Against Realism: Examining the Meaning of Local Realism

[kvantti]

kvantti, setAI: people have already responded to your assertions many times. If you don't want to be crackpots, you would do well to stop mindlessly repeating them without any regard to the received criticism.

Umm, sorry, I think this was my first message in this thread... I apologie if I have missed something (I don't follow these forums too often).

In the quantum computer: where else?

In fact, when I learned quantum computation in one of my classes, we learned it from a solidly Copenhagenist viewpoint.

Yes, you can always say something like "all the different calculations are performed simultanously as a superposition of every possible calculation", but that isn't exactly a physical description of what is happening in a quantum computer... it is more or less a description of the mathematics.

OK, let's assume that the MWI is false (which it, ofcourse, might be). How would you explain the physical behaviour of a quantum computer?

 

[kvantti]

Tell me then, how does the quantum computer in our Universe communicate with all the other 'virtual quantum computers' in the other Universes?

Through quantum interference. Different universes can interfere with each other if the quantum state of the system involved is coherent (as in a quantum computer).

Also, where is this quantum computer you speak of? This is such an abstract idea that the quantum computer calculates some of the sub-problems in other dimensions that I'm not sure it warrants a response. I'm ignorant on this subject, but what experiments have been done to confirm your quote?

IBM has the most advanced quantum computer nowadays with seven qubits. See this.

Oh and mathematically you can just say that "all the different calculations are in a superposition in the quantum computer during the calculation", if you don't want to think the MWI way.

 

[Careful]

Huh? I mean to say that I think RQM > MWI > Copenhagen > Bohm.

But what is YOUR idea about quantum mechanics ?! This entire discussion is about one's favorite color of spaghetti, while all the pasta tastes the same. It seems to be much more intelligent to place this question in the light of some problematic aspects of modern theoretical physics : (a) the problem of vacuum energy vis a vis the cosmological constant (b) the issue of realism (c) the problem of time (d) the validity of special relativity at all energy scales. Then, depending upon your answers on these (and other) issues you will find yourself confined to one of these religions, or you feel the logical need to dig deeper into QM itself. One question to start with for example is wheter one truly believes gravity to be necessary to even obtain a well defined theory of quantum electrodynamics.

Careful

 

[NateTG]

OK, let's assume that the MWI is false (which it, ofcourse, might be). How would you explain the physical behaviour of a quantum computer?

That's not a useful stipulation. MWI, Bohmian Mechanics, and the Plug and Chug interpretation all make identical predictions so the only way that MWI can be scientifically falsified is if all of the other interpretations also have the same problems.

Moreover, and this is something people love to ignore, physics does not and will never explain anything rather, physics is a collection of theories that make predictions. Statements like 'things fall because of gravity' are misleading - it would be better to say 'we call the tendency of things to fall gravity'.

As such, interpretations of quantum mechanics are (from a scientific point of view) really primarily interesting because they can lead to experimentally verifiable predictions. Otherwise, we might as well be discussing how many angels can dance on the head of a pin.

The fact is that (assuming the physics is correct) the quantum computer will work regardless of whether you think of it as a bunch of intereacting 'worlds', a winding maze of particle paths, or a black box.

 

[RandallB]

They said IF they actually make a true Quantum-Computer, it will, (IMO they won’t).

They already have. They just haven't made a "big" one.

What does that mean – something like a little bit pregnant but not really yet?

Plus even if they can come up with Q-C or other proof of MWI convincing to those that are not already convinced, that still only ‘proves’ one reality.

MWI is one thing but it doesn’t call for more than one reality!
The idea that one reality where say MWI is correct and BM is wrong;
and a second reality where say BM is right and there are no MW’s of MWI; ----- And Are both true ?

IMO that is beyond an ‘ad hominem fallacy’ it just logically ridiculous.
What argument would not fall on its own merit trying to support such a thing with anything like rational logic?

 

[AnssiH]

Well, here is a challenge for you (and for everyone who doesn't believe in the MWI) from Deutsch (as presented in his book The Fabric of Reality):

Explain where the calculations made by a quantum computer are performed when it solves the sub-problems associated with a single problem. Eg. when factoring a 250-number digit using the algorithm[/url], the number of sub-problems is about 10^500 (the number of particles in the universe is about 10^80).
And yet, a quantum computer solves all the sub-problems at the same time. So how can it solve 10^500 problems during only one calculation, if we have only one computer in only one universe?

