Scott Aaronson, "Remarks on the Physical Church-Turing Thesis" FQXi conference 2014 in Vieques
http://research.cs.queensu.ca/home/akl/cisc879/papers/PAPERS_FROM_MINDS_AND_MACHINES/VOLUME_13_NO_1/V23L84X656370574.pdf This gets quite philosophical of course
See page 561-579 in GEB