Sherlock said:
what is it that's actually seen in the macroscopic world that verifies the existence of so-called macroscopic quantum superpositions?
No, I think you have it backwards about the reasons to consider macroscopic quantum superpositions. Macroscopic quantum superpositions are simply part of the mathematical formalism of quantum theory. Maybe they don't exist, which then only means that the postulates of quantum theory are of limited use. As I tried to point out, macroscopic quantum superpositions appear naturally in the formalism of quantum theory. You have to do something to the formalism of quantum theory if you DON'T want to obtain them, and that something usually is quite "ugly", like introducing non-locality, and an unphysical interaction based upon a non-well defined concept which is "measurement".
Macroscopic superpositions are unavoidable (in the formalism) if you take the axioms of quantum theory seriously to be valid universally. There's really nothing more to it !
It is when you take your (quite understandable) "naah ! Too crazy to be true" viewpoint that you must conclude that macroscopic superpositions are, after all, not possible, and AS A CONSEQUENCE, that the postulates of quantum theory are not universally applicable. THIS IS VERY WELL POSSIBLE, I'm not denying that. But then we're left with a riddle: if the axioms of quantum theory are NOT universally valid, then what is ? Each time we do that (by introducing some "objective" mechanism of collapse, or by introducing other things, like in Bohm's theory) we seem to have a serious clash with locality, an other cherished principle.
This is why I take the viewpoint: can we not "save the day" for the axioms of quantum theory, and really take them as universally valid ? And then, what should we require ? We know that the consequence of taking the axioms of quantum theory as universally valid, is the existence of macroscopic superpositions *as theoretical constructions*. This is unavoidable in the formalism. And this clashes with your "naah, too crazy to be true". But cannot this macroscopic superposition NEVERTHELESS EXPLAIN OUR SUBJECTIVE EXPERIENCES ? The point is that if there's a way to view these superpositions as nevertheless explaining our subjective experiences, at the end of the day that's good enough for a physical theory. If that theory describes an objective world state, from which one can *derive* subjective experiences that *correspond to what we actually experience*, then this theory is good enough as a physical theory. Of course, a theory in which the subjective experiences correspond directly to the objective world state (as in classical physics) is probably intuitively more acceptable. But imagine the following situation: you're in the lab of a brain surgeon, which implants a few electrodes in your brain, and by sending certain signals in the electrodes, he can make you see pink elephants. What is now the most appropriate theory ? That there *are* pink elephants, or that the true state of the world is a manipulated brain from which we can derive that you should *experience the presence* of pink elephants ? In this case, it is probably evident that the last theory is the most useful. Which is a naive attempt, on my part, of trying to make you see that a theory that can explain all your subjective experiences is all you can require, finally, of a physical theory.
And that is what MWI does. It saves the universality of the QM postulates, it needs to postulate a non-trivial relationship between the world state and your subjective experience, but if you do so, it *explains* perfectly well those subjective experiences, and why you *have the impression* that the world is classical, and why you do not manifestly subjectively observe macroscopic objects in superposition.
The gain is the following: there is no ambiguity as to the difference between a physical phenomenon and a "measurement" (as is necessary in all collapse views) ; there is no need for non-locality and no clash with SR, and we can take the quantum state (the vector in hilbert space) as the true, objective description of the state of the world.
I don't think it's a waste of time to think about it, but the idea that there is an objective world that is revealed to us via (as opposed to being created by) our senses, and that it exists whether we happen to be sensing it or not, is the only way of thinking about it that makes any ... sense. It's the basis of physical science.
Yes, I agree with that. It is because I agree with that, that I think that one should take the "state description" of quantum theory seriously. And if you take the wavefunction as something "really out there", it cannot "disappear" at a certain macroscopic scale. Hence the necessity of the reality of macroscopic superpositions. Unless quantum theory is really fundamentally misguided (as I said, that's an option, but, in the current state of affairs, not a very helpful one).
You can take the QM formalism literally as being in 1-1 correspondence with an underlying quantum world, but how would you know?
Because it is the only coherent formalism that we have ! As I said, it is the starting point for an MWI view. I think that if you want to interpret a formalism of a physical theory, the least you can do is to take the formalism seriously.
So, for good reason, what is taught is that, as far as anybody knows, the QM formalism is not in 1-1 correspondence with an underlying quantum world.
You can take the QM formalism literally as being in 1-1 correspondence with the macroscopic world, but the difference here is that we actually experience the macroscopic world. So, for good reason, what is taught is that when the formalism says that a pointer is in a superposition of pointing at 1 and pointing at 2 for some specific preparation, then we interpret this to mean that it will either be pointing at 1 or pointing at 2 at the end of each trial, and that the probability assigned to each unique result refers to the mean average of a certain number of trials. The various pointer positions are the possible, mutually exclusive, results of individual measurements ... that's what the formalism means -- not that these things are happening simultaneously in each trial.
I'm sorry but that is NOT what the formalism says. It is what a certain interpretation of the formalism says (the Bohr view), and as such it denies any descriptive value to quantum theory. This comes about because people said "naah, too crazy!".
