Understanding Quantum Theory: A Beginner's Guide to Particle Physics

[DevilsAvocado]

You really shouldn't be posting all these "references" when you don't have a clue what they are saying.
...

Sorry Zz, you’re absolutely right – I have absolutely no idea what I’m talking about. And even worse – I dragged your name into the 'mess'. I apologize for that.

I have to explain what happened here:
This thread was first posted under PF section "Astronomy & Cosmology", with the name "Quantum theory question -- I know, wrong forum", and I did my first post #6 from there as a 'basic help' to OP. And from now on, I clicked my Reply mail to get here, not realizing that the thread had been moved to the correct section "Quantum Physics"...

Then I thought of QM question that interested me, and started the question "Since we are discussing QM ..." in post #8, still believing that we were in section "Astronomy & Cosmology" (stupid, I know ).

And then Chalnoth popped in (with whom I have long and very interesting discussions with in "Astronomy & Cosmology"), and then things got 'carried away' so to speak... (because of me, not Chalnoth).

It wasn’t until on page 2 somewhere that I realized I was posting in the 'wrong' section...

I will now wrap this 'adventure' up and return to home base "Astronomy & Cosmology", where at least I have some clues on what I’m talking about. Sorry for the 'inconvenience'...

(I will not do 'bulk' references to arXiv again unless I’m 100% sure it’s relevant and peer-reviewed.)

Just a last reply to SpectraCat, hope that’s okay...

 

[DevilsAvocado]

Actually, there are plenty of examples of "macroscopic quantum phenomena", depending on just how tightly you want to define that.
...

Thanks for the interesting reply SpectraCat.

I’m a complete novice in the QM field, but interested. As I understand this, there are things in QM that are on the border to 'magic' from a macroscopic view, as the nonlocality in the EPR paradox. The 'mechanism' in EPR, is quantum entanglement of two particles (electron, photon, etc).

Do you know if there’s any possibility to produce this entanglement on larger objects, like molecules? If it is – could it work on even larger objects like spherical fullerenes (buckyballs) or carbon nanotubes (buckytubes)? At these sizes, objects get visible under the microscope:

http://upload.wikimedia.org/wikipedia/en/2/22/CntHAADF.jpg
Electron micrograph showing a single-walled nanotube

I guess, once and for all, a visual test experiment of the EPR paradox would settle the question of nonlocality GR <> QM.

Or is this 'visual setup' theoretical impossible?

(Sorry if someone is offended by this purely speculative question.)

 

[DevilsAvocado]

That would be flatly contradicted by reality, because it is absolutely clear that gravity responds to matter.
...

Thanks for your answers Chalnoth, good as always.

 

[Chalnoth]

I guess, once and for all, a visual test experiment of the EPR paradox would settle the question of nonlocality GR <> QM.

The EPR paradox is basically solved by the many worlds interpretation of quantum mechanics, but in any case I don't know why you think it would help with the GR/QM incompatibility.

 

[DevilsAvocado]

The EPR paradox is basically solved by the many worlds interpretation of quantum mechanics, but in any case I don't know why you think it would help with the GR/QM incompatibility.

(I don’t know if I 'dare' continue to 'mumble' here in the QM section... Zz just say the word and I stop this...)

This is confusing...

Wikipedia - Nonlocality
As action at a distance, nonlocality is incompatible with relativity. However, with quantum physics nonlocality re-appeared in the form of entanglement, where its physical reality has been demonstrated experimentally together with the absence of local hidden variables. While entanglement is compatible with relativity, it prompts some of the more philosophically oriented discussions concerning quantum theory.

A more general nonlocality beyond quantum entanglement — retaining compatibility with relativity — is an active field of theoretical investigation and has yet to be observed.​

My thought was to run EPR at macroscopic level to actually see if nonlocality can be 'reproduced'...

 

[Chalnoth]

My thought was to run EPR at macroscopic level to actually see if nonlocality can be 'reproduced'...

This doesn't work.

Basically, the dynamics of quantum mechanical waves are completely and utterly local: there simply aren't any non-local effects that occur in the dynamics.

