If, as Richard Feyman insisted, nobody understands quantum mechanics

[Bob Eldritch]

...why believe that there could be a theory of everything that unifies quantum theory with general relativity?

So one of the reasons for developing a theory of quantum gravity is that it could explain the behaviour of quantum objects described in quantum mechanics that can't be explained given the present quantum theory alone.

But then suppose the fundamental reason why a successful theory of quantum gravity can't be constructed in the first place is just that quantum theory can't explain how there is such quantum behaviour described in quantum mechanics as wave, spin and entanglement?

 

[StatMechGuy]

I'm not even sure what your point is, or how "nobody understanding quantum mechanics" plays a role in unification theories. The reasons for spin are reasonably well understood as being a consequence of having a relativistically covariant theory of quantum mechanics, and entanglement is a result of conservation laws and our poor understanding of how measurement works (or at least, that's my somewhat naive view of it).

Again, what you're saying doesn't really seem to make a lot of sense.

 

[Bob Eldritch]

I'm not even sure what your point is, or how "nobody understanding quantum mechanics" plays a role in unification theories. The reasons for spin are reasonably well understood as being a consequence of having a relativistically covariant theory of quantum mechanics, and entanglement is a result of conservation laws and our poor understanding of how measurement works (or at least, that's my somewhat naive view of it).

Again, what you're saying doesn't really seem to make a lot of sense.

But then 'having a relativistically covariant theory of quantum mechanics' isn't an explanation of how the spin behaviour occurs in the first place. How is it that quantum objects don't spin like tennis balls?

Also, you don't need to 'understand how measurent works' to understand how gravity produces its effects. And the fact is that no known effects of the forces are like those of quantum entanglement, which have no measurable strength, have not been measured to vary in any way with increasing distance between objects and to occur at much faster than the speed of light.

 

[lalbatros]

Physics is not about understanding.
Understanding seems to be an anthropomorphic concept: relating facts to the archaïc human experience.

Physics really is about relating facts together in theories able to predict experimental outcome on the basis of a minimal amount of information.

 

[StatMechGuy]

But then 'having a relativistically covariant theory of quantum mechanics' isn't an explanation of how the spin behaviour occurs in the first place. How is it that quantum objects don't spin like tennis balls?

Sure it is. If you want special relativity to apply to quantum mechanics, you find all manner of paradoxes if you only look at the wave function as a scalar quantity. In order to make it Lorentz covariant, then you end up with an additional effect, which is spin and anti-matter.

Also, as for the tennis balls, our current theory treats the elementary particles (leptons are probably what we're going to focus on here) as point particles. No matter how fast they "spin like tennis balls", the angular momentum from this spinning will be zero. If you try to compute how fast an electron is spinning if its "radius" is smaller than our current resolution on measuring radii, you find that the electron is rotating at nigh-relativistic speeds or beyond. Tell me that's not a problem.

 

[Bob Eldritch]

Physics is not about understanding.
Understanding seems to be an anthropomorphic concept: relating facts to the archaïc human experience.

Physics really is about relating facts together in theories able to predict experimental outcome on the basis of a minimal amount of information.

Absolutely not. In all science other than in the standard model of quantum and partiicle physics, theory has been about the discovery of just that which already exists in the world as a cause or causes that can be described in enough clear detail to explain the observed findings. And so you can instist that this is essentially what a factual understanding is all about.

Physicists have abandoned the idea of cause and effect as a means of understanding quantum mechanics only because, given all their previous theoretical methodology that explains the effects of the known forces, it seems as though quantum wave, spin and entanglement have no cause.

Whereas what they should be doing is seeking a new understanding that is based upon a cause that is something like the quantum potential that is described in Bohmian mechanics. And only this could lead towards a genuine understanding of how matter exists and can persist in all its forms despite the action of the forces.

 

[JesseM]

Absolutely not. In all science other than in the standard model of quantum and partiicle physics, theory has been about the discovery of just about that which is already in the world as a cause or causes that can be described in enough clear detail to explain the observed findings.

Where in any theory of physics do you see anything aside from a mathematical description of a relationship between various measurable quantities? For example, in Newtonian physics, can you name a situation where you see definite "cause and effect" (it's hard to see how, since the fundamental Newtonian laws are all time-symmetric), or an ultimate explanation for why systems obey the particular equations they do? In Feynman's book The Character of Physical Law, in the chapter "The Relation of Mathematics to Physics", Feynman actually argued we should not expect physics to be about anything more than mathematical relationships, using Newtonian gravity as an example:

On the other hand, take Newton's law for gravitation, which has the aspects I discussed last time. I gave you the equation:

F=Gmm'/r^2

just to impress you with the speed with which mathematical symbols can convey information. I said that the force was proportional to the product of the masses of two objects, and inversely as the square of the distance between them, and also that bodies react to forces by changing their speeds, or changing their motions, in the direction of the force by amounts proportional to the force and inversely proportional to their masses. Those are words all right, and I did not necessarily have to write the equation. Nevertheless it is kind of mathematical, and we wonder how this can be a fundamental law. What does the planet do? Does it look at the sun, see how far away it is, and decide to calculate on its internal adding machine the inverse of the square of the distance, which tells it how much to move? This is certainly no explanation of the machinery of gravitation! You might want to look further, and various people have tried to look further. Newton was originally asked about his theory--'But it doesn't mean anything--it doesn't tell us anything'. He said, 'It tells you how it moves. That should be enough. I have told you how it moves, not why.' But people are often unsatisfied without a mechanism, and I would like to describe one theory which has been invented, among others, of the type you migh want. This theory suggests that this effect is the result of large numbers of actions, which would explain why it is mathematical.

Suppose that in the world everywhere there are a lot of particles, flying through us at very high speed. They come equally in all directions--just shooting by--and once in a while they hit us in a bombardment. We, and the sun, are practically transparent for them, practically but not completely, and some of them hit. ... If the sun were not there, particles would be bombarding the Earth from all sides, giving little impuleses by the rattle, bang, bang of the few that hit. This will not shake the Earth in any particular direction, because there are as many coming from one side as from the other, from top as from bottom. However, when the sun is there the particles which are coming from that direction are partially absorbed by the sun, because some of them hit the sun and do not go through. Therefore the number coming from the sun's direction towards the Earth is less than the number coming from the other sides, because they meet an obstacle, the sun. It is easy to see that the farther the sun is away, of all the possible directions in which particles can come, a smaller proportion of the particles are being taken out. The sun will appear smaller--in fact inversely as the square of the distance. Therefore there will be an impulse on the Earth towards the sun that varies inversely as the square of the distance. And this will be the result of a large number of very simple operations, just hits, one after the other, from all directions. Therefore the strangeness of the mathematical relation will be very much reduced, because the fundamental operation is much simpler than calculating the inverse of the square of the distance. This design, with the particles bouncing, does the calculation.

