Why is there an arrow of time; that is, why is the future so much different from the past?
If conditions were constant or uniform, then most of physics is reversible, but that doesn’t appear to be the sort of reality we exist in.
It seems that this reality we find ourselves in had a beginning, a start point, when it was very small, and very hot.
Then it started to expand and cool.
It seems it might still be expanding and cooling.
It seems that our notion of time (such as it is, illusory as it is), has something fundamental to do with this aspect of fundamental change.
That aspect of expansion and cooling seems to have allowed matter to form from energy.
As the universe continued to cool, matter formed into atoms, then molecules – initially only hydrogen and helium with tiny traces of lithium.
Tiny fluctuations in distributions from early quantum effects seem to have led to gravity being able to form stars and galaxies.
The first generation of stars were massive things, that burned brightly with their nuclear fires then collapsed as they ran out of hydrogen, and exploded as the gravitational thump of collapse triggered a star sized A-Bomb. Most of the matter we recognise seems to have formed in such stars, and got distributed by their explosive deaths.
And it is this ongoing process, of new things happening, at many different levels, that makes the future so different from the past.
The evolution of life on this earth seems to have been one such recursive series of things.
To get the first precursors of life on this planet was probably a chance mixing of poly aromatic hydrocarbons with ribo neucleic acids (RNAs) some 4 billion years ago, in an environment that by our standards could easily be described as hellish – no oxygen, methane ammonia atmosphere, volcanism everywhere, the moon only 1/16th of the distance away that it is now, the earth spinning far faster (a 6 hour day, 3 hours light, 3 dark), 300 ft tides every hour and a half anywhere near the coast (heating and cooling as they crossed lava fields).
From those early replicators, nothing at all like what we would recognise as life today, to us, seems to have taken a process of evolution by natural selection, folding back on itself about 20 times as new levels of cooperative strategies emerged from the random exploration of possibilities, creating so many levels of amazing complexity.
About half of those levels of complexity have occurred in just the last few thousand years of human social evolution, as our models of the world (which we largely inherit from our cultures) have continued to become more complex, and better, representations of the amazing complexity within which we seem to find ourselves.
It seems that as self aware entities, we are software beings, living in a software model of reality inside the brains of our bodies. The nature of that software model seems to be partly determined by our genetic inheritance, partly from the percepts and concepts implicit in whatever culture we happened to be born into, and partly from our life experience and the lessons we learn from it, and partly from a bit of pure random chance and dumb luck, and partly from choice.
So part of the change in our experience does seem to genuinely come from changes in the external reality that we exist in, and it seems most likely that the greatest change in our experience of being comes from changes internal to each of us in the model of reality that our brains create that we get to experience as reality.
So both of these aspects of change seem to be very important to our experience of time, and our place in it.
And there is, of course, the very real sense, that our only experience is of the eternal now, even if that now is a software model of something – it seems to be all the now we can have, in a very real sense. And in as much as we do inhabit a reality we share with other entities, we seem to have some sort of shadow of that reality in our models, at least to the degree that we do.
This seems to be what the arrow of time is about.
Small fractals of order that we seem to be, extracting information and energy from our surroundings.