- #71
PeterDonis
Mentor
- 47,601
- 23,877
The idealized GR models we are considering do not have to have observers with functioning memories; they just have to have worldlines with proper time that increases in a consistent direction, either towards the singularity (black hole) or away from it (white hole).stevendaryl said:to have an observer with functioning memories, you have to have an entropy difference between the two ends of time.
In the idealized GR models we are considering, the entropy is zero everywhere, since the microstate of the universe at every event is exactly known.stevendaryl said:The gross structure of spacetime doesn't determine which direction is low entropy.
You can add entropy and observers with functioning memories to the model, but then we are talking about a more complicated model, where you have to include some more detailed description of the matter that allows entropy to increase in one direction or the other based on some kind of coarse-graining of the matter's state space. Then the question would be whether such a more complicated matter model can be consistently added to both of the idealized models (black hole and white hole) or only to one (presumably the black hole). I don't see why it wouldn't be possible for both (since we already expect that we can construct consistent models in which entropy increases when things fall into black holes, and since the white hole exploding into a star model, at least in its matter portion, is just like an expanding FRW universe, and we already expect that we can also construct consistent models in which entropy increases in the direction of time in which the universe is expanding). But that still doesn't make them the same model; they are still physically different.