The only trouble with this scheme is that it does not work, for other reasons. Every theory that you make up has to be analysed against all possible consequences, to see if it predicts anything else. And this does predict something else. If the Earth is moving, more particles will hit it from in front than from behind. (If you are running in the rain, more rain hits you in the front of the face than in the back of the head, because you are running into the rain.) So, if the Earth is moving it is running into the particles coming towards it and away from the ones that are chasing it from behind. So more particles will hit it from the front than from the back, and there will be a force opposing any motion. This force would slow the Earth up in its orbit, and it certainly would not have lasted the three of four billion years (at least) that it has been going around the sun. So that is the end of that theory. 'Well,' you say, 'it was a good one, and I got rid of the mathematics for a while. Maybe I could invent a better one.' Maybe you can, because nobody knows the ultimate. But up to today, from the time of Newton, no one has invented another theoretical description of the mathematical machinery behind this law which does not either say the same thing over again, or make the mathematics harder, or predict some wrong phenomena. So there is no model of the theory of gravity today, other than the mathematical form.

If this were the only law of this character it would be interesting and rather annoying. But what turns out to be true is that the more we investigate, the more laws we find, and the deeper we penetrate nature, the more this disease persists. Every one of our laws is a purely mathematical statement in rather complex and abstruse mathematics.

...[A] question is whether, when trying to guess new laws, we should use seat-of-the-pants feelings and philosophical principles--'I don't like the minimum principle', or 'I do like the minimum principle', 'I don't like action at a distance', or 'I do like action at a distance'. To what extent do models help? It is interesting that very often models do help, and most physics teachers try to teach how to use models and to get a good physical feel for how things are going to work out. But it always turns out that the greatest discoveries abstract away from the model and the model never does any good. Maxwell's discovery of electrodynamics was made with a lot of imaginary wheels and idlers in space. But when you get rid of all the idlers and things in space the thing is O.K. Dirac discovered the correct laws for relativity quantum mechanics simply by guessing the equation. The method of guessing the equation seems to be a pretty effective way of guessing new laws. This shows again that mathematics is a deep way of expressing nature, and any attempt to express nature in philosophical principles, or in seat-of-the-pants mechanical feelings, is not an efficient way.

It always bothers me that, according to the laws as we understand them today, it takes a computing machine an infinite number of logical operations to figure out what goes on in no matter how tiny a region of space, and no matter how tiny a region of time. How can all that be going on in that tiny space? Why should it take an infinite amount of logic to figure out what one tiny piece of space/time is going to do? So I have often made the hypothesis that ultimately physics will not require a mathematical statement, that in the end the machinery will be revealed, and the laws will turn out to be simple, like the chequer board with all its apparent complexities. But this speculation is of the same nature as those other people make--'I like it', 'I don't like it',--and it is not good to be too prejudiced about these things.

Whereas what they should be doing is seeking a new understanding that is based upon a cause that is something like the quantum potential that is described in Bohmian mechanics.

And what makes you say the potential is the "cause" even in Bohmian mechanics? There is the potential evolving over time and there are particles whose positions change over time, and there is a mathematical relationship between the two.

 

[lalbatros]

Bob,

Whereas what they should be doing is seeking a new understanding that is based upon a cause that is something like the quantum potential that is described in Bohmian mechanics.

If you tried something "like" physics, mathematics and logic, you would recognize easily that the Bohmian mechanics is totally and exactly equivalent to the Schrödinger equation. There is nothing new in Bohmian mechanics with respect to the work of Schrodinger. I like this derivation quite a lot for its pedagogical aspects and for it illustrates quite clearly where the differences are between CM and QM and when these differences vanish (see for example: http://en.wikipedia.org/wiki/Bohm_interpretation).

However, in a sense the whole Bohmian mechanics has been built-in QM by Schrodinger: he derived his masterpiece on the foundations of classical mechanics, as surprising as that can be. The people who made that possible are from the classical times, to name a few think of Hamilton, Jacobi and Lagrange.

Finally note that the Bohmian formulation is probably the best way to illustrate to non-causal aspects of QM. It is just highlighted in the quantum potential term. To transform that in a causal view would imply much more than a new interpretation. This would imply certainly new experimental data, without these the current QM would be more than sufficient. A naïve possibility that I could introduce, as a joke, would be that the quantum potential is related to cosmological entanglement, but more work is needed ... I would not engage the scientific community to invest all his time on this idea ... There are already more than enough unproductive speculative works these days, I think ...

 

[rewebster]

Where in any theory of physics do you see anything aside from a mathematical description of a relationship between various measurable quantities? For example, in Newtonian physics, can you name a situation where you see definite "cause and effect" (it's hard to see how, since the fundamental Newtonian laws are all time-symmetric), or an ultimate explanation for why systems obey the particular equations they do?

but isn't the mathematical description just the description of what has been observed?

Without trying to understand the "cause and effect" of the dropping apple, would Newton not come up with his theories, equations, etc.?

I consider math to be secondary to the original idea or reason. A mathematical description is the utilization of the understanding of the initial and correct (or accepted) observation and measurement put into terms that can be usable.

Bob Eldritch: "...why believe that there could be a theory of everything that unifies quantum theory with general relativity?"

I think that it's just a very hard thing to be creative in this area and it work with EVERYTHING that is known/accepted.

To quote bush from one of the presidential debates: "IT'S HARD WORK" (squared).

 

[tpm]

umm..my question is If:

- 'NObody Understands Quantum Mechanics'

-'Everybody Understands (or at least can) Classical Mechanics'

Then What happens with Semiclassical WKB Quantum mechanics ??... :bigrin:

 

[Demystifier]

Allegedly, nobody understands quantum mechanics (QM) and everybody understands classical mechanics (CM). Why is that? Because QM is conceptually very different from CM. But is it really? The Bohmian interpretation of QM mentioned above makes it conceptually very similar to CM, which makes QM much easier to comprehend.

But there is also an opposite way, to reinterpret CM such that it looks more similar to QM:
http://arxiv.org/abs/quant-ph/0505143
With such an interpretation, I wonder if anybody understands CM.

 

[Demystifier]

umm..my question is If:

- 'NObody Understands Quantum Mechanics'

-'Everybody Understands (or at least can) Classical Mechanics'

Then What happens with Semiclassical WKB Quantum mechanics ??... :bigrin:

Nobody understands it either, but whenever you do not understand it, you can comfort yourself that you do not need to understand it as it is only an approximation anyway.

 

[Bertrand]

I think that there are many fundamentals problems with quantum mechanics, that make it an original theory in Physics.

Among these particularities, and may be the most important, is the "circularity" of the theory. That is, the fact that QM needs its limit theory, Classical Mechanics, for its own definition. This problem has been clearly exposed by Landau and Lifschitz in "Quantum Mechanics - non relativistic theory".

