Having trouble understanding the wave function collapse.

[Hurkyl]

Thanks to the miracle of relative states, and reasonable assumptions about decoherence, at the macroscopic scale one would expects quantum states to, for all practical purposes, decompose into statistical mixtures -- a classical kind of indeterminacy -- each of which corresponds to a macroscopic outcome. (Of course, many different components can correspond to the same outcome)

This means the indeterminacy is not an inherently quantum thing -- we can think about it in purely classical terms. The only novel thing that QM adds is that the components can split (and recombine too, although thermodynamics tells us that would be very unlikely), which means we can't simply shrug them off as probabilities of knowledge rather than probabilities in reality.

 

[kote]

Let me answer your question with a question. How can distances be physically meaningful in Galilean relativity, despite the fact position has no physical meaning?

I only have a basic familiarity with Galilean relativity, so you'll have to excuse me for missing the point of your question. Does MWI deny all meaning to synthetic propositions or have I missed something?

Can I ever say "in my branch, is the cat alive or dead?" It was denied earlier in the thread that MWI allows that question. As a follow up, if that question isn't allowed, can there be any such thing as truth in MWI?

 

[Hurkyl]

Can I ever say "in my branch, is the cat alive or dead?" It was denied earlier in the thread that MWI allows that question.

It sounds reasonable to me. If someone said that, then I imagine they had this caveat on mind:

"My branch" is not a constant fixed thing -- it depends on the branch. The answer to your question, likewise, is a branch-dependent.​

As a follow up, if that question isn't allowed, can there be any such thing as truth in MWI?

"Truth" is not a simple concept, and people have lots of strong opinions about it that aren't really warranted. I'm not going to say much because I'll derail the thread.

The fact of the matter is, to have a physically useful notion of truth, one doesn't need to have propositions like "The cat is dead" -- it is sufficient to have propositions like "Given that the cat is X, the cat is X" or "Given past observations Y, the cat is X". (Where X and Y are indeterminates)

Alternatively, one can adopt semantics where the truth values are not simply the constants "true" and "false", but instead they are {"true", "false"}-valued functions of branches. (or some reasonable facsimile)

 

[kote]

It sounds reasonable to me. If someone said that, then I imagine they had this caveat on mind:

"My branch" is not a constant fixed thing -- it depends on the branch. The answer to your question, likewise, is a branch-dependent.​

The fact of the matter is, to have a physically useful notion of truth, one doesn't need to have propositions like "The cat is dead" -- it is sufficient to have propositions like "Given that the cat is X, the cat is X" or "Given past observations Y, the cat is X". (Where X and Y are indeterminates)

Sure; that's what i was talking about with P(x) vs P before. If we can still have those qualified truth statements then we can still talk about the probability that in branch X, where X is the branch I'm in right now, the cat is alive or dead. This explanation makes complete sense to me. I'm just not sure, in this case, what the (unfalsifiable?) supposition of other branches adds in terms of metaphysical value, but that's another thread.

I think the issue is that I was asking about the condition earlier where I can't even ask about branch X. This interpretation seems to cause issues for truth that seem untenable to me. There would be no truth, that I can tell, even conditionally. Everything happens and doesn't happen simultaneously. All synthetic propositions, even conditional, would be rendered meaningless.

I was trying to clarify whether or not I had missed anything in that second version of things.

 

[SergioPL]

Maybe the EPR paradox is not a paradox.

Feymann diagrams show that a particle should travel "virtually" towards and backwards in time for all the possible paths until it makes the "real" travel from a point to other (usually in experiments from the sender to the receiver).

The "real travel" is never faster than light and fits the relativity frame but if we sum all the virtual travels, the speed required could be infinite.

This could mean that the particle "feels the presence" of other near, but not local, particles and it determines the way it takes. This "feeling" does not require energy transfer and could be considered instantaneous.

So, relativity would only apply to those particles that transport energy or momentum but the rules that show how will behave a particle could be instantaneous.

I don't think this rules may let us to transport information faster than light since we need some carrier to transmit this information and the faster one would be photons.

...maybe entangled particles could change this but I don't know.

 

[Fredrik]

I am afraid you're a victim of a very common misconception about MWI. People ask: ok, there are 2 outcomes, cat alive and cat dead, but why *I* see only dead one (or alive one)? For *me* it is random, right?

This is wrong. In MWI an observer is not a line, it is a tree. It 'splits' when being decoherenced with an outcome. So MWI predicts that in one branch one observer is asking "but why cat is dead?" and in another one "but why cat is alive?"

Why would you define "me" as the entire tree rather than a short unbranched segment of that tree? I mean, the other branches of "you" might not even be asking themselves if the cat is dead or alive. If I'm asking myself that, then I wouldn't consider some version of me who isn't asking himself that question "me".

I don't have any objections at all to phrases like "to me, the cat is dead". They just need to be interpreted correctly. First of all, "me" needs to be as I described above. Second, the rest of the statement should be interpreted as saying that the probability that a measurement on the cat will give us the result "dead" is 1.

That's what it means. Nothing more, nothing less. Statements such as "the cat is dead" are only "neither true nor false" when the probability of measuring that result is neither 1 nor 0.

 

[Dmitry67]

I agree that my definition is "me" is fuzzy. Any observer is a tree, but as the same time there are 2 major effects affecting our consciousness:

1. We can remember the past, not the future
2. We can not 'feel' what our 'other' copies in another branches are experiencing (because of the decoherence)

So, all the functionality of my brain is based on just one 'slice' of the reality. My consioucness is always backwards-oriented, so I don't feel branching at all.

So an observer is a tree which is severily impared in terms of the communication between the branches. To the extent that every branch thinks that is is absolutely alone.

Well, in any case, it is just wording... I think we don't have a serious disagreement, right?

 

[Zarqon]

All this talk about trees and "feel the branching" seems strange to me, given the answers I got to a question earlier.

Is the analogy of a tree really accurate to describe MWI? Given what I have read here earlier, wouldn't a better analogy be parallel racetracks, that in total sum up all possibilities that could ever happen. But in each "track" you live alone without connection to the other tracks, and in fact, if the cat is observered to be dead in "your" branch, then it was always doomed to die for you, you just didn't know it yet.

A tree that branches as time goes on implies decisions being made continuously, and I don't see that being consistent with determinism. Is this analogy a better description, or did I missunderstand something earlier?

 

[Dmitry67]

1 Is the analogy of a tree really accurate to describe MWI?

2 But in each "track" you live alone without connection to the other tracks, and in fact, if the cat is observered to be dead in "your" branch, then it was always doomed to die for you, you just didn't know it yet.

3 A tree that branches as time goes on implies decisions being made continuously, and I don't see that being consistent with determinism. Is this analogy a better description, or did I missunderstand something earlier?

1 not really, it is a simplification

2 the word "you" is not well-defined in MWI. Depending on the clarification, answer to your question might be negative or positive

Regarding the "you just don't know it yet" - note that in modern MWI "splitting" is well-defined physical process (decoherence). In case of an ideal box, before you opened it, you are not decoherenced with what is inside, so for you there are really 1/2 alive cat + 1/2 dead cat inside! Only when you open it and exchange few photons you are decoherenced with it and "split". So (another misconception about MWI) quantum events don't split the whole universe. "Splitting" propagates at ligth speed (but it can be much much slower).

3 This is important. What decisions are you talking about? Decisions for cat being dead or alive? there are no such decisions, as all outcomes do occur.