Interpretations of Quantum Mechanics

[Fredrik]

hello all (first post here!)

Hello, and welcome to Physics Forums.

I have an issue with the MWI that I didn't see while skiming through this thread, and that regards energy conservation. It seems to me very strange that the outcome of any quantum experiment would create a new world, simply because of the fact that this new world and all the people that live their lives knowing a different result of the experiment than we, has to be sustained by some energy.

If new worlds keeps being created, and we assume that energy conservation is valid with respect to the whole universe, then where does the energy to sustain the new worlds come from? Although I haven't seen this answered anywhere, it seems to obvious a question to have been ignored, so I've probably just missed it. But still, I would appreciate to get an answer on it.

There's nothing in "the MWI" (which I assume means Everett's MWI) that says that measurements lead to the creation of new universes. I can't tell you exactly what it says, and I don't think anyone can, because no one seems to have been able to write down a consistent set of axioms that defines the MWI. I would say that there is no MWI. (Comments to anyone who's offended by that: 1. Feel free to prove me wrong. 2. Don't even think about saying that all you have to do is to remove the Born rule from the standard formulation).

The best I can do is tell you that any MWI proponent would say that there's a Hilbert space with the property that any point in it represents a possible state of all the universes. Penrose calls this physical system "the omnium". The Schrödinger equation describes the time evolution of the omnium in the following sense. Given a state of the omnium (a point in the Hilbert space), the Schrödinger equation defines a unique continuous curve through that point. That curve tells you the state of the omnium at any time.

What I just described is often called the "bird's view" of time evolution. That part of the MWI is well-defined. The ill-defined part has to do with the "frog's view", i.e. a human observer's description of the time evolution of his "world". The idea is that in some subspaces* of the Hilbert space of the omnium, a short segment of the time evolution curve can be interpreted as describing a universe where an experiment is performed. Those subspaces have subspaces of their own, and some of them can be interpreted as describing a universe where the result of the experiment is A, and others can be interpreted as describing a universe where the result of the experiment is B. (If someone feels that I got this part wrong, let me know).

So there's no actual "creation" of different worlds. All that happens is that the time evolution in the bird's view is constantly changing which of the subspaces should be interpreted as different frog's views.

*) If you're unfamiliar with Hilbert spaces and other vector spaces, just think about a cartesian coordinate system for $\mathbb R^3$. Call the axes x,y and z. The x-y plane is a good example of a "subspace", and so is the y-z plane, the x-z plane, and the plane defined by the equation x+y+z=1. Those are all 2-dimensional subspaces. The x-axis and the line that consists of multiples of the vector (1,1,1) are examples of 1-dimensional subspaces. The subspaces I'm talking about are subspaces in the same sense, but they are infinite-dimensional subspaces of infinite-dimensional vector spaces.

 

[WaveJumper]

I would argue for "shut up and calculate!"

I wonder, if for at least part of its proponents, this strictly calculatory mathematical approach is implying that it's all there is and Hilbert space is reality, and as such is incomprehensible?

 

[kote]

I wonder, if for at least part of its proponents, this strictly calculatory mathematical approach is implying that it's all there is and Hilbert space is reality, and as such is incomprehensible?

"Shut up and calculate" is typically a statement of basic instrumentalism or logical positivism.

From Wikipedia: "A 1929 pamphlet written by Neurath, Hahn, and Rudolf Carnap summarized the doctrines of the Vienna Circle at that time. These included: the opposition to all metaphysics, especially ontology and synthetic a priori propositions; the rejection of metaphysics not as wrong but as having no meaning; a criterion of meaning based on Ludwig Wittgenstein's early work; the idea that all knowledge should be codifiable in a single standard language of science; and above all the project of "rational reconstruction", in which ordinary-language concepts were gradually to be replaced by more precise equivalents in that standard language."

Asking what your model means or what it represents is simply a misuse of language, according to this view. There is no meaning to the question "what is real" in ordinary-language. The Copenhagen Interpretation is simply the next step in the positivist program. It redefines reality, replacing the ordinary-language requirement that reality be separably objective with a "meaningful" definition of reality based only on experiential empirical results.

This, by the way, is the basis of the entire interpretational debate. Whether "reality" should be defined empirically or if it should be presumed to have a separable objective ontology. Bohr and Bohm both agree that all physics is necessarily instrumentalist with respect to "the real essence of phenomena" or "deeper reality."

 

[RUTA]

"Shut up and calculate" is typically a statement of basic instrumentalism or logical positivism.