The only apparent non-locality comes in with the collapse of the wavefunction. But since there is no wavefunction collapse in the many worlds interpretation, the paradox is trivially solved.

 

[DevilsAvocado]

... The only apparent non-locality comes in with the collapse of the wavefunction. But since there is no wavefunction collapse in the many worlds interpretation, the paradox is trivially solved.

Please correct me if I’m wrong, but the many worlds interpretation is one of several interpretations of QM, right? Although MWI is the most supported by (60%?) leading scientist, right?

I know you explain things very well, so if you could explain MWI in the context of EPR (without math), I would be most thankful.

Take this practical EPR experiment using Bell inequality:

In Geneva 1997 Bell test experiments showed that light sent in fiber optic cables, on a distances over 10 kilometers, did not destroy the entanglement of the photons. The Bell test experiment was successful.

To a layman as me, this means that the photons had spin (up/down, left/right, ect) and since they were entangled, they must possesses the opposite spin of their entangled 'twin'. Which spin the photons actually have is, of course, completely unknown and 100% random before the measurement takes place.

When one entangled photon is measured, several kilometers from its 'twin', the other photon instantaneously 'obtains' the opposite spin. Instantaneously means faster than light, but the outcome is pure random, and therefore do not violate information FTL.

Bell's theorem shows that there are no local hidden variables involved (i.e. the spin of the photons where set from the start by hidden variables, before the measurement).​

So, how do we explain this 'phenomena' in MWI, to get rid of nonlocality? Is it just a pure coincidence that you, me and everybody else in this forum always happens to live in one of the MWI branches where the outcome of the Bell test experiments is what it is...? Even repeated a hundred thousand times, or more...?

For a guy that doesn’t understand the mathematics behind MWI, this is as close to science fiction you ever could come – with Nobel laureates (Richard Feynman) and the geniuses (Stephen Hawking) as promoters.

To me, nonlocality seems like 'kindergarten' compared to MWI and the Universal Wave Function...??

And when Wikipedia 'explains' MWI like this...

"The quantum-mechanical "Schrödinger's cat" paradox according to the many-worlds interpretation. In this interpretation every event is a branch point; the cat is both alive and dead, even before the box is opened, but the "alive" and "dead" cats are in different branches of the universe, both of which are equally real, but which cannot interact with each other."

...I feel true sadness for physics & science (unless this isn’t a giant mistake by Wikipedia) because this means we live in a deterministic world, with 'non-deterministic' branches, and the rest is a mess.

Has anyone done a real calculation of how many real branches there might be out there in the Multiverse? How many particles, energy, etc...? In 13.8 billion years there’s a lot of branching to do for every particle, and branched particle, and branched branched particle, and so on...

Please, help me with this. Everything in 'this branch' tells me Richard Feynman & Stephen Hawking are right and I am wrong, but I can’t understand how this works in the real world...??

Sir Roger Penrose is also an extremely intelligent man, and he agrees with Hawking that QM applied to the universe implies MWI, although he considers the current lack of a successful theory of quantum gravity negates the claimed universality of conventional QM...

This saves my nerves for awhile.


Finally, no offense Chalnoth, is your solution to the 'problem' really 'solid'? Not that my speculations ever is going to work, but if we know there is a inconsistency between GR & QM, and we have repeatable physical experiments, can we then just say – this doesn’t matter according to one of these theories – when we know one of them must be wrong/incomplete (or both)...?

 

[Chalnoth]

Please correct me if I’m wrong, but the many worlds interpretation is one of several interpretations of QM, right? Although MWI is the most supported by (60%?) leading scientist, right?

Well, honestly I don't care much about who supports it. Basically interpretation that doesn't simply reduce to MWI means adding unnecessary features to the theory. Here's the basic basic argument:

1. First, we prepare a system in a superposition of two states:
|\psi\rangle = |1\rangle + |2\rangle

2. Then we make a measurement of the state. This is done by some interaction with an observer state, which becomes:
|O\rangle |\psi\rangle = |O\rangle|1\rangle + |O\rangle|2\rangle

This has the interesting property that the time it takes for states 1 and 2 to oscillate becomes extremely large after the interaction. So what you get is a superposition of one state where the observer sees state 1, and a different state where the observer sees state 2. And the physics ensures that the information about one observer is effectively not available to the other.

