OK, so here we go with an answer to the first of the Five Questions in Series 5 that I posed about a week ago.

Question 1: What is time?
Well from a technical, scientific, point of view if I knew the answer I would have a Nobel Prize. Yes, this is one of the most intransigent, but most important, questions in the whole of physics. The answer is critically inter-related with our understanding of the whole of cosmology and the structure of the universe. If we knew exactly what time was, and why it appears to move only in one direction, we would likely have a theory of everything. Yes, scientifically it is that important. But despite the best efforts of the best brains in theoretical physics, we basically have very few clues.
At a more prosaic level there are all sorts of constructs around what time is. One of the best that I can come up with is that it is an artificial construct for distinguishing past, present and future in a vaguely, but also artificially, quantum way.
At an everyday level we divide time into years, months, weeks, days, hours, minutes and seconds. All are essentially artificial, although years, months and days do have a more or less tight relationship with astronomical events. But weeks, hours, minutes and seconds are essentially arbitrary and historic divisions of time. Why are there 24 hours in a day, and not 10, 20, 25 or 100? And one can ask a similar question of weeks, minutes and seconds.
And essentially, non-scientifically, we treat these divisions of time in a quantum-ish sort of way. Either a second has passed or it hasn’t. Although we know that these time divisions are not really quantised at all. If they were we would never be able to time the 100m dash in the way we do.
That doesn’t mean that time cannot be quantised. Physicists think it may well be quantised, but at a much finer level that we can currently measure, ie. with quanta smaller than 10^-15 seconds.
But time is even stranger than that. Scientists tell us that time ticks along at an absolutely constant rate, which is what our clocks tell us. But maybe this is only because scientists have defined it that way? And so our measuring systems reflect that.
At a very personal level we know that time does not progress linearly. Some mornings we get up, shower, dress, breakfast and are ready to leave for work at 7.30. Other days we do exactly the same only to find that it’s 8.00 and we’re half an hour late. We’ve all experienced this. We know intuitively that time does not pass at a constant rate.
How can this be? We don’t know. Some think this is a function of the way our brains work. But is it not at least possible — though scientists will deny this — that time really is non-linear and somehow these imperfections are embedded deep in the underlying structure of the cosmos? Well who knows? But quantum effects have found equally strange and unexpected effects.
So then, what is time? Well only God (who or whatever he or she may or may not be) knows. And she’s not telling us!
I’ll leave you with a couple of thoughts from greater luminaries than me:

Some people are old at 18 and some are young at 90 … time is a concept that humans created.
[Yoko Ono]

To us, the moment 8:17 AM means something — something very important, if it happens to be the starting time of our daily train. To our ancestors, such an odd eccentric instant was without significance — did not even exist. In inventing the locomotive, Watt and Stevenson were part inventors of time.
[Aldous Huxley]