(It would take classical computer 10^500 times the time of a quantum computer to perform the same operation)

And for the question why the MWI has become "some sort of religion": it gives a coherent, local and deterministic description of reality and in my point of view, it would be illogical that there would be only one universe. Didn't you think that there are "other dimensions" when you were a kid?

Like I've said in about every post to this thread, we make certain assumptions about "what exists" in reality and about how they behave to come to explain how the reality works to our selves. The above questions, about where the "calculations" happen and so on, only make sense in so far that you want to talk about "calculations" (and the related entities and concepts that you imagine is performing these "calculations", in this case in multiple universes) and MWI makes sense only in so far that you make certain assumptions about such things as the reality of a photon and interference between universes.

In fact, if you take a look at the opening post here:
https://www.physicsforums.com/showthread.php?t=130623
you'll notice how one can say the "calculations" happen or rather simply "exist" in static manner in spacetime just by insisting that spacetime really exists the way Einstein believed it to exist (Of course it will be difficult to explain why there seems to exist any "moments" at all, but that's a different discussion). One way to put it would be to say that the photon moves back and forth in time so that all its possible trajectories interfere, but this is wrong vocabulary because there is no motion in spacetime.

I am confident that all interpretations have their own answers about how the phenomena happen so to come to predict the exact same observable phenomena. So after all is said and done, many-worlds interpretation is just that, an "interpretation". At this stage we cannot pick and choose any QM interpretation to be the real deal.

Also if you have followed my posts in this thread you might have noticed how I've asserted that - quite likely - the reason why QM seems so damn odd to us is that we are trying to explain a behaviour of such entities that do not exist in such manner as we imagine them to exist. It is like explaining the behaviour of a rainbow after asserting it is an object which originates from a pot of gold, and only once one realizes how rainbow is rather the interference pattern on the surface of the observer, its behaviour starts to make sense.

Likewise, if you insist on the information between atoms to travel in the form of tiny billiard balls and on top of that imagine the motion of those billiard balls to exist in Newtonian sense (only look at it from one inertial frame) you may be forced to assert that the photon exists in many worlds so to exhibit the behaviour we observe. We may have to question the nature of many things to come up with more accurate answers, like the nature of space, matter, light, motion(/time), energy... Maybe even discard these concepts to understand this system we call "reality" from a completely new angle.

(On a related note, have people formed opinions about this idea of discrepte, stepwise spacetime expansion causing quantum behaviour?
http://www.estfound.org/
Haven't had time to really look into it, does it show some obvious weaknesses right off the bat?)

So, just what NateTG is saying, physics really is quite literally a collection of theories or rather assumptions about what exists and how they behave. And like I said before, because of how our understanding works physically, we can only deal with reality by assuming there exists such and such entities and asserting they have such and such relationships between each others. Because this is the only thing we are capable of, it seems to us that reality really is like this, but little bit of philosophy can show that this is not exactly true; reality does not actually work with concepts. The mental model we have about reality in our head really is just an expression of the real thing, and it is not accurate or even "the metaphysically correct way" to express reality. (And any math you produce is also merely describing the behaviour of some "entity" you imagine to exist metaphysically in that sense)

And btw, MWI gaining ground as the "favourite way" to describe reality in some circles doesn't mean it is true, it just means it is one of the easiest ways to "imagine/visualize" QM phenomena in your mind. Think about how you try to visualize any black-box system when you are trying to back-engineer how its behaviour comes about, and you will realize how the assumptions with which you find it easiest to imagine the functions of the black box don't mean that is actually what happens inside. There are always many ways to produce some desired behaviour. Any system builder/programmer knows this very well.

And my comments about MWI becoming a religion are referring to how I find that MWI-people are continuously asserting theirs is "the only possible interpretation". Please understand that no one is saying it "cannot be true", but that it is not something that has been proven to be true in any sense. I am certainly happy to see many sensible people at this thread who still understand this, and I hope MWI won't become so standard that considering other options will be viewed as heretical or crackpot. It is important to be honest about it being just an interpretation unless you want to see everyone believe in it like a religion. And you don't want this to happen if you believe in scientific method, yes?

