There are indeed other approaches but these are the two main attempts so far, and the history of their developments allows a fairly simple introduction to the area without too much technical detail. In any case, many of them are similar in approach albeit with different wave equations (Dirac, Yang-Mills+gravity, DeWitt-Wheeler) and don’t as yet add much to the general picture I was describing for Ken.
Yes, I am familiar with Wolfram’s idea- I have to say I am not convinced at the moment. I have no issues with spacetime geometry being an emergent phenomenon, nor indeed time being an emergent phenomenon (which I’m sure it is and I wouldn’t be surprised if it has more than one dimension), but as yet I haven’t seen anything which really deals with the problems discussed here or other problems like the violation of unitarity.
cheers,
-Paul