Understanding MWI: A Newbie's Guide to Quantum Physics and the Multiverse

[confusedashell]

No it makes all the difference in the world.
if MWI is true, you NEVER interact with another conscious human being, just for 0,0,0,0,00,01 nano second.
if single universe is true, everything is real, everyones conscious, the same persons 24/7.
the girl u marrie is the girl u marrie.
ur parents is ur parents.
conversations mean something, cause the person u speak to is actually the person who listen and speaks back, not a new individual every second...
otherwise its like I speak to you now, then tomorrow i speak to a different you and that you doesn't have any idea wtf is bein discussed because the you now is in another universe...
**** i think i need antipsychotics...KEEP MWI in secrecy, it'll extinct human race forreal

 

[xantox]

No it makes all the difference in the world.
if MWI is true, you NEVER interact with another conscious human being, just for 0,0,0,0,00,01 nano second

No, you are misunderstanding, this statement is totally wrong. Whether other branches exist or not, this makes no difference in your branch. You interact with other people and they are never disappearing anywhere, nor forgetting who you are. You live your own life where only one history happen, but all other possible histories also happen elsewhere in the multiverse. So that how possibly killing out all the other possibilities could make you feel better?

 

[confusedashell]

because then I am consistantly ME from my NATURAL ORGANIC BIRTH to my NATURAL organic death
my girl is MINE ONLY MINE not some clone freak bull**** worthless nanosecond presence who's banging others in other universes.
the world is objective...

people matters, I matter, I am unique...and waht the **** does "my branch" mean?
sounds solipsitic like u guys arent existing in same universe as me and i can go around kill people because they only live here 1nanosecond

like right now, we've had this conversation for a hour or so, the consciousness who responded in this universe in the beginning to me(my consciousness) is the same as this? or are you now another one from another branch or whatever?

 

[xantox]

because then I am consistantly ME from my NATURAL ORGANIC BIRTH to my NATURAL organic death

That is always the case. No difference here between MWI or not MWI.

im unique...

If you had a twin brother, would you like to kill him to feel unique?

and waht the **** does "my branch" mean?

It means the universe where you live. Which is one amongst many existing in the MWI multiverse.

sounds solipsitic like u guys arent existing in same universe as me and i can go around kill people because they only live here 1nanosecond

I fail to understand how you can jump to such absurd conclusions. Why should you go around kill people? And why you think they only live 1 nanosecond?

like right now, we've had this conversation for a hour or so, the consciousness who responded in this universe in the beginning to me(my consciousness) is the same as this? or are you now another one from another branch or whatever?

I am the one who had this conversation with you and the other people here.

 

[confusedashell]

yea but wf does this mean, I might be waaaay off here and thas why I started this thread.
I do not give a **** if other universes exist because they do not interfere with this (atleast that's what I thought).
no i would NOT(EDIT LOL) kill my twin, because my twin wouldn't be same as me.
i know a lot of twins and they are completely different.

so are you saying me and you are in the same branch, has been since birth will be 'til death?
So my GIRL that I met 5 years ago, is still EXACTLY TEH girl I met, not some clone and the one I met has branched off?

 

[xantox]

i kno a lot of twins are they are completely different.

Well, monozygotic twins share the same DNA. They are much more similar one compared to the other than, say, myself now, compared to myself 10 years ago.

so are you saying me and you are in the same branch, has been since birth will be 'til death?

Yes. However in MWI there are also many others around, filling the other possibilities which didn't happen here. But you don't need to think about them.

 

[confusedashell]

Yes I'm aware of this, but life experiences = create your personality in a sense and identity.
I was born single, so I do not have a twin in that sense.

So what are you really saying, we arent CONSTANTLY SPLITTING off to differnt universes and in 10seconds ago u spoke to a differnt clone of me?

Me(this version of me, this only consciousness I am) will forever remain in this branch (branch = universe?) and everyone in it will to.
So, everyone is still the same, and MWI only states that other universes EXIST but its not like the stupid.

Everytime you go right, there was a new universe made where u went left, and other people u know might have gone to that branch?cause if that's the case how the hell do you get through the night?

