CTCs: I think part of the reason I want a "causal sequence/DAG" is that closed timelike curves lead you to think "why that curve and not some other?". The equations can work, but with CTCs you potentially - or possibly even necessarily - have a bad case of the creation-of-information paradox.
I think my idea of a "causal sequence" is overloaded. One use for it is in the "entropy's arrow" thought experiment; imagine a low-entropy state at some point in time, and higher-entropy states either side. You could make a case for the causal sequence to be radiating outward from that low entropy point in both time directions. However ISTR that at least some ideas of the Big Bang[1] have the idea of a point in curved spacetime where all directions are into the future, a sort of North Pole of time... but then you could use the "causal sequence" gambit to ask what was causally prior to that.
As I say, it's not an idea I'm committed to, it's one I need to... stop thinking about and start discussing.
[1] Someone suggested a better term would be Everywhere stretch - warning! YouTube!
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Date: 2013-12-10 07:37 pm (UTC)I think my idea of a "causal sequence" is overloaded. One use for it is in the "entropy's arrow" thought experiment; imagine a low-entropy state at some point in time, and higher-entropy states either side. You could make a case for the causal sequence to be radiating outward from that low entropy point in both time directions. However ISTR that at least some ideas of the Big Bang[1] have the idea of a point in curved spacetime where all directions are into the future, a sort of North Pole of time... but then you could use the "causal sequence" gambit to ask what was causally prior to that.
As I say, it's not an idea I'm committed to, it's one I need to... stop thinking about and start discussing.
[1] Someone suggested a better term would be Everywhere stretch - warning! YouTube!