I’ve just been to the amazing Quantum Redemption conference in Sweden, organized by my friend Armin Tavakoli. I had a great time, attended plenty of interesting talks, and had plenty of productive discussions outside the talks as well. I’m not going to write about any of that, though. Having a relentlessly negative personality, I’m going to write about the talk that I didn’t like. Or rather, about its background. The talk was presenting some developing ideas and preliminary results, it was explicitly not ready for publication, so I’m not going to publish it here1. But the talk didn’t make sense because its background doesn’t make sense, and that is well-published, so it’s fair game.
I’m talking about the paper Quantum mechanics and the covariance of physical laws in quantum reference frames by my friends Flaminia, Esteban, and Časlav. The basic idea is that if you can describe a particle in a superposition from the laboratory’s reference frame, you can just as well jump to the particle’s reference frame, from which the particle is well-localized and the laboratory is in a superposition. The motivations for doing this are impeccable: the universality of quantum mechanics, and the idea that reference frames must be embodied in physical systems. The problem is that you can’t really attribute a single point of view to a superposition.
By linearity, the members of a superposition will evolve independently, so why would they have a joint identity? In general you affect some members of a superposition without affecting the others, there is no mechanism transmitting information across the superposition so that a common point of view could be achieved. The only sort of “interaction” possible is interference, and that necessitates erasing all information that differentiates the members of the superposition, so it’s rather unsatisfactory.
In any case, any reference frame worth of the name will be a complex quantum system, composed of a huge amount of atoms. It will decohere very very quickly, so any talk of interfering a superposition of reference frames is science fiction. Such gedankenexperimente can nevertheless be rather illuminating, so I’d be curious about how they describe a Wigner’s friend scenario, as there the friend is commonly described as splitting in two, and I don’t see a sensible of attributing a single point of view to the two versions. Alas, as far as I understand their quantum reference frames formalism was not meant to describe such scenarios, and as far as I can tell they have never done so.
This is all about interpretations, of course. Flaminia, Esteban, and Časlav are all devout single-worlders, and pursue with religious zeal the idea of folding back the superpositions into a single narrative. I, on the other hand, pray at the Church of the Larger Hilbert Space, so I find it heresy to see these highly-decohered independently-evolving members of a superposition as anything other than many worlds.
People often complain that all this interpretations talk has no consequences whatsoever. Well, here is a case where it unquestionably does: the choice of interpretation was crucial to their approach to quantum reference frames, which is crucial to their ultimate goal of tackling quantum gravity. Good ideas tend to be fruitful, and bad ideas sterile, so whether this research direction ultimately succeeds is an indirect test of the underlying interpretation.
You might complain that this is still on the metatheoretical level, and is anyway just a weak test. It is a weak test indeed: the Big Bang theory was famously created by a Catholic priest, presumably looking for a fiat lux moment. Notwithstanding its success, I’m still an atheist. Nevertheless, weak evidence is still evidence, and hey, if you don’t like metaphysics interpretations are really not for you. If you do like metaphysics, however, you might also be interest in metatheory ;)