I Can Decoherence Explain the Many Worlds Interpretation?

[Sayestu]

TL;DR Summary

I thought I understood it well enough. Pls halp

I've been researching the MWI, and just when I thought I was starting to get it, I got confused again. There aren't many worlds; there's one in superposition? What's the difference? When do worlds(?) split? Will someone please explain the theory in a simple way, but not so simplistically I might misconceive of it?

 

[PeterDonis]

I've been researching the MWI

What sources have you already consulted?

 

[PeterDonis]

just when I thought I was starting to get it, I got confused again

You already have a discussion of the MWI open in another thread. This post of mine in that thread, in particular, addresses at least some of the questions you are posing here:

https://www.physicsforums.com/threa...cyclopedia-of-philosophy.1056867/post-6961877

 

[Sayestu]

Edit: since I'm asking about the real world, mods, I opted not to place this in Sci-Fi. If I should have, I apologize.
Edit 2: if I should have posted this as a reply elsewhere, I'm sorry. I'm used to using Quora, and I'm unfamiliar with how people use PF.
I read this MIT explanation of the MWI. At the end, the author says time doesn't "flow" in it; rather, a world exists for each instant (however small an "instant" is; I thought of "pixels"), with those worlds containing the records of their histories. So, as I understand it, a world at time t would contain the history of t-sub-negative-one, etc, but t-plus-one would not be connected to t. The flow of time would be an illusion.

If we grant the separation of instants, could time not be advancing, with new worlds spawning at the "ends" of extant worlds? A sort of "procedural generation," if I'm using that right, not a "block universe" already completed?

 

[Halc]

TL;DR Summary: Sorry for the number of my threads today. Does the MWI allow for the "real-time generation" of worlds instead of a "block"?

I sounds like you're asking if presentism is compatible with MWI. I don't see why not. One is an interpretation of time, and the other an interpretation of QM.

I read this MIT explanation of the MWI. At the end, the author says time doesn't "flow" in it;

Sounds like an opinion. The opinion that time doesn't flow is one I with which happen to agree, but nothing that can be demonstrated one way or another. The statement that MWI somehow asserts block time needs to be supported. I don't think Everett specifically specified.

So, as I understand it, a world at time t would contain the history of t-sub-negative-one, etc, but t-plus-one would not be connected to t.

If t containing the history of t-sub-negative-one is considered a connection, then wouldn't t+1 be similarly connected to t? This of course presumes the existence of t+1, which, under the flowing time model, may or may not be true depending on which t is the preferred one.

I also am unaware of MWI requiring discreet time like that. Several complications arise from it.

If we grant the separation of instants, could time not be advancing, with new worlds spawning at the "ends" of extant worlds? A sort of "procedural generation," if I'm using that right, not a "block universe" already completed?

I think it was DeWitt that coined the concept of 'spawning' 'worlds', like superposition is an act of creation or some such. This 'procedural generation' sounds like a multi-outcome computer simulation running one step at a time, which would indeed very much be a case of presentism.

 

[Sayestu]

Thanks! So, does MWI contain this idea of "a universe at each t," or did the author just assert it themself?

 

[Halc]

So, does MWI contain this idea of "a universe at each t," or did the author just assert it themself?

MWi has but the one universe. The premise is something on the order of:
"All isolated systems evolve according to theSchrodinger equation".
That describes one system, one universe. There is no mention of discreet moments in time in that premise, nor does it follow from it.

 

[PeterDonis]

Moderator's note: @Sayestu I have merged your two MWI threads in this forum into one. Please don't keep starting new threads on the MWI; we have this one open, let's at least get this discussion resolved before starting any new ones.

 

[PeterDonis]

I read this MIT explanation of the MWI.

This is the author's particular opinion/viewpoint on the MWI. Not all MWI proponents have the same viewpoint.

 

[Sayestu]

Moderator's note: @Sayestu I have merged your two MWI threads in this forum into one. Please don't keep starting new threads on the MWI; we have this one open, let's at least get this discussion resolved before starting any new ones.

Sorry. I'm used to Quora's Q&A format. Haven't used forums in a while.

 

[Sayestu]

Thanks to the other two people. How do I use Multi-Quote so I could more easily combine this with my last reply?
Edit: might have got it.

 

[Sayestu]

MWi has but the one universe. The premise is something on the order of:
"All isolated systems evolve according to theSchrodinger equation".
That describes one system, one universe. There is no mention of discreet moments in time in that premise, nor does it follow from it.

This is the author's particular opinion/viewpoint on the MWI. Not all MWI proponents have the same viewpoint.

 

[Sayestu]

What is quantum interference? Could different worlds theoretically interfere with each other?

 

[PeterDonis]

What is quantum interference?

The simplest definition I can give is that it is the reason why QM has to use probability amplitudes, i.e., complex numbers whose squared norms give probabilities, instead of just probabilities. For example, in the double slit experiment, an amplitude is assigned for the particle to go through each slit, and those amplitudes add the way complex numbers add, which results in an interference pattern on the detector screen.

Could different worlds theoretically interfere with each other?

No. If interference is possible, then "branching" of worlds has not yet occurred. (Unfortunately many discussions in the literature are not clear on this point.)

 

[PeterDonis]

@Sayestu not sure what the point is of your post #12.

 

[Sayestu]

@Sayestu not sure what the point is of your post #12.

I meant to type something like "I figured it [multi-quoting] out."
I don't really understand decoherence, which, I gather, begins the "split" that's finalized when decoherence completes. I saw it compared to untangling. Would you please explain? And thanks for the bit about interference.

 

[PeterDonis]

I don't really understand decoherence

Then I would recommend taking the time to consult some literature on the subject. It's much too broad a topic to be properly explained in a PF thread. This review article by Schlosshauer has been referenced in other PF threads on the topic:

https://arxiv.org/abs/1911.06282

The quick five-cent description is this: say you make a measurement on a quantum system, for example measuring the spin of an electron. Making the measurement involves an interaction between the electron and your measuring apparatus that entangles the two. But both the electron and the apparatus are also interacting with the rest of the environment, so the entanglement quickly spreads out into the environment, which contains a huge number of degrees of freedom that cannot be individually tracked. That does two things: first, it encodes the information about the result of the measurement in a very large number of degrees of freedom, which means that result can be repeatedly "read off" by many different people while still staying the same; and second, it makes the measurement result irreversible, because there is no way to track all of the degrees of freedom that got entangled in the environment and undo the effects of the measurement on all of them.

 

[Sayestu]

Thanks.

 

[kered rettop]

I meant to type something like "I figured it [multi-quoting] out."
I don't really understand decoherence, which, I gather, begins the "split" that's finalized when decoherence completes. I saw it compared to untangling. Would you please explain? And thanks for the bit about interference.

Decoherence results from interaction with the environment. The environment is a complex system so the state of the object in question gets scrambled. The other side of decoherence is much more important in MWI. The very same interaction disseminates the information about the interaction irreversibly into the environment, where an instrument or an observer (or anything else) can be affected by it.

The big thing about this is that you can model it as a superposition of countless microstates each containing information about one or other of the outcomes from the original interaction/measurement/observation. The final result is that an embedded observer in any microstate almost certainly sees just one outcome even though both, in a sense, occurred. This is the worlds splitting apart. I don't think the term "untangling" is helpful.