 

[Chaos' lil bro Order]

Through quantum interference. Different universes can interfere with each other if the quantum state of the system involved is coherent (as in a quantum computer).
.

Are you talking about p-branes?

What defines two events in MWI anyways? I mean what is the smallest increment of information, whether it be spacial, temporal, or any other physical property you can think of, that must occur before the 'father' universe splits into two and the 'daughter' universe is birthed?

 

[Hurkyl]

This entire discussion is about one's favorite color of spaghetti, while all the pasta tastes the same.

Right -- but I was talking to someone who thinks spaghetti has to be purple, and thinks that my motivation for saying all pasta tastes the same is because I'm an idiot that doesn't like purple.

 

[AnssiH]

I believe the idea is rather, that all the possible universes exist "all the time" and interfere with each others. So the model is relying on certain ideas about how photons exist and how they interfere with other photons in other universes.

So, when someone is claiming this is the only way QM could work, I might just as well start asserting that when a rainbow seems to mimic the motion of the observer without delay, it can be explained in non-local terms only by assuming we are seeing a rainbow of a "different universe" every time we move.

Sure, this would make the phenomenon local, but in the case of rainbow we now know enough about how it exists to be able to see how its observed "motion" occurs in completely classical terms and within one universe, and yet there is a different rainbow visible for every observer, or rather that we should not assign identity to a rainbow; there is no rainbow at all without an observer. (This is a case of realism where something cannot exist without an observer, in completely classical sense)

Likewise with QM, the motion or the "apparent trajectory" of the photon really does depend on where it's going to be observed (one way or another), and to assume there really was a photon (with identity) in flight is already an assumption that is likely to be wrong to some extent, and will lead you to assert there must be multiple universes and we are merely observing photons from one.

So, what I'm saying is... dig deeper gentlemen.

 

[Careful]

Right -- but I was talking to someone who thinks spaghetti has to be purple, and thinks that my motivation for saying all pasta tastes the same is because I'm an idiot that doesn't like purple.

Hehe, I noticed that, but you also said that you might choose for the relational colored one, once this color would gain more popularity. My point being that whatever interpretation you pick, you keep on being stuck with some embarrasing shortcomings of the formalism itself. For example, I have to think hard about Bell inequality violation and spin statistics in Barut self field (although I could put them in by hand), but there is absolutely no problem with realism, time, special relativity and vacuum energy (tell that to some QG people - all their deep problems vanish in thin air).

Cheers,

Careful

 

[Hurkyl]

Hehe, I noticed that, but you also said that you might choose for the relational colored one, once this color would gain more popularity.

I prefer RQM over MWI now -- but if I was asked to choose between the popular interpretations I'd have to pick MWI since AFAIK, RQM isn't one of them yet.

you keep on being stuck with some embarrasing shortcomings of the formalism itself

Embarassing shortcomings are in the eye of the beholder.

 

[Careful]

I prefer RQM over MWI now -- but if I was asked to choose between the popular interpretations I'd have to pick MWI since AFAIK, RQM isn't one of them yet. Embarassing shortcomings are in the eye of the beholder.

This is exactly the miss-election attitude I was referring to in the beginning, I remember asking you what YOUR view towards QM is. Embarrasing shortcomings are not in the eyes of who perceives them, but in those minds which do not wish to adress them even when it is well known that the latter issues are definitely problematic. String theory does adress the vacuum problem by making a supersymmetric ansatz : at that moment, you can say whether you find such mechanism using exotic particles credible or not (I certainly don't). Anyway, if you do not believe some detailed matter content to be responsible for the cancellation of the vacuum energy, then the most direct step would be to question the phenomenon (in either the renormalization procedures) at its very roots.

Careful

 

[kvantti]

I was writing a long post and my computer crashed so I'm pretty frustrated at the moment and therefore I will state my matter briefly:

NateTG:

That is an instrumentalist point of view. I'm glad that not all physicists are instrumentalists.