Up to now, this problem has'nt been solved, even by Zurek.

The other, and linked problem is about the nature of the wave function. The two interpretations, the realistic-objective, supported by Einstein, Schrödinger , etc..., and the subjective informational, supported by Bohr, Born, Heisenberg, etc ... both present some difficulties, and cannot explain all situations.

Bertrand

 

[JesseM]

but isn't the mathematical description just the description of what has been observed?

Without trying to understand the "cause and effect" of the dropping apple, would Newton not come up with his theories, equations, etc.?

I don't see any explanation of "cause and effect" in Newton's gravitational laws, and according to Feynman, neither did Newton:

I said that the force was proportional to the product of the masses of two objects, and inversely as the square of the distance between them, and also that bodies react to forces by changing their speeds, or changing their motions, in the direction of the force by amounts proportional to the force and inversely proportional to their masses. Those are words all right, and I did not necessarily have to write the equation. Nevertheless it is kind of mathematical, and we wonder how this can be a fundamental law. What does the planet do? Does it look at the sun, see how far away it is, and decide to calculate on its internal adding machine the inverse of the square of the distance, which tells it how much to move? This is certainly no explanation of the machinery of gravitation! You might want to look further, and various people have tried to look further. Newton was originally asked about his theory--'But it doesn't mean anything--it doesn't tell us anything'. He said, 'It tells you how it moves. That should be enough. I have told you how it moves, not why.'

Do you see any significant difference between the following two statements?

1. There is a mathematical relationship between the acceleration of a body (like an apple, or the moon), and its distance from some other massive body of mass M (like Earth), and although we have no idea why this relationship obtains, we can write down the equation as a = GM/r^2

2. We can say that a body creates a gravitational "force field", and we can write down the relationship between the force on a body with mass m and its distance from another body of mass M as F = GMm/r^2, although we have no idea why this relationship obtains; then the acceleration of the body with mass m will be equal to a = F/m

Is the second more of a "causal explanation" than the first? Isn't the second just a way of restating the first, since this entity called "force" has no measurable properties other than determining the acceleration?

This is not to say that Newton's insight was purely a mathematical one--but his physical insight just consisted in the idea that the dropping of an apple and the orbiting of the moon could both be described by the same basic rule, he didn't have any causal explanation for why this rule holds true. Physical insight often consists of noticing symmetries or seeing how a single law might explain a variety of phenomena which were previously explained with different laws, but I don't think this is what you mean by "cause and effect".

 

[Tomsk]

The two interpretations, the realistic-objective, supported by Einstein, Schrödinger , etc..., and the subjective informational, supported by Bohr, Born, Heisenberg, etc ... both present some difficulties, and cannot explain all situations.

Out of curiosity, can you tell me the specific problems with those interpretations? I think I'm leaning towards the subjective informational side, but knowing problems with it would probably be a good thing!

Also, at the moment I feel like I understand QM more than CM! But that probably just means I don't understand either... :(

 

[Bob Eldritch]

Bob,
If you tried something "like" physics, mathematics and logic, you would recognize easily that the Bohmian mechanics is totally and exactly equivalent to the Schrödinger equation. There is nothing new in Bohmian mechanics with respect to the work of Schrodinger.

I'm afraid that's just not true at all.

Bohm's 1952 paper was called 'A suggested interpretation of the quantum theory in terms of "hidden" variables." So the whole point of this account was show what the Copenhagenists insisted was not possible and John von Neumann was supposed to have proved impossible, which was that you could, after all, account for the results of experiments in terms of a mathematically systematic and determinate description of the hidden variable behaviour of quantum objects in motion.

Whereas one problem with the Schrodinger equation just was how it could be interpreted and one way was Bohr's indeterminate account whereby it could be maintained that the Heisenberg/Schrodinger/Dirac quantum mechanics provided a complete theory of the quantum findings.

And indeed, later it was shown with computer generated diagrams how both the quantum wave and the particle trajectories in a double slit experiments could be visualised using Bohmian mechanics. Something that you couldn't do with any other quantum mechanics or interpretation.

While Bohm's theory is often called causal partly because the wave, which continually accompanies the traveling particle, causes the particle trajectory deflections that result in the interference pattern.

http://www.math.rutgers.edu/~oldstein/quote.html#M

http://www.math.rutgers.edu/~oldstein/

http://www.math.rutgers.edu/~oldstein/papers/qts/node4.html

http://plato.stanford.edu/entries/qm-bohm/


For a very clear plain English account of Bohm's theory see also Jim Baggott's book https://www.amazon.com/dp/0198529279/?tag=pfamazon01-20

 

[Bertrand]

Out of curiosity, can you tell me the specific problems with those interpretations? I think I'm leaning towards the subjective informational side, but knowing problems with it would probably be a good thing!

Also, at the moment I feel like I understand QM more than CM! But that probably just means I don't understand either... :(

Problems linked withe subjective interpretation :

1) It requires a "classical world" given a priori. In its essence, Born's priniciple requires a classical, non quantum, measurement apparatus. On the other hand, the classical theory is the limit of the quantum theory, through the correspondance principle. This means that in that case, quantum theory requires its limit theory for its own formulation. A unique case in physics. For instance, relativity doesn't need the non relativistic classical theory for its own formulation, and accepts it just as its limit theory.

2) It is difficult to interpret everything just as a subjective information. Where does the reality of matter come from in this model ? The quantum theory is necessary to explain the chemical link, for instance. However, when to hydrogen atoms merge to for an hydrogen molecule, there is no observer of any kind in the process.

3) It has difficulties to explain the non-locality : in non-local processes, there is "something" which is transmitted between the two entangled parts, and this "something" is not simply a subjective information acquired by one of the obsevre, on the distant part.

4) If you consider the whole universe as a quantum object, where does classicality come from in the first place.

Objective interprétations :

They lead either to reltivity violation, or to the multi-world hypothesis, which is totally unacceptable.

Bertrand

 

[Bob Eldritch]

Objective interprétations :

They lead either to reltivity violation, or to the multi-world hypothesis, which is totally unacceptable.

Bertrand

Nonlocality only violates relativity if something travels instantaneously or at faster than light speed between quantum objects through four dimensional spacetime.

But then if you were to describe a cause of the non-local effects of entanglement, because such effects do not vary with distance around objects, their cause could not be described as surrounding objects in 4D spacetime and so could be thought to act from one or more extra dimensions of space to the three of the world experienced, and to which relativity exclusively applies.

 

[Bertrand]

Yes Bob,

This is an interesting hypothesis.

However, although in a non-local process there is "something" that moves instantly, this "something" cannot carry any information. It is necessary to have an information going through a classical channel, to let this "something" become efficient.

So this element should although be taken into account in yout theory. Is it the case ?