Asking what your model means or what it represents is simply a misuse of language, according to this view. There is no meaning to the question "what is real" in ordinary-language. The Copenhagen Interpretation is simply the next step in the positivist program. It redefines reality, replacing the ordinary-language requirement that reality be separably objective with a "meaningful" definition of reality based only on experiential empirical results.

This, by the way, is the basis of the entire interpretational debate. Whether "reality" should be defined empirically or if it should be presumed to have a separable objective ontology. Bohr and Bohm both agree that all physics is necessarily instrumentalist with respect to "the real essence of phenomena" or "deeper reality."

These questions are not without relevance to progress in physics. Consider the method by which data is collected and analyzed in particle physics, for example.

Individual detector clicks (called “hits in the tracking chamber”) are first localized spatially (called “preprocessing”), then associated with a particular track (called “pattern recognition”). The tracks must then be parameterized to obtain dynamical characteristics (called “geometrical fitting”). See Fernow, R.C.: Introduction to experimental particle physics. Cambridge University Press, Cambridge (1986), sections 1.7.1, 1.7.2 & 1.7.3, respectively. Some assumptions are required, e.g., “Sometimes it is necessary to know the identity (i.e., the mass) of at least some of the particles resulting from an interaction (Fernow, 1986, p 17)” and “Within the errors tracks may appear to come from more than one vertex. Thus, the physics questions under study may influence how the tracks are assigned to vertices (Fernow, 1986, p 25),” but no one disputes the inference that "click-causing" particles are responsible for the trajectories in tracking chambers. This leads particle physicists to treat sets of detector clicks as the fundamental constituents in their experiments. They then function under this assumption for pattern recognition, throwing away "spurious noise hits" and stressing over whether hits really constitute tracks (Fernow, p 22).

What would happen to this enterprise if we rather believed there were no click-causing particles? I think it's safe to say particle physics would not occupy such a central position in unification. I'm not here to argue about the existence of particles, I'm simply trying to illustrate that these questions are answered explicitly or tacitly in order to collect "objective data." Thus, the answers to such questions bear directly on progress in physics.

 

[nikman]

I just looked at these papers... and I'm sorry, but they are just completely wrong. Analytic propositions can't be proven a posteriori. Experiments can't serve as mathematical proofs.

Or maybe I missed the point of the papers.

Based on my understanding of the master project -- and we're talking not only Zeilinger and Brukner but also Karl Svozil, Greg Chaitin and Chris Calude -- the missing link here is Chaitin's Omega, which is the mathematical equivalent of quantum stochasticity. (It's actually been computed to 64 digits ... probably the limit for a long time to come.)

At any rate, mathematics is just as physical as is computation. Calculation requires physical media (brain cells, pencil and paper in their various analogs including abaci). Also maths is an extrapolation from adding and subtracting natural numbers. Try this experiment: here, I'm giving you nine marbles. Give me back seven and tell me how many you have left. Two? ... that's correct.

I claim that was both a physical experiment and a mathematical proof.

Anyway:

http://arxiv.org/PS_cache/quant-ph/pdf/0402/0402197v6.pdf

http://arxiv.org/PS_cache/quant-ph/pdf/0611/0611029v2.pdf

 

[kote]

What would happen to this enterprise if we rather believed there were no click-causing particles? I think it's safe to say particle physics would not occupy such a central position in unification. I'm not here to argue about the existence of particles, I'm simply trying to illustrate that these questions are answered explicitly or tacitly in order to collect "objective data." Thus, the answers to such questions bear directly on progress in physics.

RUTA, I agree that a presumption of incompleteness is very practical. The idea that there could be a unified theory that completes or replaces QM is necessary for the hope of any progress. At the same time, however, presuming incompleteness in the current theory is admitting that its elements are not basic in nature. At best they are macroscopic approximations of basic entities, and at worst they are inconsistent with them.

Practically it's probably best to hold inconsistent views about the reality of particles .

 

[Albert V]

I think the success of quantum mechanics is remarkable. The fact that calculations often precedes observations suggests that the mathematical model is closer to reality than our mental concepts of a particle.

 

[nikman]

I think the success of quantum mechanics is remarkable. The fact that calculations often precedes observations suggests that the mathematical model is closer to reality than our mental concepts of a particle.

