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elfmotat
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What about the act of observation actually causes a particle to break the superpostion and "decide" what its state is? What property does the observer posses that changes the the way particles behaves?
elfmotat said:What about the act of observation actually causes a particle to break the superposition and "decide" what its state is? What property does the observer posses that changes the the way particles behaves?
elfmotat said:What property does the observer posses that changes the the way particles behaves?
Pio2001 said:1 core of the process remains completely mysterious.
2 We know that the process seems to obey a "fundamental randomness".
Dmitry67 said:1 no, check Quantum Decoherence
2 no, it is interpretation-dependent
Fredrik said:I still think of a measurement as an interaction that, through the process of decoherence, entangles the eigenstates of some operator with macroscopically distinguishable states of a system that for practical purposes can be considered classical. So we're not getting rid of decoherence either. I don't think decoherence has solved the "measurement problem", but it has given us the best definition we have of what a measurement is.
Isham had some very interesting comments about wavefunction collapse in his book. Suppose a silver atom prepared in the spin state |s>=|+>+|-> is sent through a Stern-Gerlach apparatus without any kind of detector to tell us where the atom has ended up. The interaction with the apparatus entangles the spin states with position eigenstates, so that the time evolution of the |position>|spin> state is
|center>|s> → |left>|+>+|right>|->
What Isham pointed out is that if we now put a measuring device (e.g. another Stern-Gerlach apparatus with a different orientation, and with particle detectors at the positions of its outgoing beams) in the position where the right "beam" comes out, we're going to have to take the state before the measurement to be |right>|->. We still haven't measured anything, but we have "collapsed" the wavefunction simply by putting the measuring device on the right. This illustrates the importance of distinguishing between state preparation and measurement, and that the "collapse" isn't a physical process at all (in this "interpretation"). It's just a selection of what to measure.
Your question implies that there is such a thing as (separable) objective reality. It's important to remember that this assumption is being made, since the influential Copenhagen Interpretation, at least, disagrees. Having said that, given your assumption of realism, the wave function represents our best knowledge of reality, but nothing described by it is actually real on a basic level. Particles, as you think of them, are at best a macroscopic approximation of reality, and as such you should not expect descriptions in terms of particles to always be accurate. All that happens with the act of observation is we uncover more about the real situation that we are modeling, imperfectly, with the wave function.elfmotat said:What about the act of observation actually causes a particle to break the superpostion and "decide" what its state is? What property does the observer posses that changes the the way particles behaves?
Fredrik said:Dmitry, don't you think QM is hard enough even without confusing students with lots of different interpretations? Especially when many of those interpretations aren't even well-defined.
Dmitry67 said:1 no, check Quantum Decoherence
Dmitry67 said:2 no, it is interpretation-dependent
Whether or not all outcomes happen has essentially nothing to do with whether one outcome is observed when we perform an experiment.Pio2001 said:but it does not provide the slightest clue about why one outcome is observed when we perform an experiment. Strictly speaking, it predicts that all outcomes exist.
Zarqon said:Hmm, what exactly is the main problem of the universe containing intrinsically random elements?
A quantum state in a superposition |0> + |1>, obviously gives a perfectly deterministic answer when measured in the (|0> + |1>), (|0> - |1>) basis. Only when measured in a "wrong" basis, like e.g. (|0>, |1>) does it give a random result. It doesn't feel so strange to me that asking the wrong question yields a weird answer. It's like asking a green spot whether it's black or white; you're bound to receive a random answer. (yes, this is very simplified, don't take it too far :P ).
My point is rather that randomness could simply be viewed as natures own built in safety-answer, for everyone asking the wrong questions. A deterministic universe on the other hand seem to suggest everything is predetermined, doesn't it? I'm trying to understand why exactly people find determinism more comforting than randomness?
Hurkyl said:Whether or not all outcomes happen has essentially nothing to do with whether one outcome is observed when we perform an experiment.
Dmitry67 said:But do you agree that MWI is deterministic, so there is no randomness?
Pio2001 said:Not in its usual form. The above event, observable by a given observer, remains random in MWI
Dmitry67 said: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?
Dmitry67 said:P is not a function of an event x. It is also a function of a branch.
That's correct. Asking if the cat is dead in MWI is exactly as nonsensical as asking if some object is at rest in SR. On their own, those questions do not make sense physically -- they require some extra scaffolding before they can be meaningful.kote said:Let's let P represent "The cat is dead." You're saying that I'm not allowed to ask, "is P true or false?"
Hurkyl said:That's correct. Asking if the cat is dead in MWI is exactly as nonsensical as asking if some object is at rest in SR. On their own, those questions do not make sense physically -- they require some extra scaffolding before they can be meaningful.
Dmitry67 said: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?"
So you see, in MWI the situation is symmetric and deterministic. No randomness. Just an illusion of it.
I'm not sure what precisely you mean here. The main point as I understand it is that it is not physically meaningful to ask unconditional questions.kote said:Correct me if I'm wrong, but it seems to me that MWI denies meaning to all synthetic propositions (if understood in a form that is not reducible to standard QM through branch specification).
Hurkyl said:I'm not sure what precisely you mean here. The main point as I understand it is that it is not physically meaningful to ask unconditional questions.
Suppose we open the box and peek inside. It is not meaningful to ask "Is the cat alive?" -- it is only meaningful to ask "Given what I've observed, is the cat alive?"
This is true, incidentally, no matter what physical theory and interpretation we use. It's just that when we use interpretations that have the "definite outcomes" feature, then absolute questions like "Is the cat alive?" and conditional questions "Given what I've observed, is the cat alive?" have the same answers -- and therefore we can safely forget that all questions are conditional.
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?kote said:My question is, how can any synthetic proposition, even when qualified, have a truth value (meaning) without definite outcomes?
Hurkyl said: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?
It sounds reasonable to me. If someone said that, then I imagine they had this caveat on mind:kote said: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.
"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.As a follow up, if that question isn't allowed, can there be any such thing as truth in MWI?
Hurkyl said: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)