I Danger for the Many-Worlds Interpretation?

[Morbert]

Definitely interesting. Thanks! Ha! Copenhagen "sort of strains credulity...it's not a convincing fundamental presentation". That's putting it mildly.

PS I'm quite sympathetic to the vanilla class of Copenhagen interpretations (DH is sometimes called a neo-Copenhagen interpretation). I just think QM is capable of returning probabilities for a wider range of statements than Copenhagen considers.

 

[zonde]

It doesn't matter; all of the observers in the group (in a particular branch, since that's what we're using "observers" and "group" to mean) can communicate with each other (since they're all in the same branch) and verify that they all observed the same outcome. This communication is a classical process and doesn't require any quantum branch points anywhere, so it doesn't raise any quantum measurement issue.

If we are testing prediction there should be a possibility that we observe something different than what is predicted (there should be possibility that prediction is wrong).
So how MWI prediction could be wrong? I see two options:
1) there is single world
2) copies of different observers observing different outcomes can communicate with each other

In first case it is obvious that we should observe that different observers report the same outcome, because outcome then is physical fact and communication is by design such that it is reliable. So we observe the same thing that is predicted. So even if prediction is wrong we would get the same result.
Second case is more complicated. Say in experiment we ask different observers to repeat what they observed (they observe quantum measurement result). So basically observers make a copy of observation. But then the experimenter too should be only a copy of experimenter that is paired with certain outcome. So in a sense he is observing outcome of quantum measurement only just trough more complicated path than more direct observation and other (copies of) observers are just complicated quantum systems that rely quantum measurement result to experimenter. So the possibility that experimenter would receive conflicting reports of quantum measurement result sounds simply inconsistent and rather fake. So possibility of receiving result different from prediction sounds simply inconsistent.

So to me it seems that test of that MWI prediction is quite questionable.

 

[Minnesota Joe]

PS I'm quite sympathetic to the vanilla class of Copenhagen interpretations (DH is sometimes called a neo-Copenhagen interpretation). I just think QM is capable of returning probabilities for a wider range of statements than Copenhagen considers.

Ah, I'm not very sympathetic to neo-CI theories as I understand it (based on the descriptions here and Wiki) on explanatory grounds, but I appreciate the links anyway and all interpretations have useful contributions. Besides, great physicists like Gell-Mann are always fascinating to hear.

I'm not sure what you mean about QM being capable of returning probabilities for a wider range of statements than Copenhagen. Do you have in mind something like Bayesian inference about propositions?

 

[Minnesota Joe]

No, I'm just pointing out that you can't use the word "observer" the way it is usually used when talking about the MWI. The same goes for many other ordinary language words, as I have already pointed out.

I think you have made good points to me earlier about this, but my difficulty is that a lot of the time MWI is speaking figuratively or adopting a birds eye view of the branch structure. Or identifying "you" as a particular branch path starting in a branch and working backward. Or adopting a psychological continuity sense of identity. (But they also rightly pause to say that "you" don't literally survive branching.) It's kind of a mess.

I just tend to give them a lot of this and admittedly slip into it myself, mostly because it isn't a problem I really want to worry about right now, not wanting to get stuck in the weeds on figurative language. Other critics (like David Albert for example) also skip lightly over this ultimately. MWI has to get it correct when they derive probabilities of course.

You have thought about this more than I obviously. Do you think there is a serious problem, a serious charge that could be leveled at MWI on identity or even language grounds or do you think they can make sense of it?

 

[PeterDonis]

how MWI prediction could be wrong?

Different observers in the same branch (which in practice just means "different observers", since different branches can't interact) could observe different results for the same measurement. But we don't observe that.

there is single world

How would you determine this experimentally? That's how you determine whether a prediction is wrong.

 

[PeterDonis]

Do you think there is a serious problem, a serious charge that could be leveled at MWI on identity or even language grounds or do you think they can make sense of it?

It's possible to exercise the required discipline when using ordinary language to talk about the MWI--either avoid altogether words that are problematic, or give precise technical definitions of them that you then strictly adhere to. It's just not easy, and many people don't do it.