Eugene Wigner maintained that the success of science is remarkable, period. Or maybe it's the success of mathematics. Or both. This from "The Unreasonable Effectiveness of Mathematics in the Natural Sciences":

THERE IS A story about two friends, who were classmates in high school, talking about their jobs. One of them became a statistician and was working on population trends. He showed a reprint to his former classmate. The reprint started, as usual, with the Gaussian distribution and the statistician explained to his former classmate the meaning of the symbols for the actual population, for the average population, and so on. His classmate was a bit incredulous and was not quite sure whether the statistician was pulling his leg. "How can you know that?" was his query. "And what is this symbol here?" "Oh," said the statistician, "this is pi." "What is that?" "The ratio of the circumference of the circle to its diameter." "Well, now you are pushing your joke too far," said the classmate, "surely the population has nothing to do with the circumference of the circle."

Naturally, we are inclined to smile about the simplicity of the classmate's approach. Nevertheless, when I heard this story, I had to admit to an eerie feeling because, surely, the reaction of the classmate betrayed only plain common sense. I was even more confused when, not many days later, someone came to me and expressed his bewilderment with the fact that we make a rather narrow selection when choosing the data on which we test our theories. "How do we know that, if we made a theory which focuses its attention on phenomena we disregard and disregards some of the phenomena now commanding our attention, that we could not build another theory which has little in common with the present one but which, nevertheless, explains just as many phenomena as the present theory?" It has to be admitted that we have no definite evidence that there is no such theory.

 

[Fredrik]

I wonder, if for at least part of its proponents, this strictly calculatory mathematical approach is implying that it's all there is and Hilbert space is reality, and as such is incomprehensible?

Aren't these two (red and blue) opposite viewpoints? The blue stuff is essentially Everett's MWI, and the red stuff can be described as "shut up and calculate", but is actually better described by my post #11.

 

[f95toli]

I wonder, if for at least part of its proponents, this strictly calculatory mathematical approach is implying that it's all there is and Hilbert space is reality, and as such is incomprehensible?

No, the whole point for most of us that prefer that approach is that it in QM (and physics/science in general) is meaningless to ask if there is anything. Period.
I.e. most of us are -strictly speaking- agnostic about whether or not there even is a reality, or indeed if if Hilbert space or any other part of QM represents something that is real (whatever that means).

I think it is wrong to say that there is any "deep" philosophical ideas behind this; it is merely a result of the opinion that the only requirements on a physical theory is that it should be able to predict the outcome of experiments; whether or not that theory has anything to do with a "reality" is irrelevant.
This is why we (or at least I) think that the debatte over various intepretations etc is more or less irrelevant, unless of course someone can come up with an experiment that can test which interpretation is correct.

 

[nikman]

Aren't these two (red and blue) opposite viewpoints? The blue stuff is essentially Everett's MWI, and the red stuff can be described as "shut up and calculate", but is actually better described by my post #11.

"Shut up and calculate" works, just as "Light the fuse and run" works.

But isn't it reasonable for people to wonder how it works and why it works the way it seems to work?

 

[RUTA]

RUTA, I agree that a presumption of incompleteness is very practical. The idea that there could be a unified theory that completes or replaces QM is necessary for the hope of any progress. At the same time, however, presuming incompleteness in the current theory is admitting that its elements are not basic in nature. At best they are macroscopic approximations of basic entities, and at worst they are inconsistent with them.

Practically it's probably best to hold inconsistent views about the reality of particles .

I agree that you should use whatever model of the theory helps you do calculations. My point concerns the experimental part of physics. Ontological assumptions determine how we look for and interpret data. Thus, changes in our ontological assumptions result in changes to our experiments. That's all I was trying to establish.

 

[nikman]

From an interview with Anton Zeilinger:

... Ultimately that implies something monstrous: namely that the particle had absolutely no characteristics before it was measured. The great Danish physicist Niels Bohr once said: no one has ever seen a chair. There is no objective reality. Only that which is measured exists. We construct reality, and only in the moment of measurement or observation.

I think you have to make a distinction: in my view there is something that exists independently of us – in physics we call that the singular event. For example the activity of a particle detector. Or the activity of a certain cell in my eye, which registers a certain number of light particles and then provokes a chemical reaction that is then registered in the brain. The images that we form on the basis of this are our constructs. Bohr's chair or on a much more abstract level, the quantum mechanics equation of states, are our concepts of an object. Of course they are very purpose-oriented, because they've been corroborated with repeated use.

So there is in fact something that exists independently of us. And the moon is also there when I'm not looking at it.

Something exists, but it is not directly accessible to us. Only indirectly. And whether this thing must really be called the "moon" is another question. That is also a construct.

But there is something up there...

... the word "there" is yet another construct. Space and time are concepts aimed at giving meaning to our world of appearances. So they are entirely reasonable constructs. By no means do I want to give the impression that I believe everything is just our imagination.

The world as a huge theatre that only plays in our heads.

