Last impressive bit of progress in the QM fundamentals I recall was Zurek’s einselection, where he claimed to show that only the copies of eigenstates survive decoherence. The approach was unusual enough to be taken seriously, even if the experimental confirmation leaves much to be desired. Carroll’s “rational observers must believe in Born rule” looks rather dubious to me, mostly because it still takes a classical observer as a fundamental entity.
Observers are treated as explanatorily fundamental, sure, just as they are in any anthropic-type explanation. But I don’t see why that’s a problem. The issue is when observers are treated as ontologically fundamental, as they are in some objective collapse interpretations, because that conflicts with the apparent fact that observers are entirely made up out of quantum-mechanical parts. Carroll’s paper faces no such conflict.
Basically the approach of Sebens and Carroll is to show that if observers are present, then they will see outcomes following the Born rule.
In that sense it seems that observers here are no more problematic than the observers of special relativity, where there are claims like if you use clocks to measure time in a moving frame, then you will see time slowing down relative to mine.
The piece that is new is showing that only equal amplitude observers can be thought of as “equivalent” for their definition of equivalent. Previously, one “obvious” thing to try to do was to weight different “worlds” equally, and this answers that objection.
But also important, its actually pretty clear. A lot of quantum foundations papers are rather poor, its nice to see a contribution thats well reasoned.
Edit to add: Just because I think its interesting and clear doesn’t mean its correct. In particular, by neglecting the small off diagonal terms in the density matrix they are doing exactly what Everett did in his original derivation, which is only valid in the limit of infinite interactions.
Last impressive bit of progress in the QM fundamentals I recall was Zurek’s einselection, where he claimed to show that only the copies of eigenstates survive decoherence. The approach was unusual enough to be taken seriously, even if the experimental confirmation leaves much to be desired. Carroll’s “rational observers must believe in Born rule” looks rather dubious to me, mostly because it still takes a classical observer as a fundamental entity.
Observers are treated as explanatorily fundamental, sure, just as they are in any anthropic-type explanation. But I don’t see why that’s a problem. The issue is when observers are treated as ontologically fundamental, as they are in some objective collapse interpretations, because that conflicts with the apparent fact that observers are entirely made up out of quantum-mechanical parts. Carroll’s paper faces no such conflict.
What pragmatist said.
Basically the approach of Sebens and Carroll is to show that if observers are present, then they will see outcomes following the Born rule.
In that sense it seems that observers here are no more problematic than the observers of special relativity, where there are claims like if you use clocks to measure time in a moving frame, then you will see time slowing down relative to mine.
The piece that is new is showing that only equal amplitude observers can be thought of as “equivalent” for their definition of equivalent. Previously, one “obvious” thing to try to do was to weight different “worlds” equally, and this answers that objection.
But also important, its actually pretty clear. A lot of quantum foundations papers are rather poor, its nice to see a contribution thats well reasoned.
Edit to add: Just because I think its interesting and clear doesn’t mean its correct. In particular, by neglecting the small off diagonal terms in the density matrix they are doing exactly what Everett did in his original derivation, which is only valid in the limit of infinite interactions.