MWI very specifically commits to the existence of a universal wavefunction. Everett’s original paper is literally titled “The Theory of the Universal Wavefunction.” If you instead only take relative states seriously, that position is much closer to relational quantum mechanics. In fact, Carlo Rovelli explicitly describes RQM as adopting Everett’s relative-state idea while rejecting the notion of a universal quantum state.
MWI claims there exists a universal quantum state, but quantum theory works perfectly well without this assumption if quantum states are taken to be fundamentally relative. Every quantum state is defined in relation to something else, which is made clear by the Wigner’s friend scenario where different observers legitimately assign different states to the same system. If states are fundamentally relative, then a “universal” quantum state makes about as much sense as a “universal velocity” in Galilean relativity.
You could arbitrarily choose a reference frame in Galilean relativity and declare it universal, but this requires an extra postulate, is unnecessary for the theory, and is completely arbitrary. Likewise, you could pick some observer’s perspective and call that the universal wavefunction, but there is no non-arbitrary reason to privilege it. That wavefunction would still be relative to that observer, just with special status assigned by fiat.
Worse, such a perspective could never truly be universal because it could not include itself. To do that you would need another external perspective, leading to infinite regress. You never obtain a quantum state that includes the entire universe. Any state you define is always relative to something within the universe, unless you define it relative to something outside of the universe, but at that point you are talking about God and not science.
The analogy to Galilean relativity actually is too kind. Galilean relativity relies on Euclidean space as a background, allowing an external viewpoint fixed to empty coordinates. Hilbert space is not a background space at all; it is always defined in terms of physical systems. You can transform perspectives in spacetime, but there is no transformation to a background perspective in Hilbert space because no such background exists. The closet that exists is a statistical transformation to different perspectives within Liouville space, but this only works for objects within the space; you cannot transform to the perspective of the background itself as it is not a background space.
gbzm@piefed.social 3 hours ago
Ah so I think I sort of conflated RQM and MWI because I thought it was all about Everett’s other paper “relative state formulation of qm”.
I thought on top of an ad hoc rehabilitation of physical realism, the universal state also did something for the consistency. Something like all the density operators may be expressed as partial traces of the operator describing the their systems’ union, in order for everything to be consistent, and the ‘largest’ operator describes the state of the universe or something. I’ll check out your sources next insomnia
bunchberry@lemmy.world 2 hours ago
Depends upon what you mean by realism. If you just mean belief in a physical reality independent of a conscious observer, I am not really of the opinion you need MWI to have a philosophically realist perspective.
For some reason, everyone intuitively accepts the relativity of time and space in special relativity as an ontological feature of the world, but when it comes to the relativity of the quantum state, people’s brains explode and they start treating it like it has to do with “consciousness” or “subjectivity” or something and that if you accept it then you’re somehow denying the existence of objective reality. I have seen this kind of mentality throughout the literature and it has never made sense to me.
Even Eugene Wigner did this, when he proposed the “Wigner’s friend” thought experiment, he points out how two different observers can come to describe the same system differently, and then concludes that proves quantum mechanics is deeply connected to “consciousness.” But we have known that two observers can describe the same system differently since Galileo first introduced the concept of relativity back in 1632. There is no reason to take it as having anything to do with consciousness or subjectivity or anything like that.
(You can also treat the wavefunction nomologically as well, and then the nomological behavior you’d expect from particles would be relative, but the ontological-nomological distinction is maybe getting too much into the weeds of philosophy here.)
I am partial to the way the physicist Francois-Igor Pris puts it. Reality exists as independently of the conscious observer, but not independently from context. You have to specify the context in which you are making an ontological claim for it to have physical meaning. This context can be that of the perspective of a conscious observer, but nothing about the observer is intrinsic here, what is intrinsic is the context, and that is just one of many possible contexts an ontological claim can be made. Two observers can describe the same train to be traveling at different velocities, not because they are conscious observers, but because they are describing the same train from different contexts.
The philosopher Jocelyn Benoist and the physicist Francois-Igor Pris have argued that the natural world does have a kind of an inherent observer-observed divide but that these terms are misleading being “subject” tends to imply a human subject and “observer” tends to imply a conscious observer, and that a lot of the confusion is cleared up once you figure out how to describe this divide in a more neutral, non-anthropomorphic way, which they settle on talking about the “reality” and the “context.” The reality of the velocity of the train will be different in different contexts. You don’t have to invoke “observer-dependence” to describe relativity. Hence, you can indeed describe quantum theory as a theory of physical reality independent of the observer.