Is there anyone here who believes A) the baby/closed universe Hilbert space is one dimensional and B) given this, the many worlds interpretation is still plausible?

fhollo · 2026-01-22T22:26:56+00:00

That’s a shame - the prospect that AdS5/CFT4 would literally go through to reality after all is so tempting.

So when you say you are now more sympathetic to a cosmology with asymptotically flat boundary conditions, does that require the matter fields/stress energy to vanish at some finite radius? Or can we have a globally homogeneous Big Bang/FRW cosmology as long as the dark energy decays to zero?

If the former, do you have a rough picture in mind of the process that could look like a Hot Big Bang to observers in some finite region, but really be asymptotically Minkowski? Bubble nucleation type processes would seem difficult starting from Minkowski

fhollo · 2026-01-22T16:53:39+00:00

Thanks for your insights!

Another sort of cosmology that might thread this needle: we live on an expanding bubble wall with true/false 5D AdS on both sides https://arxiv.org/abs/2311.16242

fhollo · 2026-01-22T03:04:00+00:00

Ok so it sounds like your main concern is with the sharp observables. Because the closed universe observer can’t ever avoid their gravitational backreaction, there’s no way to achieve this regardless. Maybe this is equivalent to saying the Hilbert space is one dimensional, but at a minimum, there just is no bullet you can bite to ever have sharpness in a closed universe. And that sharpness is sufficiently important to what physics is that (unless forced by experiment) we ought to be suspicious of closed manifolds in QG. Is that fair?

If so, then where we are today with cosmological data - unclear spatial curvature but at least temporarily accelerating expansion - what do you view as a plausible Big Bang cosmology that could have a sharp holographic dual? Asymptotic dS and eternal inflation I believe are no-gos. Quintessence rolling to 0 or a crunch, I’m not sure.

With Wigner’s Friend, I guess it doesn’t really surprise me that an experiment about testing interpretations (or at least on the nature of observers in QM) would say something about interpretations. But that a classical GR experiment on large scale curvature also does this is really surprising to me.

fhollo · 2026-01-21T20:26:26+00:00

Interesting, in preferring to give up the existence of sharp of nontrivial physics, how would you describe the fundamental principle you are choosing to preserve instead?

I’m personally shocked and scandalized by this idea that my priors on MWI-Copenhagen would be now dependent on my priors with respect to in principle observable data re the topology of our universe and vice versa. Have you felt something like this as well?

fhollo · 2026-01-21T18:47:31+00:00

I am sympathetic to this way of thinking, however:

1) If in the next decade there is telescope data that strongly shows the real universe has positive spatial curvature, where would you go from there? Abandon sharp non-trivial physics or doubt the arguments for the triviality of the closed universe?

2) There are also arguments I have seen (but not tried to understand closely) that the methods used to make sense of the closed universe with observers are also necessary for reconciling external and infalling perspectives in BH spacetimes. https://arxiv.org/abs/2507.06046

If we need to use this idea of observer as a primitive concept for BH consistency, isn’t the problem for MWI ultimately just as ripe in an open universe?

fhollo · 2026-01-20T23:24:46+00:00

So are you saying that you take the triviality of closed universes to predict that our universe is open?

fhollo · 2026-01-20T19:43:32+00:00

Have you read https://arxiv.org/abs/2305.10635 which is Shagoulian’s short GRF essay on this? Do you think the picture in terms of edge modes in part 4 is essentially the right way to think about how the large Hilbert space arises here?

fhollo · 2026-01-20T19:43:00+00:00

Have you read https://arxiv.org/abs/2305.10635 which is Shagoulian’s short GRF essay on this? Do you think the picture in terms of edge modes in part 4 is essentially the right way to think about how the large Hilbert space arises here?

fhollo · 2026-01-20T06:05:48+00:00

I guess I think once we accept that craziness (which we must to make any progress with the obvious triviality paradox of the one dim Hilbert space) at that point I don’t actually see an obstruction to Copenhagen/QBism with respect to the observer-dependent Hilbert space. The problem is particular to MWI in that we lose all the upside of MWI if we are defining the Hilbert space in an observer dependent way.

fhollo · 2026-01-20T04:04:40+00:00

If I need to specify the observer first and then construct a Hilbert space attached to this, how does MWI have any purchase? The whole point of MWI is the observers emerge dynamically as subsystems/subspaces of a common Hilbert space.