So this means that wavefunction collapse is integral to the theory of quantum mechanics and just falls straight out of the wavefunction dynamics. It makes no sense, therefore, to postulate any sort of additional collapse axiom.

I know you explain things very well, so if you could explain MWI in the context of EPR (without math), I would be most thankful.

I can try. Basically, with each measurement that is made of a system in a superposition of states, the observer splits into a superposition of states, each seeing a different outcome. The math works out such that the amplitude of the observer to see inconsistent experimental results, though, is zero. So if, for example, particles 1 and 2 are emitted through some process that demands they have opposite spin, and I measure particle 1's spin to be up, then the amplitude for me to measure particle 2's spin to be up is zero.

But perhaps stated more trivially, in the MWI non-local effects are fundamentally impossible, because it is only the wavefunction dynamics that are at work in the theory, and those are entirely local.

 

[DevilsAvocado]

... I can try. Basically, with each measurement that is made of a system in a superposition of states, the observer splits into a superposition of states, each seeing a different outcome. The math works out such that the amplitude of the observer to see inconsistent experimental results, though, is zero. So if, for example, particles 1 and 2 are emitted through some process that demands they have opposite spin, and I measure particle 1's spin to be up, then the amplitude for me to measure particle 2's spin to be up is zero.
...

If I translate this to English for a layman, it would be:

When I measure particle 1 (P1), I personally split in two real separate persons, in two real separate worlds. First version of me (M1) measure P1 as spin down (SD), and next me M2 measure P1 as spin up (SU). Then we have two separate worlds (W1 & W2).

W1 => M1 => P1 => SD
W2 => M2 => P1 => SU

Now, for the measurement on P2 the outcome is not 50/50 but 100/0, depending on P1.

W1 => M1 => P1 => SD
                  W3 => M3 => P2 => SU
                  W4 => M4 => P2 => SU

W2 => M2 => P1 => SU
                  W5 => M5 => P2 => SD
                  W6 => M6 => P2 => SD

Seriously Chalnoth, isn’t this just a 'play' with numbers...? Especially as these real worlds never can interact with each other? Everyone of me M3 thru M6 will see the nonlocality according to EPR/BT, but doesn’t care about it, because mathematically all possible outcome has happened to the other versions of me, in different worlds, which I never can speak to.

This is extremely hard for me to digest...

Personally I see difficulties in putting this much trust in one mathematical theory, when we know there are 'signs' saying – something must be wrong in at least one...

 

[Chalnoth]

Seriously Chalnoth, isn’t this just a 'play' with numbers...? Especially as these real worlds never can interact with each other?

Not at all. The boundary between collapse and no collapse isn't hard and fast. There's a continuum there. And so it's entirely possible to test the theory by carefully examining the boundary between collapse and no collapse. This has been done, and decoherence is a real phenomenon.

 

[DevilsAvocado]

... And so it's entirely possible to test the theory by carefully examining the boundary between collapse and no collapse. This has been done, and decoherence is a real phenomenon.

(long time no see...)

Thanks for your answer Chalnoth. I've been lost in the making-a-living-branch for awhile, and I got real 'dizzy' after reading "Observing the Progressive Decoherence of the Meter in a Quantum Measurement". ()

This is at least 2 nanometers above my intellectual capacity, but thanks for giving it a try!

Hope to hear from you in the "Home Base" (Astronomy & Cosmology).

 

[yoda jedi]

Thanks a lot for the explanation Chalnoth.

So the Higgs field 'provides' the mass for particles, and the graviton then mediates the gravity force, corresponding to the mass ('given' by Higgs), to other particles, like: "Hey guys look at me! I got mass from Higgs!"

Correct?

But how is QCD compatible to the Higgs field? I looked at http://www.youtube.com/watch?v=ECkG_JdodMA" (thanks DrChinese!) and Frank Wilczek is showing that mass is the result of quantum fluctuations inside the nucleons?

