ttn said:
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.
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