Bertrand

 

[Tomsk]

Problems linked withe subjective interpretation :

1) It requires a "classical world" given a priori. In its essence, Born's priniciple requires a classical, non quantum, measurement apparatus. On the other hand, the classical theory is the limit of the quantum theory, through the correspondance principle. This means that in that case, quantum theory requires its limit theory for its own formulation. A unique case in physics. For instance, relativity doesn't need the non relativistic classical theory for its own formulation, and accepts it just as its limit theory.

Have you heard of Carlo Rovelli's relational interpretation? It says that all systems are quantum systems, even macroscopic measuring devices. I guess classical mechanics can come out of it through the correspondance principle.

2) It is difficult to interpret everything just as a subjective information. Where does the reality of matter come from in this model ? The quantum theory is necessary to explain the chemical link, for instance. However, when to hydrogen atoms merge to for an hydrogen molecule, there is no observer of any kind in the process.

In RQM you don't need an outside observer, one hydrogen atom can act as an observer, so from the point of view of one H atom, it would want to combine with another H atom.

3) It has difficulties to explain the non-locality : in non-local processes, there is "something" which is transmitted between the two entangled parts, and this "something" is not simply a subjective information acquired by one of the obsevre, on the distant part.

I don't see why you need for something to be transmitted non locally. What is this something? If all you have is subjective information acquired by one observer, you still get consistency, if you allow the distant observer to exist in a superposition of states, ie. treat it quantum mechanically.

4) If you consider the whole universe as a quantum object, where does classicality come from in the first place.

There is no universal wavefunction because of locality, which splits up the universe into disconnected regions, which have to be treated separately. Classicality comes from these separate parts interacting so much that there is no chance for superpositions to build up.

 

[Bertrand]

Have you heard of Carlo Rovelli's relational interpretation? It says that all systems are quantum systems, even macroscopic measuring devices. I guess classical mechanics can come out of it through the correspondance principle.

I don't know this man in particular, but I know this family of interpretations. They are all also plagued with circularity.

In RQM you don't need an outside observer, one hydrogen atom can act as an observer, so from the point of view of one H atom, it would want to combine with another H atom.

No, an hydrogen atom cannot be considered as an observer, because it is not a classical object. In the Copengaen interpretation, the observer is necessarily classical. This is the problem

I don't see why you need for something to be transmitted non locally. What is this something? If all you have is subjective information acquired by one observer, you still get consistency, if you allow the distant observer to exist in a superposition of states, ie. treat it quantum mechanically.

Because measurement on one of the parts actually modifies the wave function of the other entangled part. This change cannot carry information in itself, this would contradict relativity, but the change is nevertheless "real" in the second part. See for instance diffraction experiments on entangled photons.

In the subjective interpretation, measurement of one of the parts brings immediate knowledge on the other parts. But this not different from classical mechanics. In non local experiments, the measurement on one of the entangled parts can change something on the other part, for instance, its diffraction pattern. However, an other element of information has to be transmitted through a classical channel.

Therefore, what has been immediately transmitted is in between subjective knowledge and reality. See for instance articles from A. Apsect on the subject.

There is no universal wavefunction because of locality, which splits up the universe into disconnected regions, which have to be treated separately. Classicality comes from these separate parts interacting so much that there is no chance for superpositions to build up.

This is at obscure to me, and should have to be precised. Have you any reference on this ?

Bertrand

 

[vanesch]

They lead either to reltivity violation, or to the multi-world hypothesis, which is totally unacceptable.

Why is the "multi-world hypothesis totally unacceptable" - apart from personal, intuitive preferences ?

 

[Bob Eldritch]

Yes Bob,

This is an interesting hypothesis.

However, although in a non-local process there is "something" that moves instantly, this "something" cannot carry any information. It is necessary to have an information going through a classical channel, to let this "something" become efficient.

So this element should although be taken into account in yout theory. Is it the case ?

Bertrand

Actually, Bertrand, given an extradimensional nonlocally acting cause of entanglement I don't think you need the idea of information or anything else being transferred from one particle to another.

Entanglement itself is just unlike any other effect between objects in that it describes the relationship beween particular forms of behaviour of objects - eg spin up in relation to spin down - being maintained or conserved.

So such an effect can be thought of as being the result of a causal property of extradimensional space itself possessing something like a memory of the behavioural relationship, and so that this memory is causally imposed upon the entangled particle components simultaneously.

 

[Bertrand]

Why is the "multi-world hypothesis totally unacceptable" - apart from personal, intuitive preferences ?

Do you really think a normal person can believe that the whole universe will split in many parts, each time a measuting process is performed ?

I found this funny joke in a John Baez (who thinks the same, that the MWI is not an acceptable theory) forum :

A plane starts burning. The pilot and the copilot have to jump to escape the fire. They have no parachute. the copilot asks the pilot : what will happen to us ?

And the pilot answers : you don't have to worry, there is a universe in which we did'nt even take this plane.

In other words, I don't think I have to justify my position, which is just what allreasonable physicists believe. You have to justify your's, if you are a MWI believer.

Bertrand

 

[Mike2]

Why is the "multi-world hypothesis totally unacceptable" - apart from personal, intuitive preferences ?

As I understand it the Many Worlds Interpretation of QM is an interpretation of Feynman's path integral formulation of QM, where the various paths are actually considered real. But I think this is a wrong interpretation because the formulation states that it takes ALL possibilities just to make one reality. So if it take all to make even one reality, then they cannot exist as separate entities.


*****************************
Quantum Mechanics derived from logic. Who would have guessed it?
http://www.sirus.com/users/mjake/Physlogic.htm

 

[Tomsk]

No, an hydrogen atom cannot be considered as an observer, because it is not a classical object. In the Copengaen interpretation, the observer is necessarily classical. This is the problem

In the relational interpretation, all systems, including any sort of observer, is considered to be a quantum system, and also any system can act as an observer. So a hydrogen atom can be considered as an 'observer', or rather, it is possible to describe other H atoms with respect to one particular H atom. No one H atom or any sort of observer is special like in the Copenhagen interpretation.

Because measurement on one of the parts actually modifies the wave function of the other entangled part. This change cannot carry information in itself, this would contradict relativity, but the change is nevertheless "real" in the second part. See for instance diffraction experiments on entangled photons.

The change in the second part is not real if you reject realism, which is what RQM does. So there is no real change of anything at the distant location, it is only a subjective change from the POV of the first observer. In other words there is no 'actual' wavefunction of anyone particle, but as many (potentially different) wavefunctions as there are observers, because "the wavefunction ψ is the coding of the information that [an observer] A has about [a system] S"(http://fr.arxiv.org/PS_cache/quant-ph/pdf/0604/0604064.pdf), and the information one observer has is not necessarily the same as the information another observer has.

This is at obscure to me, and should have to be precised. Have you any reference on this ?