AnssiH:

You obviously believe in some sort of "hidden variable" theory. Fair enough. But keep in mind that even you are interpreting quantum mechanics from your own point of view, and in my eyes it doesn't differ from the Copenhagen interpretation; you and CoI state that the formulas of quantum mechanics do not describe reality as it is, but only probabilities of observations within reality. The difference between CoI and your interpretation is that the CoI doesn't explain how the "probablities" can interfere and entangle... it just says they do. You seem to have your own vision how they actually do interfere (as we all do).

The MWI says just the opposite: the formulas of quantum mechanics describe reality as it is. What would be simpler? The concept of multiverse emerges naturally from this kind of thinking. It also explains quantum interference and entanglement in a very simple manner.

It is important to be honest about it being just an interpretation unless you want to see everyone believe in it like a religion. And you don't want this to happen if you believe in scientific method, yes?

Yes, it is "just an interpretion" of quantum mechanics. But it also a "theory" of multiverse. A theory that most quantum cosmologists and quantum computer researchers find compelling, because it offers explenations of the physical behaviour of the quantum system they study, rather than just "cold mathematics".

-- you'll notice how one can say the "calculations" happen or rather simply "exist" in static manner in spacetime just by insisting that spacetime really exists the way Einstein believed it to exist.

This isn't true in the case of quantum computers; the calculations are based upon quantum effects, such as quantum interference and entanglement, so you can't just say "it just acts as if the different calculations would interfere, but actually they don't; the calculation and the result just already exist in a static spacetime." Quantum computers actually prove that quantum interference and entanglement are real phenomenom; not just illusions of "static spacetime." And that been said, you can't say that a photon moves as if it would interfere with itself; it actually does interfere with itself.

 

[tehno]

Sorry for interrupting this discussion,I'm really no expert on this but I can't resist not to ask.
With their capability of processing information can a quantum computer one day answer the ethernal question of finite strategy games:say Chess or GO?Estimation is that there are about ~10^{60} legal chess positions.

 

[kvantti]

Are you talking about p-branes?

What defines two events in MWI anyways? I mean what is the smallest increment of information, whether it be spacial, temporal, or any other physical property you can think of, that must occur before the 'father' universe splits into two and the 'daughter' universe is birthed?

Here is a FAQ considering the MWI. You'll probably find answers for your future questions there. A single universe is irreversibly split into many universes when the quantum system involved in the splitting decoheres. But that doesn't mean the decoherence causes the splitting; the universes split all the time, be there interactions or not. Decoherence only distinguishes the universes from each other and therefore they can't interfere with each other anymore.

Sorry for interrupting this discussion,I'm really no expert on this but I can't resist not to ask.
With their capability of processing information can a quantum computer one day answer the ethernal question of finite strategy games:say Chess or GO?Estimation is that there are about ~10^{60} legal chess positions.

Yes, a quantum computer can resolve this kind of calculation very fast.

 

[NateTG]

Yes, a quantum computer can resolve this kind of calculation very fast.

Quantum computers are not proven to solve NP hard problems in polynomial time, and, in fact, are strongly expected no to.

 

[AnssiH]

You obviously believe in some sort of "hidden variable" theory. Fair enough.

No, I don't particularly believe in hidden variables or copenhagen. I believe our ideas about what exist are currently wrong at a deeper level than where any of the mainstream interpretations reach. (Like what is "motion" and so on) I believe one day someone will figure out a really simple deterministic explanation and everybody will slap on their forehead and go "doh!"

Well okay maybe there's some arguments back and forth first :)

This isn't true in the case of quantum computers; the calculations are based upon quantum effects, such as quantum interference and entanglement, so you can't just say "it just acts as if the different calculations would interfere, but actually they don't; the calculation and the result just already exist in a static spacetime." Quantum computers actually prove that quantum interference and entanglement are real phenomenom; not just illusions of "static spacetime."

Obviously to interpret this in terms of static spacetime you also have to assume that what we see as a photon is in fact just a wave-like connection between two atoms over spacetime (i.e. something with "volume", which occupies all the possible "trajectories". and in such manner that the shape of the "beginning" of the travel is affected by the shape of the "end").
If you keep thinking about photons as little balls it won't help you at all to look at it from the point of view of its own inertial frame (where "delayed choice" experiments are not "delayed choice" at all), unless of course you imagine the situation as if this little ball bounces back and forth in time over every possible trajectory. But to think about it this way you are confusing a whole new concept of "time progression" within spacetime itself, so...

