the way you think about and do science. And we should all be able to agree that that is not a good strategy.

I can read that phrase in exactly the opposite way as you intended it!

Again, taking a position of solipsism, naive realism, ... by itself is for me, not "doing philosophy". It is taking position on a philosophical question. What's important is the question, not the answer. What's important is the question of the relationship between ontology and epistemology.
So one should ask the question, and be aware that the answer to it is not as simple as one would intuitively think (the answer being that one cannot know, and hence must make a hypothesis. Making a specific hypothesis is then taking on a particular stance).

Oh, one other point about spin in Bohm's theory: your answer is funny, after all that accusing me of being a naive realist (which I'm not). You say that the way Bohm incorporates spin isn't sufficiently good, because it treats spin as "merely" a property of the wave function, not of the particles, and hence not really real -- whereas, I guess, it is just obvious to you that spin really is really real. So you're a naive realist about "spin" and that's why you think Bohm's way of accounting for the relevant *observations* isn't good enough. Sigh...

No, my point was: given that the ultimate motivation for even doing Bohmian mechanics is some form of naive realism, it is somehow a bit strange that the clearly observable "spin" is relegated to that uncertain realm of the wavefunction, of which Bohmians are a little bit annoyed giving it a status of reality (given that it lives over configuration space, and not over real space), and don't put it "hard-wired" into the particle. So even though spin really seems to be a "property of a particle", nevertheless it has been relegated to purely an effect of dynamics as given by the quantum potential.
As such, one can jokingly say that in Bohmian mechanics, there is some kind of MWI situation for spins The memory of hard disks is written in the wavefunction, and not in the position of the particles.

 

[ttn]

No, my point was: given that the ultimate motivation for even doing Bohmian mechanics is some form of naive realism, it is somehow a bit strange that the clearly observable "spin" is relegated to that uncertain realm of the wavefunction, of which Bohmians are a little bit annoyed giving it a status of reality (given that it lives over configuration space, and not over real space), and don't put it "hard-wired" into the particle. So even though spin really seems to be a "property of a particle", nevertheless it has been relegated to purely an effect of dynamics as given by the quantum potential.
As such, one can jokingly say that in Bohmian mechanics, there is some kind of MWI situation for spins The memory of hard disks is written in the wavefunction, and not in the position of the particles.

There are several deep confusions here, but (since I already said I wouldn't comment anymore!) I will only address one. But in a way it is the whole argument. You describe spin as "clearly observable." That's the whole thing. It is *not* observable. One does not observe the spin of particles any more than one observes their electric charge or weak charge. What one can observe directly is that certain spots on a piece of film in a certain kind of experimental situation turn black. One then *infers* (from this and a ton of other background knowledge, itself inferred ultimately, but not at all directly, from observations) the existence of spin. And it is completely reasonable that different theories would propose different models for what, exactly, spin is. There is no way of saying a priori that the Bohmian model of spin (which is entirely consistent with everything that is observed) is wrong, unless one is oneself being a naive realist about spin.