That is certainly not my view of things.

http://www.signandsight.com/features/614.html

 

[Fredrik]

"Shut up and calculate" works, just as "Light the fuse and run" works.

But isn't it reasonable for people to wonder how it works and why it works the way it seems to work?

Sure it is. But it isn't reasonable to assume that QM "describes what actually happens", just because it makes accurate predictions about probabilities of possible results of experiments. That last part is just the minimium requirement for falsifiability, and it doesn't in any way imply that the mathematical model used by a falsifiable theory describes reality.

Without the assumption I mentioned, there's no reason to think that QM (a standard "Copenhagenish" formulation) needs an interpretation.

 

[Hurkyl]

Without the assumption I mentioned, there's no reason to think that QM (a standard "Copenhagenish" formulation) needs an interpretation.

Every physical theory needs an interpretation. Without an interpretation to connect them to experiment, your just pushing meaningless symbols around on paper.

 

[Fredrik]

Every physical theory needs an interpretation. Without an interpretation to connect them to experiment, your just pushing meaningless symbols around on paper.

I wouldn't use the word "theory" about something that doesn't already include such an interpretation. To me this is the difference between a theory and a model. I would say that a model is just a mathematical structure, and that a theory consists of a model and a set of axioms that tells us how to interpret the mathematics as predictions about the results of experiments.

The standard ("Copenhagenish") formulation of QM is a theory, not a model. What I'm trying to say is that its underlying mathematical model (Hilbert space) doesn't need to be reinterpreted.

This touches on something else that's been on my mind for a long time. A valid alternative interpretation technically defines a different theory. I would say that it's equivalent to the original theory if it makes the same predictions. Now, a theory can always be stated as a list of axioms, but for some reason no one seems to think that an "interpretation of QM" needs to be stated that way. Everyone seems to tolerate a few poorly worded comments as a definition of an "interpretation", probably because "it's just an interpretation". I think this is just ridiculous. We shouldn't be talking about "interpretations" at all. We should be talking about different theories that may or may not be equivalent.

One last comment about how I use the word "theory" here. I would be perfectly OK with using the word "theory" for an equivalence class of what I've been calling "theories" so far. If we use that terminology, I would say that the members of an equivalence class (i.e. a "theory") are different formulations of the theory. So instead of the last sentence of the preceding paragraph, I could have said that we should be talking about different formulations of QM and about formulations of slightly different theories.

I hope I have expressed myself clearly this time.

 

[Dmitry67]

Every physical theory needs an interpretation. Without an interpretation to connect them to experiment, your just pushing meaningless symbols around on paper.

Yes, except the very last step probably:
http://arxiv.org/abs/0704.0646

All these theories have two components: mathematical equations and “baggage”, words that explain how they are connected to what we humans observe and intuitively understand. Quantum mechanics as usually presented in textbooks has both components: some equations as well as three fundamental postulates written out in plain English
...
the ratio of equations to baggage decreases as we move down the tree, dropping near zero for highly applied fields such as medicine and sociology. In contrast, theories near the top are highly mathematical, and physicists are still struggling to articulate the concepts, if any, in terms of which we can understand them.
...
However, could it ever be possible to give a description of the external reality involving no baggage? If so, our description of entities in the external reality and relations between them would have to be completely abstract, forcing any words or other symbols used to denote them to be mere labels with no preconceived meanings whatsoever. A mathematical structure is precisely this: abstract entities with relations between them.

 

[Zarqon]

The best I can do is tell you that any MWI proponent would say that there's a Hilbert space with the property that any point in it represents a possible state of all the universes. Penrose calls this physical system "the omnium". The Schrödinger equation describes the time evolution of the omnium in the following sense. Given a state of the omnium (a point in the Hilbert space), the Schrödinger equation defines a unique continuous curve through that point. That curve tells you the state of the omnium at any time.

Thanks for the answer, it does clarify my issue. However, it does seem to apply, that at the creation of the universe (the omnium I assume), it was created already including all the many different worlds that would be needed, given all the quantum experiments that would be performed during the course of its existence. Is this correct?

There can't be less worlds than would be needed or we're back to energy conservation problem. There also can't be more worlds than would be needed, or we're back to a true probability of which would be used situation. The universe/omnium would have to contain exactly the number of worlds needed for the lifetime of the universe, and to me, this seems like an awfully big assumption to make. How would this be known in advance?

 

[nikman]

Sure it is. But it isn't reasonable to assume that QM "describes what actually happens", just because it makes accurate predictions about probabilities of possible results of experiments. That last part is just the minimium requirement for falsifiability, and it doesn't in any way imply that the mathematical model used by a falsifiable theory describes reality.