Again from the Smerlak-Rovelli paper linked above:

Einstein’s reasoning requires the existence of a hypothetical super-observer that can instantaneously measure the state of A and B. It is the hypothetical existence of such nonlocal super-being, and not QM, that violates locality.

So any observer attempting to describe the whole universe completely, in one go, cannot unless non-locality is allowed. The information describing every system in the universe instantaneously cannot be held by anyone system (obeying locality).

 

[Hurkyl]

Do you really think a normal person can believe that the whole universe will split in many parts, each time a measuting process is performed ?

I am sure there are "normal" people who can believe such a thing. Of course, I would have assumed we are talking about people who are studying to understand physics, and not a "normal" person.

In other words, I don't think I have to justify my position, which is just what allreasonable physicists believe. You have to justify your's, if you are a MWI believer.

Of course you do. If you have a position, it's your burden to justify it.

FYI: the fact that you feel the need to invoke words such as "normal" and "reasonable" should be a huge red flag to yourself that you are having an emotional response, and not a rational one.


The MWI position is very easy to justify -- it's a direct consequence of the the axioms of QM, if you leave out the collapse postulate. And since the reality of collapse proves unnecessary for describing the universe...

 

[rewebster]

MWI isn't logical to me---for Many Weird Inductive reasons---but,

one of the main reasons to me, (if my limited knowledge of MWI is correct), is in that, every action of every particle initiates/creates its own new dimension/reality/(whatever) is ---

---that's a whole lot of mass/energy to be created out of nothing every nanosecond throughout history.

-----------------------------------------------
number of particles in the universe
X
the number of movements each particle has made
each nanosecond--(and that's being generous-
for a photon moves quite far in a nanoseceond)
X
the number of nanoseconds since the big bang (I have trouble with the big bang theory)

= ? (the number of new dimensions/realities/(whatevers)

anyway, that's a really whole lot of mass/energy

-----------------------------------------

AND if (and it should be, I believe) each one of those new dimensions/realities/(whatevers) HAS to follow the same principles of the original dimension/reality (whatever) in creating new dimensions/realities/(whatevers) each and every nanosecond.

--------------------------------------------

maybe all that mass/energy is created by the mystical, magical, quantum string fairy!

 

[Hurkyl]

MWI isn't logical to me---for Many Weird Inductive reasons---but,

one of the main reasons to me, (if my limited knowledge of MWI is correct), is in that, every action of every particle initiates/creates a new dimension/reality/(whatever) is ---

---that's a whole lot of mass/energy to be created out of nothing every nanosecond throughout history.

When you pass a photon through a half-silvered mirror, it is one photon in a superposition of the "transmitted" and "reflected" paths. It hasn't suddenly become "two" photons. No energy has been created, no alternate realities have been formed. It's just one photon living in a certain quantum state.

The collapose postulate restricts this phenomenon to "quantum systems". All MWI says is that this is an artificial constraint -- if QM describes the universe, then the universe should be a quantum system.

 

[vanesch]

As Hurkyl pointed out, the objections you raise against MWI - and for that matter, the only objections I've ever seen against MWI - are on the intuitive-emotional side. That doesn't mean that one has to discard them off-hand ; after all, maybe our formalism is wrong and our intuition is, after all, right ! But you have to be aware that it is not a rational, well-constructed *argument*, but just an intuitive, emotional argument.
Look at your statements:

Do you really think a normal person can believe that the whole universe will split in many parts, each time a measuting process is performed ?

First of all, this is NOT what MWI claims, but I've explained this at least a dozen times here on PF. (do a search on MWI and my name, and you'll find gazillions of posts on the topic )

But the point is that making references to "normal person" and "believe" is not what constitutes a rational argumentation.

I found this funny joke in a John Baez (who thinks the same, that the MWI is not an acceptable theory) forum :

A plane starts burning. The pilot and the copilot have to jump to escape the fire. They have no parachute. the copilot asks the pilot : what will happen to us ?

And the pilot answers : you don't have to worry, there is a universe in which we did'nt even take this plane.

I know that post, it's a funny joke, and I even saw it when it appeared and as far as I understand, John Baez is not unfriendly towards MWI - at a certain point, he even declared on s.p.r. to be an adherent of it - though he might have changed his mind in the mean time, and also, "John Baez" (no matter how smart he is and how much I respect him) is an argument of authority.

In other words, I don't think I have to justify my position, which is just what all reasonable physicists believe. You have to justify your's, if you are a MWI believer.

Bertrand

So by classifying people on your position to be "normal", which "can't believe" it, by thinking (IMO erroneously) that some smart guy is making fun of it, and by classifying physicists on your side as "reasonable" you think you've given a water-tight argument on your side, and now it is only to the other side to give an (in your opinion hopeless) justification...

But you're not alone. I've repeated this often: I've never seen any OTHER argument than an emotional-intuitive one against MWI.

That doesn't mean that MWI is "right" (I wouldn't even claim that myself!), but it is not as "lunatic and obviously unacceptable" as you might think. As such, it shouldn't be out of hand dismissed as a path to resolve potentially the meta-physical difficulties that quantum-theory points out.

EDIT: some reading (by the hand of John Baez) on MWI: http://math.ucr.edu/home/baez/week27.html

and in this s.p.r. post, JB is actually arguing on the Everett side against Arnold Neumaier (who is rather against it):
http://www.lepp.cornell.edu/spr/2003-05/msg0051265.html

(and this one is really funny http://www.lepp.cornell.edu/spr/2003-05/msg0050982.html )

 

[rewebster]

... No energy has been created, no alternate realities have been formed.

"DeWitt admits that this constant splitting of worlds whenever the states of
systems become correlated is counterintuitive:
I still recall vividly the shock I experienced on first encountering this
multiworld concept. The idea of 10^100 slightly imperfect copies of oneself all
constantly spitting into further copies, which ultimately become
unrecognizable, is not easy to reconcile with common sense. Here is
schizophrenia with a vengeance (1973, p. 161). "

http://plato.stanford.edu/entries/qm-everett/


And to me, "Pure Math" may be rational, and there may be an enormous amount of properties known about the photon and the wave function (wave-particle function)---but, even the speed of light is still an approximation when you consider the ability to measure a second.

http://news.bbc.co.uk/2/hi/science/nature/3486160.stm


Are we supposed to inarguably 'believe' the 'math' (of MWI) when we still do not definitively 'what' the photon is, exactly, (to the point when we still can't explain some of its properties, e.g. the double slit), and go with the math that is so non-exact (an approximation, e.g. the second) ?

I know applied physicists may say 'we know a lot about ...'; and yes, I agree, and we 'have to' adhere to 'some' type of 'acceptable' knowledge, otherwise we wouldn't have been able to go to the moon, but...

