I now have a 3500-word post on that question up at NOVA’s “Nature of Reality” blog. If you’ve been reading Shtetl-Optimized religiously for the past decade (why?), there won’t be much new to you there, but if not, well, I hope you like it! Comments are welcome, either here or there. Thanks so much to Kate Becker at NOVA for commissioning this piece, and for her help editing it.