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Hi there Max, I immensely enjoyed your Mathematical Universe! It has been giving me food for thought for quite some time now. Something that keeps popping up in my mind (and the minds of my doppelgangers in the Level I Multiverse):

In the chapter "Are We Living in a Simulation?", on page 349, in the footnote, you write:

*Indeed, as pointed out by Ken Wharton in his paper "The Universe Is Not a Computer," at http://arxiv.org/pdf/1211.7081.pdf, our laws of physics may be such that the past doesn't uniquely determine the future, so the idea that our Universe can be simulated even in principle is a hypothesis that shouldn't be taken for granted.

My questions/thoughts, then, are:

If the world is considered a 4D object, what is causality? Can I write up the analogy and compare the slope of a mountain to the speed of a falling raindrop? The slope can be estimated around a known point, or you can find some geological rule to help you, but it certainly can't be predicted/simulated all the way from valley to peak!

Is the change of an object through time analogous to the change of an object through space?

Suggestions for further reading would also be very much appreciated, hilsen fra Norge:)