The mysterious nature of time

[3DMonkey]

Physics in the Real Universe: Time and Spacetime

My favorite parts:

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However the constraints on what future can emerge at a given here-now are not pointwise
constraints but (in relation to any local coordinates) constraints involving spatial
derivatives, or, roughly speaking, neighbouring points. So if evolution takes place
pointwise, it still involves a degree of spatial coordination between neighbouring
points, even though the neighbouring point might not “yet exist” relative to a different
here-now until it lies in the past.



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A key result then is that no unique choice for these world lines needs to be made in
the standard GR situation with simple equations of state; the ADM theory says we
locally get same result for the evolving spacetime, whatever world lines are chosen.
You can choose any time lines you like to show how things will have evolved at
different places (that is, on different observer’s world lines) at different times (that is,
at various proper times along those world lines). But this view has no foundationally
preferred status: you could have chosen different world lines, corresponding to
different shift vectors, and a different relation between times on the world lines,
corresponding to different choices of the laps function; the resulting four dimensional
spacetime is the same. In any specific situation, some of those descriptions will be
more natural and easier to use and understand than others; but this is just a
convenience, and any other surfaces and world lines could have been chosen.

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5.5 Issues of Ontology
The hidden issue underlying all this discussion is the question of the ontological
nature of spacetime: does spacetime indeed exist as a real physical entity, or is it just
a convenient way of describing relationships between physical objects, which in the
end are all that really exist at a fundamental level? Is it absolute or relational? Could
it after all be an emergent property of interacting fields and forces (Laughlin 2005), or
from deeper quantum or pre-quantum structure (Ashtekar 2005: Chapters 11-17)?

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For the record, my vote is for "just a convenient way of describing relationships", "relational"