He mostly talked about Aumann's agreement theorem:
Aumann's agreement theorem says that two people acting rationally (in a certain precise sense) and with common knowledge of each other's beliefs cannot agree to disagree. More specifically, if two people are genuine Bayesian rationalists with common priors, and if they each have common knowledge of their individual posteriors, then their posteriors must be equal.Then Aaronson tried to apply it to his professional disagreements:
A question arises whether such an agreement can be reached in a reasonable time and, from a mathematical perspective, whether this can be done efficiently. Scott Aaronson has shown that this is indeed the case.
The disagreements with which I have maybe the most real-life experience are about quantum computing. This is a proposed technology that would use quantum mechanics to solve certain problems a lot faster than we know how to today. The laws of quantum mechanics, as we understand them now, seem very unambiguously to allow this.Here is my Concise argument against quantum computing. I fully accept quantum mechanics. When the theory says that an electron is not truly a particle and has a particular probability of being observed as a particle in a region and time, then I support that. Experiments have confirmed such experiments over and over.
There are experimental groups all over the world that are actually trying to build this. Some of them are very optimistic that they may have useful devices within a decade or two decades or something, but this field has also engendered a lot of skepticism, including, some of it by physicists, some by computer scientists. Some of them will say this is all just completely a sham, this is something that can never work.
I've been very, very interested, maybe more than most of my colleagues have been, in interacting with these people and just trying to understand, where does the skepticism come from? Because my default position would be, I hope that the skeptics are right. Because if they're right, then that means that there is something wrong with our current understanding of quantum mechanics.
If there is really a deep reason why a quantum computer can't be built -- I don't mean that just it's too hard, that the money will run out or something like that, but it's really fundamentally impossible -- then there's really something that we don't understand about quantum physics itself.
But when you say that there is a mysterious quantum nonlocality that allows an unobserved electron to be in multiple places at once in order to facilitate super-Turing computation, then that is way beyond anything that has been demonstrated, and I don't believe it.
Aaronson concedes that quantum computing is an open question. Maybe it is possible, and maybe it is not. To me, that alone justifies a skeptical view. Carl Sagan said that extraordinary claims require extraordinary evidence. Some mathematicians are skeptical about the Riemann Hypothesis. Positivists only accept what is known to be true.
But Aaronson is like the theologian who admits that he cannot prove God exists, but insists that no one should identify as an atheist unless he can prove that there is no God.
Aaronson says that if quantum computing is impossible, then there must be something we don't understand about quantum mechanics. The latter is conventional wisdom among physicists and others. Whenever people talk about interpretations of quantum mechanics, such as many-worlds (MWI), or giving a Nobel Prize for Bell test experiments, or quantum gravity, or black hole firewalls, or cosmic holograms, they are always making an argument that there is something we don't understand about quantum mechanics. So most of his colleagues are already convinced!
His book lists 11 objections to quantum computing. I do not think he does them justice, but he is now writing a new book on the subject.