the trouble with physics by lee smolin

Lee Smolin works just down the road from me. Well that's a slight exaggeration - I live and work in Toronto, while Lee Smolin works out of the Perimeter Institute in Waterloo, about 100 km west of Toronto. The PI was founded in 1999 with help from the Canadian company Research in Motion (RIM), also based in Waterloo and best known for the BlackBerry.

Smolin is one of the originators of loop quantum theory, which is basically a rival to string theory. Reading The Trouble with Physics, published in 2006, I kept wishing for more about loop quantum theory, but I'll just have to get myself a copy of Three Roads to Quantum Gravity, and hope it's not too out of date (it was published in 2001).

Early in The Trouble with Physics, Smolin lays out the five big remaining problems in physics:

1. Quantum + gravity unification
2. Quantum foundations
3. Particle + force unification
4. Freely tunable parameters (in particle physics)
5. Cosmological discrepancies (dark matter and dark energy)

If I understand correctly, the reason Smolin favours more "foundational" methods - for example, to unify the quantum world with Einstein's gravity world - has to do with a preference for "background-independent" methods. Whereas Newton's laws play out on a fixed background of Cartesian (or Euclidian) space and an equably flowing arrow of time, Einstein's general relativity is well-known to define the very space and time that the events of the universe unfold across. So theorists who continue in the spirit of Einstein are like artists who do not depend on a predefined canvas but create everything from scratch. The ultimate appears to be a theory in which not only particles and forces can be seen as emergent properties of a vacuum, but space and time can themselves emerge - possibly in some quantized state. Needless to say, theories such as loop quantum gravity are background-independent whereas quantum mechanics, the standard model of particle physics, and (super)string theories are background-dependent. Now we all know that superstring theories require extra dimensions that are supposedly curled up so we can't see them. But I guess this is like an artist having a technique that requires a canvas with various quirky features, rather than a different artist whose canvas emerges as a natural part of his or her art.

A seemingly bigger issue is the fact that there are a huge number of string theories that can apparently be devised by changing any of a large number of free parameters. The "super" in superstring means proposing that fermions and bosons have supersymmetric partners, so a selectron is a boson partner to an electron, while a gluino is a fermion partner to a gluon. None of these particles has been observed, but with all those free parameters it is easy to just propose that they are way too massive to have been created in any accelerators. Abstruse mathematical models can be proved to be consistent (although even this is pretty hard at this level) but they do not necessarily correspond to reality.

Smolin talks a lot towards the end of his book about the sociology of today's physics departments, with hiring practices that favour those who will work on the now-popular string theory approach. But I think the more fundamental issue is the one he alludes to earlier concerning the adoption of the anthropic principle. Since it has become hard to test these theories experimentally, the anthropic argument implies that a physicist should pick a model universe in which he/she could exist, and which can be rigged to not look obviously different from our world. But this is hard to do, and perhaps these physicists have become just as intrigued with worlds that are mathematically possible rather than truly anthropic. In this case, theoretical physics departments may have turned into specialized mathematics departments, and string theory is now preferred precisely because it is such a rich source of mathematical worlds, even if none of them even remotely corresponds to our world!