Of course no one can say which idea of photons is more correct metaphysically, but this is just what I'm saying about how our ideas about what exists and in what manner are just assumptions, and they are bound to remain as assumptions forever, although I believe we can still peel much deeper than where we currently are.

And that been said, you can't say that a photon moves as if it would interfere with itself; it actually does interfere with itself.

Of course it does, although again, if you talk about the phenomenon in different terms, it may become meaningless to say it metaphysically interferes with itself, because it is meaningless to assign any identity to the photon in the first place. If you consider the so-called "photon" to literally be the "beep" that the atom does (i.e. when we say "the atom received a photon"), and the beep to be caused by a wave-like energy finding its way over all the possible routes to that atom (note that this is different from a wave propagating to all directions), you could only say there is wave-like energy that interfered with itself and no photon with identity ever made any journey.

But the trick is of course that you'd have to consider this to happen not over space but over spacetime. And let it be said that my confidence in the existence of spacetime is not particularly high either, I think spacetime too is a concept that is very much incorrect from what really exists.

So, if I have to make a bet about an interpretation, just for fun, I bet they are all going to be considered ridiculous when some assumption about something very fundamental will fall in place and show us more accurate view of all the phenomena we observe. Something akin to the shift from relative motion to relative time in relativity. (here I go again, praising relativity... silly me :P )

You may be tempted to say "maybe MWI is just this idea", but to me MWI is like all the other interpretations, and they are basically arguing about whether everything is made out of "earth, air, water and fire" or from "solid, liquid and gas", or perhaps the fundamentals are "opaque" and "transparent" matterpieces, when they should be concentrating on much much deeper issues. Something like, how could inertia be fundamental? Think about that.

 

[Chaos' lil bro Order]

Here is a FAQ considering the MWI. You'll probably find answers for your future questions there. A single universe is irreversibly split into many universes when the quantum system involved in the splitting decoheres. But that doesn't mean the decoherence causes the splitting; the universes split all the time, be there interactions or not. Decoherence only distinguishes the universes from each other and therefore they can't interfere with each other anymore.



Yes, a quantum computer can resolve this kind of calculation very fast.

'A single universe is irreversibly split into many universes when the quantum system involved in the splitting decoheres.'

Fair enough. Now tell me the variables of the 'quantum system' and what constitutes 'decoherence'. I'm tired of all these big words being thrown around without any real meaning behind them. This is not true understanding IMO, its akin to cut and pasting a wikipedia entry or repeating popular phrases found in books like 'The elegant Universe'. Can this thread get down the to nitty-gritty already, its becoming muddied with pop-science and its really pissing me off now.

 

[Chaos' lil bro Order]

kvanti

Upon reading some of the text from your link I found this explanation on why universes split according to MWI:

'The precise moment/location of the split is not sharply defined due to the subjective nature of irreversibility, but can be considered complete when much more than kT of energy has been released in an uncontrolled fashion into the environment. At this stage the event has become irreversible.'

It appears that MWI defines the splitting of a universe as when an 'irreversible' event takes place in it. The example given was the irreversibility of thermodynamic processes. But the statement above says that the splitting event isn't even clearly defined due to the subjective nature of irreversibility. My word, this MWI theory is turning out to be a big joke the more I read about it. I mean heck, it can't even make sharp concrete predictions even within its own framework, so why should anyone take it seriously. It seems like just another theory that defers what we do not understand about our universe to some sort of mystical idea that's no more falsifiable than god. It hinges on a subjective process, lmao!

 

[AnssiH]

Perhaps I'll throw in couple of questions myself. I don't ask these in the intent to show MWI false, but to find out some details of the model.

From FAQ:
For two branches or worlds to interfere with each other all the atoms, subatomic particles, photons and other degrees of freedom in each world have to be in the same state, which usually means they all must be in the same place or significantly overlap in both worlds, simultaneously.

"All the other atoms etc..." refers to what? All the atoms in the universes? Or just all the atoms of some smaller system? I.e. is it enough if in two worlds there just happens to exist an identical geiger-counter in the same "place" for them to interfere, even if the rest of the atoms in these worlds are different?