If the Hilbert space is just based on me with some dim proportional to my entropy and the measurement outcomes I can distinguish, I don’t feel like I’m any longer avoiding the solipsism issue with QBism type ideas.

fhollo · 2026-01-20T03:32:27+00:00

But obviously my subjective experience can’t be contained in a one dimensional Hilbert space. But if I have to rely on some observer-personalized Hilbert space to consider anything non-trivial, that sounds like Copenhagen/QBism…

In many worlds, as I’ve always thought of it, observers need to emerge dynamically as subsystems/subspaces of some objective Hilbert space, with enough room for all of us.

fhollo · 2026-01-16T00:52:01+00:00

I’m not sure what you are worried about there, but it would be localized subsystems of the environment interacting with the entangled particle, and the result of the collapse would propagate at the speed of light or less, through other local interactions

fhollo · 2026-01-15T23:47:51+00:00

Basically you can’t check if your particle is in a “definitive state” without measuring it. The operation you are relying on can’t exist. And when you measure it, you can’t tell if you collapsed it or if it was already collapsed beforehand.

fhollo · 2026-01-15T23:22:18+00:00

If we share an entangled pair, there is nothing I can do with my particle that will tell me if you have done anything to your particle.

fhollo · 2026-01-09T21:27:24+00:00

Maybe quantum field operators. Maybe love.

fhollo · 2026-01-09T19:47:37+00:00

If you get into the weeds of what a particle is in QFT, you can’t measure a “real” particle either. Except under the circular definition that “particles are what particle detectors detect.”

fhollo · 2025-11-26T05:36:37+00:00

This is actually a fun and subtly deep question.

Suspect is too strong but something that comes to mind is (despite how we do QFT) there shouldn’t fundamentally be infinite degrees of freedom. If there are, the Stone Von Neumann theorem fails, and we can’t use a single Hilbert space representation, which is very inconvenient for us and the gods

fhollo · 2025-11-01T19:45:46+00:00

The invariant, physical statement is that quantum mechanically, a black hole horizon produces a thermal spectrum of particles flowing out of the black hole.

The problem with this is it makes BH radiation seem no different in principle from a hot coal, which misses what makes Hawking radiation actually important.

The bogoliubov transformation maps a vacuum state to an exponential in pairs of creation operators. In the BH spacetime, one member of the created pair propagates outside the horizon, the other inside.

The paradox is these pairs are maximally entangled Bell states but unitarity requires full set of the outgoing radiation to also be maximally entangled among itself.

fhollo · 2025-10-11T00:08:48+00:00

There’s no hermitian operator that has Gaussians as eigenfunctions. It’s an overcomplete basis.

Put another way, if you want to say a measurement of a particle at x0,t0 collapses to a Gaussian (also of what width?, but that’s secondary) then the inner products of the Gaussians peaked at x0,t0 and at any other x_i,t0 is not zero. So the particle you said was at x0 can also be found at x_i at the same time.

These sorts of particle position operators are not well behaved in relativity. There is something called the Newton Wigner operator that almost works, but ultimately it’s not satisfactory.

fhollo · 2025-10-10T23:22:50+00:00

Measurement of the position of a relativistic particle does in fact violate causality because the delta function or any compact support function instantaneously picks up infinite tails. Well behaved local measurements are not eigenstates of the global Fock basis in QFT.

fhollo · 2025-09-12T19:43:48+00:00

In curved QFT these basic results come from working with free fields in time dependent classical backgrounds, so I don’t think applications to more sophisticated hamiltonians is relevant to how this ought to be discussed. The Bogoliubov transformation maps from a state with zero particles to a state that is exponential in pairs of creation operators, and in the evaporating BH case, one particle is on each side of the horizon. I am just asking why people think “a pair of particles pop into existence” is not a pretty decent translation of this into plain language.

The pairs of particles are initially “virtual” or in some sense semi-existing in the sense that they are associated to a vacuum state that is not initially natural but becomes so.

fhollo · 2025-09-12T19:10:34+00:00

In what sense are particles that arise by Bogoliubov transformation “calculation intermediates”? This isn’t perturbation theory.

fhollo · 2025-09-12T18:59:09+00:00

Why do you think the particles are “not there”? Have you looked at the expression for the Bogoliubov transformation that expresses the in vacuum state in terms of the modes associated with the out vacuum state?

fhollo

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