(QCD seems very cool, baryons is only 3 'RGB quarks'... I know what that is, I have RGB on my computer! )

And how is all this compatible to GR? Gravity Probe B has shown (2007) that gravity is spacetime curvature??

AND what happens with all this, if the Higgs boson is discovered at LHC this year...!?

or maybe not...
nor 2011, 2012, 2013, 2014... etc
then we have the rhison model.

 

[yoda jedi]

And when Wikipedia 'explains' MWI like this...


...I feel true sadness for physics & science (unless this isn’t a giant mistake by Wikipedia) because this means we live in a deterministic world, with 'non-deterministic' branches, and the rest is a mess.

Has anyone done a real calculation of how many real branches there might be out there in the Multiverse? How many particles, energy, etc...? In 13.8 billion years there’s a lot of branching to do for every particle, and branched particle, and branched branched particle, and so on...

Please, help me with this. Everything in 'this branch' tells me Richard Feynman & Stephen Hawking are right and I am wrong, but I can’t understand how this works in the real world...??

Sir Roger Penrose is also an extremely intelligent man, and he agrees with Hawking that QM applied to the universe implies MWI, although he considers the current lack of a successful theory of quantum gravity negates the claimed universality of conventional QM...

This saves my nerves for awhile.


Finally, no offense Chalnoth, is your solution to the 'problem' really 'solid'? Not that my speculations ever is going to work, but if we know there is a inconsistency between GR & QM, and we have repeatable physical experiments, can we then just say – this doesn’t matter according to one of these theories – when we know one of them must be wrong/incomplete (or both)...?

or a nonlinear quantum mechanics (Singh, or Hansson, or Elze, or Zloshchastiev...or a CSL model) collapse the superposition and there is not MWI.

 

[Chalnoth]

or maybe not...
nor 2011, 2012, 2013, 2014... etc
then we have the rhison model.

Well, I think it's rather likely that we will find a Higgs at the LHC (though not finding it would be just as interesting). It's just that it will take a while to get the luminosity up, and then a while after that to make sure all of the data analysis is done properly. Now that the collider is up and running at higher energies than we've seen previously, we should expect to see a Higgs within 1-2 years of operation or so, if there is one to be found.

 

[DevilsAvocado]

... Basically, with each measurement that is made of a system in a superposition of states, the observer splits into a superposition of states, each seeing a different outcome. The math works out such that the amplitude of the observer to see inconsistent experimental results, though, is zero. So if, for example, particles 1 and 2 are emitted through some process that demands they have opposite spin, and I measure particle 1's spin to be up, then the amplitude for me to measure particle 2's spin to be up is zero.

But perhaps stated more trivially, in the MWI non-local effects are fundamentally impossible, because it is only the wavefunction dynamics that are at work in the theory, and those are entirely local.

(my emphasis)

Chalnoth, I just can’t get this out of my head... if you could explain the following theoretical dilemma, I would be thankful.

The two observers A & B are going to run the EPR paradox. A stays at measuring apparatus M1, and B travels 2 light-years to the other measuring apparatus M2. The quantum entangled particles P1 and P2 are fired halfway in opposite directions, by assistant C.

After 1 year – A measure P1 to be spin up at M1, and B measure P2 to be spin down at M2, and B sends a radio-message to A with the result.

In the MWI we will also have another branch with the following scenario:

After 1 year – A measure P1 to be spin down at M1, and B measure P2 to be spin up at M2, and B sends a radio-message to A with the result.


My question:
How and when is the MWI branching done in this scenario? My guess it’s at the measuring moment, but if this is the case; how do MWI put A & B in the accurate branch instantly? They are separated by 2 light-years? This is must be a clear violation of "non-local behavior"?

If the MWI branching is done when P1 & P2 are fired off, we have the same problem in putting A & B in the same branch instantly.

If the MWI branching is done when B sends the radio-message, we have the same problem in putting A & B in the same branch instantly.

I don’t get this??

 

[Chalnoth]

(my emphasis)

Chalnoth, I just can’t get this out of my head... if you could explain the following theoretical dilemma, I would be thankful.

