Scott Aaronson has an interesting new essay online about whether there is a notion of ‘free will’ compatible with modern physics. The argument has a couple of key elements:

• change the problem: Aaronson actually analyzes the question of whether an individual’s behavior is predictable in principle, given sufficent information about the individuals brain.
• Aaronson distinguishes between “garden-variety” probablistic uncertainty in future outcomes, versus fundamental inability to induce a meaningful distribution over possible future outcomes (what he calls Knightian uncertainty).
• free-bits: Aaronson’s term for influences (e.g. photons) from the early universe that have avoided any interactions that would render their state knowable even without direct observation, and are therefore a source of true uncertainty in predicting the future course of the universe (at least at the quantum scale).

Aaronson seems to conclude that free-bits offer likely the only route by which human behavior is in principle unpredictable. (Of course in practice there are many technical difficulties, but thats not the issue). Entertaining stuff!