Biography : Scott Aaronson is an Associate Professor of Electrical Engineering and Computer Science at MIT. His research interests center around the capabilities and limits of quantum computers, and computational complexity theory more generally. He also has written about consciousness and personal identity and the relevance of quantum mechanics to these issues.

Michael Cerullo: Thanks for taking the time to talk with me. Given the recent advances in brain preservation, questions of personal identity are moving from merely academic to extremely practical questions. I want to focus on your ideas related to the relevance of quantum mechanics to consciousness and personal identity which are found in your paper “Ghost in the Quantum Turing Machine” ( http://arxiv.org/abs/1306.0159 ), your blog “Could a Quantum Computer Have Subjective Experience?” ( http://www.scottaaronson.com/blog/?p=1951 ), and your book “Quantum Computing since Democritus” ( http://www.scottaaronson.com/democritus/) .

Before we get to your own speculations in this field I want to review some of the prior work of Roger Penrose and Stuart Hameroff ( http://www.quantumconsciousness.org/content/hameroff-penrose-review-orch-or-theory ). Let me try to summarize some of the criticism of their work (including some of your own critiques of their theory). Penrose and Hameroff abandon conventional wisdom in neuroscience (i.e. that neurons are the essential computational element in the brain) and instead posit that the microtubules (which conventional neuroscience tell us are involved in nucleic and cell division, organization of intracellular structure, and intracellular transport, as well as ciliary and flagellar motility) are an essential part of the computational structure of the brain. Specifically, they claim the microtubules are quantum computers that grant a person the ability to perform non-computable computations (and Penrose claims these kinds of computations are necessary for things like mathematical understanding). The main critiques of their theory are: it relies on future results in quantum gravity that don’t exist; there is no empirical evidence that microtubules are relevant to the function of the brain; work in quantum decoherence also makes it extremely unlikely that the brain is a quatum computer; even if a brain could somehow compute non-computable functions it isn’t clear what this has to do with consciousness. Would you say these are fair criticisms of their theory and are there any other criticisms you see as relevant?

Scott Aaronson: Yes, I think all four of those are fair criticisms! I could add a fifth criticism: Penrose’s case for the brain having non-computational abilities relies on an appeal to Gödel’s Incompleteness Theorem, to the idea that no machine working within a fixed formal system can prove the system’s consistency, whereas a human can “just see” that it’s consistent. But like most mathematicians and computer scientists, I don’t agree with that argument, because I think a machine could show all the same external behavior as a human who “sees” a formal system’s consistency. So then, the argument devolves into one about indescribable inner experiences, of “just seeing” (for example) that set theory is consistent. But if we wanted to rest the case on indescribable inner experiences, then why not forget about Gödel’s Theorem, and just talk about less abstruse things like the experience of falling in love or tasting strawberries or whatever?