This new, standardized, elegant enough name of the Maldacena-Susskind proposal that I used in the title already exceeds the price of this blog entry that you had to pay. ;-)
Yesterday, there was a one-page critical paper by Hrvoje Nikolić of Zagreb, Croatia, EU trying to criticize the ER-EPR correspondence. When I am looking at similar articles, I am often ashamed to be a European: Nikolić attacking MS reminds me of an angry and hungry dog attacking Wolfgang Amadeus Mozart and Ludwig van Beethoven except that in those good old times, the composers were European and the dogs were American. This shame largely evaporates as soon as I see similarly dumb, low-quality articles inkspilled inside other continents.
Like a five-year-old spoiled boy, Nikolić screams that he has to be given some quantitative comparisons of ER and EPR correlators to trust such a thing. But it's simply not true that this is the only possible or allowed kind of evidence that the correspondence is valid. Moreover, it's obvious that once we allow the spaces with the ER bridges at all, the observables on the opposite sides of the throat will be entangled just like the rules of entanglement require; after all, despite the apparent huge distance through the normal space, they're close to each other due to the throat. In the absence of very high energy quanta, field operators at nearby points are almost equal to each other.
Also, none of the two sides of the ER-EPR correspondence is a particular dynamical theory – at least until we add some more data – so it's nonsensical to talk about particular correlators. The correspondence is a conceptual one. We may turn the ER side into something specific but just like in the AdS/CFT correspondence, the description based on the other side, the EPR entanglement built upon a space of ordinary topology, will be the only way to truly accurately describe the quantum physics of the ER bridges. With this definition of the quantum physics on an ER bridge, the ER-EPR correspondence is tautologically true.
Similarly, he screams that the ER bridges can't be equivalent to EPR because EPR produces all the shocking correlations that have stunned Einstein. This opinion held by Nikolić reflects the fact that he is spending his career by writing misguided papers on Bohmian mechanics. If he insists on a classical description of Nature, no wonder that all quantum phenomena have to look shocking to him forever.
However Maldacena and Susskind, like any contemporary physicist who doesn't completely suck, respect the postulates of quantum mechanics at every step so none of the generic correlations predicted by quantum mechanics are shocking for them. They're just giving a new (spacetime) geometric interpretation to some particular entangled states and/or projection operators upon them; they're linking an entangled state (a superposition of non-entangled states) to a geometric configuration that used to be associated with a linearly independent state (orthogonal to the visualizations of the non-entangled states) because of its different topology but it is actually not independent.
But even with this wormhole-like spacetime, one has to do quantum mechanics on top of it and it predicts the same entanglement-like correlations like any other example of a quantum mechanical theory! Nikolić's opposition is a great example of the wisdom that whatever (correct) progress would materialize in the foundations of quantum mechanics would make the picture of the world even more quantum, not less. That's just happening because we're abandoning some incorrect assumptions about the spacetime topology that were rooted in our experience with classical physics (namely the wrong assumption that the spacetime topology is a well-defined observable on the Hilbert space). No wonder that people like Nikolić who are obsessed with the dream to undo the quantum revolution feel threatened by another step in the opposite direction.
To dramatically improve the average quality (and length) of papers that refer to the ER-EPR correspondence, Mark Van Raamsdonk – the forefather of the "entanglement as a glue" paradigm – released a new, 18-page-long note:
This process therefore looks like a gradual disappearance (evaporation) of the firewall. Such a result is ironic because AMPS employed entanglement as a tool to erect the firewall and to make it impenetrable. On the contrary, Van Raamsdonk shows that the entanglement has the effect of tearing down this firewall. Mark wouldn't dare to choose this Reaganite terminology in the title of his paper because the left-wing apparatchiks filling most of the Western universities would send him to Gulag for his anti-socialist jokes.
Mark also dedicates several pages to his comments on the nice Papadodimas-Raju paper that appeared in November 2012. I think that what Mark is saying is particularly compatible with the Papadodimas-Raju paper. For example, the precise measurement of the internal microstate performed from the outside is what Papadodimas and Raju would classify as "too many measurements" and that's what terminates the validity of a local effective QFT on the opposite side (inside the black hole, in this case) in their framework.
The figures in Mark's paper (e.g. Figure 8) largely reflect a lore or an understanding of "tracing over other degrees of freedom" and "coarse/fine grained observables" that has already been adopted by several others. As the Wheeler-DeWitt equation suggests, a whole causal diamond worth of operators should be assigned to a slice and we may trace over other degrees of freedom geometrically associated with horizontally separated diamonds.
I won't offer my new results on these issues because I would be giving them away at a price that is far below the fair one and I have some very bad experience with similar (or any) violations of the rules of the free markets.