Thank you for the quote! (I tried to read the article, but after a few pages it seemed to me the author makes too many digressions, and I didn't want to know his opinions on everything, only on the technical problems with scanning brains.)
Do I understand it correctly that the question is, essentially, whether there exists a more efficient way of modelling the brain than modelling all particles of the brain?
Because if there is no such efficient way, we can probably forget about running the uploaded brains in real time.
Then, even assuming we could successfully scan the brains, we could get some kind of immortality, but we could not get greater speed, or make life cheaper... which is necessary for the predicted economical consequences of "ems".
Some smaller economical impacts could still be possible, for example if a person would be so miraculously productive, that even running them at 100× slower speed and 1000× higher costs could be meaningful. (Not easy to imagine, but technically not impossible.) Or perhaps if the quality of life increases globally, the costs of real humans could grow faster than costs of emulated humans, so at some moment emulation could be economically meaningful.
Still, my guess is that there probably is a way to emulate brain more efficiently, because it is a biological mechanism made by evolution, so it has a lot of backwards compatibility and chemistry (all those neurons have metabolism).
No his thesis is that it is possible that even a maximal upload wouldn't be human in the same way. His main argument goes like this:
a) There is no way to find out the universe's initial state, thanks to no-cloning, the requirement of low entropy, and there being only one copy.
b) So we have to talk about uncertainty about wavefunctions - something he calls Knightian uncertainty (roughly, a probability distribution over probability distributions).
c) It is conceivable that particles in which the Knightian uncertainties linger (ie they have never spoken to any...
Scott Aaronson has a new 85 page essay up, titled "The Ghost in the Quantum Turing Machine". (Abstract here.) In Section 2.11 (Singulatarianism) he explicitly mentions Eliezer as an influence. But that's just a starting point, and he then moves in a direction that's very far from any kind of LW consensus. Among other things, he suggests that a crucial qualitative difference between a person and a digital upload is that the laws of physics prohibit making perfect copies of a person. Personally, I find the arguments completely unconvincing, but Aaronson is always thought-provoking and fun to read, and this is a good excuse to read about things like (I quote the abstract) "the No-Cloning Theorem, the measurement problem, decoherence, chaos, the arrow of time, the holographic principle, Newcomb's paradox, Boltzmann brains, algorithmic information theory, and the Common Prior Assumption". This is not just a shopping list of buzzwords, these are all important components of the author's main argument. It unfortunately still seems weak to me, but the time spent reading it is not wasted at all.