How do we identify "place", or rather, what justifies us to even assume that space is like an invisible backdrop where each location has identity?

What do we mean with the parts being "simultaneously" at the same place. Does this mean they not only have to exist in the same "place" but also in the same inertial frame? (Since in relativity any object with volume will not occupy the same "place" simultaneously from two different inertial frames)

Worlds irrevocably "split" at the sites of measurement-like interactions associated with thermodynamically irreversible processes

What does "at the sites of measurements-like interactions" mean here. Is it just the "site" that splits or the whole universe? I.e. Only the parts in the geiger-counter it that moved irreversibly, or everything?

If it is latter, when do they split? If you are observing the geiger-counter from 10 lightseconds away, will you split 10 light-seconds after counter or simultaneously?

If latter, simultanously in which inertial frame and why?

If it's in your own simultaneity, what if you are approaching the geiger-counter at great speeds when the split occurs at the counter, and then brake into the same frame as the counter so that the split hasn't happened yet in this frame. Will the split reverse too?

Contact between a system and a heat sink is equivalent to increasing the dimensionality of the state space, because the description of the system has to be extended to include all parts of the environment in causal contact with it.

Are not all things causally connected to everything in the universe? Does MWI assume that "causal connection" is something that exists metaphysically, instead of just a loose semantical concept? If so, how does it exists?

And also, does MWI comment on what causes the photon to take one or another route in the beam splitter within one universe, or is it still just "non-caused", arbitrary thing?

(btw, I don't much appreciate how the FAQ states "There is no other quantum theory, besides many-worlds, that is scientific". This is just the kind of definition bending that you'd expect from religious people.

 

[kvantti]

AnssiH:

OK. Let's forget about photons. Electron is a particle that has an inertial frame and it can interfere with itself in the double slit. And yet, the electron is completely localized particle according to the Heisenberg picture. So classically thinking, for it to interfere with itself, it would have to go thru both slits at the same time and travel to all the possible directions after that. Then for some magical reason, the electron is observed only in one place.

I'll reply to your questions in another post, if no one else has before that.

Now tell me the variables of the 'quantum system' and what constitutes 'decoherence'.

Varying observables: spin, momentum, position, energy, time. These are all that I can recall.

Decoherence: the loss or absence of interference within a quantum system due to interaction (and therefore entanglement) with another quantum system. I can't explain it simpler. You should just study the concepts.

'The precise moment/location of the split is not sharply defined due to the subjective nature of irreversibility, but can be considered complete when much more than kT of energy has been released in an uncontrolled fashion into the environment. At this stage the event has become irreversible.'

You misinterpreted this. The splitting isn't sharply defined in one universe, because a single universe splits all the time in the multiverse, and you can't tell in which universe you are until the splitting has become irreversible due to decoherence within the quantum system that has been split. In other words, before you observe the state of the quantum system, you can be in every possible universe where the quantum state hasn't decohered yet. That is what the author means when he writes "the split is not sharply defined due to the subjective nature of irreversibility."

Don't jump into conclusions.

 

[AnssiH]

AnssiH:

OK. Let's forget about photons. Electron is a particle that has an inertial frame and it can interfere with itself in the double slit. And yet, the electron is completely localized particle according to the Heisenberg picture. So classically thinking, for it to interfere with itself, it would have to go thru both slits at the same time and travel to all the possible directions after that. Then for some magical reason, the electron is observed only in one place.

Well I wouldn't consider any sub-atomic element in the atomic model to be a point-like particle at all, because our idea of a point-like things are based on stable pieces of matter. The stable elements that bring about this spatial stability as their emergent feature are probably not stable in the same manner at all. I don't have any answer as to the better description of what "electron" is, but let it be said that this question is indoubtedly going to be an important one (and incidentally it is one question that, AFAIK, troubled Einstein's mind greatly at his later years)

As an interesting philosophical excercise, consider Milo Wolff's spherical standing waves. Disregarding if his idea can be real or not, it is interesting to note how radically different fundamentals could bring about so similar measurable effects.