 

[OOO]

Either MWI is false and no real physicist truly believes it or the myth that scientist are narcissitic emotional dead humans seems to fit:P

Suppose your phone number was 11131719. Suppose you had had this number for a very long time, so you have a certain emotional relationship to it (yes, of course, no way the same as to your girlfriend). How would you memorize that number ? I'd simply throw in: primes between 10 and 20.

Suppose you had actually adopted this mnemonic over the years. But now a friend comes to you and says: "nonono, this is simply not true ! These are totally random numbers. You only have them by accident. If you are using this scheme, you're once again proving that you are an overly emotional character."

A few remarks about this:

1) Obviously neither your emotional relationship to your phone number nor your emotional behaviour in general has anything to do with that mnemonic. It might be that you're overly emotional, so what ?

2) Probably for you the mnemonic is just a mnemonic. You've created it in order to come to grips with your limited resources. Thus the main point for you is that the scheme is simple and "explains" the number completely. That's an allegory of Occam's razor.

3) If you really insisted on your mnemonic being a "true" law, this claim could not be verified by looking at the number alone. The sequence is much too short, and no experimenting is possible because it doesn't change. It's like in those IQ tests where you have to complete a sequence of numbers: they only test how well you were conditioned to common primitive algorithmic thinking (or should I say how well you can create untestable theories ?).

4) In order to verify that the mnemonic is "true", it should make predictions about things you don't yet know. The difficulty is, that the only input information you have is your actual phone number. There's an awful lot of possible theories that could extend your mnemonic to something that makes predictions, but most of them will certainly be wrong.

Will this all stop you from using your mnemonic ?

What about this little helper: 1+1+1 = 3, 1+7+1 = 9 ?

 

[confusedashell]

So here you are implying what?

That I'm overly attached to my friends/loved ones and overly emotional?
Might be, I'm very emotional, but hell we live in a ****n world where its survival of the fittest, one shot, one life, any sec you could drop dead, you need human connections to survive n have meaning.

Sorry my brain is melted as you can probably see from this desperate post.
Sorry guys:P

Either way, MWI might as well NOT be true? there is no evidence? I keep eharing about Deutsch claiming there is evidence because he has seen some **** he thinks he has seen or some ****.
For me I think his hallucinating, this guy insist on MWI so much it's like a religion for him.

Now i understand the MWi is a serious Quantum Interpretation, and more likely than copenhagen interpretation which Einstein pretty much "explained away" sensible to me in 6 setences.

so, what other interpretations is there out there? that would support the world view of most people that this universe is THE only universe?
Is there any? or does QM INSIST there must other universes?

To me, since MWI can't be proven or atleast hasn't been proven yet, I'd like to live in ignorance and believe in some other interpretation, but now as I've read so much about MWI it's hard to just dismiss it without replacing it with some other view.
So if anyone got a alternative which supports more "single objective universe independant of the observer type of common sense realism ****", please show me even if you don't agree with it.

 

[xantox]

Yes I'm aware of this, but life experiences = create your personality in a sense and identity.I was born single, so I do not have a twin in that sense.

True, but according to MWI, in some other branch you was really born with a twin! In that branch, you have indeed a slightly different personality and identity than in this one.

So what are you really saying, we arent CONSTANTLY SPLITTING off to differnt universes and in 10seconds ago u spoke to a differnt clone of me?

You may picture it this way, you live only one "movie" with one scenario ad all characters stay there with you. The other instances exist in other "movies" with different scenarios than yours.

Everytime you go right, there was a new universe made where u went left, and other people u know might have gone to that branch?

Everytime you go right, you continue to stay in your own movie where you go right and your friends are there too and all see you go right. However in another movie also existing in the multiverse, a copy of you go left, and a copy of your friends see him go left.

I hope this helps and that you will not have any nightmares thinking about these issues.

 

[confusedashell]

Thanks, think I need to give my brain a cryonic hypernation for awhile lol.
This really tears the greymatter apart:P

You a true believer in MWI or ?

Im like this: if I fear it, I believe it lol not very rational but i got a disorder called "pureo" who gets "hooked" on **** and blows **** outta proportion.

