I have always appreciated that in comparison to other top physicists, Leonard Susskind was among those who deserved to be called heuristic thinkers, creative mavericks, talkers rather than calculators, and to a large extent, I have found this spirit inspiring. Susskind was always a role model for me. The number and diversity of ideas he helped to emerge from the darkness was amazing.
Of course, as the well-known lore says, physics depends on the delicate balance between the hot, bold, philosophical speculations on one side; and the cold, hard, boring facts on the other. When this balance is broken, physics degenerates either to philosophy or to botany (or stamp collecting). This lore is not meant to be an insult when it says that philosophy and botany are inferior because they self-evidently are inferior.
Well, the first outcome seems to be a good description of Susskind's new paper
He says that not only general relativity and quantum mechanics don't have any tension. (They do. In the spacetime, they may be considered separately and theories that respect both class of principles are really hugely constrained – to the extent that they must be string theory.) They are not only compatible, Susskind's gospel continues. They are inseparable, they are the same thing. Some stories about wormholes are meant to support this statement but they don't.
Susskind also says lots of his usual wrong statements resulting from a deep misunderstanding of quantum mechanics – e.g. that "quantum mechanics is the same as a classical simulation of it". A classical system, a simulation or otherwise, can never be equivalent to a quantum mechanical theory. The former really doesn't obey the uncertainty principle, allows objective facts; the latter requires an observer and is a framework to calculate probabilities of statements that are only meaningful relatively to a chosen observer's observations.
At the popular level, the suggestion that "geometry and quantum mechanics" may turn out to be the same thing in the future has been said for decades. For example, in Brian Greene's The Elegant Universe which was published last century, Edward Witten and Cumrun Vafa made similar comments during interviews mentioned in the end notes.
But in the recent decade, physicists have made progress beyond these vague statements. In particular, the ER-EPR correspondence is important real science because it not only emits these hippie qubitzers' statements of the type "all of us are brothers, all concepts in science are equivalent". It makes more precise statements that also include some "negative" implications.
For example, in the basic ER-EPR correspondence, it's important that the wormhole is non-traversable. An analogous equivalence statement that would talk about traversable wormholes wouldn't be true. Generalizations are being constructed where a traversable wormhole is used on the GR side. But something analogous must be modified on the QM side, too.
Almost everyone knows that many insights in the revolution involving entanglement in quantum gravity has led us to think about basic quantum mechanics of simple Hilbert spaces; and physical systems involving dynamical curved space to be a duality of a sort, a pair of two "opposite" perspectives to look at the same thing.
But the devil is in the details and the devil matters. And the details, the devil, seems to be missing in Susskind's letter.
So this letter that really pays no attention to any of these details has reinforced my idea about the actual birth of the ER-EPR correspondence. Juan Maldacena discovered it, went to Lenny, told him "don't you want to join? You've been saying similar things" and Lenny agreed because he found the paper to be "in the right direction". But all the actual work needed to verify that the structures on both sides match and why they wouldn't match if something were different seem to be Juan's. Lenny seems to be confused – or question – way too many things that are essential for these principles to work.
It seems clear to me that if only Lenny were writing the ER-EPR paper, it would end up look like this hippie qubitizers' letter. Even some four years later, he is still at this unrefined level. I think that the cutting-edge research is well ahead of these proclamations that resemble the dreams by the laymen who are high right now. And make no mistake about it. Physicists in California have often been high and this state of the mind has been important for certain important papers by important physicists to be written.
After all, if you study the community how it looked a few decades ago, you will find amazing interactions between Leonard Susskind and e.g. Jack Sarfatti, a hardcore crackpot who believes to have been abducted by extraterrestrial aliens, among other things. This is Northern California. Everyone is high over there. But that's not quite enough for progress in physics. Thankfully, the world has also had Argentina, the Garbage State around Princeton, and other places.