--------------------------------------------------------

Here's something that doesn't relate, but if anyone is interested in Einstein memorabilia:

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&ih=010&sspagename=STRK%3AMEWA%3AIT&viewitem=&item=200099492028&rd=1&rd=1

 

[Bertrand]

OK, OK guys,

Let's leave apart unrational arguments. Then :

- do you agree with K. Popper's argument about falsiability, that is, a theory can be considered as scientific only if some proof can be provided that it is worng ?

- If the answer is yes, then is MWI a scientific theory, according to Popper's principle ?

Besides, I will take some time to read your trillion articles about MWI, then I'll come back. By trillion, do you mean you include all versions of your articles in all universes ?

is there a universe in which you don't believe in MWI ?

Sorry, these were my last stupid jokes ...

Bertrand

 

[Tomsk]

I think the thing about MWI is, it's just an interpretation, so it is falsifiable because quantum mechanics is falsifiable. Even if you don't like to look at QM in that way, it is still possible to, as you get the same answers. In a way it is just faith to say that MWI is the *correct* interpretation of QM. But I like Carlo Rovelli's argument that if QM can be derived from physical principles, then the interpretation of it would not be an issue any more, because how it works would become obvious.

 

[JesseM]

I think the thing about MWI is, it's just an interpretation, so it is falsifiable because quantum mechanics is falsifiable.

Well, certain aspects of MWI can be falsified, but those are precisely the elements which are common to all the other interpretations because they are part of the basic structure of quantum mechanics; the things that make each interpretation unique, like the multiple versions of the same experimenter in the MWI or the hidden variables of Bohmian mechanics or the "offer waves" of Cramer's transactional interpretation, are unfalsifiable because they do not lead to any empirical predictions which are different than any other interpretation. That's exactly why they're all called "intepretations", not "theories". I think the only hope of really settling these foundational issues is if quantum gravity modifies quantum mechanics in such a way that it leads to a modified version of one of these interpretations that actually makes specific empirical predictions.

 

[rewebster]

I think the only hope of really settling these foundational issues is if quantum gravity modifies quantum mechanics in such a way that it leads to a modified version of one of these interpretations that actually makes specific empirical predictions.

OR, come up with a new 'theory' to satisfy it all (or, at least better than what is presently 'out there'*).


*(PS: 'out there'---or way, way out there)

-------------------------------------------------

just a note on the DeWitt comment--"Here is
schizophrenia with a vengeance."

I think he may of meant "split-personality disorder" (multiple personality disorder, 'Dissociative identity disorder') ---the correct definition of schizophrenia really doesn't fit the analogy. It's funny that a 'scientist' doesn't know the right definition, and uses the 'popular' erroneous one. I guess we all have 'errors'.

 

[Bob Eldritch]

Well, certain aspects of MWI can be falsified, but those are precisely the elements which are common to all the other interpretations because they are part of the basic structure of quantum mechanics; the things that make each interpretation unique, like the multiple versions of the same experimenter in the MWI or the hidden variables of Bohmian mechanics or the "offer waves" of Cramer's transactional interpretation, are unfalsifiable because they do not lead to any empirical predictions which are different than any other interpretation. That's exactly why they're all called "intepretations", not "theories". I think the only hope of really settling these foundational issues is if quantum gravity modifies quantum mechanics in such a way that it leads to a modified version of one of these interpretations that actually makes specific empirical predictions.

There could also be a way of falsifying all quantum interpretations except Bohmian mechanics. This being the only mechanics that describes the quantum wave in terms of a cause (called the quantum potential) acting in addition to the forces and its effects upon particles in motion.

So that Bohm's mechanics would be developed into a quantum hypothesis that justified and represented further details of the quantum potential cause from its effects upon particles. This hypothesis was found to be such that large scale observable natural evidence would be found of where the cause could also be justified and described as acting. This then allowed a general theory to be developed that made unique and testable predictions.

 

[Demystifier]

Why is the "multi-world hypothesis totally unacceptable" - apart from personal, intuitive preferences ?

Nothing is totally unacceptable, not even MWI.
Seriously, although MWI is also a logical possibility, there do exist arguments against it that are not purely emotional or intuitive. Perhaps the best argument is the one based on Poper falsifiability. Also, with an appropriate definition of simplicity (for example, one world is simpler than an infinite number of worlds), it can be argued that it is not in agreement with the Ocam's razor.

 

[vanesch]

Nothing is totally unacceptable, not even MWI.
Seriously, although MWI is also a logical possibility, there do exist arguments against it that are not purely emotional or intuitive. Perhaps the best argument is the one based on Poper falsifiability.

As JesseM pointed out, MWI is a *metaphysical* interpretation of a formalism, that is, it attaches ontological meanings to formal elements. In the same way as the metaphysical assumption of the existence of "spacetime" is an interpretation of the formalism of general relativity. Metaphysical concepts do not need to be falsifiable - in fact they can't. Only formalisms can be falsified because only formalisms make observational predictions. Interpretations are what comes metaphysically on top of it, and as such, two different metaphysical interpretations of the same formalism are totally empirically equivalent - by definition.

If you want to restrict yourself to Popper falsifiability, you should refrain yourself from giving ANY metaphysical interpretation of any theory. As such, this cannot be an argument against such an interpretation!

Also, with an appropriate definition of simplicity (for example, one world is simpler than an infinite number of worlds), it can be argued that it is not in agreement with the Ocam's razor.

This is a well-known argument against MWI, but it depends on what exactly one understands by Occam's razor. Usually, by Occam's razor, one doesn't consider the complexity of the *solution* but the complexity of the *axiom set*. As such, MWI has a far simpler axiom set (it is simply the unitary part of quantum theory, together with a rule (Born rule) of what will be subjectively observed) than, say Bohmian mechanics (Bohmian mechanics includes unitary QM, and has on top of it an entire Newtonian particle dynamics) ; it is also simpler than Copenhagen, because no independent classical world with its classical dynamics needs to be axiomatically postulated.

Consider Newtonian dynamics as compared to Ptolomean planetary dynamics. Ptolomean planetary dynamics has a SIMPLER solution set (namely just exactly those sets of epicycles that correspond to actual planets ; no other planets 'are possible'), than the solution set of Newtonian dynamics which contains ALL THINKABLE orbits around the sun. The solution set of Newtonian dynamics has an infinite amount of possible solutions, while the Ptolomean view has a finite number of orbits. According your view of Occam's razor, one should prefer Ptolomy's view.
But one prefers of course Newton's view, because of the *axiomatic simplicity*. You can derive that huge set of potential orbits just with a few axioms (Newton's law F = m.a, the universal law of gravity F = -G m1m2/r^2 and some other esthetics).

So from this PoV, MWI is by far the *simplest* formulation of quantum theory. No dichotomy classical/quantum, no extra dynamical elements (particles, guide fields ...). That its solution set is huge is just a consequence of its axioms, but that's not what's aimed for by Occam's razor. Occam goes for the simplest *set of rules*, not for the simplest *solution set*.