Without the assumption I mentioned, there's no reason to think that QM (a standard "Copenhagenish" formulation) needs an interpretation.

We're probably not in disagreement here. Bohr stressed two things: (1) We have no choice but to measure the quantum world with classical instruments and describe it in classical terms, and (2) What we measure and describe is not reality any more than a pencil sketch made at midnight of a mountain is a mountain.

Couple that with complementarity, uncertainty and the realization that to start dissecting the measurement process leads to infinite regress, and it seems to me you have a pretty profound philosophical interpretation of whatever it is that philosophical interpretations are meant to philosophically interpret. (Or maybe we should substitute "foundational" for "philosophical" here.)

Neo-Copenhagenists like Brukner and Zeilinger are busy trying to marry the above to avant-garde informatics and create a new synthesis based on the principle that in the deepest possible sense you can't separate a thing from what you know about that thing. I for one say more power to them.

 

[WaveJumper]

Thanks for the answer, it does clarify my issue. However, it does seem to apply, that at the creation of the universe (the omnium I assume), it was created already including all the many different worlds that would be needed, given all the quantum experiments that would be performed during the course of its existence. Is this correct?

There can't be less worlds than would be needed or we're back to energy conservation problem. There also can't be more worlds than would be needed, or we're back to a true probability of which would be used situation. The universe/omnium would have to contain exactly the number of worlds needed for the lifetime of the universe, and to me, this seems like an awfully big assumption to make. How would this be known in advance?

All this talk of a world is misleading. There is NO world except for our perceptions of it. How old is the world? 13.7 billion years? Wrong. There is no privileged frame of reference to ascertain that the universe is 13.7 b. years old. There are almost infinite frames of reference where the universe is 1000 years old, 1 billion years old, etc., etc., etc.

How big is the universe? 70 billion light years across? Wrong. Such an answer is homocentric. Unless you believe we are something extremely special, the size of the universe through the different FOR's vary from zero to 100s of light years in diameter.

What does cosmology say about the world? Metric expansion of the universe means the outside diameter of the universe has stayed the same since the Big Bang, because space expands(is created) between points, and so the universe is expanding into itself, not into nothingness. Let's move on - Quantum mechanics introduced even more uncertainty. In the subatomic realm, the entire concept of fixed particles in time and space fuzzes out into an ever-shifting haze of probabilities. As Harvard physicist Andrew Strominger says: "space-time is an uncertain concept, so you've lost your firm footing. And that is a deep conceptual issue we have not yet come to grips with."

Let's see what string theories are saying about the world:

"Space and time may be doomed" - Ed Witten

"I am almost certain that space and time are illusions. These are primitive notions that will be replaced by something more sophisticated." - string theorist Nathan Seiberg

"When we talk about space and time, we think there is something there, and we live in it, the idea that space-time is an illusion is very disturbing. Where are we? When are we?" - David Gross

"The notion of space-time is something we've cherished for thousands of years, and it's clearly something we're going to have to give up" - Andrew Strominger of Harvard University"The real change that's around the corner is in the way we think about space and time. We haven't come to grips with what Einstein taught us. But that's coming. And that will make the world around us seem much stranger than any of us can imagine." - David Gross
"All my attempts to adapt the theoretical foundation of physics to this new type of knowledge (Quantum Theory) failed completely. It was as if the ground had been pulled out from under one, with no firm foundation to be seen anywhere, upon which one could have built." - Albert Einstein

Personally, I hold a similar view as to that of Lee Smolin - "From the most sceptical critics to the most strenuous advocates of string theory, you hear the same thing: We are missing something Big."Without this missing Big, we don't know what we are talking about and all this talk of interpreting reality from what we currently know of it from physics, is nothing but a pipe dream.

 

[nikman]

Personally, I hold a similar view as to that of Lee Smolin - "From the most sceptical critics to the most strenuous advocates of string theory, you hear the same thing: We are missing something Big."

Without this missing Big, we don't know what we are talking about and all this talk of interpreting reality from what we currently know of it from physics, is nothing but a pipe dream.

Smolin's also suggested that the problem may be that you can't genuinely observe a universe from inside that universe:

"But, beyond even this, a quantum theory of gravity must be a theory of cosmology. As such, it must also tell us how to describe the whole universe from the point of view of observers who live in it — for by definition there are no observers outside the universe. This leads directly to the main issues we're now struggling with, because it seems very difficult to understand how quantum theory could be extended from a description of atoms and molecules to a theory of the whole universe. As Bohr and Heisenberg taught us, quantum theory seems to make sense only when it's understood to be the description of something small and isolated from its observer — the observer is outside of it. For this reason, the merging of quantum theory and relativity into a single theory must also affect our understanding of the quantum theory. More generally, to solve the problem of quantum gravity we'll have to invent a good answer to the question: How can we, as observers who live inside the universe, construct a complete and objective description of it?"