But basically MWI could be wrong and this universe is all ther eis right? It's not like anything goes against it as for evidence yet?

theres just something with the whole concept I don't get like, if it were true, you would actually be running around ****ing EVERY dog in the world (cause if u start from birth) it IS physically possible.
Since we do not see such serial animalsex predators or any other extreme **** in this universe it leads me to doubt MWI.
It makes sense on the paper, but in real world "nature is as nature is" or whatever the quote is.

 

[OOO]

So here you are implying what?

I wanted to imply that MWI is a mnemonic, that it's untestable, and that emotions have nothing to do with it. Nothing about your girlfriend.

 

[cristo]

But basically MWI could be wrong and this universe is all ther eis right? It's not like anything goes against it as for evidence yet?

I don't see how there can ever be any evidence either way. The important point is that you only live in one world: all these other worlds may be splitting off every time you make a decision, but you don't get to go to these worlds as well.

 

[confusedashell]

I understand, one world, but is it like string theory whos says this universe doesn't SPLIT off and is completely isolated from others?

I love my girlfriend, but this shti has really made me doubt her being the REAL girl I fell inloev with lol.
Might sound superficial and stupid, but truly :P

it's just if I(alone) liev in this universe then my girlfriend does not just her clone.
This is something my brain cannot deal with no matter how good "replacement" she would hypthethically be.

 

[setAI]

I don't see how there can ever be any evidence either way. The important point is that you only live in one world: all these other worlds may be splitting off every time you make a decision, but you don't get to go to these worlds as well.

parallel universes interfere with each other- this interference determines the Born probabilities- so any understanding of the sum-over-histories which shape the world we observe and our consciousness of it requires an understanding of parallel universes and how they interfere with each other-

this interference can be harnessed by technology- as with quantum computers which are just scratching the surface of what may be possible- information may be shared between sets of similar parallel universes- so it appears that complex realtionships between parallel universes will be a defining aspect of future technology and thus the future of life and intelligence-

the implications of parallel universes that interfere- however subtly- go beyond what we humble apes can even envisage at this point-

 

[JesseM]

So your basically saying our lives is less than worthless pointless and even worth spendin time living?

I mean honestly: you believe you 24/7 meet "new" versions of your friends that's clones?

Seriously, I mean, seriously?

Makes no sense tho, cause, when someone dies, he never returns, if u were JUMPIN universes he'd surely ressurect a few times atleast... Since this is not the case, it seems to me ALL OF US remains in the same "branch" forever unless you believe ur consciousness is somewhat special and can travel separate from the rest of the physical world.

*Pray to spaghetti monster all Quantum phycisist who has faith in MWI tries the quantum suicide experiement so we don't have to hear this scifi delusionalism no more* :D

It seems like you're confusing two different issues here. One is the issue of whether everyone experiences a *consistent* history, so that if you see someone die in an accident, you're guaranteed not to later meet a version of them that survived the accident--as I already explained to you, the MWI does say that everyone's history is consistent. But then you're mixing in the issue of whether people are the "originals" or just "clones". But from the MWI perspective, there is no such thing as the "original"--all the different versions which split from a common past are on equal footing. If at some earlier time T1 we have your brain in state A, and at some later time T2 we have different versions of you A1, A2 and A3 which all remember having been in state A at time T1, but remember different things happening subsequently, what basis can there be for thinking that one of these is the original and the other two are copies?

Perhaps it would help if you imagine your consciousness as a river which can split into multiple different channels--the water running through each channel is still water that came from the original river. And if you don't like the idea of a single consciousness "splitting", you could also imagine something like the many-minds interpretation, where each brain state is actually associated with an infinite number of distinct streams of conscious experience which have identical histories up until that point, but which can later split apart and experience different subsequent brain states. If this view were right, when you interact with your girlfriend you'd actually be interacting with an infinite number of identical versions of her consciousness inhabiting the same brain, and if you interact with her again later you'd be interacting with some fraction of the versions that were present in the previous brain state, while other members of that collection had moved into different branches from the one you're in. Of course these are all philosophical perspectives, so there's no way of answering the question of which is "correct", or even being sure if the question has an answer!

 

[xantox]

But basically MWI could be wrong and this universe is all ther eis right? It's not like anything goes against it as for evidence yet?