So what's best ? A theory that DOES give you a precise description of what physically happens, and how what physically happens relates to what you subjectively experience as a consequence of it (MWI), or a theory that DOESN'T SAY ANYTHING OF WHAT'S HAPPENING, but allows you to calculate some probabilities of outcomes without explaining any underlying mechanism ?
Anyway, the superposition principle is employed because quantum theory is essentially a theory of wave mechanics. Waves of what ... where? The frequency distributions produced by the detectors. But what relationship do these waves have with the underlying quantum reality? Nobody knows exactly. But waves in any medium are still waves ... frequency distributions ... and, apparently, whatever is happening in the medium or media of the underlying quantum world is somewhat similar to the data waves produced by the detectors. That is, it seems that a wave is a wave is a wave --- no matter what medium it happens to be propagating in.
This is not true. Quantum theory started off as a kind of wave mechanics, but we're now far beyond that POV. The so-called waves are now to be seen as superpositions of position states: as a point being at different places in the same time. Spin superpositions cannot be seen as "waves".
Well it isn't actually the brainstate that we're aware of, is it ? We're aware that eg. an instrument pointer is pointing in some specific direction, not all possible directions, at a certain time.
Just as you are aware of the pink elephants when you were in the brain churgeon's room...
The EXTRA RULE is the interpretational one that MWI adds which says that the expansion refers to simultaneously existing, mutually exclusive macroscopic states. Since this "clearly isn't the case" we don't take QM literally as a hi-fidelity representation of physical reality, and thus dismiss the MWI approach.
Many people do this, but it is clearly based only upon "gut feeling". I would agree with you that a theory that *introduces* parallel macroscopic worlds just for the sake of it, would do an overkill. However, when the mathematical formalism shows them, then the overkill, to me, is to artificially remove them in certain, very unwell defined cases, and as such violate basic principles of the axiomatic structure (such as locality, unitarity, the superposition principle).
It's what is seen that is the primary evaluational basis, not what is metaphysically postulated or mathematically formulated.
Oops, then the world is still a flat disk ! I think that if you want to interpret physically a mathematical formalism, you should give priority to the mathematical structure. If it allows you to deduce what you *should see* (although that is not what "is out there"), then that's good enough for me.
My subjective experiences, and afaik everybody else's (unless they're all lying), ARE only related to ONE TERM in the above expansion for each trial.
Eh, yes, that's exactly what is postulated that you SHOULD see.
I think that physical science takes the "true state of the world" to be what is seen, and what is seen is that either there is a red ball left on table or there is a green ball right on table at a certain time.
It is difficult to let that notion go, I know. The pink elephant is really there. In fact, the notion of "what is seen is what is there" is indeed the pillar of classical physics. But I'm trying to point out that this is maybe a too strong requirement for a physical theory: if the theory tells you what you are supposed to see, and that's what you see, is that not good enough ? If the price of clinging to the requirement that what you see is the true state of the world leads to a lot of FORMAL difficulties and apparent paradoxes, isn't it perferable to go for the lesser requirement of only having the formalism to explain what you ought to see, even if there is something else is out there ?
I'm looking at my keyboard now. Pondering which key to hit. Is my brainstate in a superposition of all the possible continuations -- and does that mean that when I hit a key then I've actually hit all of the keys but because of the branching I'm only subjectively aware of hitting one key ?? ... naaahh
Apart from the intuitive strangeness, what's wrong with it ? If the same crazy theory also allows you to find out that everything will now appear to you *as if* you only hit one key ?
I don't know that that's a good way to approach understanding the adoption and retention of the projection postulate. It came, afaik, from considerations of how formulas and models from classical wave optics might be applied in the quantum theory, and it's retained because it works.
No, that's not the point. The EPR experiments relate only to optical experiments because of experimental possibilities. But the EPR situation can be set up for just any quantum system: atoms, electrons... take your pick.
The probabilities calculated using QM are only physically meaningful wrt to statistical ensembles.
This is a positivist viewpoint to which I don't subscribe, because it *completely gives up on any attempt to describe physical reality at all*. In fact, such a viewpoint is even more "metaphysical" than the MWI viewpoint, because at least, in the MWI viewpoint, there IS a description of physical reality (the wavefunction), even though it doesn't correspond 1-1 to your subjective experience and an extra rule is required to deduce the subjective experience. But in the positivist viewpoint, there IS NO REALITY AT ALL apart from your subjective experience!
The probabilistic interpretation of QM, which is the standard one because it's the one that makes the most sense, doesn't have any problem with "spooky action at a distance" --- which, iirc, you have written a nice exposition of.
It cannot have any problem with spooky action at a distance because there IS no description of physical reality in that viewpoint! The only thing that "exists" are your observations. So I find it strange that one can adhere to a viewpoint that there IS NO REALITY AT ALL, and make objections to a theory that gives you a certain description of reality, and a deduction of what are your subjective experiences from it.
Unless, of course, you take the viewpoint that there IS a reality, but that QM is simply not describing it correctly. As I said, that's a possibility. But is it fruitful to adhere to such a viewpoint if we don't have a correct description that can replace it ?