Also with experiments with electrons it becomes important to consider the reality of electromagnetic fields that are used to move the particles (and like I said, possibly the fundamental reality of motion)

More interesting but related problem, as I mentioned in the other thread myself, is that they have been able to make full atoms produce interference pattern. (I've been trying to find more details about these experiments)

Yet it doesn't seem to me at all that MWI "must be true", but instead my head is spinning with all the options that are still open and greatly unexplored (and cannot be explored without a decent army of scientists really spending time at taking the parts of this problem down and looking at it in completely new ways, like Einstein looked at the motion of light).

I'll reply to your questions in another post, if no one else has before that.

No hurry, I'll be away for about a week... Take care now!

 

[kvantti]

For two branches or worlds to interfere with each other all the atoms, subatomic particles, photons and other degrees of freedom in each world have to be in the same state, which usually means they all must be in the same place or significantly overlap in both worlds, simultaneously.

"All the other atoms etc..." refers to what? All the atoms in the universes? Or just all the atoms of some smaller system? I.e. is it enough if in two worlds there just happens to exist an identical geiger-counter in the same "place" for them to interfere, even if the rest of the atoms in these worlds are different?

The universes need to be almost identical for them to interfere. For example, when performing a double-slit experiment with single electrons, you get two significant sets of universes: in 50% of the universes the electron went thru the left slit and in another 50% it went thru the right slit. Since identical, these two sets of universes can interfere, giving the observed interference pattern after many electrons have been "shot" thru the slits.

If we observe which slit the electron goes thru, the universes decohere from each other and are not identical anymore. You get two sets of universes: in 50% of the universes the electron is observed to go thru the left slit and in another 50% of the universes thru the right slit. The universe is irreversibly split into two sets of universes that can't interfere with each other. So in this case we don't observe the interference pattern forming.

Does this mean they not only have to exist in the same "place" but also in the same inertial frame?

Yes, you just "look" the situation from the inertial frame of the quantum system in question. In the MWI, different inertial frames are just special cases of different universes. In other words, every inertial frame can be thought to be it's own universe (or rather, a different view of the same universe; likewise different universes are just different views of the multiverse). It is rather complicated, because the multiverse is a multidimensional "object". No one knows how many dimensions. String theory suggests 10 dimensions.

What does "at the sites of measurements-like interactions" mean here. Is it just the "site" that splits or the whole universe? I.e. Only the parts in the geiger-counter it that moved irreversibly, or everything?

Measurement-like interaction is any interaction that entangles the observed quantum system with the observers quantum state upon interaction. The whole universe is split, but the resulting universes don't have to differ radically; a single quantum event happening differently in two universes is enough for splitting. A double-slit experiment is a good example of this.

If it's in your own simultaneity, what if you are approaching the geiger-counter at great speeds when the split occurs at the counter, and then brake into the same frame as the counter so that the split hasn't happened yet in this frame. Will the split reverse too?

That is a question you need to ask from a quantum cosmologist; I have no clue... and I don't want to speculate.

Are not all things causally connected to everything in the universe? Does MWI assume that "causal connection" is something that exists metaphysically, instead of just a loose semantical concept? If so, how does it exists?

Every event that is causally connected with a certain quantum system is entangled with the quantum system. That is all there is to it.

And also, does MWI comment on what causes the photon to take one or another route in the beam splitter within one universe, or is it still just "non-caused", arbitrary thing?

The MWI is quite clear about it: in 50% of the universes the photon goes thru the beamsplitter and in another 50% it gets reflected from it. You end up in both universes, but because subjectivily you experience only one universe, it seems totally arbitrary. So you have 50% chance to experience one of the two sets of universes.

(btw, I don't much appreciate how the FAQ states "There is no other quantum theory, besides many-worlds, that is scientific". This is just the kind of definition bending that you'd expect from religious people.

This is true, but it is still the best FAQ about the MWI.

 

[RandallB]

Quantum computers . not proven to solve NP hard problems ... .

Couldn't find it used in other posts - What does "NP" stand for??

 

[NateTG]

Couldn't find it used in other posts - What does "NP" stand for??

NP is the set of problems whose answers can be checked in polynomial time.

 

[Chaos' lil bro Order]

Fine then, I think we disagree and nothing can be said by either to sway one's opinion on MWI.