This relates also to your totally wrong accusation that I want to use philosophy to settle advanced questions in physics, like questions about the nature of "spin" or which quantum theory is true. That is not at all the case. Philosophy is a very delimited subject. It deals only with fundamentals, not that which requires any specialized scientific knowledge. It is not up to philosophy how to interpret the black spots in the Stern-Gerlach apparatus, nor is it up to philosophy to decide how to best interpret the collective evidence for (some type of) quantum theory. What *is* up to philosophy to decide is such questions as whether we can rely on our perception of pieces of film. That is a philosophic, not a scientific, question, because an answer is *completely presupposed* by science. You simply cannot do science if you regard it still as an "open question" whether or not our senses systematically deceive us. (At least, you can't do good science -- maybe you can do rationalistic a priori "science" in the style of Plato or Descartes... or some of the contemporary string theorists!) My whole point in this thread will have to be summarized by this remark: by the time we get to such advanced questions as which version of quantum theory might be true, certain earlier, fundamental, questions are already settled and are hence no longer on the table. For example, the existence of such things as tables! And the proof that this point of view is right, is the hierarchical nature of knowledge: you literally couldn't even get to the point of *asking* which version of quantum theory might be true, if you didn't already accept all of those fundamentals (and much more that is not philosophically fundamental, but is still cognitively prior to any such debate as Bohm vs MWI, e.g., that matter is made of atoms). This is what I keep pointing out, and it is the final and fatal flaw in MWI -- its advocates *do* tacitly presuppose certain things in the very formulation of the theory, indeed, in the very formulation of the questions the theory is supposed to address... but then the theory they propose explicitly contradicts those presuppositions, destroying its own foundation and leading to logical collapse. Evidently many MWI advocates are narrow minded rationalists, and hence fail to grasp that they need all these presuppositions -- i.e., they make them tacitly only, and never bother to examine their own thinking and make the assumptions explicit. But they are there nonetheless, and clear thinkers who understand and value genuine science will see them and reject MWI because of the circularity they give rise to for MWI.

 

[vanesch]

It deals only with fundamentals, not that which requires any specialized scientific knowledge. It is not up to philosophy how to interpret the black spots in the Stern-Gerlach apparatus, nor is it up to philosophy to decide how to best interpret the collective evidence for (some type of) quantum theory. What *is* up to philosophy to decide is such questions as whether we can rely on our perception of pieces of film. That is a philosophic, not a scientific, question, because an answer is *completely presupposed* by science.

This is where I don't agree. At no point, science requires any presupposition of a philosophical *position*. Every position can, at any moment, be put in doubt, even if previously we were quite convinced that it was acquired "for good". This is called "keeping an open mind".

You simply cannot do science if you regard it still as an "open question" whether or not our senses systematically deceive us.

In fact, you can. At no point you need, in science, to take any position on an ontological question. And you are still using that charged term "deceived". There is no DECEPTION in our senses. Our senses sense what they have to sense, and it is up to us to give some meaning to them. *The way we do that* is entirely up to us.
Again, in MWI, there is not more any deception in "seeing a table" than in "seeing a movie". We honestly "see a table". It is the shortcut to "there IS a table" that might need a slight modification. Slight. And in certain cases.

"We see a table" is science. There IS a table is a hypothetical model of ontology.

In science, at any moment, anything can become again an open question. It is the very definition of science: falsification. At no point, nothing is carved in stone. Not even things that were taken for granted since ages. And certainly not philosophical positions which don't have a direct relationship with science, such as ontological positions.

by the time we get to such advanced questions as which version of quantum theory might be true, certain earlier, fundamental, questions are already settled and are hence no longer on the table. For example, the existence of such things as tables! And the proof that this point of view is right, is the hierarchical nature of knowledge: you literally couldn't even get to the point of *asking* which version of quantum theory might be true, if you didn't already accept all of those fundamentals

I don't agree with that point of view. Science is not a linear accumulation of knowledge without putting in question what was previously acquired. The only thing that is needed, is then a NEW explanation of why the old "acquired" knowledge has seemed right to us.

If it was once acquired, once and for good, that the Earth was flat, and that one could not put this in doubt (even after generations and generations of people who were convinced that the Earth was evidently flat) by doing more sophisticated things (such as go sailing), then we would be wrong guided. If, after centuries of being convinced that one can say, when one "sees a table" that there "is a table", and then it turns out that this needs a modification (but in such a way, that in most of the cases, "there is a table" still gives a good account of all the observations), then so be it.

This is what I keep pointing out, and it is the final and fatal flaw in MWI -- its advocates *do* tacitly presuppose certain things in the very formulation of the theory, indeed, in the very formulation of the questions the theory is supposed to address... but then the theory they propose explicitly contradicts those presuppositions, destroying its own foundation and leading to logical collapse.

It is not because when you set up your telescope and so on, that you still assume that LOCALLY at the site, the Earth is flat, that observations with that telescope that make you conclude that the Earth is round, are fatally flawed, because you "used the contrary hypothesis in gathering the evidence for your round-earth theory".