MWI is well established as one of the main interpretations of quantum theory, for two reasons: it is both consistent and simple. There is no evidence against it, but it might as well be not true.

 

[reilly]

Why doesn't an MWI approach go back to the origins of probability theory, particularly conditional probability -- as in chains of events--, circa the 17th century? (I'll bet it actually does, but was jettisoned, so to speak, for whatever reasons, one of which I would guess was cumbersomeness. )

Regards,
Reilly Atkinson

 

[vanesch]

Either way, MWI might as well NOT be true? there is no evidence? I keep eharing about Deutsch claiming there is evidence because he has seen some **** he thinks he has seen or some ****.
For me I think his hallucinating, this guy insist on MWI so much it's like a religion for him.

I think you're more in for a basic course on philosophy (and philosophy of science) than for a course on quantum mechanics

As I tried to point out earlier, scientific theories never talk about "what's true": there is scientifically no way to know what is "true", the only thing we can say about scientific endeavour is to say that there's a theoretical model of reality that makes correct observational predictions. But that's never a proof that the theoretical model is "true" ; only, it is a practical mnemonic (as OOO pointed out) to "tell yourself" that this is how nature "really" is. A good theoretical model doesn't only make right predictions, it also comes with a "picture" of reality.

Now, one of those theoretical models which is highly successful in explaining a tantalizing amount of observations is quantum theory. The problem with this theoretical model is that no matter how you turn it, it is difficult to get a fully consistent *picture* out of it. Most of the time (from the very beginning) people have been *fiddling with the theoretical model* in order to get out a "nicer picture", but it always gives some strange quirk. MWI has the advantage of NOT fiddling with the formalism, but to keep it the way it is built upon its elementary axioms. I find this personally the most attractive part of MWI, because it fullfills its role as a "mnemonic" best. But it is true that the picture that comes out of it is indeed very strange, and can have very weird philosophical implications.

So I consider MWI indeed as the best "mnemonic" for doing quantum theory as we know it. Whether it is "really true", I haven't gotten a clue. I don't think that MWI is the best mnemonic to study romantic relationships, for instance, but usually you don't consider romantic relationships in the framework of quantum theory.

However, there's something disturbing in your phrase, where you desperately HOPE that MWI is not true, for its implications on your view on romantic relationships. But that's the most anti-scientific attitude you can have ! This kind of attitude is what breeds creationists (evolution might just as well not be true ? Gosh, I would hate it to descend from an ape, and not from Adam and Eve...) From the moment you have emotional involvement in a possible scientific statement, you're out of science.

So, I'd say, cool down. First of all, you should study some philosophy of science, and about what can be known and not. Then you should study the formalism of quantum mechanics (and probably you'll understand where the MWI idea comes from: it comes from the axiom of superposition, applied to macroscopic states). And then you should realize that MWI is just *A* picture of A current theory. I think that picture does a great job in helping one understand that theory. In my eyes, it is the most elegant view on the quantum formalism. But I'm far from sure that 1) it is the "correct" picture and 2) that this current theory will remain so for ever - especially with gravity there is some serious issue for instance. But it might be. As always, we can't know what is "really true" in science, we can only say that we have models which give good predictions in agreement with observation.

 

[Count Iblis]

confusedashell, http://arxiv.org/abs/astro-ph/0302131"

 

[LURCH]

Oh, very nice, Count. You really are evil, aren't you?!

 

[reilly]

Jive

The fact that 1. nobody has tried to answer my questions, or 2. that nobody has suggested that my questions are dilatory helps to convince me even more that MWI is jive.

I know that my questions are not dilatory, as I've discussed them with quite a few physicists, some of whom think I'm out of my mind due to my disdain for MWI, and some who agree with me.

I'm convinced that MWI is simply a romantic attempt to get around the use of probability by ascribing some type of reality to each possible event in a probability system -- this could have been as easily done several hundred years ago as now. If that was the case , the idea lay dormant for several centuries, as it still should in my opinion. Where's Occam?


I'll stay more open minded if somebody could answer my questions, or indicate that they are nonsense.
Regards,
Reilly Atkinson

I'll ask three questions:

1. How many universes are there -- what kind of Cantorian infinity are we talking?

2. If there is no splitting, what conservation law attends to the constancy of the number of universes?

3. Can anyone explain, without contradictions, Deutsch's rather odd notion of shadow photons?

Regards,
Reilly Atkinson

 

[Count Iblis]

The fact that 1. nobody has tried to answer
...

