I On being the sole observer in MWI

[PeterDonis]

A encounters the experiment first, and branches. A must model things with his branch alone. B (who has not encountered the experiment) is still modeling it as a superposition. If B is sufficiently technologically advanced, he ought to be able to perform an interference experiment demonstrating the superposition, or even reverse all the entanglements.

And if that can be done, then according to the MWI, A never branched in the first place. The MWI says branching only occurs when the experimental result is irreversible. In other words, according to the MWI, if B can do something to demonstrate the superposition, then A, B, and the experiment are all one quantum system that does not branch at all.

here's Scott Aaronson channeling David Deutsch

Aaronson is assuming that your consciousness would somehow "branch" even though, according to the MWI, no branching occurs in this scenario (for the same reason as above). If there is still interference possible, then no branching has occurred.

The real question here is what it would be like to experience such an experiment. The answer is that we don't know, because we don't know how consciousness is implemented in the brain, or whether it is even sensitive to quantum-level properties.

 

[Talisman]

Hmm, I'm caught by the FAPP thing again.

Given that the evolution is unitary, it should be possible in theory for an extremely advanced B to demonstrate the superposition, right? I'm trying to figure out whether anyone would argue that it is impossible for (an extremely advanced...) A to do the same in principle.

If we're only talking FAPP then the "sole observer" question is irrelevant. All sufficiently complex systems are observers FAPP.

 

[PeterDonis]

Given that the evolution is unitary, it should be possible in theory for an extremely advanced B to demonstrate the superposition, right?

Not once the branches have decohered. Decoherence, at least in the current best version of the MWI (as I understand it) is what actually makes the branching irreversible. Or, more precisely, entanglement of the measured system and the measuring apparatus, followed by decoherence.

I'm not familiar enough with the literature on decoherence to know if it addresses the apparent contradiction between irreversible branching/decoherence and unitary evolution of the state as a whole, at least from the standpoint of the MWI.

 

[Talisman]

Not once the branches have decohered. Decoherence, at least in the current best version of the MWI (as I understand it) is what actually makes the branching irreversible. Or, more precisely, entanglement of the measured system and the measuring apparatus, followed by decoherence.

I'm not familiar enough with the literature on decoherence to know if it addresses the apparent contradiction between irreversible branching/decoherence and unitary evolution of the state as a whole, at least from the standpoint of the MWI.

From my reading, decoherence is always merely entanglement that is uncontrolled and irreversible "for all practical purposes," and thus branching is FAPP.

We ought to be able to overcome this by making our experimenters sufficiently technologically advanced. For them, decoherence and branching don't happen with a system as "small" as the ones we're discussing. That way, when we say that "A" branches, we just mean that she has become entangled with the system.

So, given that Alice can say "the electron is spin-up," can she also meaningfully model the larger system (that includes herself) as a superposition?

It sounds like the answer won't be settled here!

 

[AlexCaledin]

It's always the whole universe that is branching, together with the whole awareness - which, ironically, seems to be the best explanation of the "mysterious collapse" which emerges FAPP for this branch of awareness which has nothing to do with any other branches anyway whether they are "real" (in Heaven knows what sense) or not (each branch works strictly according to Einstein's relativity so it's not that spooky after all).

 

[bhobba]

Right, the point is that math cannot clarify it. The problem is that probability in the usual sense doesn't make

That's a rather long bow to pull I think.

What is probability? You ask a mathematician and they will refer you to the Kolmogorov axioms. But as with any mathematical theory what it means when applied is another matter. John Baez thinks that is at the root of many of the interpretative 'discussions:' about QM:
http://math.ucr.edu/home/baez/bayes.html

In MW its based more on a decision theory view of probability:
https://people.kth.se/~soh/decisiontheory.pdf

Its akin to Bayesian - but a bit different. One of the issues with these kind of views is probability is a subjective belief a rational being has - that is a minefield of all sorts of issues such as - if rational beings were not around does it still exist? These are unresolved philosophical issues in the philosophy of probability.

Yet, actuaries especially, use it, and base decisions about many millions of dollars - perhaps even billions on it. It obviously works - so maybe, as physicists, and not philosophers, we simply consign such worries to those interested in such things, namely philosophers, and say - well in many areas of applied math they have no worries with it - so maybe I shouldn't.

Thanks
Bill

 

[JamieSalaor]

You're not alone. In my experience this is one of the biggest issues people have with the MWI.
I agree with this argument; in fact it's one I've tried to make before (not here on PF, but in other discussions I've had over the years). Basically this amounts to asking how you derive the Born rule (which is basically what we use in practice to justify inferring particular quantum states from measurement results) in the MWI. There is quite a bit of literature on this, but I don't find any of it convincing (although many MWI proponents do).

So does this imply that MWI is a solipsistic interpretation? Where only one observer exists at anyone time? I always thought MWI was very sure about there being other observers...

 

[JamieSalaor]

Sorry for reviving an old thread I'm doing research for a bit of coursework! Looking at why people don't like MWI

 

[PeterDonis]

does this imply that MWI is a solipsistic interpretation?

No. "Sole observer" is probably a bad choice of words for what the OP of the thread was trying to describe. It doesn't mean Experimenter 1 is not an observer in the sense you are thinking.

 

[JamieSalaor]

No. "Sole observer" is probably a bad choice of words for what the OP of the thread was trying to describe. It doesn't mean Experimenter 1 is not an observer in the sense you are thinking.

Right I thought so was just double checking.
Thanks Peter!

 

[JamieSalaor]

Also the OP states Solipsistic Principal. I assume the author is implying a sort of I think therefore I am kind of thing? Where you must be the indivual experiencing that experience otherwise you wouldn't be
Not that Many Worlds is solipsism?

Thanks again

 

[akvadrako]

I find the term multi-solipsism sometimes useful when trying to understand the MWI. Once you accept that there are multiple worlds there is no reason to doubt other observers exist, even if exactly one viewpoint corresponds to one exact world. Further, even considering this multi-solipsism, the observers you interact with will be in worlds that almost complete overlap with your world; the semi-classical picture is basically the same.

 

[JamieSalaor]

I find the term multi-solipsism sometimes useful when trying to understand the MWI. Once you accept that there are multiple worlds there is no reason to doubt other observers exist, even if exactly one viewpoint corresponds to one exact world. Further, even considering this multi-solipsism, the observers you interact with will be in worlds that almost complete overlap with your world; the semi-classical picture is basically the same.

Multi-solipsism? As in the Many Minds interpretation?

 

[akvadrako]

Multi-solipsism? As in the Many Minds interpretation?

Maybe; I've never quite figured out how many-minds differs from other variants of MWI. I'm not sure where I first heard the term though I saved this from a few years ago: http://philsci-archive.pitt.edu/5515/

 

[JamieSalaor]

Maybe; I've never quite figured out how many-minds differs from other variants of MWI. I'm not sure where I first heard the term though I saved this from a few years ago: http://philsci-archive.pitt.edu/5515/

Aaaah yes I read that paper a while ago.
Its an interesting take, Soltau does follow Many Minds, this contradicts with Deutsch and Wallace's interpretation of MWI. Wallace doesn't really think Many Minds caught on, and to be honest I agree with his reasoning . But it is definitely worth thinking about and an interesting take..