In the same way, it is not because of the good agreement in most cases between observations "I see a table" and "there is a table" which are used for the practical setup of experiments on quantum theory, that this undermines it rethinking of an ontology.

It is your good right not to like MWI. I can understand people not liking it. But it is a flaw to think that your not liking becomes a logical flaw. The thing you point out is not a logical flaw at all. Out of MWI, one can derive all of classical mechanics just as well as in any other view on quantum theory. As such, this shows that observations in MWI are in most cases entirely compatible with the formalism of classical mechanics (in the right limits: macroscopic bodies in interaction with their environment), and as such, within MWI, one can use, in those right limits, all hypotheses (including ontology hypotheses) that went with classical mechanics. It will give the SAME OUTCOMES OF OBSERVATIONS. So MWI predicts exactly the same behaviour of a "table" as in classical mechanics. As such, all things that we derived "naively" using classical mechanics, concerning tables, ARE NOT CONTRADICTED in MWI. Tables (in the sense of: the coherent set of observations that correspond to what we usually call a "table") act in exactly the same way in MWI as in classical mechanics. All the properties of tables needed to do experiments, are also in agreement with the MWI view on "tables". As such, MWI doesn't (as you claim) render the observations done using tables invalid.

Again, what we call "table" is just a kind of name for an association of different observations (when it looks like a table, when it smells like a table, when it feels like a table, when we see it burn like a table ... , well, we CALL it a table). At no point, we need anything more to do science with tables.
MWI reproduces exactly this set of observations. Hence, in MWI, there are these coherent sets of observations which we call "tables", just as much as they are present in classical mechanics. So at no point, MWI is in contradiction with the table we needed to do our quantum experiments on.

This is analogous to the astronomer who found out that the Earth was round. It was not because, in setting up his telescope, he made locally the hypothesis of a flat Earth (and initially in his mind, the Earth was a large, flat disk), that his observations leading to his conclusion are undermined. However, he DOES need to provide for the LINK between the old paradigm (flat earth) and the new one (spherical earth): he needs to show that in the new paradigm (the spherical earth), one still HAS THE IMPRESSION of a flat earth, locally. The link is the big ratio between the human scale and the scale of the earth. In a series development around the place of observation, on the human scale, the Earth still looks quite flat.

So, observations made by using the old paradigm of a flat earth, led the man to conclude that his old paradigm needed a review. However, he understands that within the new paradigm, most of the old conclusions drawn from the old paradigm are still valid, except for a huge "difference in principle". He now also understands that most of his old observations (which made people conclude that the Earth was flat initially) are entirely compatible with TWO DIFFERENT "ontologies" (flat Earth and round earth). As such, the chosen ontology has, for most (human) purposes, not much influence. Actually, for most human activities on small scale, it is STILL A MORE USEFUL hypothesis to take on the flat Earth hypothesis. But for SOME observations (such as go sailing), one is better with the new paradigm.

It is exactly the same for MWI. There is no LOGICAL flaw.

 

[ueit]

My main issue with MWI is the fact that it doesn't give an answer to perfectly valid scientific questions like "why I measured electron’s spin as being up?".
The theory only says that there is another copy of me that measured the spin "down" but this is not an answer, not for me, neither for my copy.
If we accept this kind of "explanation" then we can safely say that we know everything.

Why the gravitational constant has the value it has?
Nothing easier. There are many worlds, each with a different g. We happen to live in this one, by chance.

I don't think that MWI is contradictory. It may be even true. But I don't see anything useful coming out of it.

The argument that we are forced to accept MWI because of EPR-Bell and relativity is wrong. As Vanesch pointed out in another thread, superdeterminism (which is nothing but the plain old determinism) is a valid approach. As a deterministic theory, Bohm's interpretation may therefore allow a local reformulation.

