Question: Are Tegmark's ideas more entertaining than Cornwell 1's physics pornography? Or, are they more of the same? :chin:

Multiverses..,total speculation. When there is even one piece of observational evidence,
I will take it seriously. Right now it seems to be an atheist's delusion, nothing more.

As far as I can see it, the pornography of Tegmark is a perversion. There cannot be copies of you and me because the universe is finite. The closed propagators before inflation contained elementary particles that were in a superposition of all energies and 3-momenta up to the Planck energy, not on mass shell (that's what virtual means). Just like closed photon or electron propagators can be turned into real particles by real electron-positron pairs (pair annihilation), resp. by photon pairs (positron-electron creation), the virtual gauge bubbles and fermion bubbles can get realized by inflation, somewhat reminiscent to the realization of particles in the production of Hawking radiation (though there are negative energies excited to reduce the mass inside of the hole, and these negative energies are not involved here). Cornwell1 seems a fair pornography director.

I've not read the whole thread, but responders are not always in accordance with physics. Mostly I see Tegmark's works countered with argument from incredulity, especially in the other thread that actually discusses MWI and level-4 universes. My interpretation of choice is probably RQM, not MWI.

MWI is unrelated to a level-1 multiverse. The argument that there are infinite copies of yourself seems to presume counterfactuals, which MWI does not, so right there Tegmark's stated preferences precludes the existence of an identical copy of yourself out there. So I will proceed under the premise of 1) a universe of infinite spatial extent, and 2) a quantum interpretation that supports counterfactuals, such as Bohmian mechanics.

I want to discuss his first type (I won't give my opinion about the other three as this probably gets me in trouble...). In an infinite universe, so Tegmark conjectures, there are infinite Hubble volumes and in this infinity an infinite of exact copies of you and me exist. — Cornwell1

Funny that it's worded that way. Hubble volumes have nothing to do with it. We can see objects that are currently outside our Hubble volume, so clearly those objects have a causal influence on us.

This can't be true because all Hubble volumes interact with their surroundings.

Fallicious reasoning. A given state (say current state of Earth) has a finite set of events in its past light cone, which was (as measured in proper distance) was merely the size of a grapefruit at the end of the inflation epoch, grew to a maximum proper size of nearly 6 billion light years around 7 billion years ago, and is today the size of Earth. By definition (and assuming cause and effect cannot happen faster than light), no event inside that past light cone can interact with an event inside that light cone.

So all we need for an identical Earth is for some finite volume of space (a similar light cone) out there with identical state. There are only a finite number of possible states for this finite volume, which Tegmark computes, hence there must be infinite copies given an infinite number of rolls of this finite sided die.

Suppose there is such a copy in a Hubble volume identical to the one we live in. How can this be? Near the border of our volume, there is interaction with stuff outside of the volume

Yes, which is why Hubble volume is useless. It affect the thing at the edge of the Hubble volume, but it doesn't effect you, so it matters not. Use past light cone instead of Hubble volume, and the logic works.

Tegmark's argument does not proceed along these my lines at all, and nor the one you describe. He does use a Hubble volume (like that matters. Only Earth matters). He computes that any finite volume can, at any given time, be only in some finite number of states. It's causal past is not considered. Since it is a current state, influence from the outside of the arbitrary volume is similarly moot. Infinite volumes divided by finite states makes for a vanishingly small probability of each state not appearing elsewhere.

which creates a difference between our volume and the identical volume somewhere else.

No it doesn't. It might mean that the identical (arbitrary) volume evolves subsequently differently, but that has no effect on the existence of the copy of you already at the center.

Am I right or am I left?
Worth a poll?

Polls get you opinions. The above is logic, not opinion.

Extra credit: Tegmark's argument (finite state possibilities of a given volume) is stronger than mine (worded from a causality standpoint). Can you point out why?

Personally (opinion here) I don't think there's an exact copy of me out there, but only because I don't accept all the premises I list above, not because the logic is faulty. I do accept the infinite spatial extent premise, without which the argument is meaningless. If it's finite at all, it's probably smaller than the insane large number that Tegmark computed for the nearest copy.