The two observers A & B are going to run the EPR paradox. A stays at measuring apparatus M1, and B travels 2 light-years to the other measuring apparatus M2. The quantum entangled particles P1 and P2 are fired halfway in opposite directions, by assistant C.

After 1 year – A measure P1 to be spin up at M1, and B measure P2 to be spin down at M2, and B sends a radio-message to A with the result.

In the MWI we will also have another branch with the following scenario:

After 1 year – A measure P1 to be spin down at M1, and B measure P2 to be spin up at M2, and B sends a radio-message to A with the result.My question:
How and when is the MWI branching done in this scenario? My guess it’s at the measuring moment, but if this is the case; how do MWI put A & B in the accurate branch instantly? They are separated by 2 light-years? This is must be a clear violation of "non-local behavior"?

If the MWI branching is done when P1 & P2 are fired off, we have the same problem in putting A & B in the same branch instantly.

If the MWI branching is done when B sends the radio-message, we have the same problem in putting A & B in the same branch instantly.

I don’t get this??

Okay, so, here's the basic idea:

1. Observer A measures the spin of his particle. This causes decoherence, which makes it so that observer A effectively splits into one who observes spin up, and one who observes spin down. Each component of observer A reports his result by sending out a radio signal. So we have two radio signals being sent to observer B: one that is coherent with observer A measuring spin up, and one that is coherent with observer A measuring spin down.

2. Observer B also performs his measurement, and also splits into a superposition of two states: one that measures spin up, and one that measures spin down. The one that measures spin up is necessarily coherent with the observer A that measures spin down, and thus necessarily coherent with the incoming radio signal that reports a spin down measurement, and so of the two radio signals that are sent, he is only able to measure the one that he is coherent with: the spin down result.

 

[DevilsAvocado]

... So we have two radio signals being sent to observer B ...

Fascinating... this is turning into a thriller... and I’m going to 'argue' a little bit, just to pin down MWI once and for all.

Recap from 'birds view':

State1) A & B is 2 light-years away from each other. Two quantum entangled particles are heading towards them in opposite direction.
At State1 – A & B are in the same branch (obviously).

State2.Alt1) A & B do their measurement on respective entangled particle.
At State2.Alt1 – A & B effectively splits into two separate observers A1/A2 & B1/B2.

State2.Alt2) A & B do their measurement on respective entangled particle, but something went wrong with both measuring apparatus M1 & M2. No measurement took place.
At State2.Alt2 – A & B must be in the same (old) branch still, right?

State2.Alt3) A & B do their measurement on respective entangled particle, but something went wrong with B’s measuring apparatus M2. Only one measurement took place at A.
At State2.Alt3 – A effectively splits into two separate observers A1/A2, and B must be in the 'old single' branch, right?


Questions:
To me this looks like there is instant communication over 2 light-years. Things can go wrong with the separated apparatus, and there is no other way than "spooky action at a distance" to communicate this fact?

And, what happens with the radio signal in case of State2.Alt3? A1/A2 is sending signals to B1/B2, but there is only B to receive them? Where do they go?

 

[Chalnoth]

Fascinating... this is turning into a thriller... and I’m going to 'argue' a little bit, just to pin down MWI once and for all.

Recap from 'birds view':

State1) A & B is 2 light-years away from each other. Two quantum entangled particles are heading towards them in opposite direction.
At State1 – A & B are in the same branch (obviously).

State2.Alt1) A & B do their measurement on respective entangled particle.
At State2.Alt1 – A & B effectively splits into two separate observers A1/A2 & B1/B2.

State2.Alt2) A & B do their measurement on respective entangled particle, but something went wrong with both measuring apparatus M1 & M2. No measurement took place.
At State2.Alt2 – A & B must be in the same (old) branch still, right?

State2.Alt3) A & B do their measurement on respective entangled particle, but something went wrong with B’s measuring apparatus M2. Only one measurement took place at A.
At State2.Alt3 – A effectively splits into two separate observers A1/A2, and B must be in the 'old single' branch, right?


