As I’ve written about before (here), theoretical physics has seemed stuck in recent years. Astronomical observations continue to uncover new features of the universe (like dark energy), but these mostly add new mysteries to the list which cannot be explained by current theories. The jury is also definitely still out on older ideas about the early universe (such as the inflationary scenario). Physicists await with great expectations the 2007 debut of the Large Hadron Collider, which offers the possibility of revealing new phenomena at higher energies which may assist theorists. In the meantime, there is plenty of time for physicists and philosophers of science to step back and consider conceptual issues which underlie physical theories. Here are a couple of examples of this I came across recently.
Lee Smolin has a new essay (hat tip to Peter Woit’s blog): “The case for background independence.” The context of the essay is the ongoing difficulty in constructing a theory of quantum gravity. Smolin thinks what has ultimately stymied the most popular quantum gravity research program--string/M theory-- is its use of a space-time backdrop as a “given”, rather than having space-time emerge from relations of variables within the theory. The essay starts with a historical sketch of the absolute and relational views of space-time, then summarizes how this is playing out in the different quantum gravity research programs (Smolin is one of the founders of the approach called loop quantum gravity, which attempts to preserve the background independent flavor of general relativity). Lots of interesting ideas in the essay. Also, for a debate on string theory see this post and comment thread on the excellent new team physics blog, Cosmic Variance.
As an aside, the debate between absolute and relational theories of space-time was most clearly personified in the personal rivalry between Newton and Leibniz. A fun coincidence is that I’m now working my way through the second installment (The Confusion) of Neal Stephenson’s Baroque Cycle, which features this rivalry as a minor subplot. The Confusion has a nice scene where Leibniz explains the relational view to a colleague.
On a related topic, the interpretation of quantum mechanics, I enjoyed this paper revisiting the philosophical conflict between Bohr and Einstein by N.P. Landsman (from PhilSci archive, via Brian Weatherson’s invaluable papers blog).
Einstein was famously frustrated by quantum mechanics, despite being one of the founders of the theory. Landsman summarizes his view as one where physical thought and physical laws require (classical) separability and locality (which QM doesn’t provide). Bohr accepted that while classical (objective) thinking was indeed necessary for science, the classical world in which physicists work incontrovertibly sits next to a quantum realm which has a nature which will not reduced to the classical. Landsman importantly also brings out the interesting “flip side” to Bohr’s thought, which is that it is equally true that a full quantum description of the world which leaves out the classical is impossible (many physicists and philosophers seem to miss this point). The implication is that there are necessarily two modes of reality: the quantum realm of spread-out potentials and the classical world of interactions/measurements.