I'll stay more open minded if somebody could answer my questions, or indicate that they are nonsense.
Regards,
Reilly Atkinson

Forget this "splitting", "number of universes" etc. You just have the postulates of QM without wavefunction collapse. How can an observer collapse the state of the entire universe by just observing?

 

[JesseM]

I'll ask three questions:

1. How many universes are there -- what kind of Cantorian infinity are we talking?

2. If there is no splitting, what conservation law attends to the constancy of the number of universes?

3. Can anyone explain, without contradictions, Deutsch's rather odd notion of shadow photons?

Regards,
Reilly Atkinson

How familiar are you with the mathematical structure of conventional (non-MWI) QM? Do you understand the idea that a quantum system is assigned a quantum state which evolves over time according to the Schroedinger equation, and that each quantum state involves a "superposition" of different possible eigenstates which correspond to particular measurement outcomes, with each measurement "collapsing" the system's state onto one of the eigenstates with a probability of collapsing into any eigenstate proportional to the square of its amplitude in the superposition before the measurement? If you are, then as I understand it the MWI twist on this is that there is no "collapse" on measurement, that the universe is assigned a single state which remains in a massive superposition, and that each macroscopically-distinct element of the superposition will appear as a distinct "world" to its inhabitants. So the question of the number would be somewhat subjective, depending on how coarse-grained a measure of "macroscopically-distinct" you use...the Everett FAQ says in question #11:

Q11 How many worlds are there?

The thermodynamic Planck-Boltzmann relationship, S = k*log(W), counts the branches of the wavefunction at each splitting, at the lowest, maximally refined level of Gell-Mann's many-histories tree. (See "What is many-histories?") The bottom or maximally divided level consists of microstates which can be counted by the formula W = exp (S/k), where S = entropy, k = Boltzmann's constant (approx 10^-23 Joules/Kelvin) and W = number of worlds or macrostates. The number of coarser grained worlds is lower, but still increasing with entropy by the same ratio, i.e. the number of worlds a single world splits into at the site of an irreversible event, entropy dS, is exp(dS/k). Because k is very small a great many worlds split off at each macroscopic event.

The FAQ also says in questions 6, 7 and 19 that worlds do "split" in the sense of their being multiple macroscopically-distinct later states for a single earlier state, so your question 2 wouldn't really apply. As for your own question 3, are you familiar with the Feynman path integral or sum-over-paths formalism in conventional QM, where the probability of measuring a particular outcome is calculated by doing a certain type of sum of all possible pathways leading up to that outcome, and allowing the different pathways to interfere with one another? I think Deutsch's talk about "shadow photons" is just a poetic way of discussing this, but with Deutsch believing that each path is actually taken by an alternate version of the photon.

 

[xantox]

Where's Occam?

Occam is in the fact that MWI is just that "every closed system evolves according to the Schrödinger equation".

Worlds are defined by decoherence, this was studied by Gell-Mann, Hartle and others.

 

[Hans de Vries]

I'll stay more open minded if somebody could answer my questions, or indicate that they are nonsense.
Regards,
Reilly Atkinson

Discussions like these tend to depress me especially if they start
involving solipsism, That makes me run away as fast as I can
These discussions are more appropriate in the philosophy section.
How can one claim (referring to Deutsch), that, at the same time,

A) Different worlds do not interfere.
B) Each path in a path integral belongs to a different world.

The latter means that they interfere maximally and that the single world we
live in is just undetectable noise in an interference pattern.

The total number of worlds which should exist at the same time to support
the ideas floating around during these discussions must dwarf the number
of elementary particles in the universe. Nobody bothers?
What leap of faith is needed to believe in solipsism, especially if the solipsist
never manages to prove his/her claim by performing any miracles, "acts of god"
or other such things, but instead has to argue and discuss endlessly and
mostly fruitlessly with the products of his own imagination (the other people)
to try to convince them that they just own their entire existence to his or
her consciousness? Must be frustrating...

