Thread: Decoherence

Hello again! Recently, I have been looking at the multiple worlds interpretation. I understand that it uses decoherence to show that a wavefunction doesn't really "collapse," it just appears to. However, I don't quite understand how decoherence causes different timelines to happen.

Anyways, my real question is:

Where exactly are the parallel universes that MWI describes? I always thought that they were just decohered away from our own. However, that makes me wonder where they actually, physically are. How can the same particle be in different timelines?

Also, my understanding of decoherence is a bit fuzzy. Does it just mean that the universe that we observe just isn't interacting with the universes parallel to our own?

I realize this is a lot of questions, but I'm very interested!

Originally Posted by guymillion

Hello again! Recently, I have been looking at the multiple worlds interpretation. I understand that it uses decoherence to show that a wavefunction doesn't really "collapse," it just appears to. However, I don't quite understand how decoherence causes different timelines to happen.

Anyways, my real question is:

Where exactly are the parallel universes that MWI describes? I always thought that they were just decohered away from our own. However, that makes me wonder where they actually, physically are. How can the same particle be in different timelines?

Also, my understanding of decoherence is a bit fuzzy. Does it just mean that the universe that we observe just isn't interacting with the universes parallel to our own?

I realize this is a lot of questions, but I'm very interested!

Yes, those are a lot of questions, and good ones, at that! Let's start with the meaning of "parallel" in the MWI context. It's actually a misnomer; more properly, perhaps the word "orthogonal" should be used.

As you probably know, Everett (the father of the MWI, although the name ""many worlds" didn't get attached to his work until many years later, and by others) pointed out that the Copenhagen interpretation, in which a superposition of possibilities exists until a measurement "collapses" the possibilities into an actual reality, is only one interpretation of QM. MWI yields the same experimental outcomes as QM, but sidesteps the collapse business by positing that all possibilities actually become realities. At each bifurcation, new realities emerge, but they do not interact with each other (otherwise MWI would produce different experimental outcomes than Copenhagen would describe). Mathematically, such non-interacting realities are said to be orthogonal, not parallel.

Because MWI and QM predict identical experimental outcomes, there is no way to falsify either interpretation (which is why they're called interpretations, not theories). Favoring one or the other is thus a matter of taste.

"Because MWI and QM predict identical experimental outcomes, there is no way to falsify either interpretation (which is why they're called interpretations, not theories). Favoring one or the other is thus a matter of taste."
But you can understend that thay move back in time to preduse multi world , and you recgnize them only when they interact with yours (there no place when there no feild its only consernd to interact )

Originally Posted by tk421

Yes, those are a lot of questions, and good ones, at that! Let's start with the meaning of "parallel" in the MWI context. It's actually a misnomer; more properly, perhaps the word "orthogonal" should be used.

As you probably know, Everett (the father of the MWI, although the name ""many worlds" didn't get attached to his work until many years later, and by others) pointed out that the Copenhagen interpretation, in which a superposition of possibilities exists until a measurement "collapses" the possibilities into an actual reality, is only one interpretation of QM. MWI yields the same experimental outcomes as QM, but sidesteps the collapse business by positing that all possibilities actually become realities. At each bifurcation, new realities emerge, but they do not interact with each other (otherwise MWI would produce different experimental outcomes than Copenhagen would describe). Mathematically, such non-interacting realities are said to be orthogonal, not parallel.

Because MWI and QM predict identical experimental outcomes, there is no way to falsify either interpretation (which is why they're called interpretations, not theories). Favoring one or the other is thus a matter of taste.

Thank you so much! This post does a very good job of describing MWI and explaining the difference between it and Copenhagen!

I have thought that there could be a universe that is "orthogonal" to our own where the big bang has not happened yet. 13.5 billion years ago, our universe split. In one universe, the big bang did not happen, and in another, it did. This could explain why the universe exists at all. There could be other orthogonal universes with different initial conditions, which means that they could have different types of spacetime, and different matter.