 

[Morbert]

I'm not sure what you mean about QM being capable of returning probabilities for a wider range of statements than Copenhagen. Do you have in mind something like Bayesian inference about propositions?

Copenhagen traditionally limits itself to probabilities for possible measurement outcomes of some experimental procedure carried out by an observer external to the quantum system. DH is concerned with probabilities for coarse grained histories of quantum properties of a system. The latter category includes probabilities for typical lab scenarios of Copenhagen, but also includes probabilities for retrodictive claims, for claims regarding closed systems with internal observers, for processes with a low degree of classicality etc. etc.

 

[A. Neumaier]

It predicts that the copy of the experimenter in each branch observes only one outcome. But none of those copies is "the same" as the experimenter before the measurement, so you can't say that any of the outcomes is "the" outcome that "the" experimenter observed.

This would be strange since then it would be impossible to have identity across measurements for anything, even in the branch that we actually observe...

Your "the branch that we actually observe" is a misuse of language when describing the MWI. There is no single "we" after the measurement, because "we" branch along with everything else. After the measurement there are multiple copies of "we", and each copy observes a different measurement result.

By the same token, there is also no "experimenter before the measurement", as the notion of the experimenter would have split before each time someone anywhere in the world would have performed a measurement. Thus the notion of an experimenter (or observer or detector) and even of a system becomes inapplicable and MWI becomes mute about anything in the real world.

As I have said in previous discussions of the MWI, it's hard even to talk about the MWI using ordinary language, because ordinary language has assumptions built into it that the MWI violates.

Well, if one cannot model in MWI ordinary experiments stated in ordinary language then MWI is impotent as an interpretation.

 

[bahamagreen]

While searching for an answer to my question in post #296, I found this interesting paper...

Quantum Phenomena Modeled by Interactions between Many Classical Worlds

https://journals.aps.org/prx/pdf/10.1103/PhysRevX.4.041013

 

[A. Neumaier]

Quantum Phenomena Modeled by Interactions between Many Classical Worlds

https://journals.aps.org/prx/pdf/10.1103/PhysRevX.4.041013

It is a kind of discrete version of Bohmian mechanics, and only approximately reproduces QM.

 

[akvadrako]

Again Copenhagen people wouldn't claim it does, they say QM doesn't provide a mechanism for how the history is selected. It's not that they're saying this is an explanation or satisfying in some way, they're saying the quantum formalism doesn't give such an explanation.

That's why I said in the first reply of this sub-thread that it doesn't explain anything.

 

[bahamagreen]

It is a kind of discrete version of Bohmian mechanics, and only approximately reproduces QM.

Thanks.

Still wondering about the time between splits... your linked site, manyworlds, Q7:

Q7 When do worlds split?

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.

That is the first clue I have found.
The wavefunction is for the universe, that which are subject to splits are whole worlds (universes), sufficient kT released anywhere within the universe will invoke a split of the entire universe (or is that not what the MWI means)?

I change Boltzmann's constant 10e-23 J/K to Planck units about 5e-32 Ep/7e-33 Tp about 7 Ep/Tp
Hottest observed stellar surface about 2.5e5 K about 1.75e-27 Tp
7 Ep/Tp * 1.75e-27 Tp is about 1.2e-26 Ep about the range of x-ray photons
Convert to Planck time units
= hbar/Ep
= 1 / 1.2e-26 Ep
= 8e25 tp

= 4.3e-18s ...order of attoseconds between world splits... ?

 

[A. Neumaier]

Thanks.

Still wondering about the time between splits... your linked site, manyworlds, Q7:

Q7 When do worlds split?

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.

That is the first clue I have found.

The contents of that link is mainly nonsense, as my theoretical physics FAQ makes clear.

 

[EPR]

That's why I said in the first reply of this sub-thread that it doesn't explain anything.

Still much better than to add unsupported assertions and fairytales like many worlds and magical guiding waves which give the appearance of classicality(you can't explain that which you observe by simply stating that it's what you observe- not a good starting point). This is why the CI is in the textbooks and MWI and BM are not.