 

[vanesch]

OK, OK guys,

Let's leave apart unrational arguments. Then :

- do you agree with K. Popper's argument about falsiability, that is, a theory can be considered as scientific only if some proof can be provided that it is worng ?

- If the answer is yes, then is MWI a scientific theory, according to Popper's principle ?

I answered that in another post (MWI is not a scientific theory, but a metaphysical view on a scientific formalism, and as such, Popper doesn't apply to it).

Besides, I will take some time to read your trillion articles about MWI, then I'll come back. By trillion, do you mean you include all versions of your articles in all universes ?

is there a universe in which you don't believe in MWI ?

I don't *believe* in MWI, not more than I *believe* in a static spacetime (or in Newtonian absolute space, for that matter). I think that these are, nevertheless, good metaphysical hypotheses that go very well with the scientific theories that we have today. The reason for that is that they start from the formalism, and take "as metaphysically real" what the formalism gives us, and not the other way around, where we try to re-interpret a formalism as a function of what we intuitively WOULD LIKE things to be like. As such, these interpretations (MWI for QM, and static spacetime for GR) give us a good mental PICTURE to develop a good INTUITION for the workings of the formalism. Call it "applied philosophy" to the service of physics.

I am totally agnostic as to whether the physical theories we have today contain in them any form of "absolute truth" concerning the world around us - although I trust them a bit more than my own intuition of which I'm pretty sure that it is misleading.

 

[Demystifier]

As such, MWI has a far simpler axiom set (it is simply the unitary part of quantum theory, together with a rule (Born rule) of what will be subjectively observed) than, say Bohmian mechanics (Bohmian mechanics includes unitary QM, and has on top of it an entire Newtonian particle dynamics) ; it is also simpler than Copenhagen, because no independent classical world with its classical dynamics needs to be axiomatically postulated.

I wouldn't agree. As far I can see, both interpretations have essentially only two axioms. One is their common axiom representing the unitary part. The other axiom is either the axiom on probabilities of subjective experiences or the axiom that determines particle trajectories.

 

[Bob Eldritch]

I answered that in another post (MWI is not a scientific theory, but a metaphysical view on a scientific formalism, and as such, Popper doesn't apply to it).

I don't *believe* in MWI, not more than I *believe* in a static spacetime (or in Newtonian absolute space, for that matter). I think that these are, nevertheless, good metaphysical hypotheses that go very well with the scientific theories that we have today. The reason for that is that they start from the formalism, and take "as metaphysically real" what the formalism gives us, and not the other way around, where we try to re-interpret a formalism as a function of what we intuitively WOULD LIKE things to be like. As such, these interpretations (MWI for QM, and static spacetime for GR) give us a good mental PICTURE to develop a good INTUITION for the workings of the formalism. Call it "applied philosophy" to the service of physics.

I am totally agnostic as to whether the physical theories we have today contain in them any form of "absolute truth" concerning the world around us - although I trust them a bit more than my own intuition of which I'm pretty sure that it is misleading.

Trouble is, all quantum interpretations bar one disallow any possibility of a quantum hypthesis that could be developed into a fully justified causal theory. So that Bohmian mechanics is the only accoumt that, as I have said, descrbes a distinct cause that acts in addition to the forces. And one could think if only for this reason, the de Broglie-Bohm account is the one correct interpretation of quantum physics.

But why believe that the quantum potential is a cause that acts in the real world in addition to the forces?

Well, despite all its successes at prediction and explaining the chemical, electromagnetic and nuclear properties of many forms of matter, there is one universal feature of matter that the quantum theory of the standard model can't adequately explain, which is how matter can be and remain in its forms as atoms and molecules and organised out of its subatomic components and while the forces act just as they have been measured and described.

And it can be reasoned that even though a cause of quantum entanglement can't be measured and described in terms of it strength of effect, entanglement describes a fixed composite relationship between certain forms of behaviour such as the spin up in relation to spin down of electrons and protons. And a cause could thus be described as acting upon these objects so as to maintain or conserve this organised relationship. Also, by preventing electrons from falling into nuclei the wave property of electrons would be described as maintaining or conserving the form of atoms and molecules.

And then you could suppose that such a cause could act elsewhere in the natural world and on the large scale...

Any less likely than a many worlds interpretation?

 

[nutshell]

Hi guys I just discovered this forum and randomly wandered when I came to this extremely interesting topic (well actually they all are, as long as they concern science right ?)

Anyway it seems to me that that you guys believe (and I believe that too, to some extent) that physics can not only describe how things are but also WHY they are (ultimately this goes to the question why there is an Universe)

However, we must also consider the fact that maybe, physics cannot do that. Maybe there is no rationnal explanation at all to why things are.

Of course, one could mention the anthropic principle to get away from this difficulty...

I don't know if what I want to say is very clear but I would like to have your opinion on this.

THX

 

[NateTG]

Anyway it seems to me that that you guys believe (and I believe that too, to some extent) that physics can not only describe how things are but also WHY they are (ultimately this goes to the question why there is an Universe)

However, we must also consider the fact that maybe, physics cannot do that. Maybe there is no rationnal explanation at all to why things are.

Strictly speaking, there is no maybe about it. Physics, as a science, is about testable theories. Interpretations that deal with the why of things are primarily interesting because they can lead to testable predictions - that is, notions of how,when, or where' things happen. Science does not answer questions about why things happen.

 

[Mike2]

Strictly speaking, there is no maybe about it. Physics, as a science, is about testable theories. Interpretations that deal with the why of things are primarily interesting because they can lead to testable predictions - that is, notions of how,when, or where' things happen. Science does not answer questions about why things happen.

As I understand it. Physics can't tell us what the facts actually are. Physical theories only tell us (at best) that IF there were to exist particular circumstances, THEN particular results will follow as a consequence. In other words, physics may only be able to tell us WHY. Measurement tells us what facts exists; physical theory tells us how and why facts change.

***************************************
Can quantum mechanics be derived from logic alone?
http://www.sirus.com/users/mjake/Physlogic.htm

 

[reilly]

Seems to me that MWI is nothing more than the belief that a probability tree describes reality. There is nothing inherently quantum about this idea; works just as well for a poker game, stock market, what ever. Most, if not all, workers in math and probability for centuries never bothered with such a concept. Suggests nobody took the idea seriously -- as soon as you get to conditional probability, you have probability trees, which go back a few years.

My first reaction upon hearing of MWI is not printable. MWI seems to me to be very contrived, and an effort to get around probability -- back to the 19th century. I think this MWI stuff presents huge complications. Born may not be perfect, but no one has come up with anything practical that's better than Born.