 

[vanesch]

My main issue with MWI is the fact that it doesn't give an answer to perfectly valid scientific questions like "why I measured electron’s spin as being up?".

In as much as this *sounds* like a scientific question, it isn't actually one. The scientific question is always "will I observe that...".

The theory only says that there is another copy of me that measured the spin "down" but this is not an answer, not for me, neither for my copy.
If we accept this kind of "explanation" then we can safely say that we know everything.

Why are you the person you are, and aren't you George Bush ?

Why the gravitational constant has the value it has?
Nothing easier. There are many worlds, each with a different g. We happen to live in this one, by chance.

This is in fact not MWI, but rather the Landscape in string theory.

I don't think that MWI is contradictory. It may be even true. But I don't see anything useful coming out of it.

The only thing "useful" that comes out of it, is to have an ontology which fits perfectly with the formalism of quantum theory as we know it, without any desire to fiddle with it. It is a kind of tranquilizer which helps you come to peace with the quantum formalism - and to help you develop some intuition for it. That's what "interpretations" are for. Peace of mind.

The argument that we are forced to accept MWI because of EPR-Bell and relativity is wrong. As Vanesch pointed out in another thread, superdeterminism (which is nothing but the plain old determinism) is a valid approach. As a deterministic theory, Bohm's interpretation may therefore allow a local reformulation.

We are absolutely NOT forced to accept MWI because of EPR-Bell! We are not forced AT ALL to accept any interpretation (and certainly not MWI). Only, EPR-Bell is there in the first place because of the quantum formalism. So why look for *another* explanation if we have a machinery already on paper which has made us put up with the situation in the first place! The reason I mention this is different. If all this discussion were not there, and if, from the start, one was to have an MWI-kind of view on quantum theory, then one could easily see an EPR-experiment as a kind of "confirmation" of quantum interference with macroscopic bodies: you entangle two macroscopic systems with different spin decompositions of a pair of entangled particles, and you do this at spacelike separation in order to be sure that they don't decohere immediately with one another. Then you let them "interfere" (calculate the correlations), and you show that this is statistically not possible without quantum effects.
That's typically the kind of experiment you do to make quantum effects manifest (which means: to demonstrate "superposition"). With small objects, you do this with a kind of double-slit experiment, but you can do other kinds of "interferrometry", like this one.
Of course it doesn't *prove* MWI (you can't prove any interpretational scheme). It would just be a kind of indication that "superposition of macroscopic systems seems to work too".

If, in Newtonian mechanics, you calculate the force on a planet, and find, from those calculations, that the orbit is then going to be an ellipse, then why would you want to look for *another* mechanism which could produce elliptic orbits ? The very formalism that gave you the orbit (the Newtonian force of gravity) can also serve as the explanation or the mechanism. So you imagine with your mind's eye that some "force" is pulling on a planet, and that as such, it follows the orbit it should. If you do Newtonian mechanics, it wouldn't come to your mind to think of that as a kind of "bending of spacetime" or "crystal ellipsoids on which the planets roll" or whatever: if you do Newtonian mechanics, you take "as real" the elements of the theory. You don't lie awake at night of what "mechanism" might be responsible for elliptical orbits which come out of the mathematical formalism.
Well, to me, MWI is just a similar kind of reasoning held wrt the unitary quantum formalism. That's all. It's just a picture to keep in mind, when working with a formalism. A kind of "image of a reality" that will do the trick. But the nice thing is that because it sticks closely to the formalism, that it helps you reason intuitively in quantum theory.

MWI will learn you nothing more than what is already in the quantum formalism. To me, it is the kind of "minimalistic" ontological picture that one can have. It simply gives you the "ease of mind" not to have the desire to change the formalism. In exactly the same way as the usual view of Newtonian mechanics doesn't give you any desire to change it.

 

[tehno]

http://video.google.com/videoplay?docid=-45154219728824809&sourceid=searchfeed%20

Lisa Randall talks about her book .She even answers some of really profund questions touching differences between physics and philosophy that you are discussing.Watch ,have fun and learn.