There are no actual branches in these interpretations, that’s just a popular simplification that is taken way too seriously. Every part of the wavefunction contributes to the ongoing evolution of the wavefunction, continuously and unitarily.
One apparently nitpicky but actually very serious objection is that for large messy heavily decoherent criteria like “WW2 happened”, we can’t even hope to define in quantum terms what sort of measurement would correspond to an outcome “it did happen” or “it did not happen”. It’s on a massively separated level of abstraction from any quantum properties.
However from various macroscopic principles, we can be pretty sure that any such measurement would overwhelmingly have greater measure on “it didn’t happen”. The probability would have too many 9′s to count after the decimal point, and the exact number would depend upon the exact details of what measurement is conceived of for determining whether “WW2 happened”.
In the MWI interpretation, we[1] occupy an ill-defined microscopic blob within a microscopic dot on a microscopic dot in the continuous wavefunction. Despite that, the future of “WW2 happened” states is almost entirely determined by the present of “WW2 happened” states due to the near-perfect symmetry of decoherence. There is effectively zero net contribution from the vastly greater parts of the quantum wavefunction, leaving only a tiny coherent contribution from something approximating the classical past.
So the same property that allows us to have a classical-like past at all, almost certainly prevents us from measuring anything about the parts of the past that are decoherent from us—not counterfactual, because in MWI interpretations these things did happen—including things like “Earth never formed” or “humans never existed on Earth” or “human history was the same up until 1930 but WW2 never happened”. To the extent that MWI is accurate, those things are still happening right here and right now. Not in some “parallel universe”, but right here in ours, just undetectable. They’re just so perfectly balanced in complex phase that—like the staggeringly immense positive and negative charges in ordinary matter—the net effect on us is neutrality.
The meaning of “we” is also highly ill-defined from a quantum point of view, but should include all states that are somewhat compatible with given subjective experiences (such as memories of evidence that WW2 happened) and not just a single point in state space.
To the extent that MWI applies then nothing is counterfactual so might as well use that as a synonym for “decoherent” to bridge the differences between ontologies.
The past is not exactly classical and to the extent it is “merely” classical-like that data extraction hope is possible.
The “WW2 never happened” portion would have its own classical-like past so calling it “right now” doesn’t seem obviously proper. Sure the crosstalk parties make more sense to be existing on the same level rather than betwen a real and not real party. But like I would not count neutrinos passing through me as part of my body I would not count that “other side of the wavefunction” to be part of my immediate experience.
I do wonder if somebody wanted to maximise the amount of crosstalk possible what would be the limiting factors.
There are no actual branches in these interpretations, that’s just a popular simplification that is taken way too seriously. Every part of the wavefunction contributes to the ongoing evolution of the wavefunction, continuously and unitarily.
But to very varying extents, so that decoherence can occur for all practical purposes.
There are no actual branches in these interpretations, that’s just a popular simplification that is taken way too seriously. Every part of the wavefunction contributes to the ongoing evolution of the wavefunction, continuously and unitarily.
One apparently nitpicky but actually very serious objection is that for large messy heavily decoherent criteria like “WW2 happened”, we can’t even hope to define in quantum terms what sort of measurement would correspond to an outcome “it did happen” or “it did not happen”. It’s on a massively separated level of abstraction from any quantum properties.
However from various macroscopic principles, we can be pretty sure that any such measurement would overwhelmingly have greater measure on “it didn’t happen”. The probability would have too many 9′s to count after the decimal point, and the exact number would depend upon the exact details of what measurement is conceived of for determining whether “WW2 happened”.
In the MWI interpretation, we[1] occupy an ill-defined microscopic blob within a microscopic dot on a microscopic dot in the continuous wavefunction. Despite that, the future of “WW2 happened” states is almost entirely determined by the present of “WW2 happened” states due to the near-perfect symmetry of decoherence. There is effectively zero net contribution from the vastly greater parts of the quantum wavefunction, leaving only a tiny coherent contribution from something approximating the classical past.
So the same property that allows us to have a classical-like past at all, almost certainly prevents us from measuring anything about the parts of the past that are decoherent from us—not counterfactual, because in MWI interpretations these things did happen—including things like “Earth never formed” or “humans never existed on Earth” or “human history was the same up until 1930 but WW2 never happened”. To the extent that MWI is accurate, those things are still happening right here and right now. Not in some “parallel universe”, but right here in ours, just undetectable. They’re just so perfectly balanced in complex phase that—like the staggeringly immense positive and negative charges in ordinary matter—the net effect on us is neutrality.
The meaning of “we” is also highly ill-defined from a quantum point of view, but should include all states that are somewhat compatible with given subjective experiences (such as memories of evidence that WW2 happened) and not just a single point in state space.
To the extent that MWI applies then nothing is counterfactual so might as well use that as a synonym for “decoherent” to bridge the differences between ontologies.
The past is not exactly classical and to the extent it is “merely” classical-like that data extraction hope is possible.
The “WW2 never happened” portion would have its own classical-like past so calling it “right now” doesn’t seem obviously proper. Sure the crosstalk parties make more sense to be existing on the same level rather than betwen a real and not real party. But like I would not count neutrinos passing through me as part of my body I would not count that “other side of the wavefunction” to be part of my immediate experience.
I do wonder if somebody wanted to maximise the amount of crosstalk possible what would be the limiting factors.
But to very varying extents, so that decoherence can occur for all practical purposes.