Questions:
To me this looks like there is instant communication over 2 light-years. Things can go wrong with the separated apparatus, and there is no other way than "spooky action at a distance" to communicate this fact?

And, what happens with the radio signal in case of State2.Alt3? A1/A2 is sending signals to B1/B2, but there is only B to receive them? Where do they go?

Well, at that last step, the observation of the signals from B1/B2 would cause A to decohere into A1/A2.

 

[DevilsAvocado]

Well, at that last step, the observation of the signals from B1/B2 would cause A to decohere into A1/A2.

Okay, (I think you switched A/B in the last step, but that’s irrelevant) when particle P2 is close to B’s M2, both A & B are the same branch, 2 light-years away from each other.

In the exact moment B performs his measurement on P2 in M2, B splits into B1/B2.

A is supposed to do the same thing, but something went wrong, and A is now completely 'innocent'. A has not touched/alter the superposition of states, the particle P1 went right thru the apparatus M1.

To me – now the only way for B1/B2 to split A (2 light-years away) into A1/A2, is "spooky action at a distance"?

Or did I misinterpret you? Do you mean when the tow radio signals from B1/B2 reach A, he is split into A1/A2?


If the later is the case, consider this:
A & B are sending their respectively radio signal to each other. In this case A will send one radio signal to B, to inform about the goof, but B doesn’t exist anymore – he is split into B1/B2.

For A’s single radio signal to arrive at both B1 and B2 – the radio signal must be split in two.

Where and when is this done? When A sends his radio signal, he is not split (yet)??

 

[Chalnoth]

Or did I misinterpret you? Do you mean when the tow radio signals from B1/B2 reach A, he is split into A1/A2?

Yes, that's what I mean.

If the later is the case, consider this:
A & B are sending their respectively radio signal to each other. In this case A will send one radio signal to B, to inform about the goof, but B doesn’t exist anymore – he is split into B1/B2.

B1/B2 would still be coherent with A that sent the single signal, and so both B1/B2 would see the same signal from A.

 

[DevilsAvocado]

... B1/B2 would still be coherent with A that sent the single signal, and so both B1/B2 would see the same signal from A.

To make things even 'worse', consider this:
This is a theoretical thought experiment; therefore A & B has built an optical fiber cable to send the radio signal digitally...

To me this fiber cable doesn’t fit my understanding of 'reality', since it must exist in duplicate versions, transmitting physically the same information...?

How does this work in MWI??

 

[Chalnoth]

Well, if the interaction with the light signal is significant enough, it would progressively decohere as a decohered signal passes through the cable.

This really isn't that tough: interaction of an object with a signal that has decohered causes further decoherence.

 

[DevilsAvocado]

Well, if the interaction with the light signal is significant enough, it would progressively decohere as a decohered signal passes through the cable.

Okay Chalnoth, A and B1/B2 sends their digital signals in opposite direction.

At B1/B2 we now have 2 physical fiber cables, a start.
At A there’s only 1 physical fiber cable, a start.

This results in 3 digital signals, on their way in 3 physical separate fiber cables A/B1/B2.

After 1 light-year signal A 'meets' signal B1 & B2 halfway. The question is, in what cable? Or are the cables 'merged/split' by MWI, so that all works out at the endpoints? If so why? No one is doing observations at halfway? What kind of 'merge/split' would that be??


If you can answer the questions above, I’m going to make things much worse²:
A & B has planned for A not to observe the digital signal from B right away. A is saving the digital signal on a USB stick, and starts travel towards B (with a bottle of champagne ). The idea is that they are going to play the digital radio signal, with their respectively result, simultaneously at same location, and celebrate.

A goofed his measurement. A did not listen to the digital radio signal from B. A must be in the same 'old' single branch, when starting his journey towards B.

Questions:
Where do A go? B doesn’t exist anymore; he is split into B1/B2??

If A is split anyway (by some function in MWI), Ax and Ay does not know what’s on the USB stick.
How do MWI couple Ax/Ay with the right B1/B2??

 

[Chalnoth]

Okay Chalnoth, A and B1/B2 sends their digital signals in opposite direction.

