So, to get this clear (being well outside my comfort zone here), once a split into two branches has occurred, they no longer influence each other? The integration over all possibilities is something that happens in only one of the many worlds? (My recent understanding is based on “Everything that can happen does happen” by Cox & Forshaw).
There are no discrete “worlds” and “branches” in quantum physics as such. Once two regions in state space are sufficiently separated to no longer significantly influence each other they might be considered split, which makes the answer to your question “yes” by definition.
There are no discrete “worlds” and “branches” in quantum physics as such.
This seems to conflict with references to “many worlds” and “branch points” in other comments, or is the key word “discrete”? In other words, the states are a continuum with markedly varying density so that if you zoom out there is the appearance of branches? I could understand that expect for cases like Schroedinger’s cat where there seems to be a pretty clear branch (at the point where the box is opened, i.e. from the point of view of a particular state if that is the right terminology).
Once two regions in state space are sufficiently separated to no longer significantly influence each other...
From the big bang there are an unimaginably large number of regions in state space each having an unimaginably small influence. It’s not obvious, but I can perfectly well believe that the net effect is dominated by the smallness of influence, so I’ll take your word for it.
In other words, the states are a continuum with markedly varying density so that if you zoom out there is the appearance of branches?
Yes, but it’s still continuous. There’s always some influence, it can just get arbitrarily small. I’m unsure if this hypothetically allows MWI to be experimentally confirmed.
(The thesis of mangled-worlds seems to be that, in fact, in some cases that doesn’t happen—that is, world A’s influence on world B stays large.)
Schroedinger’s cat
If it helps, think of half-silvered mirrors. Those are actually symmetric, letting through half the light either way; the trick is that the ambient lighting on the “reflective” side is orders of magnitude brighter, so the light shining through from the dark side is simply washed out.
To apply that to quantum mechanics, consider that the two branches—cat dead and not-dead—can still affect each other, but as if through a 99.9-whatever number of nines-silvered mirror. By the time a divergence gets to human scale, it’ll be very, very close to an absolute separation.
Thanks, so to get back to the original question of how to describe the different effects of divergence and convergence in the context of MW, here’s how it’s seeming to me. (The terminology is probably in need of refinement).
Considering this in terms of the LW-preferred Many Worlds interpretation of quantum mechanics, exact “prediction” is possible in principle but the prediction is of the indexical uncertainty of an array of outcomes. (The indexical uncertainty governs the probability of a particular outcome if one is considered at random.) Whether a process is convergent or divergent on a macro scale makes no difference to the number of states that formally need to be included in the distribution of possible outcomes. However, in the convergent process the cases become so similar that there appears to be only one outcome at the macro scale; whereas in a divergent process the “density of probability” (in the above sense) becomes so vanishingly small for some states that at a macro scale the outcomes appear to split into separate branches. (They have become decoherent.) Any one such branch appears to an observer within that branch to be the only outcome, and so such an observer could not have known what to “expect”—only the probability distribution of what to expect. This can be described as a condition of subjective unpredictability, in the sense that there is no subjective expectation that can be formed before the divergent process which can be reliably expected to coincident with observation after the process.
So, to get this clear (being well outside my comfort zone here), once a split into two branches has occurred, they no longer influence each other? The integration over all possibilities is something that happens in only one of the many worlds? (My recent understanding is based on “Everything that can happen does happen” by Cox & Forshaw).
There are no discrete “worlds” and “branches” in quantum physics as such. Once two regions in state space are sufficiently separated to no longer significantly influence each other they might be considered split, which makes the answer to your question “yes” by definition.
This seems to conflict with references to “many worlds” and “branch points” in other comments, or is the key word “discrete”? In other words, the states are a continuum with markedly varying density so that if you zoom out there is the appearance of branches? I could understand that expect for cases like Schroedinger’s cat where there seems to be a pretty clear branch (at the point where the box is opened, i.e. from the point of view of a particular state if that is the right terminology).
From the big bang there are an unimaginably large number of regions in state space each having an unimaginably small influence. It’s not obvious, but I can perfectly well believe that the net effect is dominated by the smallness of influence, so I’ll take your word for it.
Yes, but it’s still continuous. There’s always some influence, it can just get arbitrarily small. I’m unsure if this hypothetically allows MWI to be experimentally confirmed.
(The thesis of mangled-worlds seems to be that, in fact, in some cases that doesn’t happen—that is, world A’s influence on world B stays large.)
If it helps, think of half-silvered mirrors. Those are actually symmetric, letting through half the light either way; the trick is that the ambient lighting on the “reflective” side is orders of magnitude brighter, so the light shining through from the dark side is simply washed out.
To apply that to quantum mechanics, consider that the two branches—cat dead and not-dead—can still affect each other, but as if through a 99.9-whatever number of nines-silvered mirror. By the time a divergence gets to human scale, it’ll be very, very close to an absolute separation.
Thanks, so to get back to the original question of how to describe the different effects of divergence and convergence in the context of MW, here’s how it’s seeming to me. (The terminology is probably in need of refinement).
Considering this in terms of the LW-preferred Many Worlds interpretation of quantum mechanics, exact “prediction” is possible in principle but the prediction is of the indexical uncertainty of an array of outcomes. (The indexical uncertainty governs the probability of a particular outcome if one is considered at random.) Whether a process is convergent or divergent on a macro scale makes no difference to the number of states that formally need to be included in the distribution of possible outcomes. However, in the convergent process the cases become so similar that there appears to be only one outcome at the macro scale; whereas in a divergent process the “density of probability” (in the above sense) becomes so vanishingly small for some states that at a macro scale the outcomes appear to split into separate branches. (They have become decoherent.) Any one such branch appears to an observer within that branch to be the only outcome, and so such an observer could not have known what to “expect”—only the probability distribution of what to expect. This can be described as a condition of subjective unpredictability, in the sense that there is no subjective expectation that can be formed before the divergent process which can be reliably expected to coincident with observation after the process.
With the caveat that I’m not a physicist, and don’t understand much of the math involved—yes, this seems to be correct.
Though note that quantum physics operates on phase space; if two outcomes are the same in every respect, then they really are the same outcome.