One last question. What is the smallest event that can split a universe in two?

 

[kvantti]

Well I wouldn't consider any sub-atomic element in the atomic model to be a point-like particle at all, because our idea of a point-like things are based on stable pieces of matter. The stable elements that bring about this spatial stability as their emergent feature are probably not stable in the same manner at all. I don't have any answer as to the better description of what "electron" is, but let it be said that this question is indoubtedly going to be an important one (and incidentally it is one question that, AFAIK, troubled Einstein's mind greatly at his later years)

Particles are point-like in the way that they don't have any spatial size. In our most fundamental theory of reality (the quantum field theory) particles are quantas of energy of different quantum fields. How could energy have any "size"? The classical "size" of any particle is defined by its Compton wavelenght.

As an interesting philosophical excercise, consider Milo Wolff's spherical standing waves. Disregarding if his idea can be real or not, it is interesting to note how radically different fundamentals could bring about so similar measurable effects.

Same for the string theory. It is consistent with every observation because it doesn't make any new predictions at the energy scale we can handle nowadays with our most powerful particle accelerators. It isn't falsifiable.

Also with experiments with electrons it becomes important to consider the reality of electromagnetic fields that are used to move the particles (and like I said, possibly the fundamental reality of motion)

I see no problem seeing motion as an illusion... then again, I think the whole universe we perceive is an illusion of some sort.

More interesting but related problem, as I mentioned in the other thread myself, is that they have been able to make full atoms produce interference pattern. (I've been trying to find more details about these experiments)

Not just full atoms, but whole molecules too! In the '99 a research team was able to observe the self-interference of a http://wwwcsif.cs.ucdavis.edu/~cs175/Images/Buckyball.png is an article about the experiment.

So we have little balls of matter interfering with themselves. Nice.

Yet it doesn't seem to me at all that MWI "must be true", but instead my head is spinning with all the options that are still open and greatly unexplored (and cannot be explored without a decent army of scientists really spending time at taking the parts of this problem down and looking at it in completely new ways, like Einstein looked at the motion of light).

Yes, the MWI doesn't have to be true, but it is the simplest way to explain the physical behaviour of quantum phenomenom, like the self-interference of buckyballs. Thus, it is favoured by the Occam's razor and this is the main reason I prefer it over other interpretations.

 

[tehno]

Quantum computers are not proven to solve NP hard problems in polynomial time, and, in fact, are strongly expected no to.

I admit I'm dissapointed to hear that "secret" of a chess game will remain
undiscovered for a long time then.
I really thought that quantuum computer would resolve final status of the game in the near future...

 

[charlylebeaugosse]

Travis Norsen has written an article entitled "Against Realism". In it, he argues that the phrase "local realism" is not meaningful.

Against Realism (2006)

Abstract:
"We examine the prevalent use of the phrase “local realism” in the context of Bell’s Theorem and associated experiments, with a focus on the question: what exactly is the “realism” in “local realism” supposed to mean? Carefully surveying several possible meanings, we argue that all of them are flawed in one way or another as attempts to point out a second premise (in addition to locality) on which the Bell inequalities rest, and (hence) which might be rejected in the face of empirical data violating the inequalities. We thus suggest that this vague and abused phrase “local realism” should be banned from future discussions of these issues, and urge physicists to revisit the foundational questions behind Bell’s Theorem."

-----

My questions for your consideration:

1. What does realism mean to you?

2. Einstein said: "I think that a particle must have a separate reality independent of the measurements. That is: an electron has spin, location and so forth even when it is not being measured. I like to think that the moon is there even if I am not looking at it." Is this realism?

3. In your opinion, is "realism" an assumption of Bell's Theorem? If so, where does it arise?

==========
For such matters, see
Einstein, Tolman, and Podolsky, Physical Review 37, (1931) 780-781
to wash out many errors that tend to make Einstein much more naive than he really was.

 

[setAI]

Quantum computers are not proven to solve NP hard problems in polynomial time, and, in fact, are strongly expected no to.

ehem:

using quantum computation to solve NP complete problems:

http://www.arxiv.org/abs/quant-ph/9912100
http://www.arxiv.org/abs/quant-ph/9801041
http://www.arxiv.org/abs/quant-ph/0508177