At B1/B2 we now have 2 physical fiber cables, a start.
At A there’s only 1 physical fiber cable, a start.

This results in 3 digital signals, on their way in 3 physical separate fiber cables A/B1/B2.

You're making the mistake of assuming that the entire fiber cable must decohere at the exact same time. This isn't the case. You're also making the mistake of thinking of A/B1/B2 as being different cables: this is not the case, they are just different components of the same wavefunction. Furthermore, since A2=B1 and A1=B2, A is just a superposition of B1 and B2, and the only thing that changes as the B signal passes through is that the coherence is lost between A1 and A2.

 

[DevilsAvocado]

... You're also making the mistake of thinking of A/B1/B2 as being different cables: this is not the case, ...

When P2, which is in a superposition of states, is measured by B, what happens to B?

Is B physically split into B1 and B2, seeing different outcome?

Or, is B put in some other 'superposition of states', leading to the same physical person seeing two contradictive outcomes, parallel?? (...unreal...)

(This is so weird... Wikipedia: "decoherence refers to the untangling of quantum states to produce a single macroscopic reality"...)


P.S. I have no problem accepting microscopic superposition of states, but - macroscopic superposition of states? Has this ever been observed??

 

[Chalnoth]

At this point, I don't know how to make things more clear. I think you're just confusing yourself by considering more and more convoluted scenarios. The mechanism itself is exceedingly simple.

First, you start with a system in a superposition of states. Second, you interact this system with another (perhaps to make an observation, perhaps not). The interaction causes the superposition of states to lose coherence, meaning that the two states can no longer interact.

That's all there is to it.

 

[DevilsAvocado]

... I think you're just confusing yourself by considering more and more convoluted scenarios ...

You’re right Chalnoth.

I just have this final simple question:
Does MWI stipulate that a macroscopic object (like a human observer) can be in a superposition of states?

Yes or No?

(Promise, no more questions after this)

 

[Chalnoth]

You’re right Chalnoth.

I just have this final simple question:
Does MWI stipulate that a macroscopic object (like a human observer) can be in a superposition of states?

Yes or No?

(Promise, no more questions after this)

Not a coherent superposition. The continual interactions of large objects with their environment tends to make coherent superpositions very difficult to maintain.

 

[DevilsAvocado]

Not a coherent superposition.

Chalnoth, thanks a lot for taking the time and having patience with my "entangled-brain-activities".

This is a very interesting and amazing discussion for me. We are talking in rather basic words, about a rather basic setup A/B+P1/P2. To you; this is perfectly clear and, as I understand, almost as basic as 1+1=2.

To me; the 'red lights' are flashing all over the place – it’s a mystery and complete contradiction.

I now think I know where my understanding 'breaks down'. I have a slight feeling it lays in understanding the difference between "wavefunction" and "superposition"... but I’m definitely not sure (about that either).

Therefore I shall read the arXiv paper http://arxiv.org/abs/quant-ph/0312059" by Maximilian Schlosshauer.

I’ll be back in a couple of days; either expressing my frustration, or exclaiming – The 'Red Lights' are out!

Thanks again.

 

[DevilsAvocado]

Thrilling Update:

It looks like my question in https://www.physicsforums.com/showpost.php?p=2555746&postcount=21" about macroscopic EPR is just on the edge of being realized:

http://www.sciencenews.org/view/generic/id/57385/title/Physicists_observe_quantum_properties_in_the_world_of_objects"
Potential applications, he says, include using arrays of these resonators to control multiple quantum systems in information processing or to test predictions about “Schrödinger cat” states — named for a hypothetical feline simultaneously alive and dead — in which a system exists in a mix of states known as a superposition. Cleland’s team showed, somewhat indirectly, that a form of superposition existed inside their resonator. If the researchers could make a resonator with longer-lasting vibrations, scientists might be able to test superposition on the macroscopic scale.

A 60µm resonator isn’t directly a "human observer", but it is definitely macroscopic!

http://www.sciencenews.org/view/download/id/57383/name/Quantum_object.jpg
Amazing!