Thanks to references on mathematician Peter Woit's blog, I learned there was a powerhouse physics conference last week (“The Future of Physics” at the Kavli Institute for Theoretical Physics) and the talks were made available on-line. An undercurrent of some of the big-picture talks by luminaries such as Steven Weinberg, Edward Witten, and David Gross was a sense of disappointment over the slowing of progress in theoretical physics.
The remarkable achievements of 20th century physics were relativity theory and quantum mechanics. Relativity theory explained gravity in terms of a geometry of space-time. Out of quantum mechanics grew quantum field theory which was the framework for the standard model of particle physics which made great strides in explaining nature at the microscopic level.
A natural next step was a theory which combined these successes and explained gravity in a way consistent with quantum physics. However, this remains unaccomplished. String theory and its variant mathematical models has been the largest focus of this work (the name comes from the characterization of fundamental constituents as one-dimensional strings, although the theory as moved well beyond that). As far as I have understood (not very far when you are ignorant of the mathematics), string theory was a generalization of quantum field theory which appears to naturally incorporate gravity. Problems include the lack of experimental evidence (hard to come by given the tiny scales of nature where the theory would matter) and the fact that there are a gazillion versions of the theory. A conceptual problem has been the fact that, like quantum theory, string theory presupposed a backdrop of space and time. General relativity incorporated space-time as a dynamical field within the theory itself. There have been more recent efforts which show that space could be said to be an emergent phenomenon within a string theory, but time is still not explained (A theory of dynamics by definition takes place in time, so how could it explain time?).
Loop quantum gravity (LQG) is an alternative program (one might hope it is complementary to string theory, as Abhay Ashtekar said in his brief talk). It goes more directly at the problem of quantizing general relativity in a way which is independent of background space-time, but at the cost of losing as an intrinsic ingredient the successful particle physics theory we already have. Interestingly, the concept of time is also problematic in LQG (on the question of time see my previous post).
In Witten’s talk, he mentioned that string theory doesn’t have the kind of conceptual “core idea” guiding researchers of the sort the principle of equivalence was for Einstein. Some great new insight is needed to jumpstart the future of physics. I venture to suggest that such an insight may come in addressing the still mysterious question of how time enters into physics.