Can the "many-worlds" in QM inter-communicate?

[Bharath Siva]

My layman understanding of the many-worlds interpretation of QM is that if a person had to make a decision between A and B and he chose A, then the universe splits into 2 branches, one where A was chosen and another where B was chosen. First of all, is this correct?

Secondly, regarding the many-worlds interpretation of quantum physics, is there any theory that says that these many worlds can actually communicate with each other? If the universe split into 2 because I made a decision to eat chocolate ice cream instead of vanilla, then is there a way that the "chocolate ice cream me" can communicate with "vanilla ice cream me"?

Thirdly, could we ever end up with a multiverse scenario like this: In universe X, Alice and Bob exist and they are both at an ice-cream shop that sells only vanilla or chocolate ice cream. They both make their ice-cream choice at the exact same snapshot in time. Alice chooses chocolate and causes the universe to split into X1 (Alice Chocolate universe) and X2 (Alice Vanilla universe). Meanwhile, Bob chooses Vanilla and causes the universe to split into X3 (Bob Vanilla universe) and X4 (Bob chocolate universe). Could we end up in a scenario where X3 and X1 could actually end up being the exact same universe? Or does the theory say that these 4 universes should be distinct?

 

[Orodruin]

First of all, the MWI is not about persons making decisions, it is about processes occurring at the quantum level.

Second, there simply is no such thing as "the exact same instance of time". In special relativity there are many different possibilities for defining this and it is therefore not a well defined statement.

And finally, no, the worlds cannot communicate. The MWI is simply an interpretation of quantum mechanics which make no testable predictions, it is not a scientific theory.

 

[Demystifier]

My layman understanding of the many-worlds interpretation of QM is that if a person had to make a decision between A and B and he chose A, then the universe splits into 2 branches, one where A was chosen and another where B was chosen. First of all, is this correct?

It is not correct. The splitting does not depend on person's choices. In some versions of MWI person's decisions are used to justify the Born rule, but that's another issue.

Secondly, regarding the many-worlds interpretation of quantum physics, is there any theory that says that these many worlds can actually communicate with each other? If the universe split into 2 because I made a decision to eat chocolate ice cream instead of vanilla, then is there a way that the "chocolate ice cream me" can communicate with "vanilla ice cream me"?

The splited branches cannot communicate. Strictly speaking, the splitting is never absolutely perfect so a small communication always exists, but that's negligible for all practical purposes.

 

[Bharath Siva]

The splited branches cannot communicate. Strictly speaking, the splitting is never absolutely perfect so a small communication always exists, but that's negligible for all practical purposes.

Can you elaborate on what you mean by "a small communication always exists"? How can you detect this communication?

 

[bhobba]

Can you elaborate on what you mean by "a small communication always exists"? How can you detect this communication?

Its because interference effects decay very very quickly to way way below detectability, but theoretically are never zero.

I can guess your next question, but first please cognate on way way below detectability.

Thanks
Bill

 

[Demystifier]

Can you elaborate on what you mean by "a small communication always exists"? How can you detect this communication?

You cannot detect it. As I said, it's negligible. The existence can only be "seen" through the theory.

 

[mfb]

Alice chooses chocolate and causes the universe to split into X1 (Alice Chocolate universe) and X2 (Alice Vanilla universe). Meanwhile, Bob chooses Vanilla and causes the universe to split into X3 (Bob Vanilla universe) and X4 (Bob chocolate universe). Could we end up in a scenario where X3 and X1 could actually end up being the exact same universe? Or does the theory say that these 4 universes should be distinct?

We have to replace the persons by quantum processes, but let's call those processes Alice and Bob and the result chocolate and vanilla. You would get four worlds, (Alice chocolate, Bob chocolate), (Alice chocolate, Bob vanilla), (Alice vanilla, Bob chocolate) and (Alice vanilla, Bob vanilla). This is independent of the time or time ordering of the quantum processes.

 

[.Scott]

You cannot detect it. As I said, it's negligible. The existence can only be "seen" through the theory.

There's a big difference between "negligible" and "cannot detect it". If MWI creates an effect that can be tested, then it is not an interpretation any more - it would be a theory. If MWI does not create an effect that can be tested, then it does not create that effect - and it remains an interpretation. Hugh Everett's MWI did not anticipate any interactions among the worlds.

 

[Demystifier]

There's a big difference between "negligible" and "cannot detect it". If MWI creates an effect that can be tested, then it is not an interpretation any more - it would be a theory. If MWI does not create an effect that can be tested, then it does not create that effect - and it remains an interpretation. Hugh Everett's MWI did not anticipate any interactions among the worlds.

Then let me be slightly more precise: The "interactions" between the words can be detected in principle, but not in practice.

 

[.Scott]

Then let me be slightly more precise: The interactions between the worlds can be detected in principle, but not in practice.

Is that because you are treating MWI as a form of superpositioning? MWI is suppose to be an alternate interpretation to "collapse". Are you saying that since collapse isn't a well-defined event, neither is the splitting of worlds in MWI?

 

[Demystifier]

Is that because you are treating MWI as a form of superpositioning? MWI is suppose to be an alternate interpretation to "collapse". Are you saying that since collapse isn't a well-defined event, neither is the splitting of worlds in MWI?

No. What I am saying is that there is always a small overlap between branches in MWI.

 

[.Scott]

No. What I am saying is that there is always a small overlap between branches in MWI.

I don't think Everett ever said that. Where does this overlap come from?

 

[Demystifier]

I don't think Everett ever said that.

He didn't.

Where does this overlap come from?

From the theory of decoherence (which was not known at the time of Everett).

 

[.Scott]

From the theory of decoherence (which was not known at the time of Everett).

So, since decoherence says that there will be some effect of what didn't happen in the world that eventually did happen, then MWI will too. But isn't that just more superpositioning?

 

[Demystifier]

So, since decoherence says that there will be some effect of what didn't happen in the world that eventually did happen, then MWI will too. But isn't that just more superpositioning?

I guess it is. Do you see a problem with it?

 

[.Scott]

I guess it is. Do you see a problem with it?

IT was one of my first guesses back in post #10 of this thread.
An interpretation is semantics. Now we have an interpretation that can be extended in more than one way to cover decoherence - so we have yet another layer of semantics.
So, can "many-worlds" in QM, intercommunicate? It seems to me that it depends on whether you define many-worlds to allow that.

 

[mfb]

You don't have to define anything, that is the beauty of many-worlds. You just let the wavefunction evolve. For every reasonable choice of basis, you'll see how a given starting condition, subject to the laws of quantum mechanics, can split up into separate "regions" with large amplitude, with a tiny amplitude in between. This tiny amplitude is decaying with extremely short timescales, so the influence of the region between "regions" is negligible even nanoseconds after decoherence (which is a process that takes some time) happened. Those "regions" are called worlds and they do not interact in any measurable way.
After decoherence happened, you can introduce wavefunction collapses which remove all but one world with relative probabilities according to the squared amplitude, then you are back at the Copenhagen interpretation. If you try to move the collapse to the time before decoherence happened, you get results that are not in agreement with measurements.