MWI is, to me, very strange, almost like magic not physics, but it's a free country. Believe MWI if you want, but why?
Regards,
Reilly Atkinson

 

[JesseM]

Seems to me that MWI is nothing more than the belief that a probability tree describes reality. There is nothing inherently quantum about this idea; works just as well for a poker game, stock market, what ever.

The inherently quantum reason for liking the MWI is that it prevents you from having to treat measurements as fundamentally separate from the normal evolution of the wavefunction in between measurements--no need for each measurement to discontinuously "collapse" the wavefunction into an eigenstate, and this also means that classical measuring-devices are obeying the same fundamental laws that quantum systems are. Additionally, the MWI gives you a way of explaining the results of experiments on entangled particles which violate Bell inequalities without invoking nonlocal (faster than light/backwards in time) effects.

 

[Tomsk]

I don't see the problem with wavefunction collapse. It represents the reception of information describing a system by an observer. There could well be a different description of this interaction, by a second observer who does not receive the same information that the first one does. I don't see why you need parallel universes for that.

 

[JesseM]

I don't see the problem with wavefunction collapse. It represents the reception of information describing a system by an observer.

What is the physical definition of "an observer"? Assuming the observer is a large collection of quantum-sized particles interacting, why isn't the observer's behavior guided by the laws of quantum mechanics? What laws do govern the observer's behavior, exactly?

With every non-quantum theory of physics, you can have a complete description of a hypothetical universe guided by those laws, viewed "from the outside" with a Godlike perspective. With QM under the Copenhagen interpretation you can only describe individual systems being measured by some external classical system, and the behavior of the classical system, including which sets of noncommuting variables it measures, has to be put in by hand. So if our fundamental theories of physics are quantum ones, it's difficult to say what it would mean for the entire universe to be governed by those laws, with no external measuring-device. This is one of the basic problems that various "interpretations" such as the MWI and Bohmian mechanics are supposed to resolve.

There could well be a different description of this interaction, by a second observer who does not receive the same information that the first one does. I don't see why you need parallel universes for that.

If variables don't commute, there'll be uncertainty relations between them, and these apply just as much to measurements made by different observers as to those made by a single observer. If I measure a particle's spin on one axis, then measure again on the same axis, I'll get the same answer; but if I make my measurement, then unbeknownst to me someone else measures on a different axis, then I measure again on the same axis I did the first time, the spin will be randomized in just the same way as if I had made the middle measurement.

 

[vanesch]

Seems to me that MWI is nothing more than the belief that a probability tree describes reality. There is nothing inherently quantum about this idea; works just as well for a poker game, stock market, what ever. Most, if not all, workers in math and probability for centuries never bothered with such a concept. Suggests nobody took the idea seriously -- as soon as you get to conditional probability, you have probability trees, which go back a few years.

The point is, in classical probability, you can, if you want to, introduce also a "many outcomes" view, but you can also use state variables (as is done in classical statistical mechanics) which follow ONE of the several "probable" paths. There is no distinction between the "ignorance which path, but there's only one" and the "all paths simultaneously, and I only see one" view. In other words, the specific distribution of probabilities doesn't interfere with the dynamical evolution of the system: the different possible paths have their independent dynamical evolution, un-influenced by the existence, or not, of the neighbouring paths.
So, although it is entirely possible in classical probability, to have a "many outcomes-of-which-I-only-perceive-one" view, it doesn't bring in anything.
This is of course also the critique on MWI on *fully decohered branches*, but...

In quantum theory, as you know it, the wavefunction, at least as long as there is coherence, cannot be seen as an "ignorance" description: it contains essential dynamical information. This is what appears in quantum interference between "different possible paths" and follows from the fact that the probability is given by |psi|^2 and not by psi itself, psi itself containing an essential phase. As such, we cannot see |a> + |b> as simply an ignorance on our part of whether the system is in state |a> or in state |b> ; it comes down to the difference between a (coherent) superposition and a mixture (in a mixture, we can find again our traditional "ignorance" description).

This, just to outline the difference between probability as ignorance in classical theory (in which case the "many outcomes" view doesn't bring in anything new, although it can of course be considered), and the superposition in quantum theory.

My first reaction upon hearing of MWI is not printable. MWI seems to me to be very contrived, and an effort to get around probability -- back to the 19th century. I think this MWI stuff presents huge complications. Born may not be perfect, but no one has come up with anything practical that's better than Born.

I agree with you here ; I think that many MWI proponents got lost in trying to derive Born from some OTHER probability measure, such as uniform distribution. My personal opinion, like yours, is that this complicates overly the task, and is not essential, given that we'd still have to explain where this OTHER probability distribution (the uniform distribution, and state counting and so on) comes from. But to me, that's not the essence of MWI. The point is not that we can derive the Born rule from another distribution (such as uniform distribution over decohered states). The point is that there is a probability distribution which describes the observation from the state. As Born works fine, I have no problems with keeping Born! I don't see why one should first go to another distribution (uniform) and then derive Born from there. So this, to me, is a *different* aspect, and not essential to MWI.

MWI is, to me, very strange, almost like magic not physics, but it's a free country. Believe MWI if you want, but why?

I simply see MWI (with the Born rule concerning subjective observation) as the "natural" view on quantum theory as it is usually presented, in the same way as considering a static spacetime manifold is the natural way of looking upon relativity, as it is usually presented. It's a picture, that fits nicely with the ideas behind the formalism. Whether it is "true" or not, and whether one should "believe" in it or not are not considerations I want to delve into, because they are not fruitfull. As JesseM pointed out, the MWI view "explains" some apparent paradoxes in quantum theory, such as EPR/Bell and quantum erasers (simply because MWI sticks to the spirit of quantum theory). In the same way, a static spacetime view "explains" different apparent paradoxes in relativity, also because a static spacetime view sticks to the formalism. That's all. It is not a religion !
That doesn't mean that other views cannot give also satisfactory views on things. But MWI also has its merits.

 

[Demystifier]

I simply see MWI (with the Born rule concerning subjective observation) as the "natural" view on quantum theory as it is usually presented, in the same way as considering a static spacetime manifold is the natural way of looking upon relativity, as it is usually presented. It's a picture, that fits nicely with the ideas behind the formalism. Whether it is "true" or not, and whether one should "believe" in it or not are not considerations I want to delve into, because they are not fruitfull. As JesseM pointed out, the MWI view "explains" some apparent paradoxes in quantum theory, such as EPR/Bell and quantum erasers (simply because MWI sticks to the spirit of quantum theory). In the same way, a static spacetime view "explains" different apparent paradoxes in relativity, also because a static spacetime view sticks to the formalism. That's all. It is not a religion !
That doesn't mean that other views cannot give also satisfactory views on things. But MWI also has its merits.

That is perhaps one of the best explanations of MWI. I particularly like your analogy with static spacetime. Is that analogy invented by you, or first introduced by someone else? If the latter is correct, can you give me the reference where this analogy has been originally introduced?