And then, when the solipsist finally manages to convince a product of his/her
imagination about solipsism, then it is only to convince it that it itself is the
sole source of the universe, and, the poor solipsist, well, he/she just becomes
degraded to a product of imagination...

Amusing when two solipsist aid each other in a discussion. At some point
in time one would expect the two to get bitterly fighting about who
is the real "source of the universe" and who is a product of imagination.Regards, Hans

 

[JesseM]

How can one claim (referring to Deutsch), that, at the same time,

A) Different worlds do not interfere.

When does Deutsch say this? http://findarticles.com/p/articles/mi_m1511/is_n10_v16/ai_17449599/pg_2 suggests otherwise:

An extremely large number--perhaps an infinite number--of these parallel worlds exist, says Deutsch. They reveal their existence whenever a particle has the opportunity to follow more than one path--which, of course, is essentially always the case. When a photon, for example, goes through one of the two slits, something exceedingly strange happens. In our universe, Deutsch says, the photon goes through one slit. But in some other universe, it goes through the other. These alternate realities thereafter continue to exist, with identical pasts but different futures. ("There is another universe that is as real as ours," Deutsch said during an interview, "in which I fail to get through to you today, and we'll only be talking tomorrow.") The interference pattern we see in the two-slit experiment, says Deutsch, arises because the two universes interact.

Such interference between worlds, he points out, is detectable only under very carefully controlled conditions. The interference is a rare example of two universes that have briefly diverged--in this case when the photons went through different slits--and then merged into one reality, leaving only the interference pattern as evidence of their once independent status. Normally, outside the bounds of an experiment specifically designed to create interference, particles that split off into separate realities will go off and collide with various other particles, and these with still others, in an unending, diverging cascade. The chance that any two of these many different worlds, each marching to the beat of its own capricious quantum drummer, will subsequently evolve along identical paths is vanishingly small. So while many David Deutsches occupy these assorted worlds, they will never meet. Only the evanescent warp and woof of quantum mechanical interactions stitch these universes together.

The total number of worlds which should exist at the same time to support
the ideas floating around during these discussions must dwarf the number
of elementary particles in the universe. Nobody bothers?

This seems more like an "argument from incredulity" than an actual scientific or philosophical argument.

 

[Hans de Vries]

When does Deutsch say this? http://findarticles.com/p/articles/mi_m1511/is_n10_v16/ai_17449599/pg_2 suggests otherwise:

I'm aware what Deutsch says. I've seen his video's and studied his
experimental setups, which can be explained entirely from classical
optics without any need for MWI claims at all.

He claims the interference is extremely small so that entirely different
worlds can coexist without notably disturbing each other. That is,
you don't run into a car passing in another world while swimming in
the pool.

These ideas have little or nothing to do with what the path integral
formalism says and they defy quantum mechanics and most of its
applications.

There's a huge industry using molecular modeling algorithms based on
the notion that the wave function is a distributed charge/spin density
and that it is the whole wavefunction which contributes to the electric
and spin /angular momentum magnetic fields which determine the
properties of molecules and solid state materials.

In David Deutsch's picture the atom is a core with a classical particle
rotating around it, taking one path in one world and a different path
in another world. This is not correct.

This seems more like an "argument from incredulity" than an actual scientific or philosophical argument.

With a single human body containing ~10^30 elementary particles
with all of them splitting up into endless numbers of paths at the
femtometer/attosecond scale, the number of different worlds add
up very fast, for a single human, let alone for an entire world.Regards, Hans

 

[Count Iblis]

Hmmm, I thought that the wavefunction of the entire muliverse is supposed to be in an eigenstate of the Hamiltonian, so we can put H|psi> = 0, so it doesn't evolve in time, it doesn't split, it is static.

 

[JesseM]

I'm aware what Deutsch says. I've seen his video's and studied his
experimental setups, which can be explained entirely from classical
optics without any need for MWI claims at all.

Plenty of quantum phenomena can't be explained with classical optics, like entanglement or quantum computing.

He claims the interference is extremely small so that entirely different
worlds can coexist without notably disturbing each other. That is,
you don't run into a car passing in another world while swimming in
the pool.

These ideas have little or nothing to do with what the path integral
formalism says and they defy quantum mechanics and most of it's
applications.