 

[Fredrik]

All this talk of a world is misleading. There is NO world except for our perceptions of it.

That makes about as much sense as it would for me to say that you didn't actually post that. I do however perceive it to be nonsense.

He asked specifically about the many-worlds interpretation of quantum mechanics. Your answer has nothing to do with the MWI.

How old is the world? 13.7 billion years? Wrong. There is no privileged frame of reference to ascertain that the universe is 13.7 b. years old. There are almost infinite frames of reference where the universe is 1000 years old, 1 billion years old, etc., etc., etc.

This is true, since you can define coordinate systems almost any way you want. But 13.7 billion years is the maximum possible proper time for any timelike curve in spacetime with one endpoint at the event "right here, right now". It's also close to the proper time of a curve that represents motion from the big bang to "right here, right now", such that any physical observer temporarily moving that way would perceive the background radiation as homogeneous and isotropic. (Every galaxy in the universe moves that way, to a good approximation). It's not like people don't have a good reason to call that number "the age of the universe".

Andrew Strominger
Ed Witten
Nathan Seiberg
David Gross
Andrew Strominger
David Gross
Albert Einstein
Lee Smolin

What's with all the quote mining? That's a really bad way to argue for something.

 

[jambaugh]

Every physical theory needs an interpretation. Without an interpretation to connect them to experiment, your just pushing meaningless symbols around on paper.

There are two levels of interpretation. (You've heard me go over this before!)
Distinguish ontological interpretation (what is being debated here) with praxic interpretation what you are talking about being necessary. QM in CI has its praxic interpetation:

Born's probability law and the Eigen-value principle
(plus what operators and hilbert spaces we associate with given physical observables and systems)

That I think is all that is necessary to do an experiment in the lab and see if it matches what the theory predicts.

You are pulling a fast one trying to confuse this type of interpretation with the metaphysical speculations of the various ontological interpretatons.

 

[Fredrik]

Thanks for the answer, it does clarify my issue. However, it does seem to apply, that at the creation of the universe (the omnium I assume), it was created already including all the many different worlds that would be needed, given all the quantum experiments that would be performed during the course of its existence. Is this correct?

No need to mention words like "creation". That only complicates things. What we need to know is that this "omnium" can be identified with a particular mathematical structure, a complex infinite-dimensional separable Hilbert space.

There can't be less worlds than would be needed or we're back to energy conservation problem.

The "worlds" are subspaces of that Hilbert space, or equivalence classes of subspaces, so no need to worry about energy.

There also can't be more worlds than would be needed,

The worlds are subspaces by definition, so no need to worry about worlds that aren't subspaces.

The universe/omnium would have to contain exactly the number of worlds needed for the lifetime of the universe, and to me, this seems like an awfully big assumption to make.

It's not. All the complex infinite-dimensional separable Hilbert spaces are isomorphic to each other (i.e. they are essentially the same space, described in different ways). So we don't have to worry about which Hilbert space represents the states of the omnium.

 

[kote]

QM in CI has its praxic interpetation:

I've never heard of praxic vs ontological aspects of theories, and I can't seem to find a good link. The word "praxic" isn't used anywhere in the Stanford Encyclopedia of Philosophy. Do you have a link or suggested source on this? Is there some more popular theory it would be similar or equivalent to?

Thanks.

 

[nikman]

The word "praxic" isn't used anywhere in the Stanford Encyclopedia of Philosophy.

I hope that every time you mention the SEP you make a note to send them at least one buck at some later date. Seriously. Where would the "world" be without them? Also if you donate maybe they'll stop badgering me all the time.

But not even Ed Zalta's on top of everything.

"The Praxic Age is a period of time in Arbre history, from about -500 to 0, immediately following the Old Mathic Age. This time is marked with great and rapid praxic advances, culminating in the Terrible Events and the Reconstitution. During this time, the scholars lived extramuros with the saeculars."

http://anathem.wikia.com/wiki/Praxic_Age

 

[DrChinese]

I hope that every time you mention the SEP you make a note to send them at least one buck at some later date. Seriously. Where would the "world" be without them? Also if you donate maybe they'll stop badgering me all the time.

But not even Ed Zalta's on top of everything.

