It seems to me that 'the arrow of time' is all about the direction of causality.
Time seems to go in the direction it does, because events in one temporal direction (that we call the 'past') seemingly determine events in the other temporal direction (the 'future').
I'm unclear what connection gravity has to the direction of causation. Perhaps somebody might be able to produce an argument from general relativity for that.
I'm kind of unmoved by the thing that CC posted about gravity and complexity. For one thing, it seems to be about time-metrics, rods and clocks, which doesn't seem to address the question of why time has a direction in the first place. Another problem I see is that a hot featureless soup of subatomic particles right after the big bang might arguably be more complex than our universe, where matter is colder and clumped together. If complexity is measured by the amount of information necessary to describe something, then a state of the universe in which we would have to individually describe every subatomic particle would seem to be extraordinarily complex.
And given Barbour's assumption that the Big Bang is the least interesting point in the universe, wouldn't the universe's subsequent evolution be a massive decrease in the universe's entropy? I thought that is uncool in thermodynamics. (Not sure why.)
(Apr 2, 2015 12:18 PM)Mr Doodlebug Wrote: What would happen at the big bang if time went the other way?
Are you thinking about the Big Bang being a Big Crunch, as the universe contracts into a point? Or are you thinking about the Big Bang exploding a universe in the other temporal direction, into the past?
As I suggested up above, I think that the direction of time is the direction of causality. And it seems to me that causality propagates away from the Big Bang. So the Big Bang might seemingly always be in the past, any way one looks at it. If we hypothesize about what happened before the big bang, we might find a second universe expanding in that direction too, except to that universe's inhabitants, their universe's origin will be in their past, just as ours is to us.
Sometimes I like to put on my sci-fi metaphysician's cap and speculate that the ground state of being itself (the super-universe in which ours is embedded) is a quantum-ish kind of chaos where causality propagates equally in both directions. There isn't any past or future in that realm, paradoxical time-loops prevail on all scales where the 'past' determines the 'future' as the 'future' is simultaneously determining the 'past' that determined it. There isn't any concrete and coherent reality and everything exists as a hugely complicated quantum-style superimposed probability state.
Then something happens, perhaps just some chance coming together of possibilities, that causes a cosmic explosion that we call the Big Bang. All of the causal chains in the blast's vicinity move away from it, so that events suddenly seem to propagate in a past -> future direction. Time as we know it comes into being. I imagine an expanding shock-wave that we call the 'present', trailed in the pastward direction by the causal chains that determined it, spreading into the surrounding quantum chaos, into the superimposed possibility states, triggering the universe's 'collapsing' into the coherent reality that we see around us.
Putting us on the surface of a reality bubble expanding into macroscopic chaos. (Maybe that's why the future seems to consist of unrealized possibilities.)