The Edge annual question for this year, asked of over 100 scientists, journalists, and assorted intellectual types was “What have you changed your mind about and why?” I checked out the responses of some of the physicists who participated.
Lee Smolin’s entry discusses the impact that his evolving views about time have had on his quantum gravity work. His earlier research was on loop quantum gravity (LQG), which as it seeks to quantize the geometry described by general relativity results in a basically “timeless” theory (unlike quantum mechanics itself, where a background time is part of the picture). Now, however, Smolin has come to believe that time, in the guise of causality, needs to be a fundamental element of a theory. This leads him to be interested in theories where causality is built in at the ground level and where the more familiar “laws of physics” (general relativity and quantum field theory) are emergent features which may themselves evolve in time (for more see my previous post on Smolin).
It was interesting to me to note a contrast with fellow loop quantum gravity pioneer Carlo Rovelli. Rovelli’s entry was about his realization that the standard interpretation of quantum mechanics made sense, but not if you tried to apply it to the whole universe. The insight led to his formulation of relational quantum mechanics a bit over 10 years ago. Now, from my outsider’s perspective, it seems that Rovelli’s relational qm is philosophically in harmony with Smolin’s interest in “emergent” quantum gravity approaches which start with a causal network of quantum systems at the fundamental level. However, while Rovelli says that relational qm has “affected substantially” his quantum gravity work, in his case, it appears this involves inspiring the ongoing extensions to the loop program rather than working on approaches which have a different fundamental starting point.
Finally, relevant to this topic is John Baez on why he decided to stop working on quantum gravity.