nick11
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"MWI having a fixed phase space that doesn't actually increase in size over time."
(1) That assumes we are already simulating the entire universe from the Big Bang forward, which is already preposterously infeasible (not to mention that we don't know the starting state).
(2) It doesn't model the central events in QM, namely the nondeterministic events which in MWI are interpreted as which "world" we "find ourselves" in.
Of course in real QM work simulations are what they are, independently of interpretations, they evolve the wavefunction, or a computationally more efficient but less accurate version of same, to the desired elaboration (which is radically different for different applications).... (read more)
That's an easy one -- objective collapse QM predicts that with astronomically^astronomically^astronomically high probability objects large enough to be seen at that distance (or even objects merely the size of ourselves) don't cease to exist. Like everything else they continue to follow the laws of objective collapse QM whether we observe them or not.
The hypo is radically different from believing in an infinitely expanding infinity of parallel "worlds", none of which we have ever observed, either directly or indirectly, and none of which are necessary for a coherent and objective QM theory.
"This happens when, way up at the macroscopic level, we 'measure' something."
vs. in objective collapse, when the collapse occurs has no necessary relationship to measurement. "Measurement" is a Copenhagen thing.
"So the wavefunction knows when we 'measure' it. What exactly is a 'measurement'? How does the wavefunction know we're here? What happened before humans were around to measure things?"
Again, this describes Copenhagen (or even Conscious Collapse, which is even worse). Objective collapse depends on neither measurements nor measurers.
Much of the rest of this parody might be characterized as a preposterously unfair roast of collapse theories, objective or otherwise, but the trouble is all the valid criticisms also apply... (read more)
It doesn't matter whether branching occurs at a point of or at during some blob of time, probabilistic or otherwise, it's a central part of MWI and you need an equation to describe when it happens. And that equation should agree with the Born probabilities up to our observational limits. Likewise for collapse in theories that invoke collapse. Otherwise it's just hand-waving not science.
"Imagine a universe containing an infinite line of apples."
If we did I would imagine it, but we don't. In QM we don't observe infinite anything, we observe discrete events. That some of the math to model this involves infinities may be merely a matter of convenience to deal with a universe that may merely have a very large but finite number of voxels (or similar), as suggested by Planck length and similar ideas.
"It's reasonable to assume run time is important, but problematic to formalize."
Run time complexity theory (and also memory space complexity, which also grows at least exponentially in MWI) is much easier to apply than Kolmogorov complexity in... (read more)
Every interpretation is "adding something." Just because interpreters choose to bundle their extra mechanisms in vague English language “interpretations” rather than mathematical models does not mean they aren’t extra mechanisms. Copenhagen adds an incoherent and subjective entity called "the observer." MWI adds a preposterous amount of mechanism involving an infinite and ever-exponentially-expanding number of completely unobservable clone universes. Copenhagen grossly violates objectivity and MWI grossly violates Occam's Razor. Also, MWI needs a way to determine when a "world" splits, or to shove the issue under the rug, every bit as much as collapse theories need to figure out or... (read more)
Emphasize the "almost". I'm advocating objective collapse, not Copenhagen.
"Eliezer's argument is that multiple worlds require no additions to the length of the theory if it was formally expressed, whereas a 'deleting worlds' function is additional. It's also unclear where it would kick in, what 'counts' as a sufficiently fixed function to chop off the other bit."
Run time is at least as important as length. If we want to simulate evolution of the wavefunction on a computer, do we get a more accurate answer of more phenomena by computing an exploding tree of alternatives that don't actually significantly influence anything that we can ever observe, or does the algorithm explain more by pruning these irrelevant branches and elaborating the branches... (read more)
Let me join all those observing that these are great explanations of QM. But I don't get why we need to invoke MWI and the Ebborians. If the wavefunction evolves into
(Human-LEFT Sensor-LEFT Atom-LEFT) + (Human-RIGHT Sensor-RIGHT Atom-RIGHT)
but we only observe
(Human-LEFT Sensor-LEFT Atom-LEFT)
then it makes far more sense to me that, rather than conjuring up a completely unobservable universe with clones of ourselves where (Human-RIGHT Sensor-RIGHT Atom-RIGHT) happened, a far more empirical explanation is that it simply didn't happen. Half of the wavefunction disappears, nondeterministically. Why, as Occam might say, multiply trees beyond necessity? Prune them instead. Multiple "worlds" strike me as no more necessary than the aether or absolute space.
BTW, it's MWI that adds extra postulates. In both MWI and collapse, parts of the wavefunction effectively disappear from the observable universe (or as MWI folks like to say "the world I find myself in.") MWI adds the extra and completely gratuitous postulate that this portion of the wave function magically re-appears in another, imaginary, completely unobservable "world", on top of the gratuitous extra postulate that these new worlds are magically created, and all of us magically cloned, such that the copy of myself I experience finds me in one "world" but not another. And all that just to explain why we observe a nondeterministic event, one random eigenstate out of the infinity of eigenstates derived from the wavefunction and operator, instead of observing all of them.
Why not just admit that quantum events are objectively nondeterministic and be done with it? What's so hard about that?