Are you familiar with the phenomenon of decoherence in ordinary QM? If you have a small subsystem A interacting thermally with another system B, the interactions can make it so that the reduced state of the subsystem A becomes arbitrarily close to a "mixed state" in which there is virtually no interference between the different elements of the superposition for A (see my post #15 on this thread for more). As I understand it, this is basically how the many-worlds interpretation explains why macroscopically different "worlds" don't noticeably interfere with one another.

In David Deutsch's picture the atom is a core with a classical particle
rotating around it, taking one path in one world and a different path
in another world. This is not correct.

Why "classical particle"? The paths in the path-integral picture don't behave like the paths of classical particles. Presumably you could in principle make correct predictions about the probability an electron will be detected at different positions around the nucleus by summing all possible paths, I think Deutsch is just adding the idea that each of these paths is a real electron, a philosophical gloss which shouldn't change the physical analysis.

With a single human body containing ~10^30 elementary particles
which each splitting up into endless numbers of paths at the femtometer,
attosecond scale, the number of different worlds add up very fast,
for a single human, let alone for an entire world.

Yes, I get that. But again, why is this anything more than an argument from incredulity, i.e. "I find it incredible there could be so many versions of me"? If the universe is spatially infinite there would also be an infinite number of slightly different versions of you at sufficiently great spatial distances, is this a good scientific or philosophical argument for believing space must be finite?

 

[xantox]

In David Deutsch's picture the atom is a core with a classical particle rotating around it, taking one path in one world and a different path in another world. This is not correct.

Attributing this view to Deutsch seem incorrect too. Do you have a quote where he did actually state this?

With a single human body containing ~10^30 elementary particles with all of them splitting up into endless numbers of paths at the femtometer/attosecond scale, the number of different worlds add up very fast, for a single human, let alone for an entire world.

Since in MWI worlds are just an emergent feature of the wavefunction, arguing on the basis of their number does not seem quite significant.

 

[Hans de Vries]

Plenty of quantum phenomena can't be explained with classical optics, like entanglement or quantum computing.

The quantum computing videos from David Deutsch I've seen could
be explained by classical optics. You might see people agreeing here
including those who consider MWI to have attractive sides (vanesch).

Are you familiar with the phenomenon of decoherence in ordinary QM? If you have a small subsystem A interacting thermally with another system B, the interactions can make it so that the reduced state of the subsystem A becomes arbitrarily close to a "mixed state" in which there is virtually no interference between the different elements of the superposition for A (see my post #15 on this thread for more). As I understand it, this is basically how the many-worlds interpretation explains why macroscopically different "worlds" don't noticeably interfere with one another.

It's hard to see how there can't be interference, non interference occurs
only in orthogonal states, 90 degrees for bosons and 180 degrees for fermions.
This means there are at most two different states per particle which do
not interfere.

For the rest, everything is based on interference. From propagator theory:
A wavefunction moves in a certain direction because all other directions
are interfered out destructively, that is, there's no motion without interference.

Why "classical particle"? The paths in the path-integral picture don't behave like the paths of classical particles. Presumably you could in principle make correct predictions about the probability an electron will be detected at different positions around the nucleus by summing all possible paths, I think Deutsch is just adding the idea that each of these paths is a real electron, a philosophical gloss which shouldn't change the physical analysis.

The propagators of massive particles reflect continuously (the interacting
left and right chiral components), but this is not the picture Deutsch gives
with "In our world the particle goes through one split and in another world
it goes through another split" This is a classical picture.

Yes, I get that. But again, why is this anything more than an argument from incredulity, i.e. "I find it incredible there could be so many versions of me"? If the universe is spatially infinite there would also be an infinite number of slightly different versions of you at sufficiently great spatial distances, is this a good scientific or philosophical argument for believing space must be finite?

What bothers me is that the different worlds do not interact as predicted
by quantum mechanics, but rather they all exist mostly independently without
disturbing each other. Especially since older ideas which claimed that particles
can only interfere with them self and not with other particles of the same kind
have been proven wrong by experiment.

Regards, Hans

 

[JesseM]

The quantum computing videos from David Deutsch I've seen could
be explained by classical optics. You might see people agreeing here
including those who consider MWI to have attractive sides (vanesch).