"The Praxic Age is a period of time in Arbre history, from about -500 to 0, immediately following the Old Mathic Age. This time is marked with great and rapid praxic advances, culminating in the Terrible Events and the Reconstitution. During this time, the scholars lived extramuros with the saeculars."

http://anathem.wikia.com/wiki/Praxic_Age

ROTFL...

And all this time I thought praxic was a home remedy for gout.

 

[jambaugh]

I've never heard of praxic vs ontological aspects of theories, and I can't seem to find a good link. The word "praxic" isn't used anywhere in the Stanford Encyclopedia of Philosophy. Do you have a link or suggested source on this? Is there some more popular theory it would be similar or equivalent to?

Thanks.

Pardon the obscure usage, Replace "praxic" with "operational" in this context.

The root is from the Greek praxis(πρᾱξις) and I used it in the context of:
http://en.wikipedia.org/wiki/Praxis_%28process%29"

[edit: changed to better link + added greek unicode for "flavor"]
[Edit: PS I used it here to parallel the greek ὄντος root to ontology ]

 

[WaveJumper]

That makes about as much sense as it would for me to say that you didn't actually post that. I do however perceive it to be nonsense.

That was a misrepresentation of my post, i said "there is no world apart from our perception of it", not "nothing exists but perception". The thread is about how qm should be interpreted in respect to how we perceive the world. Are you saying that outside the human interface(the human body), there exists a thing that you are insisting on calling a 'world'? What do you mean by 'world'? Do you know the nature of time? Can you elaborate if you do? How does Time fit into your idea of a 'world' in GR and QM? I am very interested to know why you'd call whatever exists outside of our perception(if it exists at all) a 'world'? What law in physics implies that something exists apart from our perceptions of it? It's definitely not something that has to do with passage of time.

And yes, I am extremely skeptical of the outside world as we perceive it.

He asked specifically about the many-worlds interpretation of quantum mechanics. Your answer has nothing to do with the MWI.

But my answer had to do with the impossibility to make an interpretation that would make sense.

What's with all the quote mining? That's a really bad way to argue for something.

I am definitely not a string theorist, i am not even that interested in ST, unless they make a prediction that may be tested. But those guys are some of the best physicists on the planet today, there is no question about it. And since their points are relevant to the point i was trying to make and i have to take their statements at face value, i posted them so others could comment and find faults with them(if there are any). But feel free to correct Ed Witten and co., i will refrain as i lack knowledge into their field.

I see my post has caused confusion so i'll sum it up in one sentence:

In my opinion, all interpretations of QM are religion in disguise. A description of reality that makes sense, is impossible with our current knowledge of QM.(that's one of the reasons i resorted to quotes by physicists working on a TOE)

 

[Fredrik]

WaveJumper, I think we actually agree about most of these things. I don't really consider questions about what "exists" meaningful, at least not without a definition of what that word means. (It really bugs me when people think it's intuitively obvious). To me, a theory is a set of statements that tells us how to compute probabilities of possible results of experiments, and science is the process of finding new theories and doing experiments to find out how accurate their predictions are. So the only way I can make scienctific sense of the question "Does X exist?" is to interpret it as

"Does the theory that defines X mathematically make accurate predictions? And in particular, does it make more accurate predictions than theories that don't include a definition of X?".

That's my definition of what it means for something to "exist". A good example is photons. They are defined by QED. The alternative theory is classical electrodynamics, i.e. Maxwell's equations. When we say that photons "exist", what we really mean is that there are experiments for which QED can make approximately correct predictions, but classical electrodynamics can't. (Note that the definition is still kind of ambiguous, since we can't e.g. rule out that we will find a theory that makes even better predictions than QED without using the concept of photons).

This doesn't mean that I think we should stop saying that photons exist. Our language would be much too awkward if we didn't allow ourselves to simplify things like that, but it's important to be aware of what such claims really mean.

So far we probably agree. However, your claim that "there is no world apart from our perception of it" is far too strong. That statement is almost crazy as it stands, since you say it as if you know it to be a fact.

 

[kote]

So far we probably agree. However, your claim that "there is no world apart from our perception of it" is far too strong. That statement is almost crazy as it stands, since you say it as if you know it to be a fact.

Playing Devil's advocate a bit here... but what evidence do you have of a world besides your perception of it? We're getting into semantics here (okay, so we've been on semantics for a while), but with no possible evidence of anything beyond our perceptions, isn't it more accurate to call only that which is within the limits of our knowledge "real," while leaving the rest to metaphysics?

