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 ignore when collapse occurs. If as many “interpreters” like to claim QM itself is just the wavefunction, then collapse and world-splits are both extra mechanisms.
But QM is not just the wavefunction. QM is also the Born probabilities. The wavefunction predicts nothing if we do not square it to find the probabilities of the events we actually observe. Of all the interpretations, objective collapse adds the least to quantum mechanics as it is actually practiced. Everybody who uses QM for practical purposes uses the Born probabilities or the direct consequences thereof (e.g. spectra). Thus—despite the many who shudder at the nondeterminism of the universe and thus come up with interpretations like Copenhagen and MWI to try to turn inherent nondeterminism into mere subjective ignorance—the nondeterministic quantum event whereby a superposition of eigenvectors reduces to a single eigenvector (and the various other isomorphic ways this can be mathematically represented) is every bit as central to QM as the nominally deterministic wavefunction. The Born probabilities are not in any way “extra mechanism” they are central to QM. Even more central than the wavefunction, because all that we observe directly are the Born random events. The wavefunction we never observe directly, but only infer it as defining the probability distribution of the nondeterministic events we do observe.
Thus any interpretation of QM as it is actually practiced must take the Born probabilities as being at least as objective and physical as the wavefunction. If the Born probabilities are objective, we have objective collapse, and neither Copenhagen nor MWI are true.
Wikipedia has a bare-bones description of objective collapse:
Further experimental evidence: if the Born probabilities do not represent an objective and physical randomness that is inherent to the universe, then the EPR/Bell/Aspect/et,. al. work tells us that FTL signaling (and more importantly a variety of related paradoxes, FTL signaling not itself being paradoxical in QM) is possible. QM is not special relativity. Special relativity can’t explain the small scale or even certain macroscale effects like diffraction that QM explains. Special relativity is just an emergent large-scale special case of QM (specifically of QFT), it is QM that is fundamental. QM itself, in the EPR/et. al. line of work, tells is that it is the objective and physical randomness inherent in the universe, not causal locality, that stands in the way of FTL signaling and its associated paradoxes.
MWI adds a preposterous amount of mechanism involving an infinite and ever-exponentially-expanding number of completely unobservable clone universes.
There’s no mechanism to it other than the mechanism that every interpretation of QM already has for describing the evolution of non-macroscopic quantum systems. MWI just says that large systems and small systems aren’t separate magisteria with different laws.
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 ignore when collapse occurs.
“Worlds” and “branching” are epiphenomenal concepts; they’re simplifications of what MWI actually talks about (see Decoherence is Pointless).
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.
What is or is not a “branch” is unimportant. If you have read the link you’ll know that a “branch” is not a point mass but a blob spread out in configuration space. All MWI needs is “the probability density of finding oneself in point x in the wavefunction is the amplitude squared at that point”. It’s standard probability theory then to integrate over a “branch” to find your probability of being in that branch. But the only reason to care about “branches” is because the world looks precisely identical to an observer at every point in that branch.
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 ignore when collapse occurs. If as many “interpreters” like to claim QM itself is just the wavefunction, then collapse and world-splits are both extra mechanisms.
But QM is not just the wavefunction. QM is also the Born probabilities. The wavefunction predicts nothing if we do not square it to find the probabilities of the events we actually observe. Of all the interpretations, objective collapse adds the least to quantum mechanics as it is actually practiced. Everybody who uses QM for practical purposes uses the Born probabilities or the direct consequences thereof (e.g. spectra). Thus—despite the many who shudder at the nondeterminism of the universe and thus come up with interpretations like Copenhagen and MWI to try to turn inherent nondeterminism into mere subjective ignorance—the nondeterministic quantum event whereby a superposition of eigenvectors reduces to a single eigenvector (and the various other isomorphic ways this can be mathematically represented) is every bit as central to QM as the nominally deterministic wavefunction. The Born probabilities are not in any way “extra mechanism” they are central to QM. Even more central than the wavefunction, because all that we observe directly are the Born random events. The wavefunction we never observe directly, but only infer it as defining the probability distribution of the nondeterministic events we do observe.
Thus any interpretation of QM as it is actually practiced must take the Born probabilities as being at least as objective and physical as the wavefunction. If the Born probabilities are objective, we have objective collapse, and neither Copenhagen nor MWI are true.
Wikipedia has a bare-bones description of objective collapse:
http://en.wikipedia.org/wiki/Objective_collapse_theory
Further experimental evidence: if the Born probabilities do not represent an objective and physical randomness that is inherent to the universe, then the EPR/Bell/Aspect/et,. al. work tells us that FTL signaling (and more importantly a variety of related paradoxes, FTL signaling not itself being paradoxical in QM) is possible. QM is not special relativity. Special relativity can’t explain the small scale or even certain macroscale effects like diffraction that QM explains. Special relativity is just an emergent large-scale special case of QM (specifically of QFT), it is QM that is fundamental. QM itself, in the EPR/et. al. line of work, tells is that it is the objective and physical randomness inherent in the universe, not causal locality, that stands in the way of FTL signaling and its associated paradoxes.
There’s no mechanism to it other than the mechanism that every interpretation of QM already has for describing the evolution of non-macroscopic quantum systems. MWI just says that large systems and small systems aren’t separate magisteria with different laws.
“Worlds” and “branching” are epiphenomenal concepts; they’re simplifications of what MWI actually talks about (see Decoherence is Pointless).
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.
What is or is not a “branch” is unimportant. If you have read the link you’ll know that a “branch” is not a point mass but a blob spread out in configuration space. All MWI needs is “the probability density of finding oneself in point x in the wavefunction is the amplitude squared at that point”. It’s standard probability theory then to integrate over a “branch” to find your probability of being in that branch. But the only reason to care about “branches” is because the world looks precisely identical to an observer at every point in that branch.