Do you remember a post by vanesch where he discusses this? In any case, surely you're not arguing that all quantum phenomena can be explained by classical optics (violations of the Bell inequality obviously can't, for example), so if you agree the quantum formalism is needed for certain situations, then whatever your interpretation of the quantum formalism, wouldn't you apply the same interpretation to any situation which physicists analyze using QM, like the double-slit experiment?

It's hard to see how there can't be interference

From what I remember, the interference terms in the "reduced state" for a certain subsystem (which are apparently the off-diagonal terms in the density matrix) never actually disappear completely, but they do decay exponentially. For example, this paper says:

The quantum decoherence process is elegantly expressed in the framework of the reduced density matrix of the quantum register. When no coupling to the environment is present, the reduced density matrix simply follows a Heisenberg-type evolution. As soon as the coupling to the environment is introduced, the off-diagonal terms of the reduced density matrix of the register decay with respect to time. This is often referred to as phase damping. In the simplest case of a single two level system connected to an environment, the off-diagonal elements of the reduced density matrix decay exponentially in time as ~e^−q(t) , where t is the time and the function q(t) depends on the strength of the coupling to the environment.

I have never studied decoherence formally so I don't claim to understand why this is true or even precisely what it means, I'd suggest you might at least want to do some of your own reading on the subject instead of dismissing it based on my layman's summary, as far as I know decoherence is a widely-accepted consequence of applying the rules of QM to the problem of a quantum system which is in thermal interaction with a larger environment.

The propagators of massive particles reflect continuously (the interacting
left and right chiral components), but this is not the picture Deutsch gives
with "In our world the particle goes through one split and in another world
it goes through another split" This is a classical picture.

I don't quite understand your objection here, are you just objecting that he makes it sound like there are only two paths involved? If so he'd probably say he was simplifying for a general audience, in fact you have to integrate over an infinite number of distinct paths through each slit.

What bothers me is that the different worlds do not interact as predicted
by quantum mechanics, but rather they all exist mostly independently without
disturbing each other.

Again, I think you really need to delve into the theory of decoherence to understand why many-worlds advocates say the different worlds interact only weakly (they don't say that they aren't interacting at all, I've seen a quote by Deutsch where he points out that the interference terms never disappear completely even with decoherence).

 

[xantox]

What bothers me is that the different worlds do not interact as predicted by quantum mechanics, but rather they all exist mostly independently without disturbing each other.

The classical picture is an approximation, Deutsch considers worlds unsharp and affecting each other. To discuss his views, it is perhaps more appropriate to refer to his actual papers, eg. "The Structure of the Multiverse", arxiv:quant-ph/0104033

"if reality – which in this context is called the multiverse – is indeed literally quantum-mechanical, then it must have a great deal more structure than merely a collection of entities each resembling the universe of classical physics.[...] "

"Since a generic quantum computational network does not perform anything like a classical computation on a substantial proportion of its qubits for many computational steps, it may seem that when we extend the above conclusions to the multiverse at large, we should expect parallelism (ensemble-like systems) to be confined to spatially and temporally small, scattered pockets. The reason why these systems in fact extend over the whole of spacetime with the exception of some small regions (such as the interiors of atoms and quantum computers), and why they approximately obey classical laws of physics, is studied in the theory of decoherence (see Zurek 1981, Hartle 1991)."

"For present purposes, note only that although most of the descriptors of physical systems throughout spacetime do not obey anything like classical physics, the ones that do, form a system that, to a good approximation, is not only causally autonomous but can store information for extended periods and carry it over great distances. It is therefore that system which is most easily accessible to our senses – indeed, it includes all the information processing performed by our sense organs and brains. It has the approximate structure of a classical ensemble comprising ‘the universe’ that we subjectively perceive and participate in, and other ‘parallel’ universes."

 

[Hans de Vries]

The quantum computing videos from David Deutsch I've seen could
be explained by classical optics. You might see people agreeing here
including those who consider MWI to have attractive sides (vanesch).

Do you remember a post by vanesch where he discusses this?

https://www.physicsforums.com/showthread.php?p=1070970#post1070970


Regards, Hans