And, I'm still wondering what it is that is actually seen superposed in the experiments that purport to produce macroscopic quantum superpositions.
If you take the viewpoint that quantum theory does not describe reality (but only in some way is capable of producing right statistical descriptions), then *nothing* corresponds to "superposition". It is just part of the calculational algorithm of probabilities of your observations. This is the problem (in my opinion) of the positivist viewpoint.
Nothing in the formalism corresponds to anything "out there", it is just an algorithm. It is very difficult (in my view) to devellop a "physical intuition" for such a formalism - while the MWI viewpoint allows you exactly that: to devellop a physical intuition of the formalism (a strange intuition all right, but the funny thing is that you get used to it!). To me, the positivist viewpoint reduces quantum theory to a kind of black box that spits out numbers that are probabilities. As such, it is a "non-interpretation" of the theory. It doesn't interpret the elements of the formalism as physical things.
It seems to me that there is a much less problematic way to interpret quantum theory, and if adopting the MWI approach is, ultimately, really just a matter of taste ...
Up to a point, of course it is a matter of taste and intellectual entertainment. But I think one has to make the choice between the following fundamental viewpoints:
A) there exists an objective reality that can be described by a physical theory
B) there doesn't exist such an objective reality.
Ok, there is a bizarre possibility,
C) there exists an objective reality, but it is not compatible with any mathematical theory (most religions somehow fit into this class)
Now, if we split this up further:
A)
1) quantum theory describes this objective reality
2) quantum theory does not describe this objective reality
B)
Quantum theory (as any theory) does not describe the non-existing objective reality
C)
As objective reality is not describable by a mathematical theory, the quantum formalism will not describe it either.
Your point of view is compatible with A2) and with B) and with C).
MWI is (I think) the only viewpoint that is compatible with A1). I'm open to A2), but I don't know of any formulation that doesn't have big problems in one or another way. I'm not open to B: I think that before giving up all together on an objective reality, we should think again. I don't even understand very well C because it means that we are living in an a-logical universe, maybe dictated by the will of the gods or whatever.
I'm only pointing out that A1) is perfectly possible - something which seems to be dismissed out of hand because of the outlandishness of the concept (and not because of the contradiction with observation). Given that it is *possible* to assume that quantum theory describes physical reality, I'd argue that the best interpretation of the formalism is exactly that. If it leaves you with the gut feeling "naaah!", then I find that not a sufficient argument.
People have tried hard to think of A2, but I don't find any approach as yet promising ; the local realists deny EPR situations, and Bell's theorem dictates that if you accept the quantum predictions also in EPR situations, that you will have big difficulties with locality and the principle of relativity.
Of course, from the day that a nice theory explains us why quantum theory seemed to work the way we thought, we'll know more, and we could then possibly dismiss this "subjective experience' viewpoint. But I'm pointing out that we don't HAVE such a formalism yet, so it is a bit difficult to take it as an interpretation of a theory we DO HAVE. Because one should be open to the possibility that the formalism we have is ultimately right, too and that this desired-for theory does not exist.
Nevertheless, I stick with my claim that the MWI view gives you the best possible *intuition* about the formalism (whether it ultimately will prove right or wrong doesn't matter). I tried several times to show how naturally one can interpret "weird experimental results" using this view. That by itself, I find already sufficient reason to consider it.
It is a bit as if discussions about free will would interfere with the (evident) interpretation of classical deterministic physics. I think that assuming determinism is part of the interpretational scheme of classical physics, and pointing out the "evident" problem of free will with such a scheme doesn't really help you devellop a feeling for the formalism of classical physics. In the same way, I view the MWI viewpoint as the most natural one sticking to the formalism of quantum theory ; and pointing out evident problems of "naah! Too crazy" doesn't help you in anything understanding the formalism and getting a feeling for it.
we have to ressort to intuition and guesswork.
. It is maybe the most accurate interpretation of QM 
), let us first ask what it means, in a classical context, when "something is seen with the naked eye". Clearly, at the end of the day, what counts is a certain state of the particles and fields in your brain. So what corresponds to the (subjective) notion of "I see a red ball on the left side of the table", is a certain physical configuration of your brain. Which can, physiologically, be traced back to certain nerves in the visual nerve firing, which can be traced back to a certain image impinging on your retina, which can be traced back to EM radiation being emitted from a thermal light bulb, scattering off the red ball. But at the end of the day, it is the state of your brain which makes you have your subjective experience "I see a red ball on the left side of the table". We're so much used to doing this tracing back, that we do not even realize it, and associate immediately to the subjective experience "I see a red ball on the left side of the table" that there *IS* a red ball on the left side of the table. This is what one could call the very reasonable hypothesis of an objective world which corresponds to our subjective experiences. But it is good to keep in mind that this is nothing else but a good working hypothesis, stimulated by that tracing back as described above, and which works in a lot of everyday situations. And in classical physics, indeed, the relationship is so evidently 1-1, that it is - by a lot of physicists - considered a total waste of time to even think about it.