 

[RUTA]

I see my post has caused confusion so i'll sum it up in one sentence:

In my opinion, all interpretations of QM are religion in disguise. A description of reality that makes sense, is impossible with our current knowledge of QM.(that's one of the reasons i resorted to quotes by physicists working on a TOE)

“Many scientists are deeply religious in one way or another, but all of them have a certain rather peculiar faith – they have a faith in the underlying simplicity of nature; a belief that nature is, after all, comprehensible and that one should strive to understand it as much as we can. Now this faith in simplicity, that there are simple rules – a few elementary particles, a few quantum rules to explain the structure of the world – is completely irrational and completely unjustifiable. It is therefore a religion.” Sheldon Glashow in The Quantum Universe, co-produced by WETA-TV and The Smithsonian Institution, 1990.

 

[Dmitry67]

WaveJumper, I think we actually agree about most of these things. I don't really consider questions about what "exists" meaningful, at least not without a definition of what that word means. (It really bugs me when people think it's intuitively obvious). To me, a theory is a set of statements that tells us how to compute probabilities of possible results of experiments, and science is the process of finding new theories and doing experiments to find out how accurate their predictions are. So the only way I can make scienctific sense of the question "Does X exist?" is to interpret it as

"Does the theory that defines X mathematically make accurate predictions? And in particular, does it make more accurate predictions than theories that don't include a definition of X?".

That's my definition of what it means for something to "exist". A good example is photons. They are defined by QED. The alternative theory is classical electrodynamics, i.e. Maxwell's equations. When we say that photons "exist", what we really mean is that there are experiments for which QED can make approximately correct predictions, but classical electrodynamics can't. (Note that the definition is still kind of ambiguous, since we can't e.g. rule out that we will find a theory that makes even better predictions than QED without using the concept of photons).

Thank you, I really like your definition.
Especially because virtual particles are "real" based on that definition (and I agree with it)

 

[Fredrik]

Playing Devil's advocate a bit here... but what evidence do you have of a world besides your perception of it? We're getting into semantics here (okay, so we've been on semantics for a while), but with no possible evidence of anything beyond our perceptions, isn't it more accurate to call only that which is within the limits of our knowledge "real," while leaving the rest to metaphysics?

It would be more accurate, but much more awkward. I think it's OK to sacrifice some accuracy to get read of the awkwardness when we talk about these things, at least when we all know what the simplified statements really mean. Unfortunately the stuff I've been talking about isn't taught at universities, and very few people have really thought this through, so most people won't understand it unless you explain it to them.

If you read the text you quoted again, you should see that what I'm objecting to isn't what you're talking about. WaveJumper didn't "call only that which is within the limits of our knowledge real". He stated something unknowable as a fact.

 

[WaveJumper]

If you read the text you quoted again, you should see that what I'm objecting to isn't what you're talking about. WaveJumper didn't "call only that which is within the limits of our knowledge real". He stated something unknowable as a fact.

I didn't say what we know and experience was unreal, i said that the world we experience does not exist apart from our perception of it(perception is real). It exists only in our, err, perception of it.

But if you want to make a tangent to what is real(barely ontopic), why would you consider what we experience real, when more than 95% of the mass/energy content of the proton/neutron comes from virtual particles that pop in and out of existence from the quantum vacuum where space and time break down?(latest 2008 data puts the figure at 99%). The other <5% of the mass is theorized to come from virtual Higgs bosons that derive their energy from virtual processes as well.

So, how real is real? If time and space break down at the Planck scale and all reality appear to stem from there, is that a real reality or is what you are experienceing as spatially differentiated objects in space not that real, but only marginally real at best(when the universe is looked at from a bird's perspective, inferred from our findings in QCD)?

Would you call the singularity at the centre of a black hole real? The theory of GR and its equations come up with a singularity, so according to your definition of "real", the singularity is real.

I would say, that "real" only pertains to our perceptions of it, not to something that physics can define without resorting to the human interface(body and consciousness). And that generally has always been a problem in physics - the human baggage - space, time, infinity, continuity and the contradictions they raise.

What do you think of general relativity's General covariance? How do you interpret it wrt to our experience?

Are space and time fundamental in your opinion? (this is a pretty central point in discussing what is real)
Can there be a theory of the universe's boundary conditions? (another pretty central point wrt what is real)


I'd say that it's likely that the future theory of quantum gravity will re-define how we view 'something' and 'nothing' in a radical way(there are already good idications about that).

At various academic conferences in the last years on quantum gravity, (reportedly)one can find philosophers at physicists' gatherings and physicists at philosopher's events. What, in your opinion, can explain this trend except that we are nearing a new revolution in physics that will turn our understanding of nature and reality upside down?